Crystallization and Breakdown of MetasomaticPhases in Graphite-bearing Peridotite Xenolithsfrom Marsabit (Kenya)

BENJAMIN KAESER1 ANGELIKA KALT1 AND THOMAS PETTKE2

1INSTITUT DE GEcurren OLOGIE ET DrsquoHYDROGEcurren OLOGIE UNIVERSITEcurren DE NEUCHAldquo TEL RUE EMILE-ARGAND 11 CH-2009

NEUCHAldquo TEL SWITZERLAND2INSTITUTE OF GEOLOGICAL SCIENCES UNIVERSITY OF BERN BALTZERSTRASSE 1 CH-3012 BERN SWITZERLAND

Mantle-derived xenoliths from the Marsabit shield volcano (eastern

flank of the Kenya rift) include porphyroclastic spinel peridotites

characterized by variable styles of metasomatism The petrography

of the xenoliths indicates a transition from primary clinopyroxene-

bearing cryptically metasomatized harzburgite (light rare earth

element U andTh enrichment in clinopyroxene) to modally meta-

somatized clinopyroxene-free harzburgite and dunite The metaso-

matic phases include amphibole (low-Ti Mg-katophorite) Na-rich

phlogopite apatite graphite and metasomatic low-Al orthopyroxene

Transitional samples show that metasomatism led to replacement of

clinopyroxene by amphibole In all modally metasomatized xenoliths

melt pockets (silicate glass containing silicate and oxide micro-

phenocrysts carbonates and empty vugs) occur in close textural rela-

tionship with the earlier metasomatic phasesThe petrography major

and trace element data together with constraints from thermobaro-

metry and fO2 calculations indicate that the cryptic and modal

metasomatism are the result of a single event of interaction between

peridotite and an orthopyroxene-saturated volatile-rich silicate melt

The unusual style of metasomatism (composition of amphibole

presence of graphite formation of orthopyroxene) reflects low P ^Tconditions (850^10008C at515 GPa) in the wall-rocks during

impregnation and locally low oxygen fugacities The latter allowed

the precipitation of graphite from CO2The inferred melt was possi-

bly derived from alkaline basic melts by melt^rock reaction during

the development of the Tertiary^Quaternary Kenya rift Glass-

bearing melt pockets formed at the expense of the early phases

mainly through incongruent melting of amphibole and orthopyroxene

triggered by infiltration of a CO2-rich fluid and heating related to

the magmatic activity that ultimately sampled and transported the

xenoliths to the surface

KEY WORDS graphite peridotite xenoliths Kenya Rift modal

metasomatism silicate glass

I NTRODUCTIONDuring recent decades numerous studies have describeddifferent styles of metasomatism in the Earthrsquos uppermantle These studies in combination with experimentalwork show that the agents responsible for mantle metaso-matism include mafic silicate melts carbonate melts orC^O^H-rich fluids (eg reviews by Menzies ampHawkesworth 1987 Luth 2003) Integration of texturalmineralogical and geochemical data from mantle sampleshighlights the complexity of mantle metasomatism Thedata show that metasomatic features are controlled by(1) the composition of the initial metasomatizing agent(2) the pre-metasomatic composition and heterogeneity ofthe mantle rock (3) the evolution of the physico-chemicalparameters during metasomatism (eg porosity fO2 PT)(4) element fractionation processes during melt (fluid)^rock interaction (5) mineral reactions Several recentstudies have demonstrated that compositional differ-ences between metasomatic products in a single suiteof mantle rocks do not necessarily imply different metaso-matizing events (eg Bedini et al 1997 Ionov et al 20022006 Bodinier et al 2004 Rivalenti et al 2004)Processes such as chromatographic fractionation andmeltfluid^rock reaction can generate compositionallydistinct products starting from one initial melt or fluid

Corresponding author Present address School of GeoSciences University of Edinburgh West Mains Road Edinburgh EH9 3JW UK

Telephone 0044 (0) 1316507339 Fax 0044 (0) 131668 3184E-mail v1bkaserstaffmailedacuk

Published in Journal of Petrology 48 issue 9 1725-1760 2007which should be used for any reference to this work

1

(eg Navon amp Stolper1987 Harte et al1993 Godard et al1995 Vernie res et al 1997)We present a study of metasomatized graphite-bearing

xenoliths from Marsabit (northern Kenya) to investigatemelt^rock reaction and fractionation processes The pro-ducts of metasomatism are texturally and compositionallyvariable and unusual An early Na^Si-rich assemblagecomprises volatile-bearing phases (amphibole phlogopiteapatite) and metasomatic orthopyroxene as well as gra-phite Associated with these phases is a second metaso-matic assemblage of carbonate-bearing silicate glasspatches (lsquomelt pocketsrsquo) similar to those describedfrom many other xenolith suites (eg Frey amp Green1974 Dautria et al 1992 Ionov et al 1993 1994Zinngrebe amp Foley 1995 Chazot et al 1996a Draper ampGreen 1997 Yaxley et al 1997 Coltorti et al 2000Laurora et al 2001 Bali et al 2002 Ban et al 2005)Our approach is based on detailed investigation oftrace element variation in minerals and integration oftextural constraints It emphasizes that the lsquoexoticrsquonature of the metasomatic products reflects a particularpre-existing physical and chemical environmentccedilielow ambient temperatures (and probably pressures) andrelatively low oxygen fugacitiesccedilrather than an exoticinitial metasomatic agent We will show that meta-somatism in this case does not necessitate multiplemetasomatic events of different nature but simply reflectsthe product of melt^rock reaction possibly initiatedby common continental rift-related basanites Wefurther present mass-balance calculations which showthat the melt pockets were formed by in situ partialmelting of the early assemblage (mainly amphiboleand orthopyroxene) triggered by infiltration of aCO2-rich fluid

SAMPLE CONTEXTThe investigated mantle xenoliths were collected fromscoriae of basanitic to alkali-basaltic Quaternary cindercones of the Marsabit shield volcano (Volker 1990Henjes-Kunst amp Altherr 1992) Magmatic activity inMarsabit started in the late Miocene whereas rocks fromthe shield volcano yield Pleistocene to Quaternary ages(eg Key et al 1987) Magmatism is thus clearly relatedto the development of the Kenya rift (ie the easternbranch of the East African Rift system EARS seeFig 1) The Marsabit shield volcano is however locatedeastward of the main axis of the EARS within the AnzaGraben the eastward continuation of the Turkana depres-sion The evolution of the lithosphere in this regionincludes multiple accretion during Pan-African orogenesis(formation of the Pan-African Mozambique beltfrom 720 to 550 Ma eg Meert 2003) followed by severalphases of continental rifting including formation of theMesozoic^Paleogene Anza Graben later cross-cut by the

Tertiary^Quaternary EARS [see Fig 1 and Morley (1999)for a review] A more detailed description of thegeological and geodynamic context has been given byKaeser et al (2006)In our previous study (Kaeser et al 2006) we used the

Marsabit peridotite xenoliths to constrain the evolution ofthe lithospheric mantle beneath Marsabit The peridotiteswere subdivided into four groups including the porphyro-clastic spinel harzburgites and dunites (Group III)investigated here (Table 1) For detailed descriptions ofthe textures and mineral chemistry of the primaryassemblages of all four groups the reader is referred toKaeser et al (2006)Thermobarometry calculations revealed that all

the porphyroclastic rocks experienced decompressionand cooling from high pressures and temperatures

Fig 1 Simplified geological map showing the distribution of volcanicrocks associated with the Kenyan sector of the East African riftsystem compiled on the basis of the maps of King (1970) andBaker et al (1971) Stars indicate other off-craton xenolith localities(Henjes-Kunst amp Altherr 1992 Bedini et al 1997) The approximateposition of the Anza Graben is taken from Henjes-Kunst ampAltherr (1992)

2

(mainly preserved in formerly garnet-bearing Group Ilherzolites see Fig 2) to low mantle P^T conditions of59008C at 515GPa (Kaeser et al 2006) The presentstudy focuses on metasomatic features in the low-tempera-ture Group III xenoliths Textural and compositionalcharacteristics suggest a genetic relationship with lateEARS-related heating and cryptic metasomatismleading to U^Th^Nb and light rare earth element(LREE) enrichment in clinopyroxene in the recrystallizedGroup II lherzolites The latter were interpreted toreflect metasomatism by percolation of alkaline maficmelts possibly parental to the Quaternary magmasof the Marsabit volcano (Kaeser et al 2006) post-dating rift-related decompression and deformationIn contrast metasomatic phases in the porphyroclasticGroup I lherzolites (ie Ti-pargasite and rarephlogopite Kaeser et al 2006) are texturally andcompositionally distinct suggesting a different eventThe trace element signatures of Ti-pargasite suggestthat this metasomatic assemblage formed earlierprior to rift-related decompression and deformation(Kaeser et al 2006)

ANALYT ICAL METHODSModal compositions (Table 1) were determined fromdigitized images of thin sections and rock slice scans(for details see Kaeser et al 2006) Minerals were analysedfor major elements using a CAMECA SX 50 electronmicroprobe equipped with four wavelength-dispersivespectrometers (Mineralogisch-Petrographisches InstitutUniversitalaquo t Bern) and a CAMECA SX 51 electron micro-probe with five wavelength-dispersive spectrometers(Mineralogisches Institut Universitalaquo t Heidelberg)Pyroxene olivine and spinel were analysed using routineoperating conditions (focused beam accelerating voltageof 15 kV beam current of 20 nA and counting times of20 s for most elements) Silicate glass was analysed witha larger beam size (10^15 mm) and a lower beam current(15 nA) to avoid Na loss during analysis Natural andsynthetic silicates and oxides were used as standardsCarbonates were analysed with a beam current of 10 nAand a beam size of 10 mm using natural carbonatesand sulfates as standards The concentrations of CO2

(in carbonates) and Fe3thorn (in spinel) were calculated

Table 1 Textures and mineralogical composition of the investigated peridotite xenoliths

Sample no Type Lithology Pre-metasomatic and early metasomatic phases (vol) Melt

pockets

Modal composition of melt pockets

Former

garnet1

Ol Opx Cpx Spl Am Phl Graphite

apatite

Glass Dim Olm Chrm Mg-Cc Vugs

Ke 19651 III-a porphyroclastic

spl harzburgite

15 708 229 39 09 mdash mdash trmdash mdash mdash mdash mdash mdash mdash mdash

Ke 195915 III-b porphyroclastic

spl harzburgite

50 630 260 52 07 02 mdash mdashtr 540 218 234 277 49 812 76

Ke 196515 III-b porphyroclastic

spl harzburgite

mdash 719 177 68 04 02 01 010tr 30 214 340 237 28 160 22

Ke 785 III-c porphyroclastic

spl harzburgite

mdash 640 1873 mdash 064 11 09 023mdash 144 219 270 218 57 67 70

Ke 19653 III-c porphyroclastic

spl dunite

mdash 801 323 mdash 074 mdash tr mdashtr 160 227 213 295 66 1285 74

Ke 196525 III-c porphyroclastic

amphibole

dunite

mdash 536 mdash mdash mdash 120 tr 025mdash 342 nd nd nd nd nd nd

Ke 195927 III-c porphyroclastic

dunite

mdash 885 tr3 mdash mdash mdash mdash mdash 115 nd nd nd nd nd nd

Ke 19706 III-d recrystallized spl

harzburgite

mdash 872 953 276 064 mdash mdash mdash mdash mdash mdash mdash mdash mdash mdash

Ol olivine Opx orthopyroxene Cpx clinopyroxene Am amphibole Phl phlogopite Dim Olm Chrm diopside olivineand chromite micro-phenocrysts in melt pockets Mg-Cc Mg-calcite 1Splndashopxndashcpx symplectites 2Dolomite instead ofcalcite 3Metasomatic low-Al orthopyroxene (Opxm)

4Compositionally identical to Chrm5Highly variable (24ndash197 vol)

in individual melt pockets 6Compositionally similar to glass-related Dim mdash not detected tr traces nd not determined

3

based on stoichiometry The raw data of all microprobeanalyses were corrected using routine PAP softwareTrace element contents in minerals were analysed in-situ

on polished thin sections (40^50 mm thick) using a laserablation (LA) instrument equipped with a 193 nm ArFexcimer laser (Lambda Physik Germany) coupled to anELAN 6100 (Perkin Elmer Canada) quadrupole induc-tively coupled plasma mass spectrometer (ICPMS) atthe Institut fulaquo r Isotopengeologie und MineralischeRohstoffe ETH Zulaquo rich [see Pettke et al (2004) and refer-ences therein for the instrumental setup capabilities andoperating conditions] Raw data were reduced using theLAMTRACE program Laser pit diameters were between14 and 110 mm depending on grain size and the absenceor presence of mineral fluid or melt inclusions andof exsolution lamellae

PETROGRAPHYGroup III harzburgites and dunites display two texturallydifferent types of modal metasomatic modificationThe first type of metasomatism formed clusters

and veinlets of volatile-bearing minerals (amphibolephlogopite graphite apatite Fig 3a^e) associatedwith metasomatic orthopyroxene The second type ischaracterized by the occurrence of lsquomelt pocketsrsquo(eg Frey amp Green 1974 Ionov et al 1994) patchesand networks of silicate glass associated with micro-phenocrysts of clinopyroxene olivine and chromite(Fig 4a^f) Glass further contains globules of carbonatesand vugs (Figs 4a^e and 5a b) Melt pockets form at leastpartly at the expense of volatile-bearing mineralsTherefore the volatile-bearing assemblage is referred toin the following as lsquoearly metasomaticrsquo (including metaso-matic orthopyroxene referred to as Opxm) The later glass-bearing assemblage is termed lsquolate metasomaticrsquo (Na-richCr diopside Dim olivine Olm chromite Chrm) The pre-metasomatic peridotitic phases are termed lsquoprimaryrsquo(olivine ol-I clinopyroxene cpx-I orthopyroxene opx-Ispinel spl-I)Based on petrographic investigations we subdivided

Group III peridotites into four types (Type III-a to -d)Textural and geochemical features show that the transitionfrom Type III-a to Type III-c (thornd) mainly correspondsto the transformation of modally non-metasomatizedspinel harzburgite to pervasively overprinted amphiboleharzburgite^dunite Twenty out of 27 Group III samplesinvestigated petrographically show evidence for modalmetasomatism (ie they belong toType III-b or -c)

Type III-aType III-a is classified as porphyroclastic cpx-I-bearingspl harzburgite devoid of volatile-bearing phases butshowing geochemical evidence of cryptic metasomatism(Kaeser et al 2006 and this study) This rock typewas originally garnet-bearing and was subjected todecompression as is indicated by the presence of rarespl^opx^cpx symplectites (Table 1 and Kaeser et al 2006)

Type III-bXenoliths of Type III-b are cpx-I-bearing spinel harzbur-gites however they contain metasomatic volatile-bearingphases (Table 1) One xenolith (Ke 196515) contains vein-lets of pale green amphibole phlogopite graphite apatite(Figs 3e and 4b) Graphite occurs as macroscopic flakes(up to 500 mm long Fig 3e) similar to those describedfrom cratonic kimberlite-hosted peridotite xenoliths(Pearson et al 1994) Cpx-I in the vicinity of veinlets isstrongly resorbed and develops patches or lamellae ofTi-poor amphibole indicating metasomatic replacement(Fig 4b) In all Type III-b samples amphibole and clino-pyroxene are partially replaced by the late glass-bearingassemblage (eg Fig 4a) Cpx-I is overgrown by a second-ary green Na-rich Cr diopside (Dim) occasionally contain-ing fluid glass andor carbonate inclusions (Fig 2c)Dim further occurs as micro-phenocrysts in melt pocketssometimes enclosing (resorbed) orthopyroxene

Fig 2 Pressure^temperature evolution recorded by peridotite xeno-liths from Marsabit (light grey fields summarized from Kaeser et al2006) together with reactions relevant for this study plagioclase^spinel transition in the CFMAS system (Gasparik 1987) appearanceof garnet (grt) in model lherzolite (Klemme amp OrsquoNeill 2000) maxi-mum stability of Ti-pargasite (parg) in model lherzolite (Niida ampGreen 1999) appearance of chlorite (chl) in model lherzolite underfluid-saturated conditions (Fumagalli amp Poli 2005) solidus for primi-tive mantle under water-saturated conditions (perthornH2OGrove et al 2006) solidus of natural carbonated lherzolite from Leeamp Wyllie (2000) subsolidus decarbonation (opxthorndolfrac14olthorn cpxthornC)from Woermann amp Rosenhauer (1985) at fO2 below the carbon^carbon oxide (CCO) buffer The bold arrow schematically indicatesthe infiltrating and cooling metasomatizing Si-rich melt at supersoli-dus and derivative fluid at subsolidus conditions (See text for furtherexplanation)

4

Type III-cType III-c samples are harzburgites and dunites devoid ofcpx-I Samples transitional betweenType III-b and -c stillcontain relics of cpx-I Metasomatic products inType III-cxenoliths generally occur as clusters of amphibole

phlogopite graphite and rare apatite The primaryassemblage is dominated by ol-I GreenTi-poor amphiboleand phlogopite typically form aggregates around relic ske-letal spl-I (Fig 3c and d) Dunite xenolith Ke 196525 isunusual as it contains centimetre-scale clusters of large

Fig 3 Photomicrographs showing the textural characteristics of metasomatism in the porphyroclastic spinel harzburgites and dunites fromMarsabit (a) Twinned euhedral Mg-katophorite inType III-c amphibole duniteThe irregular grain boundaries with glassthornmicrolite-bearingmelt pockets should be noted (sample Ke 196525 crossed polars) (b) metasomatic low-Al orthopyroxene (Opxm) in aType III-c spinel harz-burgite with euhedral phlogopite inclusions (early metasomatic assemblage) Irregular concave grain boundaries indicating later resorption byolivine should be noted (sample Ke 785 crossed polarizers) (c) Type III-c Mg-katophorite and phlogopite clustering around partly resorbedspinel (spl-I) and metasomatic orthopyroxene (Opxm) in contact with partly included graphite (sample Ke 785) (d) Details of Type III-c earlymetasomatic Mg-katophorite and phlogopite the latter containing graphite inclusions (sample Ke 785) (e) Type III-b early metasomaticassemblage including interstitial phlogopite graphite and clinopyroxene (cpx-I) becoming replaced by Mg-katophorite lamellae (sample Ke196515) (f) Type III-d spl harzburgite characterized by annealed textures and clusters of chromite enclosed by green Na-rich clinopyroxene(cpx sample Ke 19706)

5

euhedral twinned amphibole crystals (Fig 3a) withsmall phlogopite inclusions Graphite is associatedwith amphibole and phlogopite (sometimes found asinclusions in the latter Fig 3d) or orthopyroxene (Fig 3c)Straight contacts between amphibole phlogopite and ol-Iindicate textural equilibrium Phlogopite also occurs as

euhedral randomly dispersed flakes in orthopyroxene(Fig 3b) This together with compositional arguments(eg very low Al contents see below) suggests thatorthopyroxene is related to metasomatism (Opxm)Sample Ke 19653 also contains early metasomaticinterstitial apatite

Fig 4 Back-scattered electron images showing textural characteristics and relationships between early and late metasomatic assemblages(a) Relic amphibole (edenite^Mg-katophorite) in a typical melt pocket containing clinopyroxene (Dim) olivine (Olm) and chromite (Chrm)micro-phenocrysts as well as silicate glass with vugs (sometimes lined or filled with dolomitic carbonate (Type III-b xenolith Ke 195915)(b) Type III-b spinel harzburgite showing the textural characteristics of early (graphite apatite Mg-katophorite replacing cpx-I) and late(glass Mg-calcite Dim) metasomatic assemblages (sample Ke 196515) (c) Formation of glass and micro-phenocrysts (Olm and Dim) at theexpense of metasomatic orthopyroxene (Opxm) It should be noted also that graphite is overgrown by Mg-calcite when no longer in contactwith Opxm (Type III-b spinel harzburgite Ke 196515) (d) Phlogopite and residual Mg-katophorite in a melt pocket in aType III-c spl harzbur-gite The more regular grain boundaries of phlogopite compared with Mg-katophorite should be noted (the latter is further rimmed by irregu-lar kaersutitic amphibole sample Ke 785) (e) Silicate glass Mg-calcite and microlites of Na-rich Cr diopside (Dim) olivine (Olm) andchromite (Chrm) in a Type III-c spinel harzburgite The grey tone variation in Dim indicating zoning should be noted (sample Ke 19653)(f) Host-related basanitic glass (containing minute clinopyroxene quench crystals) in the peculiar Type III-c xenolith Ke 195927 TheTi-augitereaction rims on Na-rich Cr diopside (Dim) micro-phenocrysts and the resorbed grain boundaries of chromite (Chrm) should be noted

6

Late metasomatic assemblages are ubiquitous inType III-c xenoliths They form melt pockets intercon-nected by glass films along olivine grain boundariesEuhedral apple-green Na-rich Cr diopside micro-phenocrysts (Dim up to 03mm long) in individual meltpockets commonly have the same crystallographic orienta-tion (Fig 4a) suggesting growth at the expense of an ear-lier phase Back-scattered electron (BSE) imaging revealspatchy zoning (Figs 4e and 5a) Melt pockets in severalsamples enclose relic resorbed amphibole grains (Fig 4d)Phlogopite (eg in Ke 785) appears to be less resorbed(Fig 4d) Early amphibole is rimmed by a kaersutitic sel-vage (Fig 4d) indicating that the early amphibole is notin chemical equilibrium with the silicate glass When incontact with glass Opxm is also strongly resorbed Severalfeatures suggest incongruent breakdown by a process simi-lar to the reaction opxthornmeltolthorn cpxthornmodifiedmelt (eg Shaw et al 1998) thin glass films are presenton orthopyroxene cleavage planes late euhedralOlmthornDimChrm nucleated within orthopyroxene andOlm overgrows orthopyroxene (Fig 3b development ofstrongly concave opx grain boundaries)Glass inType III-b and -c samples contains considerable

amounts of rounded carbonates (eg Figs 4e and 5a)texturally similar to interstitial glass-related carbonateglobules from other xenolith suites (eg Ionov et al 19931996 Norman 1998 Lee et al 2000 Laurora et al 2001Bali et al 2002 Demecurren ny et al 2004) BSE imaging andqualitative energy-dispersive spectrometry mappinghighlight that carbonate globules consist of clusters ofsometimes euhedral Mg-calcite or dolomite crystals occa-sionally showing oscillatory zoning (Fig 5a and b) Insample Ke 196515 Mg-calcite is in contact with graphiteflakes (Fig 4c) indicating nucleation of carbonateon graphite Glass in several samples contains rounded

blebs of apatite (Fig 4b) as well as minor amountsof Fe^Ni sulphide globules Abundant vugs (eg Fig 4aand d) in all types of silicate glass are possibly remnantsof an exsolved vapour phaseSample Ke 195927 (Type III-c) is unusual in several

aspects It contains kaersutite phenocrysts hosted in theglass Further Dim micro-phenocrysts are overgrown byTi-augite rims (Fig 4f) and the glass is of an untypicaldark brown colour This xenolith records interaction witha melt similar to the host basanite during ascent to thesurface

Type III-dType III-d is a unique sample (Ke 19706) of metasomaticcoarse-grained spinel harzburgite (grain size 4mmFig 3f) Olivine and orthopyroxene are almost strain freeSpinel occurs as rounded black grains (chromite) sur-rounded by a cluster of apple-green Na-rich Cr diopside(Fig 3f) containing glass inclusions Volatile-bearingphases are absent Several orthopyroxene grains shownumerous melt inclusions forming continuous films ortrails along cleavage planesThese remarkable textural fea-tures can be explained by almost complete annealing of aformer Type III-c spinel harzburgite containing silicateglassthornDim micro-phenocrysts which subsequently recrys-tallized to form the clinopyroxene clusters around spineland orthopyroxene with melt inclusions

MINERAL COMPOSIT IONCompositional data (major and trace elements) for non-metasomatic primary phases from all peridotite types aswell as for Ti-pargasite and phlogopite from Group I sam-ples have been given by Kaeser et al (2006) A more com-plete dataset with respect to the metasomatic phaseassemblages discussed in this paper is available at httppetrologyoxfordjournalsorg

Fig 5 Oscillatory zoning of carbonate related to silicate glass in porphyroclastic spinel harzburgites from Marsabit (a) Zoned euhedral dolo-mite crystals (dark) rimmed by fine selvages of calcite (light) together with Na-rich Cr diopside (Dim) and chromite (Chrm sample Ke 195915)(b) Qualitative compositional map of carbonates in a melt pocket (sample Ke 19653) Lighter grey tones indicate higher Mg contentsThe euhedral shapes of the carbonate crystals should be noted The dashed white lines in (a) and (b) highlight the contact of carbonates withthe pristine silicate glass

7

(eg Navon amp Stolper1987 Harte et al1993 Godard et al1995 Vernie res et al 1997)We present a study of metasomatized graphite-bearing

xenoliths from Marsabit (northern Kenya) to investigatemelt^rock reaction and fractionation processes The pro-ducts of metasomatism are texturally and compositionallyvariable and unusual An early Na^Si-rich assemblagecomprises volatile-bearing phases (amphibole phlogopiteapatite) and metasomatic orthopyroxene as well as gra-phite Associated with these phases is a second metaso-matic assemblage of carbonate-bearing silicate glasspatches (lsquomelt pocketsrsquo) similar to those describedfrom many other xenolith suites (eg Frey amp Green1974 Dautria et al 1992 Ionov et al 1993 1994Zinngrebe amp Foley 1995 Chazot et al 1996a Draper ampGreen 1997 Yaxley et al 1997 Coltorti et al 2000Laurora et al 2001 Bali et al 2002 Ban et al 2005)Our approach is based on detailed investigation oftrace element variation in minerals and integration oftextural constraints It emphasizes that the lsquoexoticrsquonature of the metasomatic products reflects a particularpre-existing physical and chemical environmentccedilielow ambient temperatures (and probably pressures) andrelatively low oxygen fugacitiesccedilrather than an exoticinitial metasomatic agent We will show that meta-somatism in this case does not necessitate multiplemetasomatic events of different nature but simply reflectsthe product of melt^rock reaction possibly initiatedby common continental rift-related basanites Wefurther present mass-balance calculations which showthat the melt pockets were formed by in situ partialmelting of the early assemblage (mainly amphiboleand orthopyroxene) triggered by infiltration of aCO2-rich fluid

SAMPLE CONTEXTThe investigated mantle xenoliths were collected fromscoriae of basanitic to alkali-basaltic Quaternary cindercones of the Marsabit shield volcano (Volker 1990Henjes-Kunst amp Altherr 1992) Magmatic activity inMarsabit started in the late Miocene whereas rocks fromthe shield volcano yield Pleistocene to Quaternary ages(eg Key et al 1987) Magmatism is thus clearly relatedto the development of the Kenya rift (ie the easternbranch of the East African Rift system EARS seeFig 1) The Marsabit shield volcano is however locatedeastward of the main axis of the EARS within the AnzaGraben the eastward continuation of the Turkana depres-sion The evolution of the lithosphere in this regionincludes multiple accretion during Pan-African orogenesis(formation of the Pan-African Mozambique beltfrom 720 to 550 Ma eg Meert 2003) followed by severalphases of continental rifting including formation of theMesozoic^Paleogene Anza Graben later cross-cut by the

Tertiary^Quaternary EARS [see Fig 1 and Morley (1999)for a review] A more detailed description of thegeological and geodynamic context has been given byKaeser et al (2006)In our previous study (Kaeser et al 2006) we used the

Marsabit peridotite xenoliths to constrain the evolution ofthe lithospheric mantle beneath Marsabit The peridotiteswere subdivided into four groups including the porphyro-clastic spinel harzburgites and dunites (Group III)investigated here (Table 1) For detailed descriptions ofthe textures and mineral chemistry of the primaryassemblages of all four groups the reader is referred toKaeser et al (2006)Thermobarometry calculations revealed that all

the porphyroclastic rocks experienced decompressionand cooling from high pressures and temperatures

Fig 1 Simplified geological map showing the distribution of volcanicrocks associated with the Kenyan sector of the East African riftsystem compiled on the basis of the maps of King (1970) andBaker et al (1971) Stars indicate other off-craton xenolith localities(Henjes-Kunst amp Altherr 1992 Bedini et al 1997) The approximateposition of the Anza Graben is taken from Henjes-Kunst ampAltherr (1992)

2

(mainly preserved in formerly garnet-bearing Group Ilherzolites see Fig 2) to low mantle P^T conditions of59008C at 515GPa (Kaeser et al 2006) The presentstudy focuses on metasomatic features in the low-tempera-ture Group III xenoliths Textural and compositionalcharacteristics suggest a genetic relationship with lateEARS-related heating and cryptic metasomatismleading to U^Th^Nb and light rare earth element(LREE) enrichment in clinopyroxene in the recrystallizedGroup II lherzolites The latter were interpreted toreflect metasomatism by percolation of alkaline maficmelts possibly parental to the Quaternary magmasof the Marsabit volcano (Kaeser et al 2006) post-dating rift-related decompression and deformationIn contrast metasomatic phases in the porphyroclasticGroup I lherzolites (ie Ti-pargasite and rarephlogopite Kaeser et al 2006) are texturally andcompositionally distinct suggesting a different eventThe trace element signatures of Ti-pargasite suggestthat this metasomatic assemblage formed earlierprior to rift-related decompression and deformation(Kaeser et al 2006)

ANALYT ICAL METHODSModal compositions (Table 1) were determined fromdigitized images of thin sections and rock slice scans(for details see Kaeser et al 2006) Minerals were analysedfor major elements using a CAMECA SX 50 electronmicroprobe equipped with four wavelength-dispersivespectrometers (Mineralogisch-Petrographisches InstitutUniversitalaquo t Bern) and a CAMECA SX 51 electron micro-probe with five wavelength-dispersive spectrometers(Mineralogisches Institut Universitalaquo t Heidelberg)Pyroxene olivine and spinel were analysed using routineoperating conditions (focused beam accelerating voltageof 15 kV beam current of 20 nA and counting times of20 s for most elements) Silicate glass was analysed witha larger beam size (10^15 mm) and a lower beam current(15 nA) to avoid Na loss during analysis Natural andsynthetic silicates and oxides were used as standardsCarbonates were analysed with a beam current of 10 nAand a beam size of 10 mm using natural carbonatesand sulfates as standards The concentrations of CO2

(in carbonates) and Fe3thorn (in spinel) were calculated

Table 1 Textures and mineralogical composition of the investigated peridotite xenoliths

Sample no Type Lithology Pre-metasomatic and early metasomatic phases (vol) Melt

pockets

Modal composition of melt pockets

Former

garnet1

Ol Opx Cpx Spl Am Phl Graphite

apatite

Glass Dim Olm Chrm Mg-Cc Vugs

Ke 19651 III-a porphyroclastic

spl harzburgite

15 708 229 39 09 mdash mdash trmdash mdash mdash mdash mdash mdash mdash mdash

Ke 195915 III-b porphyroclastic

spl harzburgite

50 630 260 52 07 02 mdash mdashtr 540 218 234 277 49 812 76

Ke 196515 III-b porphyroclastic

spl harzburgite

mdash 719 177 68 04 02 01 010tr 30 214 340 237 28 160 22

Ke 785 III-c porphyroclastic

spl harzburgite

mdash 640 1873 mdash 064 11 09 023mdash 144 219 270 218 57 67 70

Ke 19653 III-c porphyroclastic

spl dunite

mdash 801 323 mdash 074 mdash tr mdashtr 160 227 213 295 66 1285 74

Ke 196525 III-c porphyroclastic

amphibole

dunite

mdash 536 mdash mdash mdash 120 tr 025mdash 342 nd nd nd nd nd nd

Ke 195927 III-c porphyroclastic

dunite

mdash 885 tr3 mdash mdash mdash mdash mdash 115 nd nd nd nd nd nd

Ke 19706 III-d recrystallized spl

harzburgite

mdash 872 953 276 064 mdash mdash mdash mdash mdash mdash mdash mdash mdash mdash

Ol olivine Opx orthopyroxene Cpx clinopyroxene Am amphibole Phl phlogopite Dim Olm Chrm diopside olivineand chromite micro-phenocrysts in melt pockets Mg-Cc Mg-calcite 1Splndashopxndashcpx symplectites 2Dolomite instead ofcalcite 3Metasomatic low-Al orthopyroxene (Opxm)

4Compositionally identical to Chrm5Highly variable (24ndash197 vol)

in individual melt pockets 6Compositionally similar to glass-related Dim mdash not detected tr traces nd not determined

3

based on stoichiometry The raw data of all microprobeanalyses were corrected using routine PAP softwareTrace element contents in minerals were analysed in-situ

on polished thin sections (40^50 mm thick) using a laserablation (LA) instrument equipped with a 193 nm ArFexcimer laser (Lambda Physik Germany) coupled to anELAN 6100 (Perkin Elmer Canada) quadrupole induc-tively coupled plasma mass spectrometer (ICPMS) atthe Institut fulaquo r Isotopengeologie und MineralischeRohstoffe ETH Zulaquo rich [see Pettke et al (2004) and refer-ences therein for the instrumental setup capabilities andoperating conditions] Raw data were reduced using theLAMTRACE program Laser pit diameters were between14 and 110 mm depending on grain size and the absenceor presence of mineral fluid or melt inclusions andof exsolution lamellae

PETROGRAPHYGroup III harzburgites and dunites display two texturallydifferent types of modal metasomatic modificationThe first type of metasomatism formed clusters

and veinlets of volatile-bearing minerals (amphibolephlogopite graphite apatite Fig 3a^e) associatedwith metasomatic orthopyroxene The second type ischaracterized by the occurrence of lsquomelt pocketsrsquo(eg Frey amp Green 1974 Ionov et al 1994) patchesand networks of silicate glass associated with micro-phenocrysts of clinopyroxene olivine and chromite(Fig 4a^f) Glass further contains globules of carbonatesand vugs (Figs 4a^e and 5a b) Melt pockets form at leastpartly at the expense of volatile-bearing mineralsTherefore the volatile-bearing assemblage is referred toin the following as lsquoearly metasomaticrsquo (including metaso-matic orthopyroxene referred to as Opxm) The later glass-bearing assemblage is termed lsquolate metasomaticrsquo (Na-richCr diopside Dim olivine Olm chromite Chrm) The pre-metasomatic peridotitic phases are termed lsquoprimaryrsquo(olivine ol-I clinopyroxene cpx-I orthopyroxene opx-Ispinel spl-I)Based on petrographic investigations we subdivided

Group III peridotites into four types (Type III-a to -d)Textural and geochemical features show that the transitionfrom Type III-a to Type III-c (thornd) mainly correspondsto the transformation of modally non-metasomatizedspinel harzburgite to pervasively overprinted amphiboleharzburgite^dunite Twenty out of 27 Group III samplesinvestigated petrographically show evidence for modalmetasomatism (ie they belong toType III-b or -c)

Type III-aType III-a is classified as porphyroclastic cpx-I-bearingspl harzburgite devoid of volatile-bearing phases butshowing geochemical evidence of cryptic metasomatism(Kaeser et al 2006 and this study) This rock typewas originally garnet-bearing and was subjected todecompression as is indicated by the presence of rarespl^opx^cpx symplectites (Table 1 and Kaeser et al 2006)

Type III-bXenoliths of Type III-b are cpx-I-bearing spinel harzbur-gites however they contain metasomatic volatile-bearingphases (Table 1) One xenolith (Ke 196515) contains vein-lets of pale green amphibole phlogopite graphite apatite(Figs 3e and 4b) Graphite occurs as macroscopic flakes(up to 500 mm long Fig 3e) similar to those describedfrom cratonic kimberlite-hosted peridotite xenoliths(Pearson et al 1994) Cpx-I in the vicinity of veinlets isstrongly resorbed and develops patches or lamellae ofTi-poor amphibole indicating metasomatic replacement(Fig 4b) In all Type III-b samples amphibole and clino-pyroxene are partially replaced by the late glass-bearingassemblage (eg Fig 4a) Cpx-I is overgrown by a second-ary green Na-rich Cr diopside (Dim) occasionally contain-ing fluid glass andor carbonate inclusions (Fig 2c)Dim further occurs as micro-phenocrysts in melt pocketssometimes enclosing (resorbed) orthopyroxene

Fig 2 Pressure^temperature evolution recorded by peridotite xeno-liths from Marsabit (light grey fields summarized from Kaeser et al2006) together with reactions relevant for this study plagioclase^spinel transition in the CFMAS system (Gasparik 1987) appearanceof garnet (grt) in model lherzolite (Klemme amp OrsquoNeill 2000) maxi-mum stability of Ti-pargasite (parg) in model lherzolite (Niida ampGreen 1999) appearance of chlorite (chl) in model lherzolite underfluid-saturated conditions (Fumagalli amp Poli 2005) solidus for primi-tive mantle under water-saturated conditions (perthornH2OGrove et al 2006) solidus of natural carbonated lherzolite from Leeamp Wyllie (2000) subsolidus decarbonation (opxthorndolfrac14olthorn cpxthornC)from Woermann amp Rosenhauer (1985) at fO2 below the carbon^carbon oxide (CCO) buffer The bold arrow schematically indicatesthe infiltrating and cooling metasomatizing Si-rich melt at supersoli-dus and derivative fluid at subsolidus conditions (See text for furtherexplanation)

4

Type III-cType III-c samples are harzburgites and dunites devoid ofcpx-I Samples transitional betweenType III-b and -c stillcontain relics of cpx-I Metasomatic products inType III-cxenoliths generally occur as clusters of amphibole

phlogopite graphite and rare apatite The primaryassemblage is dominated by ol-I GreenTi-poor amphiboleand phlogopite typically form aggregates around relic ske-letal spl-I (Fig 3c and d) Dunite xenolith Ke 196525 isunusual as it contains centimetre-scale clusters of large

Fig 3 Photomicrographs showing the textural characteristics of metasomatism in the porphyroclastic spinel harzburgites and dunites fromMarsabit (a) Twinned euhedral Mg-katophorite inType III-c amphibole duniteThe irregular grain boundaries with glassthornmicrolite-bearingmelt pockets should be noted (sample Ke 196525 crossed polars) (b) metasomatic low-Al orthopyroxene (Opxm) in aType III-c spinel harz-burgite with euhedral phlogopite inclusions (early metasomatic assemblage) Irregular concave grain boundaries indicating later resorption byolivine should be noted (sample Ke 785 crossed polarizers) (c) Type III-c Mg-katophorite and phlogopite clustering around partly resorbedspinel (spl-I) and metasomatic orthopyroxene (Opxm) in contact with partly included graphite (sample Ke 785) (d) Details of Type III-c earlymetasomatic Mg-katophorite and phlogopite the latter containing graphite inclusions (sample Ke 785) (e) Type III-b early metasomaticassemblage including interstitial phlogopite graphite and clinopyroxene (cpx-I) becoming replaced by Mg-katophorite lamellae (sample Ke196515) (f) Type III-d spl harzburgite characterized by annealed textures and clusters of chromite enclosed by green Na-rich clinopyroxene(cpx sample Ke 19706)

5

euhedral twinned amphibole crystals (Fig 3a) withsmall phlogopite inclusions Graphite is associatedwith amphibole and phlogopite (sometimes found asinclusions in the latter Fig 3d) or orthopyroxene (Fig 3c)Straight contacts between amphibole phlogopite and ol-Iindicate textural equilibrium Phlogopite also occurs as

euhedral randomly dispersed flakes in orthopyroxene(Fig 3b) This together with compositional arguments(eg very low Al contents see below) suggests thatorthopyroxene is related to metasomatism (Opxm)Sample Ke 19653 also contains early metasomaticinterstitial apatite

Fig 4 Back-scattered electron images showing textural characteristics and relationships between early and late metasomatic assemblages(a) Relic amphibole (edenite^Mg-katophorite) in a typical melt pocket containing clinopyroxene (Dim) olivine (Olm) and chromite (Chrm)micro-phenocrysts as well as silicate glass with vugs (sometimes lined or filled with dolomitic carbonate (Type III-b xenolith Ke 195915)(b) Type III-b spinel harzburgite showing the textural characteristics of early (graphite apatite Mg-katophorite replacing cpx-I) and late(glass Mg-calcite Dim) metasomatic assemblages (sample Ke 196515) (c) Formation of glass and micro-phenocrysts (Olm and Dim) at theexpense of metasomatic orthopyroxene (Opxm) It should be noted also that graphite is overgrown by Mg-calcite when no longer in contactwith Opxm (Type III-b spinel harzburgite Ke 196515) (d) Phlogopite and residual Mg-katophorite in a melt pocket in aType III-c spl harzbur-gite The more regular grain boundaries of phlogopite compared with Mg-katophorite should be noted (the latter is further rimmed by irregu-lar kaersutitic amphibole sample Ke 785) (e) Silicate glass Mg-calcite and microlites of Na-rich Cr diopside (Dim) olivine (Olm) andchromite (Chrm) in a Type III-c spinel harzburgite The grey tone variation in Dim indicating zoning should be noted (sample Ke 19653)(f) Host-related basanitic glass (containing minute clinopyroxene quench crystals) in the peculiar Type III-c xenolith Ke 195927 TheTi-augitereaction rims on Na-rich Cr diopside (Dim) micro-phenocrysts and the resorbed grain boundaries of chromite (Chrm) should be noted

6

Late metasomatic assemblages are ubiquitous inType III-c xenoliths They form melt pockets intercon-nected by glass films along olivine grain boundariesEuhedral apple-green Na-rich Cr diopside micro-phenocrysts (Dim up to 03mm long) in individual meltpockets commonly have the same crystallographic orienta-tion (Fig 4a) suggesting growth at the expense of an ear-lier phase Back-scattered electron (BSE) imaging revealspatchy zoning (Figs 4e and 5a) Melt pockets in severalsamples enclose relic resorbed amphibole grains (Fig 4d)Phlogopite (eg in Ke 785) appears to be less resorbed(Fig 4d) Early amphibole is rimmed by a kaersutitic sel-vage (Fig 4d) indicating that the early amphibole is notin chemical equilibrium with the silicate glass When incontact with glass Opxm is also strongly resorbed Severalfeatures suggest incongruent breakdown by a process simi-lar to the reaction opxthornmeltolthorn cpxthornmodifiedmelt (eg Shaw et al 1998) thin glass films are presenton orthopyroxene cleavage planes late euhedralOlmthornDimChrm nucleated within orthopyroxene andOlm overgrows orthopyroxene (Fig 3b development ofstrongly concave opx grain boundaries)Glass inType III-b and -c samples contains considerable

amounts of rounded carbonates (eg Figs 4e and 5a)texturally similar to interstitial glass-related carbonateglobules from other xenolith suites (eg Ionov et al 19931996 Norman 1998 Lee et al 2000 Laurora et al 2001Bali et al 2002 Demecurren ny et al 2004) BSE imaging andqualitative energy-dispersive spectrometry mappinghighlight that carbonate globules consist of clusters ofsometimes euhedral Mg-calcite or dolomite crystals occa-sionally showing oscillatory zoning (Fig 5a and b) Insample Ke 196515 Mg-calcite is in contact with graphiteflakes (Fig 4c) indicating nucleation of carbonateon graphite Glass in several samples contains rounded

blebs of apatite (Fig 4b) as well as minor amountsof Fe^Ni sulphide globules Abundant vugs (eg Fig 4aand d) in all types of silicate glass are possibly remnantsof an exsolved vapour phaseSample Ke 195927 (Type III-c) is unusual in several

aspects It contains kaersutite phenocrysts hosted in theglass Further Dim micro-phenocrysts are overgrown byTi-augite rims (Fig 4f) and the glass is of an untypicaldark brown colour This xenolith records interaction witha melt similar to the host basanite during ascent to thesurface

Type III-dType III-d is a unique sample (Ke 19706) of metasomaticcoarse-grained spinel harzburgite (grain size 4mmFig 3f) Olivine and orthopyroxene are almost strain freeSpinel occurs as rounded black grains (chromite) sur-rounded by a cluster of apple-green Na-rich Cr diopside(Fig 3f) containing glass inclusions Volatile-bearingphases are absent Several orthopyroxene grains shownumerous melt inclusions forming continuous films ortrails along cleavage planesThese remarkable textural fea-tures can be explained by almost complete annealing of aformer Type III-c spinel harzburgite containing silicateglassthornDim micro-phenocrysts which subsequently recrys-tallized to form the clinopyroxene clusters around spineland orthopyroxene with melt inclusions

MINERAL COMPOSIT IONCompositional data (major and trace elements) for non-metasomatic primary phases from all peridotite types aswell as for Ti-pargasite and phlogopite from Group I sam-ples have been given by Kaeser et al (2006) A more com-plete dataset with respect to the metasomatic phaseassemblages discussed in this paper is available at httppetrologyoxfordjournalsorg

Fig 5 Oscillatory zoning of carbonate related to silicate glass in porphyroclastic spinel harzburgites from Marsabit (a) Zoned euhedral dolo-mite crystals (dark) rimmed by fine selvages of calcite (light) together with Na-rich Cr diopside (Dim) and chromite (Chrm sample Ke 195915)(b) Qualitative compositional map of carbonates in a melt pocket (sample Ke 19653) Lighter grey tones indicate higher Mg contentsThe euhedral shapes of the carbonate crystals should be noted The dashed white lines in (a) and (b) highlight the contact of carbonates withthe pristine silicate glass

7

(mainly preserved in formerly garnet-bearing Group Ilherzolites see Fig 2) to low mantle P^T conditions of59008C at 515GPa (Kaeser et al 2006) The presentstudy focuses on metasomatic features in the low-tempera-ture Group III xenoliths Textural and compositionalcharacteristics suggest a genetic relationship with lateEARS-related heating and cryptic metasomatismleading to U^Th^Nb and light rare earth element(LREE) enrichment in clinopyroxene in the recrystallizedGroup II lherzolites The latter were interpreted toreflect metasomatism by percolation of alkaline maficmelts possibly parental to the Quaternary magmasof the Marsabit volcano (Kaeser et al 2006) post-dating rift-related decompression and deformationIn contrast metasomatic phases in the porphyroclasticGroup I lherzolites (ie Ti-pargasite and rarephlogopite Kaeser et al 2006) are texturally andcompositionally distinct suggesting a different eventThe trace element signatures of Ti-pargasite suggestthat this metasomatic assemblage formed earlierprior to rift-related decompression and deformation(Kaeser et al 2006)

ANALYT ICAL METHODSModal compositions (Table 1) were determined fromdigitized images of thin sections and rock slice scans(for details see Kaeser et al 2006) Minerals were analysedfor major elements using a CAMECA SX 50 electronmicroprobe equipped with four wavelength-dispersivespectrometers (Mineralogisch-Petrographisches InstitutUniversitalaquo t Bern) and a CAMECA SX 51 electron micro-probe with five wavelength-dispersive spectrometers(Mineralogisches Institut Universitalaquo t Heidelberg)Pyroxene olivine and spinel were analysed using routineoperating conditions (focused beam accelerating voltageof 15 kV beam current of 20 nA and counting times of20 s for most elements) Silicate glass was analysed witha larger beam size (10^15 mm) and a lower beam current(15 nA) to avoid Na loss during analysis Natural andsynthetic silicates and oxides were used as standardsCarbonates were analysed with a beam current of 10 nAand a beam size of 10 mm using natural carbonatesand sulfates as standards The concentrations of CO2

(in carbonates) and Fe3thorn (in spinel) were calculated

Table 1 Textures and mineralogical composition of the investigated peridotite xenoliths

Sample no Type Lithology Pre-metasomatic and early metasomatic phases (vol) Melt

pockets

Modal composition of melt pockets

Former

garnet1

Ol Opx Cpx Spl Am Phl Graphite

apatite

Glass Dim Olm Chrm Mg-Cc Vugs

Ke 19651 III-a porphyroclastic

spl harzburgite

15 708 229 39 09 mdash mdash trmdash mdash mdash mdash mdash mdash mdash mdash

Ke 195915 III-b porphyroclastic

spl harzburgite

50 630 260 52 07 02 mdash mdashtr 540 218 234 277 49 812 76

Ke 196515 III-b porphyroclastic

spl harzburgite

mdash 719 177 68 04 02 01 010tr 30 214 340 237 28 160 22

Ke 785 III-c porphyroclastic

spl harzburgite

mdash 640 1873 mdash 064 11 09 023mdash 144 219 270 218 57 67 70

Ke 19653 III-c porphyroclastic

spl dunite

mdash 801 323 mdash 074 mdash tr mdashtr 160 227 213 295 66 1285 74

Ke 196525 III-c porphyroclastic

amphibole

dunite

mdash 536 mdash mdash mdash 120 tr 025mdash 342 nd nd nd nd nd nd

Ke 195927 III-c porphyroclastic

dunite

mdash 885 tr3 mdash mdash mdash mdash mdash 115 nd nd nd nd nd nd

Ke 19706 III-d recrystallized spl

harzburgite

mdash 872 953 276 064 mdash mdash mdash mdash mdash mdash mdash mdash mdash mdash

Ol olivine Opx orthopyroxene Cpx clinopyroxene Am amphibole Phl phlogopite Dim Olm Chrm diopside olivineand chromite micro-phenocrysts in melt pockets Mg-Cc Mg-calcite 1Splndashopxndashcpx symplectites 2Dolomite instead ofcalcite 3Metasomatic low-Al orthopyroxene (Opxm)

4Compositionally identical to Chrm5Highly variable (24ndash197 vol)

in individual melt pockets 6Compositionally similar to glass-related Dim mdash not detected tr traces nd not determined

3

based on stoichiometry The raw data of all microprobeanalyses were corrected using routine PAP softwareTrace element contents in minerals were analysed in-situ

on polished thin sections (40^50 mm thick) using a laserablation (LA) instrument equipped with a 193 nm ArFexcimer laser (Lambda Physik Germany) coupled to anELAN 6100 (Perkin Elmer Canada) quadrupole induc-tively coupled plasma mass spectrometer (ICPMS) atthe Institut fulaquo r Isotopengeologie und MineralischeRohstoffe ETH Zulaquo rich [see Pettke et al (2004) and refer-ences therein for the instrumental setup capabilities andoperating conditions] Raw data were reduced using theLAMTRACE program Laser pit diameters were between14 and 110 mm depending on grain size and the absenceor presence of mineral fluid or melt inclusions andof exsolution lamellae

PETROGRAPHYGroup III harzburgites and dunites display two texturallydifferent types of modal metasomatic modificationThe first type of metasomatism formed clusters

and veinlets of volatile-bearing minerals (amphibolephlogopite graphite apatite Fig 3a^e) associatedwith metasomatic orthopyroxene The second type ischaracterized by the occurrence of lsquomelt pocketsrsquo(eg Frey amp Green 1974 Ionov et al 1994) patchesand networks of silicate glass associated with micro-phenocrysts of clinopyroxene olivine and chromite(Fig 4a^f) Glass further contains globules of carbonatesand vugs (Figs 4a^e and 5a b) Melt pockets form at leastpartly at the expense of volatile-bearing mineralsTherefore the volatile-bearing assemblage is referred toin the following as lsquoearly metasomaticrsquo (including metaso-matic orthopyroxene referred to as Opxm) The later glass-bearing assemblage is termed lsquolate metasomaticrsquo (Na-richCr diopside Dim olivine Olm chromite Chrm) The pre-metasomatic peridotitic phases are termed lsquoprimaryrsquo(olivine ol-I clinopyroxene cpx-I orthopyroxene opx-Ispinel spl-I)Based on petrographic investigations we subdivided

Group III peridotites into four types (Type III-a to -d)Textural and geochemical features show that the transitionfrom Type III-a to Type III-c (thornd) mainly correspondsto the transformation of modally non-metasomatizedspinel harzburgite to pervasively overprinted amphiboleharzburgite^dunite Twenty out of 27 Group III samplesinvestigated petrographically show evidence for modalmetasomatism (ie they belong toType III-b or -c)

Type III-aType III-a is classified as porphyroclastic cpx-I-bearingspl harzburgite devoid of volatile-bearing phases butshowing geochemical evidence of cryptic metasomatism(Kaeser et al 2006 and this study) This rock typewas originally garnet-bearing and was subjected todecompression as is indicated by the presence of rarespl^opx^cpx symplectites (Table 1 and Kaeser et al 2006)

Type III-bXenoliths of Type III-b are cpx-I-bearing spinel harzbur-gites however they contain metasomatic volatile-bearingphases (Table 1) One xenolith (Ke 196515) contains vein-lets of pale green amphibole phlogopite graphite apatite(Figs 3e and 4b) Graphite occurs as macroscopic flakes(up to 500 mm long Fig 3e) similar to those describedfrom cratonic kimberlite-hosted peridotite xenoliths(Pearson et al 1994) Cpx-I in the vicinity of veinlets isstrongly resorbed and develops patches or lamellae ofTi-poor amphibole indicating metasomatic replacement(Fig 4b) In all Type III-b samples amphibole and clino-pyroxene are partially replaced by the late glass-bearingassemblage (eg Fig 4a) Cpx-I is overgrown by a second-ary green Na-rich Cr diopside (Dim) occasionally contain-ing fluid glass andor carbonate inclusions (Fig 2c)Dim further occurs as micro-phenocrysts in melt pocketssometimes enclosing (resorbed) orthopyroxene

Fig 2 Pressure^temperature evolution recorded by peridotite xeno-liths from Marsabit (light grey fields summarized from Kaeser et al2006) together with reactions relevant for this study plagioclase^spinel transition in the CFMAS system (Gasparik 1987) appearanceof garnet (grt) in model lherzolite (Klemme amp OrsquoNeill 2000) maxi-mum stability of Ti-pargasite (parg) in model lherzolite (Niida ampGreen 1999) appearance of chlorite (chl) in model lherzolite underfluid-saturated conditions (Fumagalli amp Poli 2005) solidus for primi-tive mantle under water-saturated conditions (perthornH2OGrove et al 2006) solidus of natural carbonated lherzolite from Leeamp Wyllie (2000) subsolidus decarbonation (opxthorndolfrac14olthorn cpxthornC)from Woermann amp Rosenhauer (1985) at fO2 below the carbon^carbon oxide (CCO) buffer The bold arrow schematically indicatesthe infiltrating and cooling metasomatizing Si-rich melt at supersoli-dus and derivative fluid at subsolidus conditions (See text for furtherexplanation)

4

Type III-cType III-c samples are harzburgites and dunites devoid ofcpx-I Samples transitional betweenType III-b and -c stillcontain relics of cpx-I Metasomatic products inType III-cxenoliths generally occur as clusters of amphibole

phlogopite graphite and rare apatite The primaryassemblage is dominated by ol-I GreenTi-poor amphiboleand phlogopite typically form aggregates around relic ske-letal spl-I (Fig 3c and d) Dunite xenolith Ke 196525 isunusual as it contains centimetre-scale clusters of large

Fig 3 Photomicrographs showing the textural characteristics of metasomatism in the porphyroclastic spinel harzburgites and dunites fromMarsabit (a) Twinned euhedral Mg-katophorite inType III-c amphibole duniteThe irregular grain boundaries with glassthornmicrolite-bearingmelt pockets should be noted (sample Ke 196525 crossed polars) (b) metasomatic low-Al orthopyroxene (Opxm) in aType III-c spinel harz-burgite with euhedral phlogopite inclusions (early metasomatic assemblage) Irregular concave grain boundaries indicating later resorption byolivine should be noted (sample Ke 785 crossed polarizers) (c) Type III-c Mg-katophorite and phlogopite clustering around partly resorbedspinel (spl-I) and metasomatic orthopyroxene (Opxm) in contact with partly included graphite (sample Ke 785) (d) Details of Type III-c earlymetasomatic Mg-katophorite and phlogopite the latter containing graphite inclusions (sample Ke 785) (e) Type III-b early metasomaticassemblage including interstitial phlogopite graphite and clinopyroxene (cpx-I) becoming replaced by Mg-katophorite lamellae (sample Ke196515) (f) Type III-d spl harzburgite characterized by annealed textures and clusters of chromite enclosed by green Na-rich clinopyroxene(cpx sample Ke 19706)

5

euhedral twinned amphibole crystals (Fig 3a) withsmall phlogopite inclusions Graphite is associatedwith amphibole and phlogopite (sometimes found asinclusions in the latter Fig 3d) or orthopyroxene (Fig 3c)Straight contacts between amphibole phlogopite and ol-Iindicate textural equilibrium Phlogopite also occurs as

euhedral randomly dispersed flakes in orthopyroxene(Fig 3b) This together with compositional arguments(eg very low Al contents see below) suggests thatorthopyroxene is related to metasomatism (Opxm)Sample Ke 19653 also contains early metasomaticinterstitial apatite

Fig 4 Back-scattered electron images showing textural characteristics and relationships between early and late metasomatic assemblages(a) Relic amphibole (edenite^Mg-katophorite) in a typical melt pocket containing clinopyroxene (Dim) olivine (Olm) and chromite (Chrm)micro-phenocrysts as well as silicate glass with vugs (sometimes lined or filled with dolomitic carbonate (Type III-b xenolith Ke 195915)(b) Type III-b spinel harzburgite showing the textural characteristics of early (graphite apatite Mg-katophorite replacing cpx-I) and late(glass Mg-calcite Dim) metasomatic assemblages (sample Ke 196515) (c) Formation of glass and micro-phenocrysts (Olm and Dim) at theexpense of metasomatic orthopyroxene (Opxm) It should be noted also that graphite is overgrown by Mg-calcite when no longer in contactwith Opxm (Type III-b spinel harzburgite Ke 196515) (d) Phlogopite and residual Mg-katophorite in a melt pocket in aType III-c spl harzbur-gite The more regular grain boundaries of phlogopite compared with Mg-katophorite should be noted (the latter is further rimmed by irregu-lar kaersutitic amphibole sample Ke 785) (e) Silicate glass Mg-calcite and microlites of Na-rich Cr diopside (Dim) olivine (Olm) andchromite (Chrm) in a Type III-c spinel harzburgite The grey tone variation in Dim indicating zoning should be noted (sample Ke 19653)(f) Host-related basanitic glass (containing minute clinopyroxene quench crystals) in the peculiar Type III-c xenolith Ke 195927 TheTi-augitereaction rims on Na-rich Cr diopside (Dim) micro-phenocrysts and the resorbed grain boundaries of chromite (Chrm) should be noted

6

Late metasomatic assemblages are ubiquitous inType III-c xenoliths They form melt pockets intercon-nected by glass films along olivine grain boundariesEuhedral apple-green Na-rich Cr diopside micro-phenocrysts (Dim up to 03mm long) in individual meltpockets commonly have the same crystallographic orienta-tion (Fig 4a) suggesting growth at the expense of an ear-lier phase Back-scattered electron (BSE) imaging revealspatchy zoning (Figs 4e and 5a) Melt pockets in severalsamples enclose relic resorbed amphibole grains (Fig 4d)Phlogopite (eg in Ke 785) appears to be less resorbed(Fig 4d) Early amphibole is rimmed by a kaersutitic sel-vage (Fig 4d) indicating that the early amphibole is notin chemical equilibrium with the silicate glass When incontact with glass Opxm is also strongly resorbed Severalfeatures suggest incongruent breakdown by a process simi-lar to the reaction opxthornmeltolthorn cpxthornmodifiedmelt (eg Shaw et al 1998) thin glass films are presenton orthopyroxene cleavage planes late euhedralOlmthornDimChrm nucleated within orthopyroxene andOlm overgrows orthopyroxene (Fig 3b development ofstrongly concave opx grain boundaries)Glass inType III-b and -c samples contains considerable

amounts of rounded carbonates (eg Figs 4e and 5a)texturally similar to interstitial glass-related carbonateglobules from other xenolith suites (eg Ionov et al 19931996 Norman 1998 Lee et al 2000 Laurora et al 2001Bali et al 2002 Demecurren ny et al 2004) BSE imaging andqualitative energy-dispersive spectrometry mappinghighlight that carbonate globules consist of clusters ofsometimes euhedral Mg-calcite or dolomite crystals occa-sionally showing oscillatory zoning (Fig 5a and b) Insample Ke 196515 Mg-calcite is in contact with graphiteflakes (Fig 4c) indicating nucleation of carbonateon graphite Glass in several samples contains rounded

blebs of apatite (Fig 4b) as well as minor amountsof Fe^Ni sulphide globules Abundant vugs (eg Fig 4aand d) in all types of silicate glass are possibly remnantsof an exsolved vapour phaseSample Ke 195927 (Type III-c) is unusual in several

aspects It contains kaersutite phenocrysts hosted in theglass Further Dim micro-phenocrysts are overgrown byTi-augite rims (Fig 4f) and the glass is of an untypicaldark brown colour This xenolith records interaction witha melt similar to the host basanite during ascent to thesurface

Type III-dType III-d is a unique sample (Ke 19706) of metasomaticcoarse-grained spinel harzburgite (grain size 4mmFig 3f) Olivine and orthopyroxene are almost strain freeSpinel occurs as rounded black grains (chromite) sur-rounded by a cluster of apple-green Na-rich Cr diopside(Fig 3f) containing glass inclusions Volatile-bearingphases are absent Several orthopyroxene grains shownumerous melt inclusions forming continuous films ortrails along cleavage planesThese remarkable textural fea-tures can be explained by almost complete annealing of aformer Type III-c spinel harzburgite containing silicateglassthornDim micro-phenocrysts which subsequently recrys-tallized to form the clinopyroxene clusters around spineland orthopyroxene with melt inclusions

MINERAL COMPOSIT IONCompositional data (major and trace elements) for non-metasomatic primary phases from all peridotite types aswell as for Ti-pargasite and phlogopite from Group I sam-ples have been given by Kaeser et al (2006) A more com-plete dataset with respect to the metasomatic phaseassemblages discussed in this paper is available at httppetrologyoxfordjournalsorg

Fig 5 Oscillatory zoning of carbonate related to silicate glass in porphyroclastic spinel harzburgites from Marsabit (a) Zoned euhedral dolo-mite crystals (dark) rimmed by fine selvages of calcite (light) together with Na-rich Cr diopside (Dim) and chromite (Chrm sample Ke 195915)(b) Qualitative compositional map of carbonates in a melt pocket (sample Ke 19653) Lighter grey tones indicate higher Mg contentsThe euhedral shapes of the carbonate crystals should be noted The dashed white lines in (a) and (b) highlight the contact of carbonates withthe pristine silicate glass

7

based on stoichiometry The raw data of all microprobeanalyses were corrected using routine PAP softwareTrace element contents in minerals were analysed in-situ

on polished thin sections (40^50 mm thick) using a laserablation (LA) instrument equipped with a 193 nm ArFexcimer laser (Lambda Physik Germany) coupled to anELAN 6100 (Perkin Elmer Canada) quadrupole induc-tively coupled plasma mass spectrometer (ICPMS) atthe Institut fulaquo r Isotopengeologie und MineralischeRohstoffe ETH Zulaquo rich [see Pettke et al (2004) and refer-ences therein for the instrumental setup capabilities andoperating conditions] Raw data were reduced using theLAMTRACE program Laser pit diameters were between14 and 110 mm depending on grain size and the absenceor presence of mineral fluid or melt inclusions andof exsolution lamellae

PETROGRAPHYGroup III harzburgites and dunites display two texturallydifferent types of modal metasomatic modificationThe first type of metasomatism formed clusters

and veinlets of volatile-bearing minerals (amphibolephlogopite graphite apatite Fig 3a^e) associatedwith metasomatic orthopyroxene The second type ischaracterized by the occurrence of lsquomelt pocketsrsquo(eg Frey amp Green 1974 Ionov et al 1994) patchesand networks of silicate glass associated with micro-phenocrysts of clinopyroxene olivine and chromite(Fig 4a^f) Glass further contains globules of carbonatesand vugs (Figs 4a^e and 5a b) Melt pockets form at leastpartly at the expense of volatile-bearing mineralsTherefore the volatile-bearing assemblage is referred toin the following as lsquoearly metasomaticrsquo (including metaso-matic orthopyroxene referred to as Opxm) The later glass-bearing assemblage is termed lsquolate metasomaticrsquo (Na-richCr diopside Dim olivine Olm chromite Chrm) The pre-metasomatic peridotitic phases are termed lsquoprimaryrsquo(olivine ol-I clinopyroxene cpx-I orthopyroxene opx-Ispinel spl-I)Based on petrographic investigations we subdivided

Group III peridotites into four types (Type III-a to -d)Textural and geochemical features show that the transitionfrom Type III-a to Type III-c (thornd) mainly correspondsto the transformation of modally non-metasomatizedspinel harzburgite to pervasively overprinted amphiboleharzburgite^dunite Twenty out of 27 Group III samplesinvestigated petrographically show evidence for modalmetasomatism (ie they belong toType III-b or -c)

Type III-aType III-a is classified as porphyroclastic cpx-I-bearingspl harzburgite devoid of volatile-bearing phases butshowing geochemical evidence of cryptic metasomatism(Kaeser et al 2006 and this study) This rock typewas originally garnet-bearing and was subjected todecompression as is indicated by the presence of rarespl^opx^cpx symplectites (Table 1 and Kaeser et al 2006)

Type III-bXenoliths of Type III-b are cpx-I-bearing spinel harzbur-gites however they contain metasomatic volatile-bearingphases (Table 1) One xenolith (Ke 196515) contains vein-lets of pale green amphibole phlogopite graphite apatite(Figs 3e and 4b) Graphite occurs as macroscopic flakes(up to 500 mm long Fig 3e) similar to those describedfrom cratonic kimberlite-hosted peridotite xenoliths(Pearson et al 1994) Cpx-I in the vicinity of veinlets isstrongly resorbed and develops patches or lamellae ofTi-poor amphibole indicating metasomatic replacement(Fig 4b) In all Type III-b samples amphibole and clino-pyroxene are partially replaced by the late glass-bearingassemblage (eg Fig 4a) Cpx-I is overgrown by a second-ary green Na-rich Cr diopside (Dim) occasionally contain-ing fluid glass andor carbonate inclusions (Fig 2c)Dim further occurs as micro-phenocrysts in melt pocketssometimes enclosing (resorbed) orthopyroxene

Fig 2 Pressure^temperature evolution recorded by peridotite xeno-liths from Marsabit (light grey fields summarized from Kaeser et al2006) together with reactions relevant for this study plagioclase^spinel transition in the CFMAS system (Gasparik 1987) appearanceof garnet (grt) in model lherzolite (Klemme amp OrsquoNeill 2000) maxi-mum stability of Ti-pargasite (parg) in model lherzolite (Niida ampGreen 1999) appearance of chlorite (chl) in model lherzolite underfluid-saturated conditions (Fumagalli amp Poli 2005) solidus for primi-tive mantle under water-saturated conditions (perthornH2OGrove et al 2006) solidus of natural carbonated lherzolite from Leeamp Wyllie (2000) subsolidus decarbonation (opxthorndolfrac14olthorn cpxthornC)from Woermann amp Rosenhauer (1985) at fO2 below the carbon^carbon oxide (CCO) buffer The bold arrow schematically indicatesthe infiltrating and cooling metasomatizing Si-rich melt at supersoli-dus and derivative fluid at subsolidus conditions (See text for furtherexplanation)

4

Type III-cType III-c samples are harzburgites and dunites devoid ofcpx-I Samples transitional betweenType III-b and -c stillcontain relics of cpx-I Metasomatic products inType III-cxenoliths generally occur as clusters of amphibole

phlogopite graphite and rare apatite The primaryassemblage is dominated by ol-I GreenTi-poor amphiboleand phlogopite typically form aggregates around relic ske-letal spl-I (Fig 3c and d) Dunite xenolith Ke 196525 isunusual as it contains centimetre-scale clusters of large

Fig 3 Photomicrographs showing the textural characteristics of metasomatism in the porphyroclastic spinel harzburgites and dunites fromMarsabit (a) Twinned euhedral Mg-katophorite inType III-c amphibole duniteThe irregular grain boundaries with glassthornmicrolite-bearingmelt pockets should be noted (sample Ke 196525 crossed polars) (b) metasomatic low-Al orthopyroxene (Opxm) in aType III-c spinel harz-burgite with euhedral phlogopite inclusions (early metasomatic assemblage) Irregular concave grain boundaries indicating later resorption byolivine should be noted (sample Ke 785 crossed polarizers) (c) Type III-c Mg-katophorite and phlogopite clustering around partly resorbedspinel (spl-I) and metasomatic orthopyroxene (Opxm) in contact with partly included graphite (sample Ke 785) (d) Details of Type III-c earlymetasomatic Mg-katophorite and phlogopite the latter containing graphite inclusions (sample Ke 785) (e) Type III-b early metasomaticassemblage including interstitial phlogopite graphite and clinopyroxene (cpx-I) becoming replaced by Mg-katophorite lamellae (sample Ke196515) (f) Type III-d spl harzburgite characterized by annealed textures and clusters of chromite enclosed by green Na-rich clinopyroxene(cpx sample Ke 19706)

5

euhedral twinned amphibole crystals (Fig 3a) withsmall phlogopite inclusions Graphite is associatedwith amphibole and phlogopite (sometimes found asinclusions in the latter Fig 3d) or orthopyroxene (Fig 3c)Straight contacts between amphibole phlogopite and ol-Iindicate textural equilibrium Phlogopite also occurs as

euhedral randomly dispersed flakes in orthopyroxene(Fig 3b) This together with compositional arguments(eg very low Al contents see below) suggests thatorthopyroxene is related to metasomatism (Opxm)Sample Ke 19653 also contains early metasomaticinterstitial apatite

Fig 4 Back-scattered electron images showing textural characteristics and relationships between early and late metasomatic assemblages(a) Relic amphibole (edenite^Mg-katophorite) in a typical melt pocket containing clinopyroxene (Dim) olivine (Olm) and chromite (Chrm)micro-phenocrysts as well as silicate glass with vugs (sometimes lined or filled with dolomitic carbonate (Type III-b xenolith Ke 195915)(b) Type III-b spinel harzburgite showing the textural characteristics of early (graphite apatite Mg-katophorite replacing cpx-I) and late(glass Mg-calcite Dim) metasomatic assemblages (sample Ke 196515) (c) Formation of glass and micro-phenocrysts (Olm and Dim) at theexpense of metasomatic orthopyroxene (Opxm) It should be noted also that graphite is overgrown by Mg-calcite when no longer in contactwith Opxm (Type III-b spinel harzburgite Ke 196515) (d) Phlogopite and residual Mg-katophorite in a melt pocket in aType III-c spl harzbur-gite The more regular grain boundaries of phlogopite compared with Mg-katophorite should be noted (the latter is further rimmed by irregu-lar kaersutitic amphibole sample Ke 785) (e) Silicate glass Mg-calcite and microlites of Na-rich Cr diopside (Dim) olivine (Olm) andchromite (Chrm) in a Type III-c spinel harzburgite The grey tone variation in Dim indicating zoning should be noted (sample Ke 19653)(f) Host-related basanitic glass (containing minute clinopyroxene quench crystals) in the peculiar Type III-c xenolith Ke 195927 TheTi-augitereaction rims on Na-rich Cr diopside (Dim) micro-phenocrysts and the resorbed grain boundaries of chromite (Chrm) should be noted

6

Late metasomatic assemblages are ubiquitous inType III-c xenoliths They form melt pockets intercon-nected by glass films along olivine grain boundariesEuhedral apple-green Na-rich Cr diopside micro-phenocrysts (Dim up to 03mm long) in individual meltpockets commonly have the same crystallographic orienta-tion (Fig 4a) suggesting growth at the expense of an ear-lier phase Back-scattered electron (BSE) imaging revealspatchy zoning (Figs 4e and 5a) Melt pockets in severalsamples enclose relic resorbed amphibole grains (Fig 4d)Phlogopite (eg in Ke 785) appears to be less resorbed(Fig 4d) Early amphibole is rimmed by a kaersutitic sel-vage (Fig 4d) indicating that the early amphibole is notin chemical equilibrium with the silicate glass When incontact with glass Opxm is also strongly resorbed Severalfeatures suggest incongruent breakdown by a process simi-lar to the reaction opxthornmeltolthorn cpxthornmodifiedmelt (eg Shaw et al 1998) thin glass films are presenton orthopyroxene cleavage planes late euhedralOlmthornDimChrm nucleated within orthopyroxene andOlm overgrows orthopyroxene (Fig 3b development ofstrongly concave opx grain boundaries)Glass inType III-b and -c samples contains considerable

amounts of rounded carbonates (eg Figs 4e and 5a)texturally similar to interstitial glass-related carbonateglobules from other xenolith suites (eg Ionov et al 19931996 Norman 1998 Lee et al 2000 Laurora et al 2001Bali et al 2002 Demecurren ny et al 2004) BSE imaging andqualitative energy-dispersive spectrometry mappinghighlight that carbonate globules consist of clusters ofsometimes euhedral Mg-calcite or dolomite crystals occa-sionally showing oscillatory zoning (Fig 5a and b) Insample Ke 196515 Mg-calcite is in contact with graphiteflakes (Fig 4c) indicating nucleation of carbonateon graphite Glass in several samples contains rounded

blebs of apatite (Fig 4b) as well as minor amountsof Fe^Ni sulphide globules Abundant vugs (eg Fig 4aand d) in all types of silicate glass are possibly remnantsof an exsolved vapour phaseSample Ke 195927 (Type III-c) is unusual in several

aspects It contains kaersutite phenocrysts hosted in theglass Further Dim micro-phenocrysts are overgrown byTi-augite rims (Fig 4f) and the glass is of an untypicaldark brown colour This xenolith records interaction witha melt similar to the host basanite during ascent to thesurface

Type III-dType III-d is a unique sample (Ke 19706) of metasomaticcoarse-grained spinel harzburgite (grain size 4mmFig 3f) Olivine and orthopyroxene are almost strain freeSpinel occurs as rounded black grains (chromite) sur-rounded by a cluster of apple-green Na-rich Cr diopside(Fig 3f) containing glass inclusions Volatile-bearingphases are absent Several orthopyroxene grains shownumerous melt inclusions forming continuous films ortrails along cleavage planesThese remarkable textural fea-tures can be explained by almost complete annealing of aformer Type III-c spinel harzburgite containing silicateglassthornDim micro-phenocrysts which subsequently recrys-tallized to form the clinopyroxene clusters around spineland orthopyroxene with melt inclusions

MINERAL COMPOSIT IONCompositional data (major and trace elements) for non-metasomatic primary phases from all peridotite types aswell as for Ti-pargasite and phlogopite from Group I sam-ples have been given by Kaeser et al (2006) A more com-plete dataset with respect to the metasomatic phaseassemblages discussed in this paper is available at httppetrologyoxfordjournalsorg

Fig 5 Oscillatory zoning of carbonate related to silicate glass in porphyroclastic spinel harzburgites from Marsabit (a) Zoned euhedral dolo-mite crystals (dark) rimmed by fine selvages of calcite (light) together with Na-rich Cr diopside (Dim) and chromite (Chrm sample Ke 195915)(b) Qualitative compositional map of carbonates in a melt pocket (sample Ke 19653) Lighter grey tones indicate higher Mg contentsThe euhedral shapes of the carbonate crystals should be noted The dashed white lines in (a) and (b) highlight the contact of carbonates withthe pristine silicate glass

7

Type III-cType III-c samples are harzburgites and dunites devoid ofcpx-I Samples transitional betweenType III-b and -c stillcontain relics of cpx-I Metasomatic products inType III-cxenoliths generally occur as clusters of amphibole

phlogopite graphite and rare apatite The primaryassemblage is dominated by ol-I GreenTi-poor amphiboleand phlogopite typically form aggregates around relic ske-letal spl-I (Fig 3c and d) Dunite xenolith Ke 196525 isunusual as it contains centimetre-scale clusters of large

Fig 3 Photomicrographs showing the textural characteristics of metasomatism in the porphyroclastic spinel harzburgites and dunites fromMarsabit (a) Twinned euhedral Mg-katophorite inType III-c amphibole duniteThe irregular grain boundaries with glassthornmicrolite-bearingmelt pockets should be noted (sample Ke 196525 crossed polars) (b) metasomatic low-Al orthopyroxene (Opxm) in aType III-c spinel harz-burgite with euhedral phlogopite inclusions (early metasomatic assemblage) Irregular concave grain boundaries indicating later resorption byolivine should be noted (sample Ke 785 crossed polarizers) (c) Type III-c Mg-katophorite and phlogopite clustering around partly resorbedspinel (spl-I) and metasomatic orthopyroxene (Opxm) in contact with partly included graphite (sample Ke 785) (d) Details of Type III-c earlymetasomatic Mg-katophorite and phlogopite the latter containing graphite inclusions (sample Ke 785) (e) Type III-b early metasomaticassemblage including interstitial phlogopite graphite and clinopyroxene (cpx-I) becoming replaced by Mg-katophorite lamellae (sample Ke196515) (f) Type III-d spl harzburgite characterized by annealed textures and clusters of chromite enclosed by green Na-rich clinopyroxene(cpx sample Ke 19706)

5

euhedral twinned amphibole crystals (Fig 3a) withsmall phlogopite inclusions Graphite is associatedwith amphibole and phlogopite (sometimes found asinclusions in the latter Fig 3d) or orthopyroxene (Fig 3c)Straight contacts between amphibole phlogopite and ol-Iindicate textural equilibrium Phlogopite also occurs as

euhedral randomly dispersed flakes in orthopyroxene(Fig 3b) This together with compositional arguments(eg very low Al contents see below) suggests thatorthopyroxene is related to metasomatism (Opxm)Sample Ke 19653 also contains early metasomaticinterstitial apatite

Fig 4 Back-scattered electron images showing textural characteristics and relationships between early and late metasomatic assemblages(a) Relic amphibole (edenite^Mg-katophorite) in a typical melt pocket containing clinopyroxene (Dim) olivine (Olm) and chromite (Chrm)micro-phenocrysts as well as silicate glass with vugs (sometimes lined or filled with dolomitic carbonate (Type III-b xenolith Ke 195915)(b) Type III-b spinel harzburgite showing the textural characteristics of early (graphite apatite Mg-katophorite replacing cpx-I) and late(glass Mg-calcite Dim) metasomatic assemblages (sample Ke 196515) (c) Formation of glass and micro-phenocrysts (Olm and Dim) at theexpense of metasomatic orthopyroxene (Opxm) It should be noted also that graphite is overgrown by Mg-calcite when no longer in contactwith Opxm (Type III-b spinel harzburgite Ke 196515) (d) Phlogopite and residual Mg-katophorite in a melt pocket in aType III-c spl harzbur-gite The more regular grain boundaries of phlogopite compared with Mg-katophorite should be noted (the latter is further rimmed by irregu-lar kaersutitic amphibole sample Ke 785) (e) Silicate glass Mg-calcite and microlites of Na-rich Cr diopside (Dim) olivine (Olm) andchromite (Chrm) in a Type III-c spinel harzburgite The grey tone variation in Dim indicating zoning should be noted (sample Ke 19653)(f) Host-related basanitic glass (containing minute clinopyroxene quench crystals) in the peculiar Type III-c xenolith Ke 195927 TheTi-augitereaction rims on Na-rich Cr diopside (Dim) micro-phenocrysts and the resorbed grain boundaries of chromite (Chrm) should be noted

6

Late metasomatic assemblages are ubiquitous inType III-c xenoliths They form melt pockets intercon-nected by glass films along olivine grain boundariesEuhedral apple-green Na-rich Cr diopside micro-phenocrysts (Dim up to 03mm long) in individual meltpockets commonly have the same crystallographic orienta-tion (Fig 4a) suggesting growth at the expense of an ear-lier phase Back-scattered electron (BSE) imaging revealspatchy zoning (Figs 4e and 5a) Melt pockets in severalsamples enclose relic resorbed amphibole grains (Fig 4d)Phlogopite (eg in Ke 785) appears to be less resorbed(Fig 4d) Early amphibole is rimmed by a kaersutitic sel-vage (Fig 4d) indicating that the early amphibole is notin chemical equilibrium with the silicate glass When incontact with glass Opxm is also strongly resorbed Severalfeatures suggest incongruent breakdown by a process simi-lar to the reaction opxthornmeltolthorn cpxthornmodifiedmelt (eg Shaw et al 1998) thin glass films are presenton orthopyroxene cleavage planes late euhedralOlmthornDimChrm nucleated within orthopyroxene andOlm overgrows orthopyroxene (Fig 3b development ofstrongly concave opx grain boundaries)Glass inType III-b and -c samples contains considerable

amounts of rounded carbonates (eg Figs 4e and 5a)texturally similar to interstitial glass-related carbonateglobules from other xenolith suites (eg Ionov et al 19931996 Norman 1998 Lee et al 2000 Laurora et al 2001Bali et al 2002 Demecurren ny et al 2004) BSE imaging andqualitative energy-dispersive spectrometry mappinghighlight that carbonate globules consist of clusters ofsometimes euhedral Mg-calcite or dolomite crystals occa-sionally showing oscillatory zoning (Fig 5a and b) Insample Ke 196515 Mg-calcite is in contact with graphiteflakes (Fig 4c) indicating nucleation of carbonateon graphite Glass in several samples contains rounded

blebs of apatite (Fig 4b) as well as minor amountsof Fe^Ni sulphide globules Abundant vugs (eg Fig 4aand d) in all types of silicate glass are possibly remnantsof an exsolved vapour phaseSample Ke 195927 (Type III-c) is unusual in several

aspects It contains kaersutite phenocrysts hosted in theglass Further Dim micro-phenocrysts are overgrown byTi-augite rims (Fig 4f) and the glass is of an untypicaldark brown colour This xenolith records interaction witha melt similar to the host basanite during ascent to thesurface

Type III-dType III-d is a unique sample (Ke 19706) of metasomaticcoarse-grained spinel harzburgite (grain size 4mmFig 3f) Olivine and orthopyroxene are almost strain freeSpinel occurs as rounded black grains (chromite) sur-rounded by a cluster of apple-green Na-rich Cr diopside(Fig 3f) containing glass inclusions Volatile-bearingphases are absent Several orthopyroxene grains shownumerous melt inclusions forming continuous films ortrails along cleavage planesThese remarkable textural fea-tures can be explained by almost complete annealing of aformer Type III-c spinel harzburgite containing silicateglassthornDim micro-phenocrysts which subsequently recrys-tallized to form the clinopyroxene clusters around spineland orthopyroxene with melt inclusions

MINERAL COMPOSIT IONCompositional data (major and trace elements) for non-metasomatic primary phases from all peridotite types aswell as for Ti-pargasite and phlogopite from Group I sam-ples have been given by Kaeser et al (2006) A more com-plete dataset with respect to the metasomatic phaseassemblages discussed in this paper is available at httppetrologyoxfordjournalsorg

Fig 5 Oscillatory zoning of carbonate related to silicate glass in porphyroclastic spinel harzburgites from Marsabit (a) Zoned euhedral dolo-mite crystals (dark) rimmed by fine selvages of calcite (light) together with Na-rich Cr diopside (Dim) and chromite (Chrm sample Ke 195915)(b) Qualitative compositional map of carbonates in a melt pocket (sample Ke 19653) Lighter grey tones indicate higher Mg contentsThe euhedral shapes of the carbonate crystals should be noted The dashed white lines in (a) and (b) highlight the contact of carbonates withthe pristine silicate glass

7

euhedral twinned amphibole crystals (Fig 3a) withsmall phlogopite inclusions Graphite is associatedwith amphibole and phlogopite (sometimes found asinclusions in the latter Fig 3d) or orthopyroxene (Fig 3c)Straight contacts between amphibole phlogopite and ol-Iindicate textural equilibrium Phlogopite also occurs as

euhedral randomly dispersed flakes in orthopyroxene(Fig 3b) This together with compositional arguments(eg very low Al contents see below) suggests thatorthopyroxene is related to metasomatism (Opxm)Sample Ke 19653 also contains early metasomaticinterstitial apatite

Fig 4 Back-scattered electron images showing textural characteristics and relationships between early and late metasomatic assemblages(a) Relic amphibole (edenite^Mg-katophorite) in a typical melt pocket containing clinopyroxene (Dim) olivine (Olm) and chromite (Chrm)micro-phenocrysts as well as silicate glass with vugs (sometimes lined or filled with dolomitic carbonate (Type III-b xenolith Ke 195915)(b) Type III-b spinel harzburgite showing the textural characteristics of early (graphite apatite Mg-katophorite replacing cpx-I) and late(glass Mg-calcite Dim) metasomatic assemblages (sample Ke 196515) (c) Formation of glass and micro-phenocrysts (Olm and Dim) at theexpense of metasomatic orthopyroxene (Opxm) It should be noted also that graphite is overgrown by Mg-calcite when no longer in contactwith Opxm (Type III-b spinel harzburgite Ke 196515) (d) Phlogopite and residual Mg-katophorite in a melt pocket in aType III-c spl harzbur-gite The more regular grain boundaries of phlogopite compared with Mg-katophorite should be noted (the latter is further rimmed by irregu-lar kaersutitic amphibole sample Ke 785) (e) Silicate glass Mg-calcite and microlites of Na-rich Cr diopside (Dim) olivine (Olm) andchromite (Chrm) in a Type III-c spinel harzburgite The grey tone variation in Dim indicating zoning should be noted (sample Ke 19653)(f) Host-related basanitic glass (containing minute clinopyroxene quench crystals) in the peculiar Type III-c xenolith Ke 195927 TheTi-augitereaction rims on Na-rich Cr diopside (Dim) micro-phenocrysts and the resorbed grain boundaries of chromite (Chrm) should be noted

6

Late metasomatic assemblages are ubiquitous inType III-c xenoliths They form melt pockets intercon-nected by glass films along olivine grain boundariesEuhedral apple-green Na-rich Cr diopside micro-phenocrysts (Dim up to 03mm long) in individual meltpockets commonly have the same crystallographic orienta-tion (Fig 4a) suggesting growth at the expense of an ear-lier phase Back-scattered electron (BSE) imaging revealspatchy zoning (Figs 4e and 5a) Melt pockets in severalsamples enclose relic resorbed amphibole grains (Fig 4d)Phlogopite (eg in Ke 785) appears to be less resorbed(Fig 4d) Early amphibole is rimmed by a kaersutitic sel-vage (Fig 4d) indicating that the early amphibole is notin chemical equilibrium with the silicate glass When incontact with glass Opxm is also strongly resorbed Severalfeatures suggest incongruent breakdown by a process simi-lar to the reaction opxthornmeltolthorn cpxthornmodifiedmelt (eg Shaw et al 1998) thin glass films are presenton orthopyroxene cleavage planes late euhedralOlmthornDimChrm nucleated within orthopyroxene andOlm overgrows orthopyroxene (Fig 3b development ofstrongly concave opx grain boundaries)Glass inType III-b and -c samples contains considerable

amounts of rounded carbonates (eg Figs 4e and 5a)texturally similar to interstitial glass-related carbonateglobules from other xenolith suites (eg Ionov et al 19931996 Norman 1998 Lee et al 2000 Laurora et al 2001Bali et al 2002 Demecurren ny et al 2004) BSE imaging andqualitative energy-dispersive spectrometry mappinghighlight that carbonate globules consist of clusters ofsometimes euhedral Mg-calcite or dolomite crystals occa-sionally showing oscillatory zoning (Fig 5a and b) Insample Ke 196515 Mg-calcite is in contact with graphiteflakes (Fig 4c) indicating nucleation of carbonateon graphite Glass in several samples contains rounded

blebs of apatite (Fig 4b) as well as minor amountsof Fe^Ni sulphide globules Abundant vugs (eg Fig 4aand d) in all types of silicate glass are possibly remnantsof an exsolved vapour phaseSample Ke 195927 (Type III-c) is unusual in several

aspects It contains kaersutite phenocrysts hosted in theglass Further Dim micro-phenocrysts are overgrown byTi-augite rims (Fig 4f) and the glass is of an untypicaldark brown colour This xenolith records interaction witha melt similar to the host basanite during ascent to thesurface

Type III-dType III-d is a unique sample (Ke 19706) of metasomaticcoarse-grained spinel harzburgite (grain size 4mmFig 3f) Olivine and orthopyroxene are almost strain freeSpinel occurs as rounded black grains (chromite) sur-rounded by a cluster of apple-green Na-rich Cr diopside(Fig 3f) containing glass inclusions Volatile-bearingphases are absent Several orthopyroxene grains shownumerous melt inclusions forming continuous films ortrails along cleavage planesThese remarkable textural fea-tures can be explained by almost complete annealing of aformer Type III-c spinel harzburgite containing silicateglassthornDim micro-phenocrysts which subsequently recrys-tallized to form the clinopyroxene clusters around spineland orthopyroxene with melt inclusions

MINERAL COMPOSIT IONCompositional data (major and trace elements) for non-metasomatic primary phases from all peridotite types aswell as for Ti-pargasite and phlogopite from Group I sam-ples have been given by Kaeser et al (2006) A more com-plete dataset with respect to the metasomatic phaseassemblages discussed in this paper is available at httppetrologyoxfordjournalsorg

Fig 5 Oscillatory zoning of carbonate related to silicate glass in porphyroclastic spinel harzburgites from Marsabit (a) Zoned euhedral dolo-mite crystals (dark) rimmed by fine selvages of calcite (light) together with Na-rich Cr diopside (Dim) and chromite (Chrm sample Ke 195915)(b) Qualitative compositional map of carbonates in a melt pocket (sample Ke 19653) Lighter grey tones indicate higher Mg contentsThe euhedral shapes of the carbonate crystals should be noted The dashed white lines in (a) and (b) highlight the contact of carbonates withthe pristine silicate glass

7

Late metasomatic assemblages are ubiquitous inType III-c xenoliths They form melt pockets intercon-nected by glass films along olivine grain boundariesEuhedral apple-green Na-rich Cr diopside micro-phenocrysts (Dim up to 03mm long) in individual meltpockets commonly have the same crystallographic orienta-tion (Fig 4a) suggesting growth at the expense of an ear-lier phase Back-scattered electron (BSE) imaging revealspatchy zoning (Figs 4e and 5a) Melt pockets in severalsamples enclose relic resorbed amphibole grains (Fig 4d)Phlogopite (eg in Ke 785) appears to be less resorbed(Fig 4d) Early amphibole is rimmed by a kaersutitic sel-vage (Fig 4d) indicating that the early amphibole is notin chemical equilibrium with the silicate glass When incontact with glass Opxm is also strongly resorbed Severalfeatures suggest incongruent breakdown by a process simi-lar to the reaction opxthornmeltolthorn cpxthornmodifiedmelt (eg Shaw et al 1998) thin glass films are presenton orthopyroxene cleavage planes late euhedralOlmthornDimChrm nucleated within orthopyroxene andOlm overgrows orthopyroxene (Fig 3b development ofstrongly concave opx grain boundaries)Glass inType III-b and -c samples contains considerable

amounts of rounded carbonates (eg Figs 4e and 5a)texturally similar to interstitial glass-related carbonateglobules from other xenolith suites (eg Ionov et al 19931996 Norman 1998 Lee et al 2000 Laurora et al 2001Bali et al 2002 Demecurren ny et al 2004) BSE imaging andqualitative energy-dispersive spectrometry mappinghighlight that carbonate globules consist of clusters ofsometimes euhedral Mg-calcite or dolomite crystals occa-sionally showing oscillatory zoning (Fig 5a and b) Insample Ke 196515 Mg-calcite is in contact with graphiteflakes (Fig 4c) indicating nucleation of carbonateon graphite Glass in several samples contains rounded

blebs of apatite (Fig 4b) as well as minor amountsof Fe^Ni sulphide globules Abundant vugs (eg Fig 4aand d) in all types of silicate glass are possibly remnantsof an exsolved vapour phaseSample Ke 195927 (Type III-c) is unusual in several

aspects It contains kaersutite phenocrysts hosted in theglass Further Dim micro-phenocrysts are overgrown byTi-augite rims (Fig 4f) and the glass is of an untypicaldark brown colour This xenolith records interaction witha melt similar to the host basanite during ascent to thesurface

Type III-dType III-d is a unique sample (Ke 19706) of metasomaticcoarse-grained spinel harzburgite (grain size 4mmFig 3f) Olivine and orthopyroxene are almost strain freeSpinel occurs as rounded black grains (chromite) sur-rounded by a cluster of apple-green Na-rich Cr diopside(Fig 3f) containing glass inclusions Volatile-bearingphases are absent Several orthopyroxene grains shownumerous melt inclusions forming continuous films ortrails along cleavage planesThese remarkable textural fea-tures can be explained by almost complete annealing of aformer Type III-c spinel harzburgite containing silicateglassthornDim micro-phenocrysts which subsequently recrys-tallized to form the clinopyroxene clusters around spineland orthopyroxene with melt inclusions

MINERAL COMPOSIT IONCompositional data (major and trace elements) for non-metasomatic primary phases from all peridotite types aswell as for Ti-pargasite and phlogopite from Group I sam-ples have been given by Kaeser et al (2006) A more com-plete dataset with respect to the metasomatic phaseassemblages discussed in this paper is available at httppetrologyoxfordjournalsorg

Fig 5 Oscillatory zoning of carbonate related to silicate glass in porphyroclastic spinel harzburgites from Marsabit (a) Zoned euhedral dolo-mite crystals (dark) rimmed by fine selvages of calcite (light) together with Na-rich Cr diopside (Dim) and chromite (Chrm sample Ke 195915)(b) Qualitative compositional map of carbonates in a melt pocket (sample Ke 19653) Lighter grey tones indicate higher Mg contentsThe euhedral shapes of the carbonate crystals should be noted The dashed white lines in (a) and (b) highlight the contact of carbonates withthe pristine silicate glass

7

Early metasomatic assemblagesClinopyroxene

lsquoCrypticallyrsquo metasomatized cpx-I (Type III-a and -b) arediopsides (Table 2) with high Mg-numbers and lowTi con-tents typical for mantle harzburgite (eg Pearson et al2003) The major element compositions of lsquounmetasoma-tizedrsquo cpx-I from Type III-a and -b xenoliths are similarHowever Type III-b samples occasionally contain clino-pyroxene that is texturally identical to cpx-I but containssignificantly higher amounts of Na2O (up to 254wtsee Fig 6a) pointing to cryptic metasomatism Trace ele-ment analyses (Table 2) show enrichment of U Th Srand LREE relative to middle REE (MREE) variabledepletions in heavy REE (HREE) and marked negativeanomalies for high field strength elements (HFSE Nb TaZr HfTi Fig7a)Trace element enrichment is strongest inthe high-Na cpx-I (Fig 7a)

Orthopyroxene

Opx-I from Type III-a and -b xenoliths is enstatite-rich(Mg-number of 912^925) It is LREE-depleted andslightly enriched in U and Th indicating cryptic metaso-matism (Table 3) Opxm (Types III-c and -d) has higherMg-numbers than opx-I (up to 931) and is extremelyAl-poor (02^10wt Al2O3) LREE and MREE areslightly enriched relative to HFSE which is unusual formantle-derived orthopyroxene (Rampone et al 1991)

Amphibole

Green amphibole (Type III-b and -c) is mostly Ti-poor(Table 4) with very high Si and Na contents and highMg-numbers (Fig 8a and b) Low-Ti amphibole in TypeIII-c is Mg-katophorite (Fig 8a according to Leake et al1997) which is an unusual composition for mantle-derivedamphibole Type III-b samples contain less silicic edenite^Mg-katophorite (Fig 8a) Cr contents (Fig 8b) are highonly in amphiboles related to metasomatic replacement ofspl-I Fluorine and Cl were not detected by electronmicroprobe Trace element patterns show enrichment inU and Th strongly negative Hf and Zr anomaliesand enrichment of LREE over HREE (Fig 9a and b)Amphibole from sample Ke 196515 has the lowest HREEconcentrations but in contrast exhibits a conspicuouspositive Ti anomaly (Fig 9b) Nb and Ta contents varyconsiderably with the highest abundances in euhedralMg-katophorite (Table 4 Fig 9c) Compared with typicalmantle amphiboles (eg Ionov et al 1997) theMg-katophorites show a large compositional range(eg Fig 10a) They are however remarkably depleted inlarge ion lithophile elements (LILE eg 126^83 mgg Bacompared with 75^1400 mgg as reported for mantle amphi-bole from several off-craton xenolith suites see Ionov et al1997) and show lowLILELREE ratios (Fig10b)

Phlogopite

Phlogopites (Table 5) have very high Na2O contents (up to288wt) Ti concentrations are mostly very low andMg-numbers are high (493) Fluorine and Cl were notdetected by electron microprobe Generally Na-rich phlo-gopites have higher U and Th concentrations and lowerHFSE contents than phlogopite from the Group I (grt)^spllherzolites (Kaeser et al 2006 Fig 9a) Some trace elementsshow very broad ranges between samples (eg Nb rangingfrom 102mgg in Ke 785 to 479 mgg in Ke 19653)

Apatite

Considerable differences were found between the two tex-tural types of apatite (early in sample Ke 19653 and phos-phate globules associated with glass in Ke 195915)Microprobe analyses only poorly satisfy apatite stoichio-metry and show fairly low totals (Table 5) This isprobably due to the abundant inclusions which areinevitably included in the analysis Early apatite isF-rich (189^253wt F) with variable Cl contents(092^157wt) It has extremely high REE and SrO con-tents (338^524wt see Table 5) The MgO Na2O andFeOtot contents are typical for upper mantle apatites(OrsquoReilly amp Griffin 2000)In contrast apatite globules in silicate glass are Cl-rich

(296^314wt Cl) and have lower SrO and Na2O butslightly higher FeOtot contents (Table 5) Both types of apa-tite are strongly LREE enriched [(LaYb)N up to 783] andare the major host for U andTh (up to 33500 times primi-tive mantle values) whereas HFSE concentrations are verylow (Fig 11b)

Late (glass-bearing) assemblagesSilicate glass

Representative results of microprobe and LA-ICPMSanalyses of silicate glass are given in Table 6 and selectedcompositional plots are displayed in Fig 12a^f Generallyglass is siliceous (up to 66wt SiO2) and peraluminous(1245^2507wt Al2O3) with characteristics (very lowMgO and FeO high alkalis) similar to those in manyother xenolith suites (eg Ionov et al 1994 Zinngrebe ampFoley 1995 Chazot et al 1996a Neumann amp Wulff-Pedersen 1997 Draper amp Green 1997 Varela et al 1999Coltorti et al 2000 Laurora et al 2001 Shaw amp Klulaquo gel2002 Ban et al 2005) Mg-numbers are commonly high(up to 70 Fig 12b) Considerable compositional hetero-geneity can occur on a very small scale within individualsamples In sample 785 for examples glass in the vicinityof Opxm contains up to 65wt SiO2 whereas a nearbymelt pocket (at less than 5mm distance) contains glasswith 58wt SiO2With respect to trace elements glass is always LREE

enriched [(LaYb)N up to 77] at variable absolute REEconcentrations (Fig 13 YbN from 14 to 910) Zr Hf and

8

Table 2 Representative analyses of clinopyroxene (oxides in wt trace elements in mgg)

Type III-a III-b III-c III-d

Mineral cpx-I cpx-I cpx-I cpx-I cpx-I Dim Dim Dim Dim Ti-aug cpx

Sample 19651 19651 19651 196515 195915 195915 19653 785 195927 195927 19706

Texture Zone A1

core

Zone B1

rim

Zone C1

core

core core

high-Na

core micro-xx

core

micro-xx

core

micro-xx

core

micro-xx

rim

core

high-Na

SiO2 5246 5322 5325 5310 5410 5189 5433 5388 5429 4757 5390

TiO2 015 016 014 040 020 022 012 037 016 271 007

Al2O3 351 356 396 436 425 409 199 219 275 692 389

Cr2O3 135 121 112 139 137 203 545 362 224 146 284

FeO(tot) 228 179 174 196 210 204 183 206 244 334 253

MnO 004 006 002 004 007 004 009 007 014 013 004

NiO 009 na na 005 003 002 004 003 nd 005 nd

MgO 1611 1632 1609 1594 1566 1653 1585 1667 1696 1439 1604

CaO 2198 2222 2171 2030 1981 2264 1738 1970 1981 2195 1841

Na2O 127 134 151 193 241 063 292 167 172 080 232

K2O nd nd nd 003 001 001 001 nd nd 004 nd

Total 9926 9988 9954 9950 10000 9949 10001 10024 10051 9934 10002

Mg-no 9266 9419 9427 9355 9302 9353 9392 9352 9254 8848 9189

P 126 169 168 216 655 611 1135 468 611 786 129

K 664 086 985 198 186 5839 137 112 187 766 114

Sc 416 646 464 243 692 119 970 108 736 604 786

Ti 747 816 828 2027 1053 875 613 1663 1022 9622 439

V 233 245 249 275 278 261 378 525 234 385 303

Ni 287 269 277 302 294 348 368 411 316 472 329

Zn 585 587 679 914 137 440 793 108 150 293 109

Ga 232 168 239 473 na na na 440 na na 358

Rb 0015 0013 0016 016 50076 5021 0051 013 049 033 50008

Sr 528 580 118 986 106 474 687 470 218 115 484

Y 362 106 232 165 110 181 739 199 406 229 436

Zr 835 100 790 104 130 4792 226 229 535 536 174

Nb 013 0051 0045 014 014 028 358 0070 113 072 116

Cs 0003 50002 50002 0005 50032 50069 50019 50020 50018 50079 50002

Ba 060 0047 423 444 028 5063 047 038 602 5079 0085

La 097 304 681 259 139 122 884 280 882 395 217

Ce 243 503 705 548 216 377 195 137 371 146 903

Pr 045 053 056 073 166 478 239 264 693 268 161

Nd 239 242 238 357 538 196 995 133 385 167 864

Sm 078 088 071 117 110 241 184 39 129 594 214

Eu 035 037 028 043 037 122 621 136 400 225 741

Gd 085 110 079 087 110 311 143 387 122 411 164

Tb 013 024 011 010 na na na 068 na na 233

Dy 072 172 055 049 164 235 134 496 923 503 112

Ho 013 038 0094 0068 037 079 258 081 166 084 182

Er 035 118 017 015 117 124 707 209 415 186 379

Tm 0056 019 0027 0017 016 024 095 022 053 024 039

Yb 051 128 016 012 095 192 567 137 336 280 211

Lu 0087 018 0022 0015 022 028 070 0089 042 015 024

(continued)

9

Ti are depleted relative to the REE and Nb and Ta rela-tive to U and Th Trace element compositions alwaysmirror the composition of the associated earlier phases(ie amphibole in most cases but also clinopyroxeneorthopyroxene phlogopite and apatite)Glass in Group III xenoliths is clearly different com-

pared with Ti-rich subalkaline glass occasionally found inGroup I lherzolites from Marsabit (related to melting ofTi-pargasite see Kaeser 2006 and Fig 12a^f) Also glassis generally clearly distinct from the xenolith-hosting basa-nites (Fig 12a^f) The exception is glass within the TypeIII-c dunite Ke 195927 and from the margin of xenolithKe 196515 (contact with host lava) which show someimportant differences compared with glass from theother samples that is higher FeO and lower SiO2

contents similar to the composition of the host basanite(Figs 12a^f and 13)

Clinopyroxene micro-phenocrysts

Clinopyroxene micro-phenocrysts (Dim) are Na-rich Crdiopsides (up to 323wt Na2O and 645wt Cr2O3Fig 6a and b) with low TiO2 contents (505wt) andhigh Mg-numbers (up to 949) In all samples Dim arestrongly zoned with Cr and Na contents generally decreas-ing from cores to rims (Fig 6b) Dim in contact withbasanitic glass in the kaersutite-bearing dunite (Ke 195927) has resorbed rims overgrown by Ti-augite (Fig 4f)with significantly lower Na2O (51wt) contents andMg-numbers (85 seeTable 2)Trace element patterns of Dim are similar in shape to the

associated glass (except LILE Cs to U Fig 13) NegativeHFSE anomalies are mostly due to higher REEabundances that is the absolute concentrations of HFSEare similar to those of cpx-I from Type III-a and -b(see Fig 7a no real HFSE depletion) Texturally equili-brated clinopyroxene in the Type III-d spl harzburgite

(Ke 19706 Fig 3f) is a Na-rich Cr diopside withsimilar major (Fig 6a) and trace element characteristics(Fig 7b) similar to those of the glass-hosted micro-phenocryst cores

Olivine spinel kaersutite

Compared with ol-I glass-related Olm is more magnesian(Table 3 up to Fo946) and enriched in Ca Ol-I in contactwith glass develops strong compositional gradientsapproaching the composition of Olm at the rims Spinelmicro-phenocrysts (Chrm) are Mg-rich chromites (Cr-number up to 665) enriched in (calculated) Fe2O3 com-pared with spl-IKaersutite occurs as tiny rims on amphibole inType III-

b and -c and as micro-phenocrysts inType III-c sample Ke195927 Reaction rims are characterized by higherMg-numbers than the original amphibole They are toonarrow to be analysed for trace elements Kaersutitemicro-phenocrysts in sample Ke 195927 have lowMg-number (about 82) and high K2O contents (Table 4)Major element characteristics as well as trace elementpatterns (convex-upward REE patterns high Nb^Ta lowU^Th Fig 9d) are similar to those of hornblenditesand amphibole megacrysts from alkaline magmas(eg Shaw amp Eyzaguirre 2000)

Carbonates

Most glass-related carbonate globules contain Mg-calcite(Table 7) with an average Mg-number of 965 andCa(CathornMg) between 087 and 098 In sample Ke 19653 MgO correlates positively with SrO (up to 535wtMgO and 089wt SrO) Other elements show verylow concentrations (Table 7) One sample (Type III-c Ke195915) contains zoned crystals ranging from dolomiticto calcitic in composition (Fig 5a) Carbonates havelow REE contents with flat to slightly LREE-enriched

Table 2 Continued

Type III-a III-b III-c III-d

Mineral cpx-I cpx-I cpx-I cpx-I cpx-I Dim Dim Dim Dim Ti-aug cpx

Sample 19651 19651 19651 196515 195915 195915 19653 785 195927 195927 19706

Texture Zone A1

core

Zone B1

rim

Zone C1

core

core core

high-Na

core micro-xx

core

micro-xx

core

micro-xx

core

micro-xx

rim

core

high-Na

Hf 034 031 028 050 025 040 081 0056 102 242 023

Ta 0011 0007 0016 0021 50029 50080 0657 50012 0043 5019 0056

Pb 037 050 058 115 068 5035 023 5028 50089 5039 011

Th 012 041 131 073 494 038 045 0019 0062 50074 0062

U 0076 027 047 028 129 50078 0049 50018 0030 50071 0017

1See Fig 15nd not detected na not analysed

10

patterns (Fig 14) Extended trace element plots exhibit avery broad range of concentrations in different samples(eg Pb Fig 14) Large positive peaks for U Pb and Srare typical In addition U and Nb are strongly enriched

Fig 6 Major element composition of clinopyroxene in porphyro-clastic spinel harzburgite and dunite xenoliths from Marsabit(a) Plot of Cr2O3 vs Na2O (wt) in micro-phenocrysts comparedwith cryptically metasomatized cpx-I in Type III-b sampleKe 195915 texturally equilibrated Na-rich clinopyroxene in TypeIII-d sample Ke 19706 and texturally equilibrated Na-rich cpxfrom fluid-metasomatized peridotite from Tok (far-east RussiaIonov et al 2005) The dashed line outlines the compositionalrange of common low-Na micro-phenocrysts in glass from otherxenolith suites (see Ionov et al 2005 and references therein)Crosses show the compositional variation of micro-phenocrysts inthe Group III-c xenolith 195927 (ie Dim overgrown by low-Naand low-Cr Ti-augite see Fig 4f) Compositional range of primarycpx-I in all Group III xenoliths (cpx-I grey field) is from Kaeseret al (2006) (b) Na2O and Cr2O3 (wt) variation across a singleDim micro-phenocryst in Type III-c xenolith Ke 19653 (electronmicroprobe profile)

Fig 7 Trace element composition of clinopyroxene from the porphyr-oclastic spinel harzburgites from Marsabit [normalized to the primi-tive mantle values of McDonough amp Sun (1995)] (a) Trace elementpatterns of primary cryptically metasomatized clinopyroxene (cpx-I)inType III-a and -b spinel harzburgites Small arrows indicate high-Na cpx-I inType III-b sample Ke 195915 (see also Fig 6a) (b) Traceelement patterns of texturally equilibrated Na-rich Cr diopside fromthe recrystallized Type III-d spinel harzburgite The shaded fieldsindicate the compositional range of glass-related clinopyroxene micro-phenocrst (Dim)

11

Table 3 Representative analyses of orthopyroxene spinel and olivine (oxides in wt trace elements in mgg)

Type III-a III-b III-b III-c III-d III-c III-d III-c III-c III-c III-d

Mineral opx-I opx-I opx-I Opxm Opxm Chrm spl-I ol-I Olm ol-I ol-I

Sample 19651 196515 195915 19653 19706 19653 19706 785 19653 196525 19706

Texture Zone C1 core core core core core core core core core core core

SiO2 5657 5642 5615 5807 5713 012 005 4093 4135 4078 4098

TiO2 004 013 006 001 005 044 016 nd 001 001 nd

Al2O3 265 269 299 046 192 1223 1880 001 001 nd 004

Cr2O3 043 056 040 032 054 5454 5043 001 029 001 005

Fe2O3 mdash mdash mdash mdash mdash 554 338 mdash mdash mdash mdash

FeO(tot) 533 568 576 538 486 1114 1207 939 583 1133 771

MnO 013 014 014 012 012 na 005 015 013 027 007

NiO na 010 009 007 009 015 009 040 024 037 039

MgO 3473 3420 3380 3561 3464 1480 1502 4900 5182 4739 5013

CaO 051 053 048 020 052 003 000 001 008 002 008

Na2O 005 008 013 009 014 na na 001 004 nd 001

Total 10044 10052 10000 10034 10002 9910 10008 9992 9980 10018 9947

Mg-no 9208 9148 9128 9218 9271 7032 6894 9029 9406 8817 9206

Na 341 423 1136 539 1120 na 438 297 262 647 230

Al 15120 14586 17785 1817 10834 na 114911 893 821 120 1201

P 156 145 175 175 248 na 198 421 292 105 244

Ca 3163 3016 3881 1487 3895 na 55348 822 579 200 658

K 172 5044 5035 981 150 na 5086 5037 5102 198 630

Sc 805 546 173 244 124 na 280 061 432 094 286

Ti 239 619 366 449 227 na 956 156 871 170 204

V 768 906 772 149 634 na 925 104 377 096 283

Ni 568 646 585 588 748 na 1010 2686 2875 2920 2649

Zn 256 301 245 606 373 na 732 630 665 146 693

Ga 181 350 na na 133 na 280 0015 na 0035 077

Sr 024 014 044 102 167 na 0089 0017 50013 0045 052

Y 0081 0073 049 069 202 na 0025 0015 016 010 019

Zr 023 0277 051 0038 088 na 124 50005 50033 50006 015

Nb 50005 0020 50007 018 0051 na 0877 50003 0025 0008 0035

Ba 0056 50018 50021 748 0035 na 50028 50016 50071 017 013

La 0019 50002 0055 0061 0045 na 0039 0021 50004 0017 0019

Ce 0020 0005 0046 0109 027 na 0052 0023 50012 0027 015

Pr 0002 50001 0005 0007 0064 na 0024 50001 50011 0009 0019

Nd 50008 0010 0043 0039 042 na 50029 50007 50052 0018 011

Sm 0014 50009 0016 50018 019 na 0022 50006 50051 50009 0032

Eu 50003 50002 50005 0012 0077 na 0018 50002 50012 50003 0011

Gd 50014 50006 0028 0021 020 na 50026 50006 50041 50016 0035

Tb 0002 50001 na na 0045 na 50004 50001 na 50001 0004

Dy 0015 0011 0086 0058 038 na 0010 50003 50043 50009 0041

Ho 0007 0003 0012 0031 0069 na 0003 50001 50006 0004 0010

Er 50012 0008 0043 0094 022 na 50015 50006 0058 0018 0027

Tm 0002 0001 0015 0014 0031 na 50003 50001 0010 0003 0004

Yb 0027 0011 019 0170 028 na 50018 0009 0064 0060 0038

Lu 0001 0002 0026 0026 0033 na 0007 0002 0040 0018 0008

(continued)

12

Table 4 Representative analyses of amphibole (oxides in wt trace elements in mgg)

Type III-b III-c

Mineral katoph katoph katoph katoph katoph katoph katoph katoph kaersut kaersut

Sample 195915 196515 196515 196515 785 785 196525 196525 195927 195927

Texture residual

in mp

interst interst lamellae

in cpx-I

residual

in mp

residual

in mp

euhedral core euhedral rim micro-xx micro-xx

SiO2 4541 4603 4625 4887 4955 4959 4952 4953 4064 4106

TiO2 015 190 203 099 030 029 022 028 612 577

Al2O3 1365 1145 1100 915 826 807 877 794 1405 1395

Cr2O3 251 123 148 144 254 263 106 115 031 008

FeO(tot) 350 293 304 278 302 294 359 346 588 615

MnO 006 003 004 002 008 002 010 008 016 010

NiO 009 014 009 010 010 006 009 012 010 010

MgO 1883 1886 1896 1997 2067 2062 2000 2024 1576 1545

CaO 851 974 960 976 805 793 863 909 1086 1075

Na2O 474 427 440 432 548 555 529 509 284 294

K2O 049 038 041 031 027 029 028 039 139 138

Total 9811 9695 9728 9771 9837 9805 9759 9741 9817 9793

Mg-no 9055 9199 9176 9275 9243 9259 9086 9124 8269 8173

P 445 222 235 na 175 196 206 120 214 266

K 4301 3916 3637 na 2393 2654 2955 3655 13289 12762

Sc 402 704 663 na 358 345 311 352 375 270

Ti 720 9867 9917 na 1732 1898 1421 1589 32154 32071

V 216 317 332 na 289 299 172 187 469 404

Ni 726 842 828 na 811 876 737 813 638 530

Zn 298 163 159 na 188 214 451 457 463 548

Ga na 981 939 na 103 114 867 872 na na

Rb 279 184 151 na 035 050 096 150 744 713

Sr 216 325 335 na 708 667 955 117 618 668

Y 101 269 315 na 111 126 390 359 310 355

Zr 539 370 331 na 292 297 114 592 933 110

Nb 339 413 447 na 102 121 192 705 265 341

Cs 0006 50001 0005 na 0006 50009 0008 0018 50019 50010

(continued)

Table 3 Continued

Type III-a III-b III-b III-c III-d III-c III-d III-c III-c III-c III-d

Mineral opx-I opx-I opx-I Opxm Opxm Chrm spl-I ol-I Olm ol-I ol-I

Sample 19651 196515 195915 19653 19706 19653 19706 785 19653 196525 19706

Texture Zone C1 core core core core core core core core core core core

Hf 0006 0010 0014 50009 0013 na 0042 50003 50017 50009 50006

Ta 50001 50001 50004 50005 50002 na 0015 0001 50018 50001 50001

Pb 50028 0041 0047 0074 50041 na 015 0036 0075 0042 0046

Th 0009 50002 0069 0020 50002 na 0010 0050 50006 0011 50001

U 0004 50001 0026 0017 0004 na 0017 0011 50006 50001 50002

1See Fig 15nd not detected na not analysed

13

compared withTh andTa respectively Hf Zr andTi showvery low concentrations Overall major and trace elementcharacteristics of the carbonates are similar to those of car-bonates found in other peridotite xenoliths (eg Ionov1998 Lee et al 2000 Laurora et al 2001 Ionov ampHarmer 2002 see Fig 14)

FORMATION OF THE EARLYASSEMBLAGESDespite large compositional variations numerous commontextural and compositional features indicate that the earlymetasomatic assemblages in all Group III samples formedduring a single metasomatic event In the discussion belowwe argue that during a first metasomatic stage the pri-mary (pre-metasomatic) phase assemblage in Group IIIspinel harzburgites was partially replaced by metasomaticminerals finally resulting in clinopyroxene-free amphi-bole- and phlogopite-bearing dunite and harzburgite(Type III-c) The initial metasomatic agent was probably

an evolved CO2^H2O-rich melt and the transitionfrom peridotites of Type III-a to -c can be explained byprogressive melt^rock reaction

Transition from cryptic (Type III-a) tomodal (Type III-b and -c) metasomatismFormation of new phases requires (1) crystallization from amelt or fluid phase (decreasing melt or fluid mass) andor(2) melt^solid or fluid^solid reactionTextural features suchas spinel being replaced by amphibole and phlogopite(eg Fig 3c) indicate melt(fluid)^rock reaction Thegeochemical signatures observed in the different peridotitetypes thus probably reflect pre-existing compositionalheterogeneities coupled with metasomatic modificationThis is illustrated best in sample Ke 19651 (Type III-a)the least metasomatized xenolith in this study It can besubdivided into three parts containing clinopyroxenewith different REE signatures (Zones A B and C inFig 15) The LREE are increasingly enriched from ZoneA to C indicating increasing cryptic metasomatism

Table 4 Continued

Type III-b III-c

Mineral katoph katoph katoph katoph katoph katoph katoph katoph kaersut kaersut

Sample 195915 196515 196515 196515 785 785 196525 196525 195927 195927

Texture residual

in mp

interst interst lamellae

in cpx-I

residual

in mp

residual

in mp

euhedral core euhedral rim micro-xx micro-xx

Ba 781 614 540 na 259 309 846 841 331 322

La 357 116 110 na 704 824 177 113 107 138

Ce 627 225 231 na 217 248 621 426 338 448

Pr 574 208 224 na 301 349 856 623 557 724

Nd 177 720 779 na 128 151 363 296 318 398

Sm 208 143 143 na 285 316 752 740 945 111

Eu 080 058 060 na 108 115 232 252 363 384

Gd 182 085 088 na 225 241 701 692 940 980

Tb 011 013 na 039 050 120 117 na na

Dy 171 057 067 na 202 252 735 698 700 819

Ho 036 010 010 na 039 050 155 144 137 147

Er 106 024 027 na 097 114 405 382 296 336

Tm 016 0030 0033 na 012 017 060 058 027 039

Yb 120 028 030 na 077 099 400 386 243 272

Lu 019 0036 0038 na 0079 0077 058 044 033 031

Hf 015 0067 0054 na 0065 0048 028 078 331 376

Ta 0033 014 014 na 0002 0004 010 107 167 198

Pb 201 360 374 na 142 173 195 222 022 039

Th 682 150 134 na 181 196 059 028 010 020

U 212 039 039 na 040 048 019 012 0018 0048

katoph Mg-katophorite kaersut kaersutite interst interstitial mp melt pocket nd not detected na notanalysed

14

HREE abundances on the other hand are very hetero-geneous in the least metasomatized Zone A moderatelydepleted in Zone B and homogeneously low in the mostmetasomatizedZoneC (containinggraphite Fig15) similarto REE patterns in clinopyroxene from the amphibole- andphlogopite-bearingType III-b sample (Ke196515 Fig7a)The decoupling of strongly (LREETh U) from moder-

ately incompatible elements (HREE) can be explained bychromatographic fractionation during percolation ofsmall amounts of melts or fluids through an initially non-metasomatized peridotite (Navon amp Stolper 1987 Godardet al 1995 Bedini et al 1997 Vernie res et al 1997 Ionovet al 2002) Upon reaction with the wall-rock a fluidor melt will become progressively enriched in incom-patible elements as a function of increasing distancefrom its source This is due to the fact that elements withhigher Dmineralmelt values (eg HREE) are selectivelyremoved from a percolating liquid when it exchanges withthe surrounding minerals The results are faster-movingchemical fronts of incompatible elements compared withthose of more compatible elements This is consistent with

the geochemical signature of cpx-I from the least metaso-matized part of sample Ke 19651 (Zone A Fig 15) whichalready started to re-equilibrate with respect to the incom-patible elements (ie addition of Th U and LREE)whereas more compatible elements still reflect pre-metasomatic heterogeneities In this case these areextreme HREE variations probably reflecting the earliergarnet breakdown (Kaeser et al 2006) On the otherhand more intensely metasomatized parts (eg Zone Cin sample Ke 19651 and sample Ke 196515) are character-ized by clinopyroxene trace element patterns wheremildly incompatible elements have also been modified(eg homogeneously depleted HREE Figs 7a and 15)Similar fractionation processes can explain the modal

and compositional differences between Type III-b and -cCompared withType III-b higher abundances of metaso-matic phases inType III-c suggests that higher amounts offluid or melt crystallized in or passed through these rocksThe amphibole-rich dunite (Ke 196525) for examplecontains low-Ti Mg-katophorite exhibiting the lowest(LaYb)N and (LaCe)N ratios and high (NbLa)N and

Fig 8 Major element characteristics of amphiboles from porphyroclastic spinel harzburgites and dunites from Marsabit compared with com-positions of mantle-derived amphiboles from other localities (XB basalt-hosted xenoliths XS xenoliths from subduction settings O orogenicperidotites) Mg-numberfrac14Mg(MgthornFetot) in molar proportions

15

(NbTh)N ratios (Fig 10a and c) indicating a moderatelyfractionated metasomatic agent Mg-katophorite fromthe phlogopite-rich harzburgite (Ke 785 Type III-c)and amphiboles from the more cpx-rich harzburgites(Type III-b) show progressively more fractionated patterns(eg Fig 10c) and decreasing abundances in lsquoamphibole-compatiblersquoelements (eg Nb Fig 9a) Thus the transitionfrom Type III-b to -c could reflect decreasing distancebetween the site at which the xenolith was sampledand the source of the lsquoinitialrsquo metasomatizing agent (egZanetti et al 1996 Ionov et al 2002 Bodinier et al 2004)

Judging from the abundance of modal metasomatic phasesthe amphibole dunite (Ke 196525) was probably closest tothe source of the metasomatizing agent The phlogopite-rich nature of Ke 785 which indicates higher K andLILE (Cs Rb Ba) contents in the remaining agent is inagreement with a more fractionated melt or fluid at some-what greater distance from the metasomatic sourceThe Nb abundances in amphibole also agree with this

model as well For example amphibole from Type III-cKe 196525 has comparatively high Nb concentrations(Fig 9c) in line with relatively high DNb

amphmelt values

Fig 9 Trace element composition of amphibole from porphyroclastic spinel harzburgites and dunites from Marsabit [normalized to the primi-tive mantle values of McDonough amp Sun (1995)] Mg-katophorite in Type III-b and -c xenoliths (a^c) and glass-related kaersutite (d) Theshaded fields indicate the compositional variation of all Mg-katophorite analyses (light grey) and the compositional range of LREE-depletedTi-pargasite from the Group I (grt)^spinel lherzolites (dark grey Kaeser et al 2006)

16

constrained by different studies (eg Ionov amp Hofmann1995 LaTourrette et al 1995) In Ti-poor systems such asthose of this study Nb uptake by amphibole is evenenhanced as a result of increased DNb values (Tiepoloet al 2001) Consequently in an event during which amphi-bole crystallizes continuously the remaining fluid or meltwill become progressively depleted in Nb (Ionov et al2002) resulting in formation of Nb-depleted amphibolesfurther away from the source (Zanetti et al 1996 Ionovet al 2002) such Nb-depleted amphibole is found insample Ke 785 as well as inType III-b xenoliths (eg Ke195915 Fig 9a) Amphibole signatures from xenolith suitesmodelled successfully in the context of percolative traceelement fractionation (eg Spitsbergen Ionov et al 2002)show similar ranges (eg in NbTh ratios Fig10a) to thosedocumented for our samples This further supports the

hypothesis that similar percolation mechanisms acted inthe mantle beneath Marsabit

The role of pre-existing heterogeneitiesand exotic mineralsAlthough a model based on trace element fractionationduring melt percolation is supported by textures and canaccount for many compositional features some composi-tional peculiarities remain unexplained First clinopyrox-ene and amphibole in several samples (Ke 785 19651 and196515) are characterized by extremely low HREE abun-dances more than one order of magnitude lower thanin amphiboles from the strongly metasomatizedamphibole dunite Ke 196525 (Fig 9a^c) The mainphases that crystallize during metasomatism are low-TiMg-katophorite and phlogopite However DHREE

amphmelt

Fig 10 Selected trace element ratios in amphiboles from the porphyroclastic spinel harzburgites from Marsabit compared with those ofmantle-derived amphibole from other tectonic settings (XK and XB cratonic kimberlite-hosted and off-cratonic basalt-hosted xenolithsrespectively O orogenic peridotites XS xenoliths from subduction settings) Concentrations (ratios) normalized to the primitive mantlevalues of McDonough amp Sun (1995)

17

Table 5 Representative analyses of phlogopite and apatite (oxides in wt trace elements in mgg)

Type III-b III-c III-b III-c

Mineral phlog phlog phlog phlog phlog phlog phlog apatite apatite

Sample 196515 196515 19653 785 785 785 196525 195915 19653

Texture veinlet interst in opx in opx with amph

in mp

around

chromite

incl in

amph

bleb in

glass

interst

grain

SiO2 3925 3914 4171 4190 4124 4134 4230 SiO2 016 049

TiO2 313 307 009 053 039 037 027 Al2O3 001 nd

Al2O3 1544 1583 1360 1360 1384 1377 1477 P2O5 3768 3715

Cr2O3 112 105 132 147 190 197 063 FeO(tot) 037 029

FeO(tot) 350 347 271 000 289 277 340 MnO 006 nd

MnO nd 002 008 305 004 002 005 MgO 084 091

NiO 023 027 019 023 024 028 023 CaO 4750 4237

MgO 2287 2280 2577 2591 2582 2528 2525 SrO 095 397

CaO nd 002 001 nd 004 001 002 Na2O 079 133

Na2O 215 219 282 258 264 262 208 La2O3 003 007

K2O 719 707 704 706 674 684 683 Ce2O3 088 347

Nd2O3 003 007

F 139 224

Cl 287 092

Total 9488 9493 9571 9641 9587 9535 9590 Totaly 9305 9245

Mg-no 9282 9287 9443 9380 9408 9420 9298

P 369 187 480 339 206 188 362 P 129717 139419

K 71711 72575 60747 79052 70997 69992 70189 K 110 897

Sc 099 110 131 130 088 091 160 Sc 242 079

Ti 15500 16408 890 2839 2096 1807 1736 Ti 235 5294

V 255 284 888 142 108 130 788 V 365 382

Ni 1808 1727 1744 2046 1813 1773 1905 Ni 121 397

Zn 234 221 358 382 317 321 695 Zn 119 096

Ga 111 984 na 210 190 188 235 Ga na na

Rb 185 147 289 183 133 133 342 Rb 015 049

Sr 102 116 775 325 363 350 101 Sr 7138 42233

Y 010 0086 50057 50054 0021 0006 0052 Y 988 1454

Zr 019 019 043 50088 0077 0070 073 Zr 307 101

Nb 506 628 479 177 102 146 210 Nb 024 020

Cs 175 138 261 399 216 180 798 Cs 50013 50036

Ba 1212 1096 1343 298 277 219 1124 Ba 139 126

La 010 0055 0069 50029 0020 50003 50045 La 7460 19048

Ce 0027 0011 50075 50019 014 50003 0078 Ce 6844 25945

Pr 0012 0008 50067 50024 0012 50003 50019 Pr 380 2212

Nd 0019 0035 50405 50079 0065 50009 50224 Nd 749 6530

Sm 0015 50011 50474 50090 0016 50010 50132 Sm 515 747

Eu 0022 0017 50104 50026 0013 0010 50095 Eu 147 189

Gd 50010 50011 5043 5015 50011 0012 017 Gd 314 451

Tb 50002 50002 na 50027 0002 50002 50028 Tb na na

Dy 50015 50017 045 50077 50007 50006 50077 Dy 198 281

Ho 50002 50003 50043 50020 0002 50002 50028 Ho 359 511

Er 50009 50013 5025 5011 50016 50009 5011 Er 903 119

Tm 50002 50002 50088 50013 0002 0003 50018 Tm 119 138

(continued)

18

data (eg LaTourrette et al 1995) indicate that amphibolecrystallization alone is unlikely to deplete the metasomaticagent to such a high degree in HREE The same probablyaccounts for the extreme Nb depletion in amphiboles fromKe 785 relative to the otherType III-c xenolith (Ke 196525 see also Ionov et al 2002)Further clinopyroxene phlogopite and amphibole in the

Type III-b sample Ke 196515 on one hand all showstrongly fractionated REE patterns (eg high LaCe inamphibole Fig 10c) but on the other hand are character-ized by a positiveTi anomaly decoupled from other HFSE(eg Hf and Zr Fig 9b) Trace element fractionationduring amphibole crystallization (ie formation of TypeIII-c amphibole dunites) however should lower Ti con-tents in the remaining fluid or melt (eg Moine et al 2001)Based on textures mineralogy and dominant common

compositional features the hypothesis that Type III-band -c xenoliths were metasomatized by completely inde-pendent metasomatic agents can be ruled out Thereforetwo other possibilities remain to account for the inconsis-tent observations described above (1) inference of otherphases reactants or products that are able to fractionatespecific trace elements present in the mantle beneathMarsabit but not present in the few investigated samples(2) pre-metasomatic compositional heterogeneities similarto the inherited HREE variation inType III-a clinopyrox-ene (see above)Garnet would be an important sink for HREE but

metasomatism occurred in the spinel field after decom-pression with maximum pressures of 515GPa (Kaeseret al 2006) so garnet had already broken down to sym-plectites (Fig 15) Percolation of melt through garnet-bearing websterites prior to metasomatism in Ke 19651and 196515 could be an alternativeWebsterites have been

interpreted to occur in close spatial relationship with por-phyroclastic peridotites in the shallow mantle beneathMarsabit (Kaeser et al 2006) and they occasionally con-tain rutile and ilmenite which could further account forthe strong Nb and Ta depletion in some samples (eg Ke785) The available compositional data from the webster-ites however do not indicate metasomatic modification ofwebsteritic garnet (Olker 2001 B Kaeser unpublisheddata) nor compositional similarities to Group IIIperidotitesCareful study by SEM rules out the presence of exotic

phases such as zircon or Ti-oxides described fromother xenolith suites (eg Bodinier et al 1996 Rudnicket al 1999) Nevertheless it cannot be excluded that themetasomatizing agent percolated through mantle domainscontaining these phases before infiltrating the studiedrocksThe preservation of pre-metasomatic compositional het-

erogeneities is another possibility that must be consideredThat such lsquorock signalsrsquo partly overlap with the lsquometaso-matic signalrsquo is indicated by the HREE variation in TypeIII-a clinopyroxene (see above) and also by other composi-tional parameters For example both Mg-katophorite andphlogopite growing at the expense of spinel have very highCr2O3 contents whereas euhedral Mg-katophorite inthe amphibole dunite (Ke 196525) or in phlogopiteinclusions in Opxm is comparably Cr2O3-poor (Fig 8bTables 4 and 5) Pre-existing heterogeneities would essen-tially be preserved through elements that are relativelycompatible in a given mineral (ie that are not mobilizedduring metasomatism) This would be in agreement withthe preserved HREE variations in Type III-a clinopyrox-ene high Cr2O3 in amphiboles and phlogopite and Tispikes in amphibole from Ke 196515

Table 5 Continued

Type III-b III-c III-b III-c

Mineral phlog phlog phlog phlog phlog phlog phlog apatite apatite

Sample 196515 196515 19653 785 785 785 196525 195915 19653

Texture veinlet interst in opx in opx with amph

in mp

around

chromite

incl in

amph

bleb in

glass

interst

grain

Yb 50014 50011 5047 5015 50011 50019 5017 Yb 803 724

Lu 0002 50002 50061 50014 50002 50002 50020 Lu 115 742

Hf 0019 0018 5025 50053 50012 50006 5019 Hf 50023 50120

Ta 020 022 140 0031 0012 0009 023 Ta 50015 50075

Pb 546 586 125 108 083 098 143 Pb 436 577

Th 50004 50004 50080 0043 0088 0056 50021 Th 2538 1305

U 0012 0011 0069 50018 0038 0030 0031 U 572 278

interst interstitial mp melt pocket na not analysed nd not detected

19

Further very high Mg-numbers in replacive amphibolessupport melt (fluid)^rock interaction at low melt (fluid)rock ratios where Mg and Fe contents are buffered by theambient peridotite (Kelemen et al 1995 Tiepolo et al2001) The slightly lower Mg-number of euhedralamphiboles in Ke 196525 combined with their lowerCr2O3 contents (Fig 8b) suggests a higher melt (fluid)rock ratio which is further supported by considerablylower Mg-number in associated ol-I (Fo88 Table 3)

The limited number of investigated samples does notallow us to decide which of the processes described aboveacted and to what extent However given the heterogene-ity observed in the numerous non-metasomatized xenolithsfrom Marsabit (Kaeser et al 2006) pre-metasomaticfeatures certainly affect the final product of the melt^rockreaction process to a considerable degree

The nature of the metasomatizing agent(s)Graphite stability and oxygen fugacity considerations

The Marsabit xenoliths are unusual in that they occur inan off-cratonic setting yet bear macroscopic graphite Thelatter usually either occurs in kimberlite-hosted peridotites(on-craton) or in pyroxenites from alpine-type peridotitebodies (eg Pearson et al 1994) In the phlogopite perido-tites from Finero graphite has recently been reportedas nanoscopic layers in phlogopite (Ferraris et al 2004)In xenoliths from Marsabit graphite occurs in the sametextural relationship as in kimberlite-hosted peridotitexenoliths (ie millimetre-sized flakes typically associatedwith orthopyroxene see Fig 3c^e and Pearson et al 1994)Graphite inclusions in phlogopite and Opxm couldindicate crystallization during metasomatism by alsquoreducedrsquo C-bearing fluid or melt However precipitationof graphite more probably reflects the relatively reducingconditions in the uppermost mantle prior to metasoma-tism We used the oxygen barometer of Ballhaus et al(1991) based on ferric iron contents in spinel coexistingwith olivine to constrain oxygen fugacity (fO2)conditions prior to metasomatism in the Marsabitxenoliths (Fig 16) The results show that non-metasomatized Group I and IV samples reflect similarlyreducing conditions to the cryptically metasomatizedType III-a samplesAlthough assessing Fe3thornFe2thorn ratios in spinel based

on microprobe analyses can be problematic (Wood ampVirgo 1989 Ballhaus et al 1991) all microprobeanalyses were obtained using the same operationalsettings and therefore give at least the relative fO2

variation between samples The relatively reducingconditions in the pre-metasomatic cold andhighly deformed mantle well below carbon^carbonate^silicate equilibria (Mg2Si2O6thorn 05 CaMg(CO3)2frac14Mg2SiO4thorn 05 CaMgSi2O6thornCthornO2 Eggler amp Baker1982 see Fig 16) and close to the CCO oxygen buffer arein agreement with the presence of graphite in the GroupIII harzburgites Moreover the P^T conditions at whichthe Group III xenoliths partially re-equilibrated are closeto the subsolidus decarbonation of model carbonated lher-zolite (Fig 2 see Lee amp Wyllie 2000 and referencestherein) in agreement with the observed phase assemblageincluding orthopyroxene olivine clinopyroxene andgraphite

Fig 11 Trace element patterns of (a) phlogopite and (b) apatitefrom porphyroclastic spinel harzburgites and dunites from Marsabit[concentrations normalized to the primitive mantle valuesof McDonough amp Sun (1995)] The grey field in (a) indicates thecomposition of Ti-rich phlogopite in Group I (grt)^spl lherzolites(Kaeser et al 2006)

20

Table 6 Representative analyses of silicate glass (oxides in wt trace elements in mgg)

Type III-b III-c Host

Sample 195915 196515 196515 785 196525 19653 19653 19653 195927

Remark next amph next cpx xenol margin next amph next amph melt pocket next apatite next Opxm melt pocket host basanite

SiO2 5306 5775 5161 6096 5657 5938 5974 6530 4882 4410

TiO2 017 157 106 062 030 024 020 023 378 255

Al2O3 2412 2053 2025 1779 2313 1575 1245 1355 1890 1396

Cr2O3 005 001 nd 001 001 nd nd nd 002 na

FeO(tot) 188 247 626 126 252 295 374 265 509 1283

MnO 007 000 026 001 004 005 008 007 018 021

NiO nd nd nd na nd na 006 nd 001 na

MgO 182 129 116 114 122 304 382 252 244 938

CaO 506 257 283 082 044 124 101 039 810 1030

Na2O 1107 966 969 971 1299 1388 1384 1211 673 374

K2O 154 197 465 357 138 121 142 127 300 146

Cl 031 106 038 012 014 023 061 020 015 na

P2O5 041 008 044 009 012 049 233 018 100 059

Total 9956 9896 9860 9609 9887 9845 9929 9846 9821 9912

Mg-no 6329 4816 2489 6175 4632 6474 6458 6289 4552 5658

P 1061 448 1610 230 503 1519 6738 676 2994 na

K 12563 16408 39599 32515 15872 9265 11277 10071 25900 na

Sc 179 184 126 406 151 970 136 848 691 237

Ti 665 7329 4974 2953 1965 1213 1163 1055 18199 15311

V 107 265 235 796 920 210 151 134 234 na

Ni 128 371 92 5575 983 170 5199 5298 199 191

Zn 962 103 109 110 459 440 508 309 819 na

Ga na 173 273 194 220 na na na na na

Rb 181 263 124 546 626 176 398 369 577 369

Sr 427 488 1407 188 881 1644 4997 985 1062 792

Y 543 145 224 752 262 619 158 369 301 299

Zr 466 171 353 761 660 578 365 428 242 216

Nb 416 161 137 260 136 234 164 169 837 647

Cs 014 046 186 125 007 022 102 052 063 na

Ba 238 261 1064 129 168 405 1099 641 625 563

La 698 183 848 144 889 2212 1008 106 537 462

Ce 947 256 131 308 138 333 1265 164 104 823

Pr 655 194 120 330 116 304 975 160 120 na

Nd 146 603 408 101 329 993 256 564 463 400

Sm 142 048 570 179 419 141 379 679 105 851

Eu 041 035 228 082 112 438 103 241 345 267

Gd 062 051 415 158 402 106 245 566 677 729

Tb na 0086 086 020 071 na na na na 111

Dy 090 027 359 098 434 104 171 637 732 na

Ho 023 0037 105 025 095 206 446 104 117 114

Er 045 038 195 058 255 611 153 264 305 na

Tm 009 50039 033 0081 042 085 292 5014 036 052

Yb 062 023 267 044 307 667 120 487 253 220

(continued)

21

Evidence for low-temperature SiO2^Na2O^CO2^H2O-rich melts or fluids

Potential metasomatic agents inferred from the presenceof graphite are C^O^H-rich fluids carbonate melts orCO2-bearing silicate melts The resulting phase assem-blages and geochemical signatures are the result of infiltra-tion of fluid or melt into a cold (750^8508C) shallow(515GPa Fig 2) heterogeneous and locally reduced litho-spheric mantle Metasomatism under such conditions in anopen system must result in complex geochemical signa-tures that reflect the degree of fluid^rock reaction Thismakes it particularly difficult to constrain the exactnature of the lsquooriginalrsquometasomatic agentType III-c xenolith Ke 196525 contains the highest frac-

tion of metasomatic phases and the least fractionatedamphibole trace element patterns The lsquomagmaticrsquo texturesof the Mg-katophorites in this sample suggest direct crys-tallization from a melt or a fluid rather than replacementof a pre-existing phase Therefore hypothetical liquids inequilibrium with the amphiboles in this xenolith couldyield the composition of the initial metasomatic agentsFigure 17 illustrates the trace element patterns of suchmelts and fluids calculated using sets of experimentally orempirically determined mineralmelt (fluid) partitioncoefficients including carbonate melt silicate melt andhydrous fluid (for references see caption to Fig 17)Common features for all calculated metasomatic agents

are negative Zr^Hf^Ti anomalies and LREE enrichment(Fig 17) However previous studies have shown that thesesignatures are indicative of almost all types of metasoma-tism including interaction of peridotite with carbonatiticmelt (Green amp Wallace 1988 Dautria et al 1992 Hauriet al 1993 Rudnick et al 1993 Ionov et al 1996 Yaxleyet al 1998 Coltorti et al 1999) with hydrous fluids andormelts for example originating from dehydration of altered

subducting lithosphere (Maury et al 1992 Zanetti et al1999 Grecurren goire et al 2001 Laurora et al 2001 Parkinsonet al 2003) and interaction of peridotite with volatile-and Si-rich melts that evolved through reactive porousflow (Bedini et al 1997 Ionov et al 2002 2006 Bodinieret al 2004 Rivalenti et al 2004)In the case of the Marsabit xenoliths integration of the

trace element characteristics thermobarometric inferencesand mineralogical constraints favours a Na2O^SiO2^CO2^H2O-rich silicate melt Carbonated melts can prob-ably be ruled out on major element and mineralogical con-siderations First amphiboles are Ca-poor in conflict withcarbonate melt metasomatism which is generally charac-terized by Ca enrichment (Rudnick et al 1993 Neumannet al 2002) Further orthopyroxene is a stable phase andprobably even crystallizes during metasomatism in somesamples (eg Ke 785) At near-solidus conditions coexis-tence of orthopyroxene and carbonate melt is possibleonly at pressures 20GPa (see Fig 2 eg Lee amp Wyllie2000 and references therein) which is considerably higherthan the maximum pressures indicated by the compositionof spinel (515GPa) At such low pressures metasomatismby carbonated melts will convert lherzolite and harzbur-gite immediately into wehrlite which is the major perido-tite type in all xenolith suites where carbonate meltmetasomatism is convincingly demonstrated (Rudnicket al 1993 Yaxley et al 1998 Neumann et al 2002)In Marsabit wehrlites are absent howeverAn Na2O^SiO2^CO2^H2O-rich melt or fluid on the

other hand would explain the Na-rich nature of amphiboleand phlogopite as well as the presence of orthopyroxeneGrowth of metasomatic (and Al-poor) orthopyroxene hasbeen described from several xenolith localities where peri-dotites were infiltrated either by hydrous fluids orSi-rich melts (Smith et al 1999 McInnes et al 2001Morishita et al 2003) Na-rich phlogopite is commonly

Table 6 Continued

Type III-b III-c Host

Sample 195915 196515 196515 785 196525 19653 19653 19653 195927

Remark next amph next cpx xenol margin next amph next amph melt pocket next apatite next Opxm melt pocket host basanite

Lu 014 0050 026 0041 046 080 140 058 040 031

Hf 026 052 535 014 076 110 5087 124 568 504

Ta 50080 033 638 0022 123 101 784 801 394 403

Pb 541 448 280 255 840 797 260 749 493 na

Th 194 296 160 466 703 185 247 105 534 506

U 545 069 478 099 152 447 724 216 171 126

nd not detected na not analysedAverage composition of the strongly alkaline basanites from Marsabit (Volker 1990)

22

Fig 12 Total alkalis vs SiO2 (a) Harker-type co-variation diagrams (b^e) and incompatible trace element co-variation diagram (f) showingthe compositional variability of silicate glass found in the porphyroclastic spinel harzburgites and dunites from Marsabit In (b) binary mixinglines between the typical composition of the alkali-rich glass (large open diamond) and the associated microlites are shown (each increment is10wt)They illustrate that the glass does not represent the derivate of host lava infiltration and fractional crystallization Subalkaline glass inGroup I (grt)^spl lherzolites from Marsabit is shown for comparison and is interpreted to derive from incongruent melting of Ti-pargasite(Kaeser 2006)

23

found as a reaction product between anhydrous mafic toultramafic rocks and H2O-rich evolved liquids (Costaet al 2001 and references therein) or H2O-bearing fluids(eg Odling 1994) The latter study was an experiment at20GPa simulating the effects of decreasing temperatureson the phase assemblages and their composition duringmetasomatism using a model peridotitethornC^O^H fluidThe coolest part of Odlingrsquos (1994) experiment (510758C)contained a strikingly similar phase assemblage (kato-phorite phlogopite with 2wt Na2O and Al-poororthopyroxene) to the Marsabit samples interpreted tohave formed during sub-solidus fluid^solid interactionA compositionally similar phase assemblage (highamounts of opx Si-rich hornblende Na-rich phlogopite)resulted from experiments simulating peridotite interac-tion with Si-rich melt (dacite) at lower P^T conditions(9508C at 092GPa Prouteau amp Scaillet 2003)In this study the trace element signatures of Group III-c

Mg-katophorite and the calculated metasomatic agentsfurther suggest that the metasomatizing agent was a meltrather than a fluid Hypothetical fluids in equilibrium witheuhedral Mg-katophorite from sample Ke 196525 havehigh NbLILE (eg Rb Ba) and low LILEREE ratios(Fig 17) Because experimental data predict the oppositefor fluids (LILE enriched over HFSE and REE egManning 2004 and references therein) a fluid with asignature such as that indicated in Fig 17 is unrealistic

Fig 13 Trace element patterns of silicate glass in porphyroclasticspinel harzburgites and dunites from Marsabit compared withthe trace element composition of associated clinopyroxene (Dim)micro-phenocrysts xenolith-hosting basanites from Marsabit andglass interpreted to represent xenolith^host interaction [concentra-tions normalized to the primitive mantle values of McDonough ampSun (1995)]

Table 7 Representative analyses of carbonates (oxides in

wt trace elements in mgg)

Group III-b III-b III-c III-c III-c

Mineral Dol Mg-Cc Mg-Cc Mg-Cc Mg-Cc

Sample 195915 196515 785 19653 19653

Texture in glass in glass in glass in glass in glass

SiO2 na 003 na nd 003

FeO(tot) 016 011 010 010 006

MnO 001 nd nd nd nd

MgO 1922 236 190 125 517

CaO 3189 5313 5377 5391 4853

Na2O 003 nd 004 nd 003

K2O na 002 na 001 nd

SrO 003 010 008 004 021

BaO 003 nd nd na na

CO21 4618 4444 4442 4377 4391

Total 9756 10021 10032 9909 9794

Mg-no 9954 9745 9713 9554 9931

Ca-no 5439 9418 9531 9687 8709

P 220 334 137 580 589

K 970 212 642 105 155

Sc 5065 50150 027 5027 50076

Ti 975 189 242 528 196

V 5085 0481 5042 5037 021

Ni 900 673 517 5294 593

Zn 459 5089 113 296 045

Ga na 5014 5013 na na

Rb 0084 50072 50074 50060 0046

Sr 593 860 371 251 4274

Y 012 011 130 704 015

Zr 261 024 012 097 756

Nb 144 019 0056 692 185

Cs 50063 50011 50018 50039 50008

Ba 126 230 5024 029 271

La 034 057 084 136 025

Ce 032 013 134 997 023

Pr 006 006 018 095 0026

Nd 030 013 068 228 0078

Sm 5044 50091 0074 048 50018

Eu 5012 50019 0051 020 50008

Gd 5037 50090 014 051 0026

Tb na 50010 0036 na na

Dy 5019 50039 0089 058 50038

Ho 50029 0013 0044 011 50006

Er 5012 50041 0083 033 50019

Tm 50052 50013 0016 0084 50006

Yb 5043 50063 014 027 50040

Lu 50072 50010 0011 0066 50008

(continued)

24

At the relatively low maximum pressures (515GPa) asupercritical liquid is not expected either (eg Kesselet al 2005)Finally a relatively hydrous metasomatizing agent

would be consistent with formation of Mg-katophoriteswith lowTi and presumably low Fe3thorn contents (given therelatively reducing conditions) which in turn imply high(OH)^ contents In other words partial dehydrogenationtypically observed in common (Ti-rich) mantle amphibole

(eg Popp et al 1995 Tiepolo et al 2001 and referencestherein) cannot have played an important role in thiscase as H deficiency is mainly the result of coupled substi-tutions involvingTi and Fe3thorn (Popp et al 1995) In conclu-sion the metasomatizing agent was most probably aNa2O^SiO2^CO2^H2O-rich meltComparison with experimental studies further con-

strains the low-temperature regime in which metasoma-tism occurred In ultramafic (eg Niida amp Green 1999)basaltic (eg Rapp 1995) and Si-enriched systems (egtrachybasaltic Barclay amp Carmichael 2004) amphibole(in the latter study together with phlogopite) is confinedto temperatures lower than 1000^10508C at pressures520GPa (Fig 2)Thus crystallization of the early metaso-matic phase assemblage can be explained by infiltration ofa silicate melt that rapidly cooled upon contact with theambient peridotite (750^8508C prior to metasomatismFig 2) However this low-T stage is based on Ca-in-opxthermometry (see Brey amp Kolaquo hler 1990 Kaeser et al2006 Table 1) and therefore probably underestimates theeffective temperature conditions during metasomatismThis can be explained by low diffusion rates and proximityto the closure temperature for the diopside^enstatiteexchange reaction at temperatures lower than 8508C(eg Smith 1999 and references therein) The lower tem-perature limit at which silicate melt metasomatism ispossible (ie defined by the meltrsquos solidus and viscosity)depends strongly on volatile and SiO2 contentsExperiments by Draper amp Green (1997) for exampleshowed that highly silicic melts (52^64wt SiO2) canbe in equilibrium with mantle harzburgite at temperaturesas low as 8508C (at 10^20GPa under anhydrous andC^O^H-fluid-bearing conditions) These are howeverextreme compositions for mantle melts The actual SiO2

content of the metasomatizing agent in this case is difficultto constrain from the metasomatic mineral assemblage ascompositions may have varied considerably during reac-tive percolation (eg including Si-releasing reactions suchas cpx replacement by amphibole versus Si-consumingreactions such as Opxm crystallization) Moreover it isunlikely that a melt persists at temperatures muchbelow 9008C which is the solidus of fertile lherzolite(ie primitive upper mantle composition) underH2O-saturated conditions at 15GPa (Grove et al 2006see Fig 2) Below the solidus residual volatile-enrichedfluids may percolate further for example forming the lessmetasomatized Type III-c xenoliths (eg Ke 785) as wellas Type III-b and -a peridotites (see also Bailey 1987)Relatively cold conditions would also explain the veryhigh compositional gradients developed on small scales inType III-a samples (Fig 15) Given the absence of chloritein all investigated samples the lower temperature limitof such fluid metasomatism is 8008C (eg Fumagalli ampPoli 2005 see Fig 2)

Table 7 Continued

Group III-b III-b III-c III-c III-c

Mineral Dol Mg-Cc Mg-Cc Mg-Cc Mg-Cc

Sample 195915 196515 785 19653 19653

Texture in glass in glass in glass in glass in glass

Hf 5016 50038 50036 50074 50013

Ta 0048 50012 50016 50031 0008

Pb 5024 858 5021 074 018

Th 0021 50020 50017 50028 50005

U 060 094 086 600 148

Mg-Cc Mg-calcite Dol dolomite nd not detectedna not analysed Ca-numberfrac14 100Ca(CathornMg)1Calculated from stoichiometry

Fig 14 Trace element patterns of glass-hosted carbonates in TypeIII-b and -c spinel harzburgite and dunite xenoliths from Marsabit[concentrations normalized to the primitive mantle values ofMcDonough amp Sun (1995)] The grey field outlines the compositionalrange of carbonates found in other peridotite xenoliths (Ionov ampHarmer 2002 and references therein) Carbonate material inter-preted as quenched carbonatite melt found in peridotite xenolithsfrom Kerguelen (Moine et al 2004) is shown for comparison(bold line)

25

Finally the presence of apatite even though oftentaken as indicator for carbonatite-type metasomatism(eg Yaxley et al 1998) is not in conflict with metasoma-tism by silicate melts Apatite has also been shown to crys-tallize from volatile and alkali-rich derivatives formedduring reactive porous flow of originally mafic silicatemelts (Woodland et al 1996 Bedini et al 1997 Bodinieret al 2004 Ionov et al 2006)

FORMATION OF THE LATE GLASS -BEAR ING ASSEMBLAGESTextures strongly suggest that the glass-bearing assem-blages formed later and at the expense of the early metaso-matic assemblages (Fig 4a and d) Features such as thereappearance of clinopyroxene (Na-rich Cr diopsidemicro-phenocrysts) a phase replaced by the early

Fig 15 Detailed REE systematics in primary clinopyroxene (cpx-I) in a porphyroclastic cryptically metasomatized spinel harzburgite(sample Ke 19651) showing the transition fromType III-a (Zone A and B) toType III-b (Zone C) on a scale of a few centimetres The numberson the photomicrographs (crossed polars) correspond to single laser ablation microprobe pits (for further explanation see text)

26

metasomatic assemblage indicate that melt pocket forma-tion reflects a significant change in P^T^X space in partsof the mantle beneath Marsabit after earlier metasoma-tism Based on geochemical arguments and mass-balanceconsiderations we will show that (1) the late glass-bearingassemblages are closely related to early metasomaticassemblages mainly representing breakdown productsand (2) breakdown was induced by infiltration of aCO2-rich fluid in the mantle probably followed andenhanced by heating and decompression melting duringxenolith transport in the host magma

Evidence for in situ partial meltingBSE imaging reveals that the phases becoming unstableare amphibole orthopyroxene and to minor extentsspl-I phlogopite cpx-I and apatite Compositional(inter- and intra-xenolith) heterogeneity of silicate glass istexture controlled and varies as a function of theassociated precursor phases (Fig 12a^f)TiO2 for exampleshows higher concentrations in glass from xenolith Ke196515 containing Ti-enriched amphibole (Fig 12c)whereas K2O contents are highest in phlogopite-rich sam-ples (eg Ke 785 Fig 12d) Elevated P2O5 contents arefound in glass adjacent to apatite (Fig 12e) whichmirrors at the same time the highTh and LREE contents

of apatite (Fig 12f) Significant Si enrichment is found inglass coating orthopyroxene (Fig 12d) supporting incon-gruent melting of orthopyroxene (see below) Even thoughthe melt pockets are connected along grain boundaries thestrong compositional variation (on inter- and intra-xeno-lith scale) suggests that the mobility of the melt was lim-ited or that the melting event was very short-lived (fortiming constraints see below)The co-variation plots in Fig 12 further reveal consider-

able compositional differences between all types of xeno-lith glass and the host basanite These data indicate thatthe glass cannot be the product of xenolith^host lava inter-action during ascent to the surface Also back-mixing ofmicro-phenocrysts into the melt fails to derive the glassfrom a host-related melt (Fig 12b) Glass from xenolithmargins however is compositionally closer to the bulkhost basanite (eg Fig 12b) in line with a direct derivationfrom xenolith^host lava interaction Its more evolvedmajor element composition (eg higher SiO2 lowerMg-number Fig 12b) is probably the result of fractiona-tion of basanite phenocrysts (eg olivine augite andtitanomagnetite Volker 1990) Host lava^xenolith interac-tion is further supported by the trace element signature ofglass at the margin of the xenoliths which is closely similarto that of the Marsabit basanites (Fig 13 Volker 1990)The same characteristics are observed for intra-xenolithglass in the unusual kaersutite-bearing sample Ke 195927

Fig 16 Oxygen fugacities relative to the quartz^fayalite^magnetitebuffer (QFM equation of OrsquoNeill 1987) as a function of temperatureat assumed pressure of 1GPa calculated using the composition ofcoexisting spinel and olivine (ol-I^spl-I pairs and Chrm^Olm pairsrespectively) based on the oxygen barometer of Ballhaus et al (1991)The graphite^CO2 equilibrium was calculated after Frost amp Wood(1998) and the graphite^carbonate^silicate equilibrium was calcu-lated using the equation of Eggler amp Baker (1982) (See text for furtherexplanation)

Fig 17 Calculated metasomatic agents in equilibrium with the euhe-dral Mg-katophorites from the Type III-c amphibole dunite Ke 196525 Amphibolehydrous fluid and amphibolecarbonatite melt parti-tion coefficients are from Adam et al (1997) and Adam amp Green(2001) respectively Silicate melts have been calculated using amphi-boleSi-rich melt partition coefficients from Chazot et al (1996b) andamphibolebasanite partition coefficients from LaTourrette et al(1995) The compositional range of the Marsabit basanites and alkalibasalts (Volker 1990) is shown for comparison

27

Xenolith glass unrelated to the host lava and melt pock-ets similar to those reported here in association with resi-dual phases (eg amphibole or clinopyroxene) has beenreported from many xenolith localities (eg Frey ampGreen 1974 Ionov et al 1994 1996 Chazot et al 1996a1996b Yaxley et al 1997 Laurora et al 2001 Bali et al2002) Residual phases within the melt pockets stronglyindicate in situ partial melting within the xenolithsHowever a variety of processes may trigger such in situ

melting including (1) closed-system decompression melt-ing of either anhydrous (Maale amp Printzlau 1979) orhydrous phases (Frey amp Green 1974 Francis 1976) duringxenolith ascent in the host lava (2) melting during interac-tion of the xenoliths with the host basaltic melt duringxenolith ascent (eg Zinngrebe amp Foley 1995 Shaw ampKlulaquo gel 2002 Shaw et al 2006) (3) melting in the mantleprior to xenolith entrainment induced by an infiltratingfluid (ie flux melting eg Jones et al 1983 Ionov et al1994 Demecurren ny et al 2004) However most studies agreethat that partial melting occurred very shortly beforeandor during xenolith exhumation by the host magmaThus a combination of metasomatically triggered meltingfollowed andor assisted by decompression melting duringxenolith transport may be a realistic alternative in somecases (eg Dawson 2002)Recently Ban et al (2005) have shown in a very detailed

mass-balance study on glass-bearing xenoliths from theWest Eifel that the weight proportions of glass and theassociated micro-phenocrysts match the composition ofthe preserved parent amphibole We used the sameapproach and performed simple mass-balance calculations(least-squares fitting) to constrain the conclusions drawnfrom petrography and to test which phase contributedand to what extent to the formation of the melt pocketsThe results reported in Table 8 show that in most casesamphibole-melting can account successfully for the meltpocket composition and only a minor amount (520) ofother phases (orthopyroxene andor phlogopite) needs tobe added to the reactant Such a contribution of otherphases agrees with textural observation (eg Fig 4cshowing Dim growing on resorbed Opxm) as well as withthe conclusions of Ban et al (2005) who stated that moreSi-rich glass (ie 454wt) necessitates orthopyroxenecontributing to the meltTrace element patterns of the bulk glassthornmicro-

phenocryst pockets calculated using the measured traceelement abundances of glass and micro-phenocrysts andthe respective mass-balance parameters (Table 8) arebroadly identical to the measured amphibole composition(Fig 18a and b) Higher LILE (Cs Rb Ba) concentrationsin the calculated bulk melt patches in sample Ke 785 canbe balanced by adding 7wt of phlogopite (Fig 18b)which is in line with mass balance (Table 8) The dominantcontrol of amphibole to account for the trace element

budget of the melt pockets is similar to results found inother suites of glass-bearing xenoliths (Chazot et al1996a Yaxley amp Kamenetsky 1999 Laurora et al 2001)Further the obtained phase proportions for the melt pock-ets are in good agreement with the results from image ana-lysis (Table 1) In addition they are similar to the glass^cpx^ol proportions recorded in recent experiments on the

Table 8 Results of mass-balance calculations to test melt

pocket formation by incongruent melting of the early metaso-

matic assemblages

Sample 195915 785 785 196525 196525 19653

Reactant

amph 1000 1000 0820 1000 0944 mdash

opx mdash mdash 0078 mdash 0056 0406

phl mdash mdash 0102 mdash mdash mdash

glass mdash mdash mdash mdash mdash 05941

SiO2 4484 4989 4964 5129 5167 5898

TiO2 014 039 036 025 024 018

Cr2O3 232 291 258 127 121 009

Al2O3 1368 762 769 800 757 961

FeO(tot) 336 314 333 343 357 394

MnO 006 008 008 012 012 010

NiO 009 013 014 011 011 004

MgO 1844 2029 2197 2040 2122 1625

CaO 854 783 644 966 913 078

Na2O 460 500 436 504 476 843

K2O 049 032 096 042 040 060

Product

glass 0389 0322 0335 0292 0284 07032

ol 0243 0259 0315 0266 0286 0273

cpx 0288 0373 0308 0442 0431 0024

spl 0052 0032 0028 mdash mdash mdash

carb 0028 0014 0014 mdash mdash mdash

SiO2 4477 5027 4986 5207 5181 5903

TiO2 013 041 039 015 014 017

Cr2O3 213 314 272 109 106 015

Al2O3 1364 764 770 798 776 957

FeO(tot) 374 331 351 322 332 350

MnO 007 007 007 009 009 006

NiO 011 013 015 011 012 007

MgO 1834 2024 2196 2063 2141 1635

CaO 854 759 632 952 929 080

Na2O 449 385 388 499 485 860

K2O 060 096 100 015 014 091

r2 023 200 035 084 028 033

Prefix Ke omitted1Low-Si glass 2High-Si glass (see text for explanation)

28

melting behaviour of amphibole in wehrlites at tempera-tures between 1175 and 12508C and pressures of05^10GPa (Mecurren dard et al 2006) These temperatures arein good agreement with the estimates for the formation ofolivine and chromite micro-phenocrysts in the melt

pockets (Fig 16) The heat supply required for amphibolemelting in the formerly low-T metasomatized Group IIIxenoliths (T 200^3008C) may come from local heatingshortly before or during xenolith entrainment in the hostlava or from fluid infiltration that triggered melting (seediscussion below)Finally formation of the most Si-enriched melt (adjacent

to Opxm in Group III-c samples) can be modelled assum-ing progressing interaction between infiltrating melt andorthopyroxene where the latter melts incongruently

opxthornmelt1 olthornmelt2 cpx ethseeTable 8THORN eth1THORN

To calculate reaction (1) we chose the least evolved glassobserved in melt patches in sample Ke 19653 (SiO2

5921wt Mg-number 6465) to represent the initialmelt1 whereas melt2 adjacent to opx is SiO2-rich(6530wt) Trace element abundances in glass next toorthopyroxene are shifted to lower concentrations(Table 6) which is in agreement with dilution caused byaddition of a trace element-poor orthopyroxene compo-nent The resultant phase proportions are similar to thoseobserved at orthopyroxene^melt interfaces (ie higholivine and low cpx abundances) and are in agreementwith experimental studies (Shaw et al 1998)

Partial melting triggered by fluid fluxMass-balance calculations provide an explanation for meltpocket formation by in situ partial melting Thus onemight conclude that melting is simply the result of decom-pression and heating during xenolith transport in the hostmagma However there are two points that argue thatmelting was triggered by the infiltration of a fluid phaseshortly before the xenoliths were sampled by the hostmagma (1) the presence of occasionally high amounts ofcarbonate included in the melt pockets (2) the extremelyNa-rich clinopyroxene (Dim) compositions either includedas micro-phenocrysts in the melt pocket or present asequilibrated grains around chromite in the single TypeIII-d xenolith (Fig 3f) Both arguments will be discussedin the next section

Formation of carbonates

Textures and compositions of carbonate globules can beexplained by carbonate precipitation from a CO2-bearingfluid As demonstrated by several studies highCa(CathornMg) values (Table 7) and low REE concentra-tions are strong arguments that the carbonates are cumu-late crystals precipitated from a CO2-rich melt or afluid and not quenched carbonated melts (Lee et al 2000Ionov amp Harmer 2002) Oscillatory zoning (Fig 5a and b)of individual carbonate crystals within singleglobules corroborates this interpretation The roundedcontacts toward glass (globules) are also in line with thesefindings Similar globular forms were reported fromexperimental studies and do not a priori imply the

Fig 18 Calculated trace element characteristics of reactants (in mostcases amphibolethorn orthopyroxene filled symbols) and products(melt pockets open symbols) of the lsquoamphibole meltingrsquo model[see equation (1) in text] The relative phase proportions wereobtained by mass-balance calculations and are given in Table 8 Thedashed lines in (b) show that the high LILE concentrations obtainedfor the product (bulk melt pocket) can be explained by adding about7wt of phlogopite to the reactant (amphibolethorn orthopyroxeneTable 8) Further explanation is given in the text

29

presence of immiscible carbonate and silicate melts(eg Lee amp Wyllie 1996)The high amount of carbonate in some melt pockets

however (eg Fig 4e) does need further explanation asthe incongruent melting reaction (1) does not account forthe CO2 required for carbonate precipitation duringamphibole melting Therefore amphibole breakdowneither occurred in an open system with addition of at leastCO2 (eg Laurora et al 2001 Ban et al 2005) or anotherpre-existing phase acted as a source of carbon Oxidationof graphite texturally identified as a part of the earlyassemblage (eg Fig 3d) could account for some of theCO2 necessary for carbonate formation A change in fO2

conditions is also indicated when the composition of glass-related spinel (Chrm) and Olm are used for oxygen baro-metry which yields oxygen fugacities well on the oxidizedside of the carbon^carbonate^silicate buffer (Fig 16)However increasing fO2 triggered by closed-systemdecompression-induced melting alone does not appear fea-sible Also textures such as straight contacts between gra-phite and Mg-calcite (Fig 4c) suggest that carbonategrows on graphite (ie indicating local fO2 conditions cor-responding to carbon^carbonate equilibrium) rather thanreplacing it We therefore interpret melt pocket formationto be triggered by the infiltration of a CO2-rich fluid(ie flux melting see also Ionov et al 1994) This interpre-tation also accounts for the presence of abundant vugs inthe glass that could have contained a fluid (Ionov et al1994 2006) which was presumably lost during sample pre-paration However we emphasize here that late fluid infil-tration had only subtle effects on the trace elementcomposition of Type III-b and -c xenoliths

Formation of Na-rich Cr diopside

Infiltration of CO2-rich fluidwould also explain the Na-richcomposition of Dim micro-phenocryst cores (Fig 6a and b)Ionov et al (2005) noted recently that clinopyroxenes inmeltpockets are commonly characterized by Na2O contents51wt mainly as a result of low cpxmelt partition coeffi-cients at low pressures (eg Blundy et al 1995) Thus veryhighNa2Ocontents in Dimmicro-phenocrysts (Fig6a) sug-gest replacement of amphibole by Dim and eventually Olmand Chrm during reaction with a fluid at subsolidus condi-tions An origin of micro-phenocrysts by replacementrather than crystallization from a melt is indicated by theidentical crystallographic orientation of individual Dimcrystals in singlemelt pockets (Fig4a)Once the solidus of the precursor phase(s) was reached

(ie 10508C for amphibole Niida amp Green 1999) silicatemelt was generated in situ and led to the formation of clin-opyroxene rims with lower Na contents (Fig 6b) and thedevelopment of euhedral shapes in contact with melt (egFig 4a and e) Eventual immiscibility between remainingfluid and newly formed melt (eg during xenolith ascent)would then explain the presence of carbonate globules and

vugs enclosed in glass Likewise different origins of meltand carbonate were recently proposed for similar meltpockets from Hungarian xenoliths by Demecurren ny et al(2004) who found oxygen isotope disequilibrium betweencarbonate globules and the host glass Such a two-stepmodel (fluid-induced replacement followed by melt gen-eration) can be explained by CO2-fluid infiltration occur-ring within the mantle at sub-solidus conditions ultimatelyfollowed by xenolith entrainment in the host magma andtransport to the surfaceThe host magma possibly suppliedthe additional heat necessary for melt generationFast decompression would further enhance melting andcould also account for the lower Na contents in Dimmicro-phenocryst rims as a result of decreasing cpxmeltpartition coefficients at decreasing pressures (Blundyet al 1995)Rapid replacement of amphibole ( phlogopite) by an

anhydrous phase assemblage indicates that the fluid waswater-poor (eg low H2OCO2 ratio see Ionov et al2006)This is corroborated by the fact that Mg-katophorite(presumably OH-rich see above) is occasionally rimmedby a residual kaersutitic selvage (Fig 4d) Such Ti-richmantle amphiboles are commonly OH-poor as a result ofpartial dehydrogenation (eg Tiepolo et al 2001 and refer-ences therein)Finally the presence of theType III-d xenolith contain-

ing texturally equilibrated clinopyroxene of almost identi-cal composition to Dim micro-phenocrysts in melt pockets(Table 2 Figs 6a and 7b) further strengthens the argu-ment that CO2-fluid percolation started in the mantlebefore entrainment (ie enough time elapsed to allow thisrock to anneal completely after the late metasomaticassemblage had formed) A similar conclusion can bedrawn for the clearly host-related glass found in xenolithKe 195927 (eg Fig 13b) ThisType III-c sample is charac-terized by the presence of Dim micro-phenocrysts partlyovergrown by mineral rims clearly related to the host basa-nite magma (kaersutite Ti-augite and olivine withFo885 Fig 4f Tables 3 and 4 and supplementary dataat httppetrologyoxfordjournalsorg) Kaersutite hasmajor and trace element characteristics similar to typicalbasanitic and alkali basaltic amphibole phenocrysts(Fig 9d) The special phase assemblages in this xenolithindicates that host-related basanitic melts intruded someType III-c xenoliths and partly overprinted the assemblageformed slightly earlier by in situ derived alkali-rich peralu-minous melts fully preserved in other samples (eg Ke19653 or Ke 785)

ORIGIN OF METASOMATIZ INGAGENTS T IM ING AND TECTONICSETT INGAs noted above late metasomatism was probably short-lived started at mantle depths and probably continued

30

Table 2 Representative analyses of clinopyroxene (oxides in wt trace elements in mgg)

Type III-a III-b III-c III-d

Mineral cpx-I cpx-I cpx-I cpx-I cpx-I Dim Dim Dim Dim Ti-aug cpx

Sample 19651 19651 19651 196515 195915 195915 19653 785 195927 195927 19706

Texture Zone A1

core

Zone B1

rim

Zone C1

core

core core

high-Na

core micro-xx

core

micro-xx

core

micro-xx

core

micro-xx

rim

core

high-Na

SiO2 5246 5322 5325 5310 5410 5189 5433 5388 5429 4757 5390

TiO2 015 016 014 040 020 022 012 037 016 271 007

Al2O3 351 356 396 436 425 409 199 219 275 692 389

Cr2O3 135 121 112 139 137 203 545 362 224 146 284

FeO(tot) 228 179 174 196 210 204 183 206 244 334 253

MnO 004 006 002 004 007 004 009 007 014 013 004

NiO 009 na na 005 003 002 004 003 nd 005 nd

MgO 1611 1632 1609 1594 1566 1653 1585 1667 1696 1439 1604

CaO 2198 2222 2171 2030 1981 2264 1738 1970 1981 2195 1841

Na2O 127 134 151 193 241 063 292 167 172 080 232

K2O nd nd nd 003 001 001 001 nd nd 004 nd

Total 9926 9988 9954 9950 10000 9949 10001 10024 10051 9934 10002

Mg-no 9266 9419 9427 9355 9302 9353 9392 9352 9254 8848 9189

P 126 169 168 216 655 611 1135 468 611 786 129

K 664 086 985 198 186 5839 137 112 187 766 114

Sc 416 646 464 243 692 119 970 108 736 604 786

Ti 747 816 828 2027 1053 875 613 1663 1022 9622 439

V 233 245 249 275 278 261 378 525 234 385 303

Ni 287 269 277 302 294 348 368 411 316 472 329

Zn 585 587 679 914 137 440 793 108 150 293 109

Ga 232 168 239 473 na na na 440 na na 358

Rb 0015 0013 0016 016 50076 5021 0051 013 049 033 50008

Sr 528 580 118 986 106 474 687 470 218 115 484

Y 362 106 232 165 110 181 739 199 406 229 436

Zr 835 100 790 104 130 4792 226 229 535 536 174

Nb 013 0051 0045 014 014 028 358 0070 113 072 116

Cs 0003 50002 50002 0005 50032 50069 50019 50020 50018 50079 50002

Ba 060 0047 423 444 028 5063 047 038 602 5079 0085

La 097 304 681 259 139 122 884 280 882 395 217

Ce 243 503 705 548 216 377 195 137 371 146 903

Pr 045 053 056 073 166 478 239 264 693 268 161

Nd 239 242 238 357 538 196 995 133 385 167 864

Sm 078 088 071 117 110 241 184 39 129 594 214

Eu 035 037 028 043 037 122 621 136 400 225 741

Gd 085 110 079 087 110 311 143 387 122 411 164

Tb 013 024 011 010 na na na 068 na na 233

Dy 072 172 055 049 164 235 134 496 923 503 112

Ho 013 038 0094 0068 037 079 258 081 166 084 182

Er 035 118 017 015 117 124 707 209 415 186 379

Tm 0056 019 0027 0017 016 024 095 022 053 024 039

Yb 051 128 016 012 095 192 567 137 336 280 211

Lu 0087 018 0022 0015 022 028 070 0089 042 015 024

(continued)

9

Ti are depleted relative to the REE and Nb and Ta rela-tive to U and Th Trace element compositions alwaysmirror the composition of the associated earlier phases(ie amphibole in most cases but also clinopyroxeneorthopyroxene phlogopite and apatite)Glass in Group III xenoliths is clearly different com-

pared with Ti-rich subalkaline glass occasionally found inGroup I lherzolites from Marsabit (related to melting ofTi-pargasite see Kaeser 2006 and Fig 12a^f) Also glassis generally clearly distinct from the xenolith-hosting basa-nites (Fig 12a^f) The exception is glass within the TypeIII-c dunite Ke 195927 and from the margin of xenolithKe 196515 (contact with host lava) which show someimportant differences compared with glass from theother samples that is higher FeO and lower SiO2

contents similar to the composition of the host basanite(Figs 12a^f and 13)

Clinopyroxene micro-phenocrysts

Clinopyroxene micro-phenocrysts (Dim) are Na-rich Crdiopsides (up to 323wt Na2O and 645wt Cr2O3Fig 6a and b) with low TiO2 contents (505wt) andhigh Mg-numbers (up to 949) In all samples Dim arestrongly zoned with Cr and Na contents generally decreas-ing from cores to rims (Fig 6b) Dim in contact withbasanitic glass in the kaersutite-bearing dunite (Ke 195927) has resorbed rims overgrown by Ti-augite (Fig 4f)with significantly lower Na2O (51wt) contents andMg-numbers (85 seeTable 2)Trace element patterns of Dim are similar in shape to the

associated glass (except LILE Cs to U Fig 13) NegativeHFSE anomalies are mostly due to higher REEabundances that is the absolute concentrations of HFSEare similar to those of cpx-I from Type III-a and -b(see Fig 7a no real HFSE depletion) Texturally equili-brated clinopyroxene in the Type III-d spl harzburgite

(Ke 19706 Fig 3f) is a Na-rich Cr diopside withsimilar major (Fig 6a) and trace element characteristics(Fig 7b) similar to those of the glass-hosted micro-phenocryst cores

Olivine spinel kaersutite

Compared with ol-I glass-related Olm is more magnesian(Table 3 up to Fo946) and enriched in Ca Ol-I in contactwith glass develops strong compositional gradientsapproaching the composition of Olm at the rims Spinelmicro-phenocrysts (Chrm) are Mg-rich chromites (Cr-number up to 665) enriched in (calculated) Fe2O3 com-pared with spl-IKaersutite occurs as tiny rims on amphibole inType III-

b and -c and as micro-phenocrysts inType III-c sample Ke195927 Reaction rims are characterized by higherMg-numbers than the original amphibole They are toonarrow to be analysed for trace elements Kaersutitemicro-phenocrysts in sample Ke 195927 have lowMg-number (about 82) and high K2O contents (Table 4)Major element characteristics as well as trace elementpatterns (convex-upward REE patterns high Nb^Ta lowU^Th Fig 9d) are similar to those of hornblenditesand amphibole megacrysts from alkaline magmas(eg Shaw amp Eyzaguirre 2000)

Carbonates

Most glass-related carbonate globules contain Mg-calcite(Table 7) with an average Mg-number of 965 andCa(CathornMg) between 087 and 098 In sample Ke 19653 MgO correlates positively with SrO (up to 535wtMgO and 089wt SrO) Other elements show verylow concentrations (Table 7) One sample (Type III-c Ke195915) contains zoned crystals ranging from dolomiticto calcitic in composition (Fig 5a) Carbonates havelow REE contents with flat to slightly LREE-enriched

Table 2 Continued

Type III-a III-b III-c III-d

Mineral cpx-I cpx-I cpx-I cpx-I cpx-I Dim Dim Dim Dim Ti-aug cpx

Sample 19651 19651 19651 196515 195915 195915 19653 785 195927 195927 19706

Texture Zone A1

core

Zone B1

rim

Zone C1

core

core core

high-Na

core micro-xx

core

micro-xx

core

micro-xx

core

micro-xx

rim

core

high-Na

Hf 034 031 028 050 025 040 081 0056 102 242 023

Ta 0011 0007 0016 0021 50029 50080 0657 50012 0043 5019 0056

Pb 037 050 058 115 068 5035 023 5028 50089 5039 011

Th 012 041 131 073 494 038 045 0019 0062 50074 0062

U 0076 027 047 028 129 50078 0049 50018 0030 50071 0017

1See Fig 15nd not detected na not analysed

10

patterns (Fig 14) Extended trace element plots exhibit avery broad range of concentrations in different samples(eg Pb Fig 14) Large positive peaks for U Pb and Srare typical In addition U and Nb are strongly enriched

Fig 6 Major element composition of clinopyroxene in porphyro-clastic spinel harzburgite and dunite xenoliths from Marsabit(a) Plot of Cr2O3 vs Na2O (wt) in micro-phenocrysts comparedwith cryptically metasomatized cpx-I in Type III-b sampleKe 195915 texturally equilibrated Na-rich clinopyroxene in TypeIII-d sample Ke 19706 and texturally equilibrated Na-rich cpxfrom fluid-metasomatized peridotite from Tok (far-east RussiaIonov et al 2005) The dashed line outlines the compositionalrange of common low-Na micro-phenocrysts in glass from otherxenolith suites (see Ionov et al 2005 and references therein)Crosses show the compositional variation of micro-phenocrysts inthe Group III-c xenolith 195927 (ie Dim overgrown by low-Naand low-Cr Ti-augite see Fig 4f) Compositional range of primarycpx-I in all Group III xenoliths (cpx-I grey field) is from Kaeseret al (2006) (b) Na2O and Cr2O3 (wt) variation across a singleDim micro-phenocryst in Type III-c xenolith Ke 19653 (electronmicroprobe profile)

Fig 7 Trace element composition of clinopyroxene from the porphyr-oclastic spinel harzburgites from Marsabit [normalized to the primi-tive mantle values of McDonough amp Sun (1995)] (a) Trace elementpatterns of primary cryptically metasomatized clinopyroxene (cpx-I)inType III-a and -b spinel harzburgites Small arrows indicate high-Na cpx-I inType III-b sample Ke 195915 (see also Fig 6a) (b) Traceelement patterns of texturally equilibrated Na-rich Cr diopside fromthe recrystallized Type III-d spinel harzburgite The shaded fieldsindicate the compositional range of glass-related clinopyroxene micro-phenocrst (Dim)

11

Table 3 Representative analyses of orthopyroxene spinel and olivine (oxides in wt trace elements in mgg)

Type III-a III-b III-b III-c III-d III-c III-d III-c III-c III-c III-d

Mineral opx-I opx-I opx-I Opxm Opxm Chrm spl-I ol-I Olm ol-I ol-I

Sample 19651 196515 195915 19653 19706 19653 19706 785 19653 196525 19706

Texture Zone C1 core core core core core core core core core core core

SiO2 5657 5642 5615 5807 5713 012 005 4093 4135 4078 4098

TiO2 004 013 006 001 005 044 016 nd 001 001 nd

Al2O3 265 269 299 046 192 1223 1880 001 001 nd 004

Cr2O3 043 056 040 032 054 5454 5043 001 029 001 005

Fe2O3 mdash mdash mdash mdash mdash 554 338 mdash mdash mdash mdash

FeO(tot) 533 568 576 538 486 1114 1207 939 583 1133 771

MnO 013 014 014 012 012 na 005 015 013 027 007

NiO na 010 009 007 009 015 009 040 024 037 039

MgO 3473 3420 3380 3561 3464 1480 1502 4900 5182 4739 5013

CaO 051 053 048 020 052 003 000 001 008 002 008

Na2O 005 008 013 009 014 na na 001 004 nd 001

Total 10044 10052 10000 10034 10002 9910 10008 9992 9980 10018 9947

Mg-no 9208 9148 9128 9218 9271 7032 6894 9029 9406 8817 9206

Na 341 423 1136 539 1120 na 438 297 262 647 230

Al 15120 14586 17785 1817 10834 na 114911 893 821 120 1201

P 156 145 175 175 248 na 198 421 292 105 244

Ca 3163 3016 3881 1487 3895 na 55348 822 579 200 658

K 172 5044 5035 981 150 na 5086 5037 5102 198 630

Sc 805 546 173 244 124 na 280 061 432 094 286

Ti 239 619 366 449 227 na 956 156 871 170 204

V 768 906 772 149 634 na 925 104 377 096 283

Ni 568 646 585 588 748 na 1010 2686 2875 2920 2649

Zn 256 301 245 606 373 na 732 630 665 146 693

Ga 181 350 na na 133 na 280 0015 na 0035 077

Sr 024 014 044 102 167 na 0089 0017 50013 0045 052

Y 0081 0073 049 069 202 na 0025 0015 016 010 019

Zr 023 0277 051 0038 088 na 124 50005 50033 50006 015

Nb 50005 0020 50007 018 0051 na 0877 50003 0025 0008 0035

Ba 0056 50018 50021 748 0035 na 50028 50016 50071 017 013

La 0019 50002 0055 0061 0045 na 0039 0021 50004 0017 0019

Ce 0020 0005 0046 0109 027 na 0052 0023 50012 0027 015

Pr 0002 50001 0005 0007 0064 na 0024 50001 50011 0009 0019

Nd 50008 0010 0043 0039 042 na 50029 50007 50052 0018 011

Sm 0014 50009 0016 50018 019 na 0022 50006 50051 50009 0032

Eu 50003 50002 50005 0012 0077 na 0018 50002 50012 50003 0011

Gd 50014 50006 0028 0021 020 na 50026 50006 50041 50016 0035

Tb 0002 50001 na na 0045 na 50004 50001 na 50001 0004

Dy 0015 0011 0086 0058 038 na 0010 50003 50043 50009 0041

Ho 0007 0003 0012 0031 0069 na 0003 50001 50006 0004 0010

Er 50012 0008 0043 0094 022 na 50015 50006 0058 0018 0027

Tm 0002 0001 0015 0014 0031 na 50003 50001 0010 0003 0004

Yb 0027 0011 019 0170 028 na 50018 0009 0064 0060 0038

Lu 0001 0002 0026 0026 0033 na 0007 0002 0040 0018 0008

(continued)

12

Table 4 Representative analyses of amphibole (oxides in wt trace elements in mgg)

Type III-b III-c

Mineral katoph katoph katoph katoph katoph katoph katoph katoph kaersut kaersut

Sample 195915 196515 196515 196515 785 785 196525 196525 195927 195927

Texture residual

in mp

interst interst lamellae

in cpx-I

residual

in mp

residual

in mp

euhedral core euhedral rim micro-xx micro-xx

SiO2 4541 4603 4625 4887 4955 4959 4952 4953 4064 4106

TiO2 015 190 203 099 030 029 022 028 612 577

Al2O3 1365 1145 1100 915 826 807 877 794 1405 1395

Cr2O3 251 123 148 144 254 263 106 115 031 008

FeO(tot) 350 293 304 278 302 294 359 346 588 615

MnO 006 003 004 002 008 002 010 008 016 010

NiO 009 014 009 010 010 006 009 012 010 010

MgO 1883 1886 1896 1997 2067 2062 2000 2024 1576 1545

CaO 851 974 960 976 805 793 863 909 1086 1075

Na2O 474 427 440 432 548 555 529 509 284 294

K2O 049 038 041 031 027 029 028 039 139 138

Total 9811 9695 9728 9771 9837 9805 9759 9741 9817 9793

Mg-no 9055 9199 9176 9275 9243 9259 9086 9124 8269 8173

P 445 222 235 na 175 196 206 120 214 266

K 4301 3916 3637 na 2393 2654 2955 3655 13289 12762

Sc 402 704 663 na 358 345 311 352 375 270

Ti 720 9867 9917 na 1732 1898 1421 1589 32154 32071

V 216 317 332 na 289 299 172 187 469 404

Ni 726 842 828 na 811 876 737 813 638 530

Zn 298 163 159 na 188 214 451 457 463 548

Ga na 981 939 na 103 114 867 872 na na

Rb 279 184 151 na 035 050 096 150 744 713

Sr 216 325 335 na 708 667 955 117 618 668

Y 101 269 315 na 111 126 390 359 310 355

Zr 539 370 331 na 292 297 114 592 933 110

Nb 339 413 447 na 102 121 192 705 265 341

Cs 0006 50001 0005 na 0006 50009 0008 0018 50019 50010

(continued)

Table 3 Continued

Type III-a III-b III-b III-c III-d III-c III-d III-c III-c III-c III-d

Mineral opx-I opx-I opx-I Opxm Opxm Chrm spl-I ol-I Olm ol-I ol-I

Sample 19651 196515 195915 19653 19706 19653 19706 785 19653 196525 19706

Texture Zone C1 core core core core core core core core core core core

Hf 0006 0010 0014 50009 0013 na 0042 50003 50017 50009 50006

Ta 50001 50001 50004 50005 50002 na 0015 0001 50018 50001 50001

Pb 50028 0041 0047 0074 50041 na 015 0036 0075 0042 0046

Th 0009 50002 0069 0020 50002 na 0010 0050 50006 0011 50001

U 0004 50001 0026 0017 0004 na 0017 0011 50006 50001 50002

1See Fig 15nd not detected na not analysed

13

compared withTh andTa respectively Hf Zr andTi showvery low concentrations Overall major and trace elementcharacteristics of the carbonates are similar to those of car-bonates found in other peridotite xenoliths (eg Ionov1998 Lee et al 2000 Laurora et al 2001 Ionov ampHarmer 2002 see Fig 14)

FORMATION OF THE EARLYASSEMBLAGESDespite large compositional variations numerous commontextural and compositional features indicate that the earlymetasomatic assemblages in all Group III samples formedduring a single metasomatic event In the discussion belowwe argue that during a first metasomatic stage the pri-mary (pre-metasomatic) phase assemblage in Group IIIspinel harzburgites was partially replaced by metasomaticminerals finally resulting in clinopyroxene-free amphi-bole- and phlogopite-bearing dunite and harzburgite(Type III-c) The initial metasomatic agent was probably

an evolved CO2^H2O-rich melt and the transitionfrom peridotites of Type III-a to -c can be explained byprogressive melt^rock reaction

Transition from cryptic (Type III-a) tomodal (Type III-b and -c) metasomatismFormation of new phases requires (1) crystallization from amelt or fluid phase (decreasing melt or fluid mass) andor(2) melt^solid or fluid^solid reactionTextural features suchas spinel being replaced by amphibole and phlogopite(eg Fig 3c) indicate melt(fluid)^rock reaction Thegeochemical signatures observed in the different peridotitetypes thus probably reflect pre-existing compositionalheterogeneities coupled with metasomatic modificationThis is illustrated best in sample Ke 19651 (Type III-a)the least metasomatized xenolith in this study It can besubdivided into three parts containing clinopyroxenewith different REE signatures (Zones A B and C inFig 15) The LREE are increasingly enriched from ZoneA to C indicating increasing cryptic metasomatism

Table 4 Continued

Type III-b III-c

Mineral katoph katoph katoph katoph katoph katoph katoph katoph kaersut kaersut

Sample 195915 196515 196515 196515 785 785 196525 196525 195927 195927

Texture residual

in mp

interst interst lamellae

in cpx-I

residual

in mp

residual

in mp

euhedral core euhedral rim micro-xx micro-xx

Ba 781 614 540 na 259 309 846 841 331 322

La 357 116 110 na 704 824 177 113 107 138

Ce 627 225 231 na 217 248 621 426 338 448

Pr 574 208 224 na 301 349 856 623 557 724

Nd 177 720 779 na 128 151 363 296 318 398

Sm 208 143 143 na 285 316 752 740 945 111

Eu 080 058 060 na 108 115 232 252 363 384

Gd 182 085 088 na 225 241 701 692 940 980

Tb 011 013 na 039 050 120 117 na na

Dy 171 057 067 na 202 252 735 698 700 819

Ho 036 010 010 na 039 050 155 144 137 147

Er 106 024 027 na 097 114 405 382 296 336

Tm 016 0030 0033 na 012 017 060 058 027 039

Yb 120 028 030 na 077 099 400 386 243 272

Lu 019 0036 0038 na 0079 0077 058 044 033 031

Hf 015 0067 0054 na 0065 0048 028 078 331 376

Ta 0033 014 014 na 0002 0004 010 107 167 198

Pb 201 360 374 na 142 173 195 222 022 039

Th 682 150 134 na 181 196 059 028 010 020

U 212 039 039 na 040 048 019 012 0018 0048

katoph Mg-katophorite kaersut kaersutite interst interstitial mp melt pocket nd not detected na notanalysed

14

HREE abundances on the other hand are very hetero-geneous in the least metasomatized Zone A moderatelydepleted in Zone B and homogeneously low in the mostmetasomatizedZoneC (containinggraphite Fig15) similarto REE patterns in clinopyroxene from the amphibole- andphlogopite-bearingType III-b sample (Ke196515 Fig7a)The decoupling of strongly (LREETh U) from moder-

ately incompatible elements (HREE) can be explained bychromatographic fractionation during percolation ofsmall amounts of melts or fluids through an initially non-metasomatized peridotite (Navon amp Stolper 1987 Godardet al 1995 Bedini et al 1997 Vernie res et al 1997 Ionovet al 2002) Upon reaction with the wall-rock a fluidor melt will become progressively enriched in incom-patible elements as a function of increasing distancefrom its source This is due to the fact that elements withhigher Dmineralmelt values (eg HREE) are selectivelyremoved from a percolating liquid when it exchanges withthe surrounding minerals The results are faster-movingchemical fronts of incompatible elements compared withthose of more compatible elements This is consistent with

the geochemical signature of cpx-I from the least metaso-matized part of sample Ke 19651 (Zone A Fig 15) whichalready started to re-equilibrate with respect to the incom-patible elements (ie addition of Th U and LREE)whereas more compatible elements still reflect pre-metasomatic heterogeneities In this case these areextreme HREE variations probably reflecting the earliergarnet breakdown (Kaeser et al 2006) On the otherhand more intensely metasomatized parts (eg Zone Cin sample Ke 19651 and sample Ke 196515) are character-ized by clinopyroxene trace element patterns wheremildly incompatible elements have also been modified(eg homogeneously depleted HREE Figs 7a and 15)Similar fractionation processes can explain the modal

and compositional differences between Type III-b and -cCompared withType III-b higher abundances of metaso-matic phases inType III-c suggests that higher amounts offluid or melt crystallized in or passed through these rocksThe amphibole-rich dunite (Ke 196525) for examplecontains low-Ti Mg-katophorite exhibiting the lowest(LaYb)N and (LaCe)N ratios and high (NbLa)N and

Fig 8 Major element characteristics of amphiboles from porphyroclastic spinel harzburgites and dunites from Marsabit compared with com-positions of mantle-derived amphiboles from other localities (XB basalt-hosted xenoliths XS xenoliths from subduction settings O orogenicperidotites) Mg-numberfrac14Mg(MgthornFetot) in molar proportions

15

(NbTh)N ratios (Fig 10a and c) indicating a moderatelyfractionated metasomatic agent Mg-katophorite fromthe phlogopite-rich harzburgite (Ke 785 Type III-c)and amphiboles from the more cpx-rich harzburgites(Type III-b) show progressively more fractionated patterns(eg Fig 10c) and decreasing abundances in lsquoamphibole-compatiblersquoelements (eg Nb Fig 9a) Thus the transitionfrom Type III-b to -c could reflect decreasing distancebetween the site at which the xenolith was sampledand the source of the lsquoinitialrsquo metasomatizing agent (egZanetti et al 1996 Ionov et al 2002 Bodinier et al 2004)

Judging from the abundance of modal metasomatic phasesthe amphibole dunite (Ke 196525) was probably closest tothe source of the metasomatizing agent The phlogopite-rich nature of Ke 785 which indicates higher K andLILE (Cs Rb Ba) contents in the remaining agent is inagreement with a more fractionated melt or fluid at some-what greater distance from the metasomatic sourceThe Nb abundances in amphibole also agree with this

model as well For example amphibole from Type III-cKe 196525 has comparatively high Nb concentrations(Fig 9c) in line with relatively high DNb

amphmelt values

Fig 9 Trace element composition of amphibole from porphyroclastic spinel harzburgites and dunites from Marsabit [normalized to the primi-tive mantle values of McDonough amp Sun (1995)] Mg-katophorite in Type III-b and -c xenoliths (a^c) and glass-related kaersutite (d) Theshaded fields indicate the compositional variation of all Mg-katophorite analyses (light grey) and the compositional range of LREE-depletedTi-pargasite from the Group I (grt)^spinel lherzolites (dark grey Kaeser et al 2006)

16

constrained by different studies (eg Ionov amp Hofmann1995 LaTourrette et al 1995) In Ti-poor systems such asthose of this study Nb uptake by amphibole is evenenhanced as a result of increased DNb values (Tiepoloet al 2001) Consequently in an event during which amphi-bole crystallizes continuously the remaining fluid or meltwill become progressively depleted in Nb (Ionov et al2002) resulting in formation of Nb-depleted amphibolesfurther away from the source (Zanetti et al 1996 Ionovet al 2002) such Nb-depleted amphibole is found insample Ke 785 as well as inType III-b xenoliths (eg Ke195915 Fig 9a) Amphibole signatures from xenolith suitesmodelled successfully in the context of percolative traceelement fractionation (eg Spitsbergen Ionov et al 2002)show similar ranges (eg in NbTh ratios Fig10a) to thosedocumented for our samples This further supports the

hypothesis that similar percolation mechanisms acted inthe mantle beneath Marsabit

The role of pre-existing heterogeneitiesand exotic mineralsAlthough a model based on trace element fractionationduring melt percolation is supported by textures and canaccount for many compositional features some composi-tional peculiarities remain unexplained First clinopyrox-ene and amphibole in several samples (Ke 785 19651 and196515) are characterized by extremely low HREE abun-dances more than one order of magnitude lower thanin amphiboles from the strongly metasomatizedamphibole dunite Ke 196525 (Fig 9a^c) The mainphases that crystallize during metasomatism are low-TiMg-katophorite and phlogopite However DHREE

amphmelt

Fig 10 Selected trace element ratios in amphiboles from the porphyroclastic spinel harzburgites from Marsabit compared with those ofmantle-derived amphibole from other tectonic settings (XK and XB cratonic kimberlite-hosted and off-cratonic basalt-hosted xenolithsrespectively O orogenic peridotites XS xenoliths from subduction settings) Concentrations (ratios) normalized to the primitive mantlevalues of McDonough amp Sun (1995)

17

Table 5 Representative analyses of phlogopite and apatite (oxides in wt trace elements in mgg)

Type III-b III-c III-b III-c

Mineral phlog phlog phlog phlog phlog phlog phlog apatite apatite

Sample 196515 196515 19653 785 785 785 196525 195915 19653

Texture veinlet interst in opx in opx with amph

in mp

around

chromite

incl in

amph

bleb in

glass

interst

grain

SiO2 3925 3914 4171 4190 4124 4134 4230 SiO2 016 049

TiO2 313 307 009 053 039 037 027 Al2O3 001 nd

Al2O3 1544 1583 1360 1360 1384 1377 1477 P2O5 3768 3715

Cr2O3 112 105 132 147 190 197 063 FeO(tot) 037 029

FeO(tot) 350 347 271 000 289 277 340 MnO 006 nd

MnO nd 002 008 305 004 002 005 MgO 084 091

NiO 023 027 019 023 024 028 023 CaO 4750 4237

MgO 2287 2280 2577 2591 2582 2528 2525 SrO 095 397

CaO nd 002 001 nd 004 001 002 Na2O 079 133

Na2O 215 219 282 258 264 262 208 La2O3 003 007

K2O 719 707 704 706 674 684 683 Ce2O3 088 347

Nd2O3 003 007

F 139 224

Cl 287 092

Total 9488 9493 9571 9641 9587 9535 9590 Totaly 9305 9245

Mg-no 9282 9287 9443 9380 9408 9420 9298

P 369 187 480 339 206 188 362 P 129717 139419

K 71711 72575 60747 79052 70997 69992 70189 K 110 897

Sc 099 110 131 130 088 091 160 Sc 242 079

Ti 15500 16408 890 2839 2096 1807 1736 Ti 235 5294

V 255 284 888 142 108 130 788 V 365 382

Ni 1808 1727 1744 2046 1813 1773 1905 Ni 121 397

Zn 234 221 358 382 317 321 695 Zn 119 096

Ga 111 984 na 210 190 188 235 Ga na na

Rb 185 147 289 183 133 133 342 Rb 015 049

Sr 102 116 775 325 363 350 101 Sr 7138 42233

Y 010 0086 50057 50054 0021 0006 0052 Y 988 1454

Zr 019 019 043 50088 0077 0070 073 Zr 307 101

Nb 506 628 479 177 102 146 210 Nb 024 020

Cs 175 138 261 399 216 180 798 Cs 50013 50036

Ba 1212 1096 1343 298 277 219 1124 Ba 139 126

La 010 0055 0069 50029 0020 50003 50045 La 7460 19048

Ce 0027 0011 50075 50019 014 50003 0078 Ce 6844 25945

Pr 0012 0008 50067 50024 0012 50003 50019 Pr 380 2212

Nd 0019 0035 50405 50079 0065 50009 50224 Nd 749 6530

Sm 0015 50011 50474 50090 0016 50010 50132 Sm 515 747

Eu 0022 0017 50104 50026 0013 0010 50095 Eu 147 189

Gd 50010 50011 5043 5015 50011 0012 017 Gd 314 451

Tb 50002 50002 na 50027 0002 50002 50028 Tb na na

Dy 50015 50017 045 50077 50007 50006 50077 Dy 198 281

Ho 50002 50003 50043 50020 0002 50002 50028 Ho 359 511

Er 50009 50013 5025 5011 50016 50009 5011 Er 903 119

Tm 50002 50002 50088 50013 0002 0003 50018 Tm 119 138

(continued)

18

data (eg LaTourrette et al 1995) indicate that amphibolecrystallization alone is unlikely to deplete the metasomaticagent to such a high degree in HREE The same probablyaccounts for the extreme Nb depletion in amphiboles fromKe 785 relative to the otherType III-c xenolith (Ke 196525 see also Ionov et al 2002)Further clinopyroxene phlogopite and amphibole in the

Type III-b sample Ke 196515 on one hand all showstrongly fractionated REE patterns (eg high LaCe inamphibole Fig 10c) but on the other hand are character-ized by a positiveTi anomaly decoupled from other HFSE(eg Hf and Zr Fig 9b) Trace element fractionationduring amphibole crystallization (ie formation of TypeIII-c amphibole dunites) however should lower Ti con-tents in the remaining fluid or melt (eg Moine et al 2001)Based on textures mineralogy and dominant common

compositional features the hypothesis that Type III-band -c xenoliths were metasomatized by completely inde-pendent metasomatic agents can be ruled out Thereforetwo other possibilities remain to account for the inconsis-tent observations described above (1) inference of otherphases reactants or products that are able to fractionatespecific trace elements present in the mantle beneathMarsabit but not present in the few investigated samples(2) pre-metasomatic compositional heterogeneities similarto the inherited HREE variation inType III-a clinopyrox-ene (see above)Garnet would be an important sink for HREE but

metasomatism occurred in the spinel field after decom-pression with maximum pressures of 515GPa (Kaeseret al 2006) so garnet had already broken down to sym-plectites (Fig 15) Percolation of melt through garnet-bearing websterites prior to metasomatism in Ke 19651and 196515 could be an alternativeWebsterites have been

interpreted to occur in close spatial relationship with por-phyroclastic peridotites in the shallow mantle beneathMarsabit (Kaeser et al 2006) and they occasionally con-tain rutile and ilmenite which could further account forthe strong Nb and Ta depletion in some samples (eg Ke785) The available compositional data from the webster-ites however do not indicate metasomatic modification ofwebsteritic garnet (Olker 2001 B Kaeser unpublisheddata) nor compositional similarities to Group IIIperidotitesCareful study by SEM rules out the presence of exotic

phases such as zircon or Ti-oxides described fromother xenolith suites (eg Bodinier et al 1996 Rudnicket al 1999) Nevertheless it cannot be excluded that themetasomatizing agent percolated through mantle domainscontaining these phases before infiltrating the studiedrocksThe preservation of pre-metasomatic compositional het-

erogeneities is another possibility that must be consideredThat such lsquorock signalsrsquo partly overlap with the lsquometaso-matic signalrsquo is indicated by the HREE variation in TypeIII-a clinopyroxene (see above) and also by other composi-tional parameters For example both Mg-katophorite andphlogopite growing at the expense of spinel have very highCr2O3 contents whereas euhedral Mg-katophorite inthe amphibole dunite (Ke 196525) or in phlogopiteinclusions in Opxm is comparably Cr2O3-poor (Fig 8bTables 4 and 5) Pre-existing heterogeneities would essen-tially be preserved through elements that are relativelycompatible in a given mineral (ie that are not mobilizedduring metasomatism) This would be in agreement withthe preserved HREE variations in Type III-a clinopyrox-ene high Cr2O3 in amphiboles and phlogopite and Tispikes in amphibole from Ke 196515

Table 5 Continued

Type III-b III-c III-b III-c

Mineral phlog phlog phlog phlog phlog phlog phlog apatite apatite

Sample 196515 196515 19653 785 785 785 196525 195915 19653

Texture veinlet interst in opx in opx with amph

in mp

around

chromite

incl in

amph

bleb in

glass

interst

grain

Yb 50014 50011 5047 5015 50011 50019 5017 Yb 803 724

Lu 0002 50002 50061 50014 50002 50002 50020 Lu 115 742

Hf 0019 0018 5025 50053 50012 50006 5019 Hf 50023 50120

Ta 020 022 140 0031 0012 0009 023 Ta 50015 50075

Pb 546 586 125 108 083 098 143 Pb 436 577

Th 50004 50004 50080 0043 0088 0056 50021 Th 2538 1305

U 0012 0011 0069 50018 0038 0030 0031 U 572 278

interst interstitial mp melt pocket na not analysed nd not detected

19

Further very high Mg-numbers in replacive amphibolessupport melt (fluid)^rock interaction at low melt (fluid)rock ratios where Mg and Fe contents are buffered by theambient peridotite (Kelemen et al 1995 Tiepolo et al2001) The slightly lower Mg-number of euhedralamphiboles in Ke 196525 combined with their lowerCr2O3 contents (Fig 8b) suggests a higher melt (fluid)rock ratio which is further supported by considerablylower Mg-number in associated ol-I (Fo88 Table 3)

The limited number of investigated samples does notallow us to decide which of the processes described aboveacted and to what extent However given the heterogene-ity observed in the numerous non-metasomatized xenolithsfrom Marsabit (Kaeser et al 2006) pre-metasomaticfeatures certainly affect the final product of the melt^rockreaction process to a considerable degree

The nature of the metasomatizing agent(s)Graphite stability and oxygen fugacity considerations

The Marsabit xenoliths are unusual in that they occur inan off-cratonic setting yet bear macroscopic graphite Thelatter usually either occurs in kimberlite-hosted peridotites(on-craton) or in pyroxenites from alpine-type peridotitebodies (eg Pearson et al 1994) In the phlogopite perido-tites from Finero graphite has recently been reportedas nanoscopic layers in phlogopite (Ferraris et al 2004)In xenoliths from Marsabit graphite occurs in the sametextural relationship as in kimberlite-hosted peridotitexenoliths (ie millimetre-sized flakes typically associatedwith orthopyroxene see Fig 3c^e and Pearson et al 1994)Graphite inclusions in phlogopite and Opxm couldindicate crystallization during metasomatism by alsquoreducedrsquo C-bearing fluid or melt However precipitationof graphite more probably reflects the relatively reducingconditions in the uppermost mantle prior to metasoma-tism We used the oxygen barometer of Ballhaus et al(1991) based on ferric iron contents in spinel coexistingwith olivine to constrain oxygen fugacity (fO2)conditions prior to metasomatism in the Marsabitxenoliths (Fig 16) The results show that non-metasomatized Group I and IV samples reflect similarlyreducing conditions to the cryptically metasomatizedType III-a samplesAlthough assessing Fe3thornFe2thorn ratios in spinel based

on microprobe analyses can be problematic (Wood ampVirgo 1989 Ballhaus et al 1991) all microprobeanalyses were obtained using the same operationalsettings and therefore give at least the relative fO2

variation between samples The relatively reducingconditions in the pre-metasomatic cold andhighly deformed mantle well below carbon^carbonate^silicate equilibria (Mg2Si2O6thorn 05 CaMg(CO3)2frac14Mg2SiO4thorn 05 CaMgSi2O6thornCthornO2 Eggler amp Baker1982 see Fig 16) and close to the CCO oxygen buffer arein agreement with the presence of graphite in the GroupIII harzburgites Moreover the P^T conditions at whichthe Group III xenoliths partially re-equilibrated are closeto the subsolidus decarbonation of model carbonated lher-zolite (Fig 2 see Lee amp Wyllie 2000 and referencestherein) in agreement with the observed phase assemblageincluding orthopyroxene olivine clinopyroxene andgraphite

Fig 11 Trace element patterns of (a) phlogopite and (b) apatitefrom porphyroclastic spinel harzburgites and dunites from Marsabit[concentrations normalized to the primitive mantle valuesof McDonough amp Sun (1995)] The grey field in (a) indicates thecomposition of Ti-rich phlogopite in Group I (grt)^spl lherzolites(Kaeser et al 2006)

20

Table 6 Representative analyses of silicate glass (oxides in wt trace elements in mgg)

Type III-b III-c Host

Sample 195915 196515 196515 785 196525 19653 19653 19653 195927

Remark next amph next cpx xenol margin next amph next amph melt pocket next apatite next Opxm melt pocket host basanite

SiO2 5306 5775 5161 6096 5657 5938 5974 6530 4882 4410

TiO2 017 157 106 062 030 024 020 023 378 255

Al2O3 2412 2053 2025 1779 2313 1575 1245 1355 1890 1396

Cr2O3 005 001 nd 001 001 nd nd nd 002 na

FeO(tot) 188 247 626 126 252 295 374 265 509 1283

MnO 007 000 026 001 004 005 008 007 018 021

NiO nd nd nd na nd na 006 nd 001 na

MgO 182 129 116 114 122 304 382 252 244 938

CaO 506 257 283 082 044 124 101 039 810 1030

Na2O 1107 966 969 971 1299 1388 1384 1211 673 374

K2O 154 197 465 357 138 121 142 127 300 146

Cl 031 106 038 012 014 023 061 020 015 na

P2O5 041 008 044 009 012 049 233 018 100 059

Total 9956 9896 9860 9609 9887 9845 9929 9846 9821 9912

Mg-no 6329 4816 2489 6175 4632 6474 6458 6289 4552 5658

P 1061 448 1610 230 503 1519 6738 676 2994 na

K 12563 16408 39599 32515 15872 9265 11277 10071 25900 na

Sc 179 184 126 406 151 970 136 848 691 237

Ti 665 7329 4974 2953 1965 1213 1163 1055 18199 15311

V 107 265 235 796 920 210 151 134 234 na

Ni 128 371 92 5575 983 170 5199 5298 199 191

Zn 962 103 109 110 459 440 508 309 819 na

Ga na 173 273 194 220 na na na na na

Rb 181 263 124 546 626 176 398 369 577 369

Sr 427 488 1407 188 881 1644 4997 985 1062 792

Y 543 145 224 752 262 619 158 369 301 299

Zr 466 171 353 761 660 578 365 428 242 216

Nb 416 161 137 260 136 234 164 169 837 647

Cs 014 046 186 125 007 022 102 052 063 na

Ba 238 261 1064 129 168 405 1099 641 625 563

La 698 183 848 144 889 2212 1008 106 537 462

Ce 947 256 131 308 138 333 1265 164 104 823

Pr 655 194 120 330 116 304 975 160 120 na

Nd 146 603 408 101 329 993 256 564 463 400

Sm 142 048 570 179 419 141 379 679 105 851

Eu 041 035 228 082 112 438 103 241 345 267

Gd 062 051 415 158 402 106 245 566 677 729

Tb na 0086 086 020 071 na na na na 111

Dy 090 027 359 098 434 104 171 637 732 na

Ho 023 0037 105 025 095 206 446 104 117 114

Er 045 038 195 058 255 611 153 264 305 na

Tm 009 50039 033 0081 042 085 292 5014 036 052

Yb 062 023 267 044 307 667 120 487 253 220

(continued)

21

Evidence for low-temperature SiO2^Na2O^CO2^H2O-rich melts or fluids

Potential metasomatic agents inferred from the presenceof graphite are C^O^H-rich fluids carbonate melts orCO2-bearing silicate melts The resulting phase assem-blages and geochemical signatures are the result of infiltra-tion of fluid or melt into a cold (750^8508C) shallow(515GPa Fig 2) heterogeneous and locally reduced litho-spheric mantle Metasomatism under such conditions in anopen system must result in complex geochemical signa-tures that reflect the degree of fluid^rock reaction Thismakes it particularly difficult to constrain the exactnature of the lsquooriginalrsquometasomatic agentType III-c xenolith Ke 196525 contains the highest frac-

tion of metasomatic phases and the least fractionatedamphibole trace element patterns The lsquomagmaticrsquo texturesof the Mg-katophorites in this sample suggest direct crys-tallization from a melt or a fluid rather than replacementof a pre-existing phase Therefore hypothetical liquids inequilibrium with the amphiboles in this xenolith couldyield the composition of the initial metasomatic agentsFigure 17 illustrates the trace element patterns of suchmelts and fluids calculated using sets of experimentally orempirically determined mineralmelt (fluid) partitioncoefficients including carbonate melt silicate melt andhydrous fluid (for references see caption to Fig 17)Common features for all calculated metasomatic agents

are negative Zr^Hf^Ti anomalies and LREE enrichment(Fig 17) However previous studies have shown that thesesignatures are indicative of almost all types of metasoma-tism including interaction of peridotite with carbonatiticmelt (Green amp Wallace 1988 Dautria et al 1992 Hauriet al 1993 Rudnick et al 1993 Ionov et al 1996 Yaxleyet al 1998 Coltorti et al 1999) with hydrous fluids andormelts for example originating from dehydration of altered

subducting lithosphere (Maury et al 1992 Zanetti et al1999 Grecurren goire et al 2001 Laurora et al 2001 Parkinsonet al 2003) and interaction of peridotite with volatile-and Si-rich melts that evolved through reactive porousflow (Bedini et al 1997 Ionov et al 2002 2006 Bodinieret al 2004 Rivalenti et al 2004)In the case of the Marsabit xenoliths integration of the

trace element characteristics thermobarometric inferencesand mineralogical constraints favours a Na2O^SiO2^CO2^H2O-rich silicate melt Carbonated melts can prob-ably be ruled out on major element and mineralogical con-siderations First amphiboles are Ca-poor in conflict withcarbonate melt metasomatism which is generally charac-terized by Ca enrichment (Rudnick et al 1993 Neumannet al 2002) Further orthopyroxene is a stable phase andprobably even crystallizes during metasomatism in somesamples (eg Ke 785) At near-solidus conditions coexis-tence of orthopyroxene and carbonate melt is possibleonly at pressures 20GPa (see Fig 2 eg Lee amp Wyllie2000 and references therein) which is considerably higherthan the maximum pressures indicated by the compositionof spinel (515GPa) At such low pressures metasomatismby carbonated melts will convert lherzolite and harzbur-gite immediately into wehrlite which is the major perido-tite type in all xenolith suites where carbonate meltmetasomatism is convincingly demonstrated (Rudnicket al 1993 Yaxley et al 1998 Neumann et al 2002)In Marsabit wehrlites are absent howeverAn Na2O^SiO2^CO2^H2O-rich melt or fluid on the

other hand would explain the Na-rich nature of amphiboleand phlogopite as well as the presence of orthopyroxeneGrowth of metasomatic (and Al-poor) orthopyroxene hasbeen described from several xenolith localities where peri-dotites were infiltrated either by hydrous fluids orSi-rich melts (Smith et al 1999 McInnes et al 2001Morishita et al 2003) Na-rich phlogopite is commonly

Table 6 Continued

Type III-b III-c Host

Sample 195915 196515 196515 785 196525 19653 19653 19653 195927

Remark next amph next cpx xenol margin next amph next amph melt pocket next apatite next Opxm melt pocket host basanite

Lu 014 0050 026 0041 046 080 140 058 040 031

Hf 026 052 535 014 076 110 5087 124 568 504

Ta 50080 033 638 0022 123 101 784 801 394 403

Pb 541 448 280 255 840 797 260 749 493 na

Th 194 296 160 466 703 185 247 105 534 506

U 545 069 478 099 152 447 724 216 171 126

nd not detected na not analysedAverage composition of the strongly alkaline basanites from Marsabit (Volker 1990)

22

Fig 12 Total alkalis vs SiO2 (a) Harker-type co-variation diagrams (b^e) and incompatible trace element co-variation diagram (f) showingthe compositional variability of silicate glass found in the porphyroclastic spinel harzburgites and dunites from Marsabit In (b) binary mixinglines between the typical composition of the alkali-rich glass (large open diamond) and the associated microlites are shown (each increment is10wt)They illustrate that the glass does not represent the derivate of host lava infiltration and fractional crystallization Subalkaline glass inGroup I (grt)^spl lherzolites from Marsabit is shown for comparison and is interpreted to derive from incongruent melting of Ti-pargasite(Kaeser 2006)

23

found as a reaction product between anhydrous mafic toultramafic rocks and H2O-rich evolved liquids (Costaet al 2001 and references therein) or H2O-bearing fluids(eg Odling 1994) The latter study was an experiment at20GPa simulating the effects of decreasing temperatureson the phase assemblages and their composition duringmetasomatism using a model peridotitethornC^O^H fluidThe coolest part of Odlingrsquos (1994) experiment (510758C)contained a strikingly similar phase assemblage (kato-phorite phlogopite with 2wt Na2O and Al-poororthopyroxene) to the Marsabit samples interpreted tohave formed during sub-solidus fluid^solid interactionA compositionally similar phase assemblage (highamounts of opx Si-rich hornblende Na-rich phlogopite)resulted from experiments simulating peridotite interac-tion with Si-rich melt (dacite) at lower P^T conditions(9508C at 092GPa Prouteau amp Scaillet 2003)In this study the trace element signatures of Group III-c

Mg-katophorite and the calculated metasomatic agentsfurther suggest that the metasomatizing agent was a meltrather than a fluid Hypothetical fluids in equilibrium witheuhedral Mg-katophorite from sample Ke 196525 havehigh NbLILE (eg Rb Ba) and low LILEREE ratios(Fig 17) Because experimental data predict the oppositefor fluids (LILE enriched over HFSE and REE egManning 2004 and references therein) a fluid with asignature such as that indicated in Fig 17 is unrealistic

Fig 13 Trace element patterns of silicate glass in porphyroclasticspinel harzburgites and dunites from Marsabit compared withthe trace element composition of associated clinopyroxene (Dim)micro-phenocrysts xenolith-hosting basanites from Marsabit andglass interpreted to represent xenolith^host interaction [concentra-tions normalized to the primitive mantle values of McDonough ampSun (1995)]

Table 7 Representative analyses of carbonates (oxides in

wt trace elements in mgg)

Group III-b III-b III-c III-c III-c

Mineral Dol Mg-Cc Mg-Cc Mg-Cc Mg-Cc

Sample 195915 196515 785 19653 19653

Texture in glass in glass in glass in glass in glass

SiO2 na 003 na nd 003

FeO(tot) 016 011 010 010 006

MnO 001 nd nd nd nd

MgO 1922 236 190 125 517

CaO 3189 5313 5377 5391 4853

Na2O 003 nd 004 nd 003

K2O na 002 na 001 nd

SrO 003 010 008 004 021

BaO 003 nd nd na na

CO21 4618 4444 4442 4377 4391

Total 9756 10021 10032 9909 9794

Mg-no 9954 9745 9713 9554 9931

Ca-no 5439 9418 9531 9687 8709

P 220 334 137 580 589

K 970 212 642 105 155

Sc 5065 50150 027 5027 50076

Ti 975 189 242 528 196

V 5085 0481 5042 5037 021

Ni 900 673 517 5294 593

Zn 459 5089 113 296 045

Ga na 5014 5013 na na

Rb 0084 50072 50074 50060 0046

Sr 593 860 371 251 4274

Y 012 011 130 704 015

Zr 261 024 012 097 756

Nb 144 019 0056 692 185

Cs 50063 50011 50018 50039 50008

Ba 126 230 5024 029 271

La 034 057 084 136 025

Ce 032 013 134 997 023

Pr 006 006 018 095 0026

Nd 030 013 068 228 0078

Sm 5044 50091 0074 048 50018

Eu 5012 50019 0051 020 50008

Gd 5037 50090 014 051 0026

Tb na 50010 0036 na na

Dy 5019 50039 0089 058 50038

Ho 50029 0013 0044 011 50006

Er 5012 50041 0083 033 50019

Tm 50052 50013 0016 0084 50006

Yb 5043 50063 014 027 50040

Lu 50072 50010 0011 0066 50008

(continued)

24

At the relatively low maximum pressures (515GPa) asupercritical liquid is not expected either (eg Kesselet al 2005)Finally a relatively hydrous metasomatizing agent

would be consistent with formation of Mg-katophoriteswith lowTi and presumably low Fe3thorn contents (given therelatively reducing conditions) which in turn imply high(OH)^ contents In other words partial dehydrogenationtypically observed in common (Ti-rich) mantle amphibole

(eg Popp et al 1995 Tiepolo et al 2001 and referencestherein) cannot have played an important role in thiscase as H deficiency is mainly the result of coupled substi-tutions involvingTi and Fe3thorn (Popp et al 1995) In conclu-sion the metasomatizing agent was most probably aNa2O^SiO2^CO2^H2O-rich meltComparison with experimental studies further con-

strains the low-temperature regime in which metasoma-tism occurred In ultramafic (eg Niida amp Green 1999)basaltic (eg Rapp 1995) and Si-enriched systems (egtrachybasaltic Barclay amp Carmichael 2004) amphibole(in the latter study together with phlogopite) is confinedto temperatures lower than 1000^10508C at pressures520GPa (Fig 2)Thus crystallization of the early metaso-matic phase assemblage can be explained by infiltration ofa silicate melt that rapidly cooled upon contact with theambient peridotite (750^8508C prior to metasomatismFig 2) However this low-T stage is based on Ca-in-opxthermometry (see Brey amp Kolaquo hler 1990 Kaeser et al2006 Table 1) and therefore probably underestimates theeffective temperature conditions during metasomatismThis can be explained by low diffusion rates and proximityto the closure temperature for the diopside^enstatiteexchange reaction at temperatures lower than 8508C(eg Smith 1999 and references therein) The lower tem-perature limit at which silicate melt metasomatism ispossible (ie defined by the meltrsquos solidus and viscosity)depends strongly on volatile and SiO2 contentsExperiments by Draper amp Green (1997) for exampleshowed that highly silicic melts (52^64wt SiO2) canbe in equilibrium with mantle harzburgite at temperaturesas low as 8508C (at 10^20GPa under anhydrous andC^O^H-fluid-bearing conditions) These are howeverextreme compositions for mantle melts The actual SiO2

content of the metasomatizing agent in this case is difficultto constrain from the metasomatic mineral assemblage ascompositions may have varied considerably during reac-tive percolation (eg including Si-releasing reactions suchas cpx replacement by amphibole versus Si-consumingreactions such as Opxm crystallization) Moreover it isunlikely that a melt persists at temperatures muchbelow 9008C which is the solidus of fertile lherzolite(ie primitive upper mantle composition) underH2O-saturated conditions at 15GPa (Grove et al 2006see Fig 2) Below the solidus residual volatile-enrichedfluids may percolate further for example forming the lessmetasomatized Type III-c xenoliths (eg Ke 785) as wellas Type III-b and -a peridotites (see also Bailey 1987)Relatively cold conditions would also explain the veryhigh compositional gradients developed on small scales inType III-a samples (Fig 15) Given the absence of chloritein all investigated samples the lower temperature limitof such fluid metasomatism is 8008C (eg Fumagalli ampPoli 2005 see Fig 2)

Table 7 Continued

Group III-b III-b III-c III-c III-c

Mineral Dol Mg-Cc Mg-Cc Mg-Cc Mg-Cc

Sample 195915 196515 785 19653 19653

Texture in glass in glass in glass in glass in glass

Hf 5016 50038 50036 50074 50013

Ta 0048 50012 50016 50031 0008

Pb 5024 858 5021 074 018

Th 0021 50020 50017 50028 50005

U 060 094 086 600 148

Mg-Cc Mg-calcite Dol dolomite nd not detectedna not analysed Ca-numberfrac14 100Ca(CathornMg)1Calculated from stoichiometry

Fig 14 Trace element patterns of glass-hosted carbonates in TypeIII-b and -c spinel harzburgite and dunite xenoliths from Marsabit[concentrations normalized to the primitive mantle values ofMcDonough amp Sun (1995)] The grey field outlines the compositionalrange of carbonates found in other peridotite xenoliths (Ionov ampHarmer 2002 and references therein) Carbonate material inter-preted as quenched carbonatite melt found in peridotite xenolithsfrom Kerguelen (Moine et al 2004) is shown for comparison(bold line)

25

Finally the presence of apatite even though oftentaken as indicator for carbonatite-type metasomatism(eg Yaxley et al 1998) is not in conflict with metasoma-tism by silicate melts Apatite has also been shown to crys-tallize from volatile and alkali-rich derivatives formedduring reactive porous flow of originally mafic silicatemelts (Woodland et al 1996 Bedini et al 1997 Bodinieret al 2004 Ionov et al 2006)

FORMATION OF THE LATE GLASS -BEAR ING ASSEMBLAGESTextures strongly suggest that the glass-bearing assem-blages formed later and at the expense of the early metaso-matic assemblages (Fig 4a and d) Features such as thereappearance of clinopyroxene (Na-rich Cr diopsidemicro-phenocrysts) a phase replaced by the early

Fig 15 Detailed REE systematics in primary clinopyroxene (cpx-I) in a porphyroclastic cryptically metasomatized spinel harzburgite(sample Ke 19651) showing the transition fromType III-a (Zone A and B) toType III-b (Zone C) on a scale of a few centimetres The numberson the photomicrographs (crossed polars) correspond to single laser ablation microprobe pits (for further explanation see text)

26

metasomatic assemblage indicate that melt pocket forma-tion reflects a significant change in P^T^X space in partsof the mantle beneath Marsabit after earlier metasoma-tism Based on geochemical arguments and mass-balanceconsiderations we will show that (1) the late glass-bearingassemblages are closely related to early metasomaticassemblages mainly representing breakdown productsand (2) breakdown was induced by infiltration of aCO2-rich fluid in the mantle probably followed andenhanced by heating and decompression melting duringxenolith transport in the host magma

Evidence for in situ partial meltingBSE imaging reveals that the phases becoming unstableare amphibole orthopyroxene and to minor extentsspl-I phlogopite cpx-I and apatite Compositional(inter- and intra-xenolith) heterogeneity of silicate glass istexture controlled and varies as a function of theassociated precursor phases (Fig 12a^f)TiO2 for exampleshows higher concentrations in glass from xenolith Ke196515 containing Ti-enriched amphibole (Fig 12c)whereas K2O contents are highest in phlogopite-rich sam-ples (eg Ke 785 Fig 12d) Elevated P2O5 contents arefound in glass adjacent to apatite (Fig 12e) whichmirrors at the same time the highTh and LREE contents

of apatite (Fig 12f) Significant Si enrichment is found inglass coating orthopyroxene (Fig 12d) supporting incon-gruent melting of orthopyroxene (see below) Even thoughthe melt pockets are connected along grain boundaries thestrong compositional variation (on inter- and intra-xeno-lith scale) suggests that the mobility of the melt was lim-ited or that the melting event was very short-lived (fortiming constraints see below)The co-variation plots in Fig 12 further reveal consider-

able compositional differences between all types of xeno-lith glass and the host basanite These data indicate thatthe glass cannot be the product of xenolith^host lava inter-action during ascent to the surface Also back-mixing ofmicro-phenocrysts into the melt fails to derive the glassfrom a host-related melt (Fig 12b) Glass from xenolithmargins however is compositionally closer to the bulkhost basanite (eg Fig 12b) in line with a direct derivationfrom xenolith^host lava interaction Its more evolvedmajor element composition (eg higher SiO2 lowerMg-number Fig 12b) is probably the result of fractiona-tion of basanite phenocrysts (eg olivine augite andtitanomagnetite Volker 1990) Host lava^xenolith interac-tion is further supported by the trace element signature ofglass at the margin of the xenoliths which is closely similarto that of the Marsabit basanites (Fig 13 Volker 1990)The same characteristics are observed for intra-xenolithglass in the unusual kaersutite-bearing sample Ke 195927

Fig 16 Oxygen fugacities relative to the quartz^fayalite^magnetitebuffer (QFM equation of OrsquoNeill 1987) as a function of temperatureat assumed pressure of 1GPa calculated using the composition ofcoexisting spinel and olivine (ol-I^spl-I pairs and Chrm^Olm pairsrespectively) based on the oxygen barometer of Ballhaus et al (1991)The graphite^CO2 equilibrium was calculated after Frost amp Wood(1998) and the graphite^carbonate^silicate equilibrium was calcu-lated using the equation of Eggler amp Baker (1982) (See text for furtherexplanation)

Fig 17 Calculated metasomatic agents in equilibrium with the euhe-dral Mg-katophorites from the Type III-c amphibole dunite Ke 196525 Amphibolehydrous fluid and amphibolecarbonatite melt parti-tion coefficients are from Adam et al (1997) and Adam amp Green(2001) respectively Silicate melts have been calculated using amphi-boleSi-rich melt partition coefficients from Chazot et al (1996b) andamphibolebasanite partition coefficients from LaTourrette et al(1995) The compositional range of the Marsabit basanites and alkalibasalts (Volker 1990) is shown for comparison

27

Xenolith glass unrelated to the host lava and melt pock-ets similar to those reported here in association with resi-dual phases (eg amphibole or clinopyroxene) has beenreported from many xenolith localities (eg Frey ampGreen 1974 Ionov et al 1994 1996 Chazot et al 1996a1996b Yaxley et al 1997 Laurora et al 2001 Bali et al2002) Residual phases within the melt pockets stronglyindicate in situ partial melting within the xenolithsHowever a variety of processes may trigger such in situ

melting including (1) closed-system decompression melt-ing of either anhydrous (Maale amp Printzlau 1979) orhydrous phases (Frey amp Green 1974 Francis 1976) duringxenolith ascent in the host lava (2) melting during interac-tion of the xenoliths with the host basaltic melt duringxenolith ascent (eg Zinngrebe amp Foley 1995 Shaw ampKlulaquo gel 2002 Shaw et al 2006) (3) melting in the mantleprior to xenolith entrainment induced by an infiltratingfluid (ie flux melting eg Jones et al 1983 Ionov et al1994 Demecurren ny et al 2004) However most studies agreethat that partial melting occurred very shortly beforeandor during xenolith exhumation by the host magmaThus a combination of metasomatically triggered meltingfollowed andor assisted by decompression melting duringxenolith transport may be a realistic alternative in somecases (eg Dawson 2002)Recently Ban et al (2005) have shown in a very detailed

mass-balance study on glass-bearing xenoliths from theWest Eifel that the weight proportions of glass and theassociated micro-phenocrysts match the composition ofthe preserved parent amphibole We used the sameapproach and performed simple mass-balance calculations(least-squares fitting) to constrain the conclusions drawnfrom petrography and to test which phase contributedand to what extent to the formation of the melt pocketsThe results reported in Table 8 show that in most casesamphibole-melting can account successfully for the meltpocket composition and only a minor amount (520) ofother phases (orthopyroxene andor phlogopite) needs tobe added to the reactant Such a contribution of otherphases agrees with textural observation (eg Fig 4cshowing Dim growing on resorbed Opxm) as well as withthe conclusions of Ban et al (2005) who stated that moreSi-rich glass (ie 454wt) necessitates orthopyroxenecontributing to the meltTrace element patterns of the bulk glassthornmicro-

phenocryst pockets calculated using the measured traceelement abundances of glass and micro-phenocrysts andthe respective mass-balance parameters (Table 8) arebroadly identical to the measured amphibole composition(Fig 18a and b) Higher LILE (Cs Rb Ba) concentrationsin the calculated bulk melt patches in sample Ke 785 canbe balanced by adding 7wt of phlogopite (Fig 18b)which is in line with mass balance (Table 8) The dominantcontrol of amphibole to account for the trace element

budget of the melt pockets is similar to results found inother suites of glass-bearing xenoliths (Chazot et al1996a Yaxley amp Kamenetsky 1999 Laurora et al 2001)Further the obtained phase proportions for the melt pock-ets are in good agreement with the results from image ana-lysis (Table 1) In addition they are similar to the glass^cpx^ol proportions recorded in recent experiments on the

Table 8 Results of mass-balance calculations to test melt

pocket formation by incongruent melting of the early metaso-

matic assemblages

Sample 195915 785 785 196525 196525 19653

Reactant

amph 1000 1000 0820 1000 0944 mdash

opx mdash mdash 0078 mdash 0056 0406

phl mdash mdash 0102 mdash mdash mdash

glass mdash mdash mdash mdash mdash 05941

SiO2 4484 4989 4964 5129 5167 5898

TiO2 014 039 036 025 024 018

Cr2O3 232 291 258 127 121 009

Al2O3 1368 762 769 800 757 961

FeO(tot) 336 314 333 343 357 394

MnO 006 008 008 012 012 010

NiO 009 013 014 011 011 004

MgO 1844 2029 2197 2040 2122 1625

CaO 854 783 644 966 913 078

Na2O 460 500 436 504 476 843

K2O 049 032 096 042 040 060

Product

glass 0389 0322 0335 0292 0284 07032

ol 0243 0259 0315 0266 0286 0273

cpx 0288 0373 0308 0442 0431 0024

spl 0052 0032 0028 mdash mdash mdash

carb 0028 0014 0014 mdash mdash mdash

SiO2 4477 5027 4986 5207 5181 5903

TiO2 013 041 039 015 014 017

Cr2O3 213 314 272 109 106 015

Al2O3 1364 764 770 798 776 957

FeO(tot) 374 331 351 322 332 350

MnO 007 007 007 009 009 006

NiO 011 013 015 011 012 007

MgO 1834 2024 2196 2063 2141 1635

CaO 854 759 632 952 929 080

Na2O 449 385 388 499 485 860

K2O 060 096 100 015 014 091

r2 023 200 035 084 028 033

Prefix Ke omitted1Low-Si glass 2High-Si glass (see text for explanation)

28

melting behaviour of amphibole in wehrlites at tempera-tures between 1175 and 12508C and pressures of05^10GPa (Mecurren dard et al 2006) These temperatures arein good agreement with the estimates for the formation ofolivine and chromite micro-phenocrysts in the melt

pockets (Fig 16) The heat supply required for amphibolemelting in the formerly low-T metasomatized Group IIIxenoliths (T 200^3008C) may come from local heatingshortly before or during xenolith entrainment in the hostlava or from fluid infiltration that triggered melting (seediscussion below)Finally formation of the most Si-enriched melt (adjacent

to Opxm in Group III-c samples) can be modelled assum-ing progressing interaction between infiltrating melt andorthopyroxene where the latter melts incongruently

opxthornmelt1 olthornmelt2 cpx ethseeTable 8THORN eth1THORN

To calculate reaction (1) we chose the least evolved glassobserved in melt patches in sample Ke 19653 (SiO2

5921wt Mg-number 6465) to represent the initialmelt1 whereas melt2 adjacent to opx is SiO2-rich(6530wt) Trace element abundances in glass next toorthopyroxene are shifted to lower concentrations(Table 6) which is in agreement with dilution caused byaddition of a trace element-poor orthopyroxene compo-nent The resultant phase proportions are similar to thoseobserved at orthopyroxene^melt interfaces (ie higholivine and low cpx abundances) and are in agreementwith experimental studies (Shaw et al 1998)

Partial melting triggered by fluid fluxMass-balance calculations provide an explanation for meltpocket formation by in situ partial melting Thus onemight conclude that melting is simply the result of decom-pression and heating during xenolith transport in the hostmagma However there are two points that argue thatmelting was triggered by the infiltration of a fluid phaseshortly before the xenoliths were sampled by the hostmagma (1) the presence of occasionally high amounts ofcarbonate included in the melt pockets (2) the extremelyNa-rich clinopyroxene (Dim) compositions either includedas micro-phenocrysts in the melt pocket or present asequilibrated grains around chromite in the single TypeIII-d xenolith (Fig 3f) Both arguments will be discussedin the next section

Formation of carbonates

Textures and compositions of carbonate globules can beexplained by carbonate precipitation from a CO2-bearingfluid As demonstrated by several studies highCa(CathornMg) values (Table 7) and low REE concentra-tions are strong arguments that the carbonates are cumu-late crystals precipitated from a CO2-rich melt or afluid and not quenched carbonated melts (Lee et al 2000Ionov amp Harmer 2002) Oscillatory zoning (Fig 5a and b)of individual carbonate crystals within singleglobules corroborates this interpretation The roundedcontacts toward glass (globules) are also in line with thesefindings Similar globular forms were reported fromexperimental studies and do not a priori imply the

Fig 18 Calculated trace element characteristics of reactants (in mostcases amphibolethorn orthopyroxene filled symbols) and products(melt pockets open symbols) of the lsquoamphibole meltingrsquo model[see equation (1) in text] The relative phase proportions wereobtained by mass-balance calculations and are given in Table 8 Thedashed lines in (b) show that the high LILE concentrations obtainedfor the product (bulk melt pocket) can be explained by adding about7wt of phlogopite to the reactant (amphibolethorn orthopyroxeneTable 8) Further explanation is given in the text

29

presence of immiscible carbonate and silicate melts(eg Lee amp Wyllie 1996)The high amount of carbonate in some melt pockets

however (eg Fig 4e) does need further explanation asthe incongruent melting reaction (1) does not account forthe CO2 required for carbonate precipitation duringamphibole melting Therefore amphibole breakdowneither occurred in an open system with addition of at leastCO2 (eg Laurora et al 2001 Ban et al 2005) or anotherpre-existing phase acted as a source of carbon Oxidationof graphite texturally identified as a part of the earlyassemblage (eg Fig 3d) could account for some of theCO2 necessary for carbonate formation A change in fO2

conditions is also indicated when the composition of glass-related spinel (Chrm) and Olm are used for oxygen baro-metry which yields oxygen fugacities well on the oxidizedside of the carbon^carbonate^silicate buffer (Fig 16)However increasing fO2 triggered by closed-systemdecompression-induced melting alone does not appear fea-sible Also textures such as straight contacts between gra-phite and Mg-calcite (Fig 4c) suggest that carbonategrows on graphite (ie indicating local fO2 conditions cor-responding to carbon^carbonate equilibrium) rather thanreplacing it We therefore interpret melt pocket formationto be triggered by the infiltration of a CO2-rich fluid(ie flux melting see also Ionov et al 1994) This interpre-tation also accounts for the presence of abundant vugs inthe glass that could have contained a fluid (Ionov et al1994 2006) which was presumably lost during sample pre-paration However we emphasize here that late fluid infil-tration had only subtle effects on the trace elementcomposition of Type III-b and -c xenoliths

Formation of Na-rich Cr diopside

Infiltration of CO2-rich fluidwould also explain the Na-richcomposition of Dim micro-phenocryst cores (Fig 6a and b)Ionov et al (2005) noted recently that clinopyroxenes inmeltpockets are commonly characterized by Na2O contents51wt mainly as a result of low cpxmelt partition coeffi-cients at low pressures (eg Blundy et al 1995) Thus veryhighNa2Ocontents in Dimmicro-phenocrysts (Fig6a) sug-gest replacement of amphibole by Dim and eventually Olmand Chrm during reaction with a fluid at subsolidus condi-tions An origin of micro-phenocrysts by replacementrather than crystallization from a melt is indicated by theidentical crystallographic orientation of individual Dimcrystals in singlemelt pockets (Fig4a)Once the solidus of the precursor phase(s) was reached

(ie 10508C for amphibole Niida amp Green 1999) silicatemelt was generated in situ and led to the formation of clin-opyroxene rims with lower Na contents (Fig 6b) and thedevelopment of euhedral shapes in contact with melt (egFig 4a and e) Eventual immiscibility between remainingfluid and newly formed melt (eg during xenolith ascent)would then explain the presence of carbonate globules and

vugs enclosed in glass Likewise different origins of meltand carbonate were recently proposed for similar meltpockets from Hungarian xenoliths by Demecurren ny et al(2004) who found oxygen isotope disequilibrium betweencarbonate globules and the host glass Such a two-stepmodel (fluid-induced replacement followed by melt gen-eration) can be explained by CO2-fluid infiltration occur-ring within the mantle at sub-solidus conditions ultimatelyfollowed by xenolith entrainment in the host magma andtransport to the surfaceThe host magma possibly suppliedthe additional heat necessary for melt generationFast decompression would further enhance melting andcould also account for the lower Na contents in Dimmicro-phenocryst rims as a result of decreasing cpxmeltpartition coefficients at decreasing pressures (Blundyet al 1995)Rapid replacement of amphibole ( phlogopite) by an

anhydrous phase assemblage indicates that the fluid waswater-poor (eg low H2OCO2 ratio see Ionov et al2006)This is corroborated by the fact that Mg-katophorite(presumably OH-rich see above) is occasionally rimmedby a residual kaersutitic selvage (Fig 4d) Such Ti-richmantle amphiboles are commonly OH-poor as a result ofpartial dehydrogenation (eg Tiepolo et al 2001 and refer-ences therein)Finally the presence of theType III-d xenolith contain-

ing texturally equilibrated clinopyroxene of almost identi-cal composition to Dim micro-phenocrysts in melt pockets(Table 2 Figs 6a and 7b) further strengthens the argu-ment that CO2-fluid percolation started in the mantlebefore entrainment (ie enough time elapsed to allow thisrock to anneal completely after the late metasomaticassemblage had formed) A similar conclusion can bedrawn for the clearly host-related glass found in xenolithKe 195927 (eg Fig 13b) ThisType III-c sample is charac-terized by the presence of Dim micro-phenocrysts partlyovergrown by mineral rims clearly related to the host basa-nite magma (kaersutite Ti-augite and olivine withFo885 Fig 4f Tables 3 and 4 and supplementary dataat httppetrologyoxfordjournalsorg) Kaersutite hasmajor and trace element characteristics similar to typicalbasanitic and alkali basaltic amphibole phenocrysts(Fig 9d) The special phase assemblages in this xenolithindicates that host-related basanitic melts intruded someType III-c xenoliths and partly overprinted the assemblageformed slightly earlier by in situ derived alkali-rich peralu-minous melts fully preserved in other samples (eg Ke19653 or Ke 785)

ORIGIN OF METASOMATIZ INGAGENTS T IM ING AND TECTONICSETT INGAs noted above late metasomatism was probably short-lived started at mantle depths and probably continued

30

Ti are depleted relative to the REE and Nb and Ta rela-tive to U and Th Trace element compositions alwaysmirror the composition of the associated earlier phases(ie amphibole in most cases but also clinopyroxeneorthopyroxene phlogopite and apatite)Glass in Group III xenoliths is clearly different com-

pared with Ti-rich subalkaline glass occasionally found inGroup I lherzolites from Marsabit (related to melting ofTi-pargasite see Kaeser 2006 and Fig 12a^f) Also glassis generally clearly distinct from the xenolith-hosting basa-nites (Fig 12a^f) The exception is glass within the TypeIII-c dunite Ke 195927 and from the margin of xenolithKe 196515 (contact with host lava) which show someimportant differences compared with glass from theother samples that is higher FeO and lower SiO2

contents similar to the composition of the host basanite(Figs 12a^f and 13)

Clinopyroxene micro-phenocrysts

Clinopyroxene micro-phenocrysts (Dim) are Na-rich Crdiopsides (up to 323wt Na2O and 645wt Cr2O3Fig 6a and b) with low TiO2 contents (505wt) andhigh Mg-numbers (up to 949) In all samples Dim arestrongly zoned with Cr and Na contents generally decreas-ing from cores to rims (Fig 6b) Dim in contact withbasanitic glass in the kaersutite-bearing dunite (Ke 195927) has resorbed rims overgrown by Ti-augite (Fig 4f)with significantly lower Na2O (51wt) contents andMg-numbers (85 seeTable 2)Trace element patterns of Dim are similar in shape to the

associated glass (except LILE Cs to U Fig 13) NegativeHFSE anomalies are mostly due to higher REEabundances that is the absolute concentrations of HFSEare similar to those of cpx-I from Type III-a and -b(see Fig 7a no real HFSE depletion) Texturally equili-brated clinopyroxene in the Type III-d spl harzburgite

(Ke 19706 Fig 3f) is a Na-rich Cr diopside withsimilar major (Fig 6a) and trace element characteristics(Fig 7b) similar to those of the glass-hosted micro-phenocryst cores

Olivine spinel kaersutite

Compared with ol-I glass-related Olm is more magnesian(Table 3 up to Fo946) and enriched in Ca Ol-I in contactwith glass develops strong compositional gradientsapproaching the composition of Olm at the rims Spinelmicro-phenocrysts (Chrm) are Mg-rich chromites (Cr-number up to 665) enriched in (calculated) Fe2O3 com-pared with spl-IKaersutite occurs as tiny rims on amphibole inType III-

b and -c and as micro-phenocrysts inType III-c sample Ke195927 Reaction rims are characterized by higherMg-numbers than the original amphibole They are toonarrow to be analysed for trace elements Kaersutitemicro-phenocrysts in sample Ke 195927 have lowMg-number (about 82) and high K2O contents (Table 4)Major element characteristics as well as trace elementpatterns (convex-upward REE patterns high Nb^Ta lowU^Th Fig 9d) are similar to those of hornblenditesand amphibole megacrysts from alkaline magmas(eg Shaw amp Eyzaguirre 2000)

Carbonates

Most glass-related carbonate globules contain Mg-calcite(Table 7) with an average Mg-number of 965 andCa(CathornMg) between 087 and 098 In sample Ke 19653 MgO correlates positively with SrO (up to 535wtMgO and 089wt SrO) Other elements show verylow concentrations (Table 7) One sample (Type III-c Ke195915) contains zoned crystals ranging from dolomiticto calcitic in composition (Fig 5a) Carbonates havelow REE contents with flat to slightly LREE-enriched

Table 2 Continued

Type III-a III-b III-c III-d

Mineral cpx-I cpx-I cpx-I cpx-I cpx-I Dim Dim Dim Dim Ti-aug cpx

Sample 19651 19651 19651 196515 195915 195915 19653 785 195927 195927 19706

Texture Zone A1

core

Zone B1

rim

Zone C1

core

core core

high-Na

core micro-xx

core

micro-xx

core

micro-xx

core

micro-xx

rim

core

high-Na

Hf 034 031 028 050 025 040 081 0056 102 242 023

Ta 0011 0007 0016 0021 50029 50080 0657 50012 0043 5019 0056

Pb 037 050 058 115 068 5035 023 5028 50089 5039 011

Th 012 041 131 073 494 038 045 0019 0062 50074 0062

U 0076 027 047 028 129 50078 0049 50018 0030 50071 0017

1See Fig 15nd not detected na not analysed

10

patterns (Fig 14) Extended trace element plots exhibit avery broad range of concentrations in different samples(eg Pb Fig 14) Large positive peaks for U Pb and Srare typical In addition U and Nb are strongly enriched

Fig 6 Major element composition of clinopyroxene in porphyro-clastic spinel harzburgite and dunite xenoliths from Marsabit(a) Plot of Cr2O3 vs Na2O (wt) in micro-phenocrysts comparedwith cryptically metasomatized cpx-I in Type III-b sampleKe 195915 texturally equilibrated Na-rich clinopyroxene in TypeIII-d sample Ke 19706 and texturally equilibrated Na-rich cpxfrom fluid-metasomatized peridotite from Tok (far-east RussiaIonov et al 2005) The dashed line outlines the compositionalrange of common low-Na micro-phenocrysts in glass from otherxenolith suites (see Ionov et al 2005 and references therein)Crosses show the compositional variation of micro-phenocrysts inthe Group III-c xenolith 195927 (ie Dim overgrown by low-Naand low-Cr Ti-augite see Fig 4f) Compositional range of primarycpx-I in all Group III xenoliths (cpx-I grey field) is from Kaeseret al (2006) (b) Na2O and Cr2O3 (wt) variation across a singleDim micro-phenocryst in Type III-c xenolith Ke 19653 (electronmicroprobe profile)

Fig 7 Trace element composition of clinopyroxene from the porphyr-oclastic spinel harzburgites from Marsabit [normalized to the primi-tive mantle values of McDonough amp Sun (1995)] (a) Trace elementpatterns of primary cryptically metasomatized clinopyroxene (cpx-I)inType III-a and -b spinel harzburgites Small arrows indicate high-Na cpx-I inType III-b sample Ke 195915 (see also Fig 6a) (b) Traceelement patterns of texturally equilibrated Na-rich Cr diopside fromthe recrystallized Type III-d spinel harzburgite The shaded fieldsindicate the compositional range of glass-related clinopyroxene micro-phenocrst (Dim)

11

Table 3 Representative analyses of orthopyroxene spinel and olivine (oxides in wt trace elements in mgg)

Type III-a III-b III-b III-c III-d III-c III-d III-c III-c III-c III-d

Mineral opx-I opx-I opx-I Opxm Opxm Chrm spl-I ol-I Olm ol-I ol-I

Sample 19651 196515 195915 19653 19706 19653 19706 785 19653 196525 19706

Texture Zone C1 core core core core core core core core core core core

SiO2 5657 5642 5615 5807 5713 012 005 4093 4135 4078 4098

TiO2 004 013 006 001 005 044 016 nd 001 001 nd

Al2O3 265 269 299 046 192 1223 1880 001 001 nd 004

Cr2O3 043 056 040 032 054 5454 5043 001 029 001 005

Fe2O3 mdash mdash mdash mdash mdash 554 338 mdash mdash mdash mdash

FeO(tot) 533 568 576 538 486 1114 1207 939 583 1133 771

MnO 013 014 014 012 012 na 005 015 013 027 007

NiO na 010 009 007 009 015 009 040 024 037 039

MgO 3473 3420 3380 3561 3464 1480 1502 4900 5182 4739 5013

CaO 051 053 048 020 052 003 000 001 008 002 008

Na2O 005 008 013 009 014 na na 001 004 nd 001

Total 10044 10052 10000 10034 10002 9910 10008 9992 9980 10018 9947

Mg-no 9208 9148 9128 9218 9271 7032 6894 9029 9406 8817 9206

Na 341 423 1136 539 1120 na 438 297 262 647 230

Al 15120 14586 17785 1817 10834 na 114911 893 821 120 1201

P 156 145 175 175 248 na 198 421 292 105 244

Ca 3163 3016 3881 1487 3895 na 55348 822 579 200 658

K 172 5044 5035 981 150 na 5086 5037 5102 198 630

Sc 805 546 173 244 124 na 280 061 432 094 286

Ti 239 619 366 449 227 na 956 156 871 170 204

V 768 906 772 149 634 na 925 104 377 096 283

Ni 568 646 585 588 748 na 1010 2686 2875 2920 2649

Zn 256 301 245 606 373 na 732 630 665 146 693

Ga 181 350 na na 133 na 280 0015 na 0035 077

Sr 024 014 044 102 167 na 0089 0017 50013 0045 052

Y 0081 0073 049 069 202 na 0025 0015 016 010 019

Zr 023 0277 051 0038 088 na 124 50005 50033 50006 015

Nb 50005 0020 50007 018 0051 na 0877 50003 0025 0008 0035

Ba 0056 50018 50021 748 0035 na 50028 50016 50071 017 013

La 0019 50002 0055 0061 0045 na 0039 0021 50004 0017 0019

Ce 0020 0005 0046 0109 027 na 0052 0023 50012 0027 015

Pr 0002 50001 0005 0007 0064 na 0024 50001 50011 0009 0019

Nd 50008 0010 0043 0039 042 na 50029 50007 50052 0018 011

Sm 0014 50009 0016 50018 019 na 0022 50006 50051 50009 0032

Eu 50003 50002 50005 0012 0077 na 0018 50002 50012 50003 0011

Gd 50014 50006 0028 0021 020 na 50026 50006 50041 50016 0035

Tb 0002 50001 na na 0045 na 50004 50001 na 50001 0004

Dy 0015 0011 0086 0058 038 na 0010 50003 50043 50009 0041

Ho 0007 0003 0012 0031 0069 na 0003 50001 50006 0004 0010

Er 50012 0008 0043 0094 022 na 50015 50006 0058 0018 0027

Tm 0002 0001 0015 0014 0031 na 50003 50001 0010 0003 0004

Yb 0027 0011 019 0170 028 na 50018 0009 0064 0060 0038

Lu 0001 0002 0026 0026 0033 na 0007 0002 0040 0018 0008

(continued)

12

Table 4 Representative analyses of amphibole (oxides in wt trace elements in mgg)

Type III-b III-c

Mineral katoph katoph katoph katoph katoph katoph katoph katoph kaersut kaersut

Sample 195915 196515 196515 196515 785 785 196525 196525 195927 195927

Texture residual

in mp

interst interst lamellae

in cpx-I

residual

in mp

residual

in mp

euhedral core euhedral rim micro-xx micro-xx

SiO2 4541 4603 4625 4887 4955 4959 4952 4953 4064 4106

TiO2 015 190 203 099 030 029 022 028 612 577

Al2O3 1365 1145 1100 915 826 807 877 794 1405 1395

Cr2O3 251 123 148 144 254 263 106 115 031 008

FeO(tot) 350 293 304 278 302 294 359 346 588 615

MnO 006 003 004 002 008 002 010 008 016 010

NiO 009 014 009 010 010 006 009 012 010 010

MgO 1883 1886 1896 1997 2067 2062 2000 2024 1576 1545

CaO 851 974 960 976 805 793 863 909 1086 1075

Na2O 474 427 440 432 548 555 529 509 284 294

K2O 049 038 041 031 027 029 028 039 139 138

Total 9811 9695 9728 9771 9837 9805 9759 9741 9817 9793

Mg-no 9055 9199 9176 9275 9243 9259 9086 9124 8269 8173

P 445 222 235 na 175 196 206 120 214 266

K 4301 3916 3637 na 2393 2654 2955 3655 13289 12762

Sc 402 704 663 na 358 345 311 352 375 270

Ti 720 9867 9917 na 1732 1898 1421 1589 32154 32071

V 216 317 332 na 289 299 172 187 469 404

Ni 726 842 828 na 811 876 737 813 638 530

Zn 298 163 159 na 188 214 451 457 463 548

Ga na 981 939 na 103 114 867 872 na na

Rb 279 184 151 na 035 050 096 150 744 713

Sr 216 325 335 na 708 667 955 117 618 668

Y 101 269 315 na 111 126 390 359 310 355

Zr 539 370 331 na 292 297 114 592 933 110

Nb 339 413 447 na 102 121 192 705 265 341

Cs 0006 50001 0005 na 0006 50009 0008 0018 50019 50010

(continued)

Table 3 Continued

Type III-a III-b III-b III-c III-d III-c III-d III-c III-c III-c III-d

Mineral opx-I opx-I opx-I Opxm Opxm Chrm spl-I ol-I Olm ol-I ol-I

Sample 19651 196515 195915 19653 19706 19653 19706 785 19653 196525 19706

Texture Zone C1 core core core core core core core core core core core

Hf 0006 0010 0014 50009 0013 na 0042 50003 50017 50009 50006

Ta 50001 50001 50004 50005 50002 na 0015 0001 50018 50001 50001

Pb 50028 0041 0047 0074 50041 na 015 0036 0075 0042 0046

Th 0009 50002 0069 0020 50002 na 0010 0050 50006 0011 50001

U 0004 50001 0026 0017 0004 na 0017 0011 50006 50001 50002

1See Fig 15nd not detected na not analysed

13

compared withTh andTa respectively Hf Zr andTi showvery low concentrations Overall major and trace elementcharacteristics of the carbonates are similar to those of car-bonates found in other peridotite xenoliths (eg Ionov1998 Lee et al 2000 Laurora et al 2001 Ionov ampHarmer 2002 see Fig 14)

FORMATION OF THE EARLYASSEMBLAGESDespite large compositional variations numerous commontextural and compositional features indicate that the earlymetasomatic assemblages in all Group III samples formedduring a single metasomatic event In the discussion belowwe argue that during a first metasomatic stage the pri-mary (pre-metasomatic) phase assemblage in Group IIIspinel harzburgites was partially replaced by metasomaticminerals finally resulting in clinopyroxene-free amphi-bole- and phlogopite-bearing dunite and harzburgite(Type III-c) The initial metasomatic agent was probably

an evolved CO2^H2O-rich melt and the transitionfrom peridotites of Type III-a to -c can be explained byprogressive melt^rock reaction

Transition from cryptic (Type III-a) tomodal (Type III-b and -c) metasomatismFormation of new phases requires (1) crystallization from amelt or fluid phase (decreasing melt or fluid mass) andor(2) melt^solid or fluid^solid reactionTextural features suchas spinel being replaced by amphibole and phlogopite(eg Fig 3c) indicate melt(fluid)^rock reaction Thegeochemical signatures observed in the different peridotitetypes thus probably reflect pre-existing compositionalheterogeneities coupled with metasomatic modificationThis is illustrated best in sample Ke 19651 (Type III-a)the least metasomatized xenolith in this study It can besubdivided into three parts containing clinopyroxenewith different REE signatures (Zones A B and C inFig 15) The LREE are increasingly enriched from ZoneA to C indicating increasing cryptic metasomatism

Table 4 Continued

Type III-b III-c

Mineral katoph katoph katoph katoph katoph katoph katoph katoph kaersut kaersut

Sample 195915 196515 196515 196515 785 785 196525 196525 195927 195927

Texture residual

in mp

interst interst lamellae

in cpx-I

residual

in mp

residual

in mp

euhedral core euhedral rim micro-xx micro-xx

Ba 781 614 540 na 259 309 846 841 331 322

La 357 116 110 na 704 824 177 113 107 138

Ce 627 225 231 na 217 248 621 426 338 448

Pr 574 208 224 na 301 349 856 623 557 724

Nd 177 720 779 na 128 151 363 296 318 398

Sm 208 143 143 na 285 316 752 740 945 111

Eu 080 058 060 na 108 115 232 252 363 384

Gd 182 085 088 na 225 241 701 692 940 980

Tb 011 013 na 039 050 120 117 na na

Dy 171 057 067 na 202 252 735 698 700 819

Ho 036 010 010 na 039 050 155 144 137 147

Er 106 024 027 na 097 114 405 382 296 336

Tm 016 0030 0033 na 012 017 060 058 027 039

Yb 120 028 030 na 077 099 400 386 243 272

Lu 019 0036 0038 na 0079 0077 058 044 033 031

Hf 015 0067 0054 na 0065 0048 028 078 331 376

Ta 0033 014 014 na 0002 0004 010 107 167 198

Pb 201 360 374 na 142 173 195 222 022 039

Th 682 150 134 na 181 196 059 028 010 020

U 212 039 039 na 040 048 019 012 0018 0048

katoph Mg-katophorite kaersut kaersutite interst interstitial mp melt pocket nd not detected na notanalysed

14

HREE abundances on the other hand are very hetero-geneous in the least metasomatized Zone A moderatelydepleted in Zone B and homogeneously low in the mostmetasomatizedZoneC (containinggraphite Fig15) similarto REE patterns in clinopyroxene from the amphibole- andphlogopite-bearingType III-b sample (Ke196515 Fig7a)The decoupling of strongly (LREETh U) from moder-

ately incompatible elements (HREE) can be explained bychromatographic fractionation during percolation ofsmall amounts of melts or fluids through an initially non-metasomatized peridotite (Navon amp Stolper 1987 Godardet al 1995 Bedini et al 1997 Vernie res et al 1997 Ionovet al 2002) Upon reaction with the wall-rock a fluidor melt will become progressively enriched in incom-patible elements as a function of increasing distancefrom its source This is due to the fact that elements withhigher Dmineralmelt values (eg HREE) are selectivelyremoved from a percolating liquid when it exchanges withthe surrounding minerals The results are faster-movingchemical fronts of incompatible elements compared withthose of more compatible elements This is consistent with

the geochemical signature of cpx-I from the least metaso-matized part of sample Ke 19651 (Zone A Fig 15) whichalready started to re-equilibrate with respect to the incom-patible elements (ie addition of Th U and LREE)whereas more compatible elements still reflect pre-metasomatic heterogeneities In this case these areextreme HREE variations probably reflecting the earliergarnet breakdown (Kaeser et al 2006) On the otherhand more intensely metasomatized parts (eg Zone Cin sample Ke 19651 and sample Ke 196515) are character-ized by clinopyroxene trace element patterns wheremildly incompatible elements have also been modified(eg homogeneously depleted HREE Figs 7a and 15)Similar fractionation processes can explain the modal

and compositional differences between Type III-b and -cCompared withType III-b higher abundances of metaso-matic phases inType III-c suggests that higher amounts offluid or melt crystallized in or passed through these rocksThe amphibole-rich dunite (Ke 196525) for examplecontains low-Ti Mg-katophorite exhibiting the lowest(LaYb)N and (LaCe)N ratios and high (NbLa)N and

Fig 8 Major element characteristics of amphiboles from porphyroclastic spinel harzburgites and dunites from Marsabit compared with com-positions of mantle-derived amphiboles from other localities (XB basalt-hosted xenoliths XS xenoliths from subduction settings O orogenicperidotites) Mg-numberfrac14Mg(MgthornFetot) in molar proportions

15

(NbTh)N ratios (Fig 10a and c) indicating a moderatelyfractionated metasomatic agent Mg-katophorite fromthe phlogopite-rich harzburgite (Ke 785 Type III-c)and amphiboles from the more cpx-rich harzburgites(Type III-b) show progressively more fractionated patterns(eg Fig 10c) and decreasing abundances in lsquoamphibole-compatiblersquoelements (eg Nb Fig 9a) Thus the transitionfrom Type III-b to -c could reflect decreasing distancebetween the site at which the xenolith was sampledand the source of the lsquoinitialrsquo metasomatizing agent (egZanetti et al 1996 Ionov et al 2002 Bodinier et al 2004)

Judging from the abundance of modal metasomatic phasesthe amphibole dunite (Ke 196525) was probably closest tothe source of the metasomatizing agent The phlogopite-rich nature of Ke 785 which indicates higher K andLILE (Cs Rb Ba) contents in the remaining agent is inagreement with a more fractionated melt or fluid at some-what greater distance from the metasomatic sourceThe Nb abundances in amphibole also agree with this

model as well For example amphibole from Type III-cKe 196525 has comparatively high Nb concentrations(Fig 9c) in line with relatively high DNb

amphmelt values

Fig 9 Trace element composition of amphibole from porphyroclastic spinel harzburgites and dunites from Marsabit [normalized to the primi-tive mantle values of McDonough amp Sun (1995)] Mg-katophorite in Type III-b and -c xenoliths (a^c) and glass-related kaersutite (d) Theshaded fields indicate the compositional variation of all Mg-katophorite analyses (light grey) and the compositional range of LREE-depletedTi-pargasite from the Group I (grt)^spinel lherzolites (dark grey Kaeser et al 2006)

16

constrained by different studies (eg Ionov amp Hofmann1995 LaTourrette et al 1995) In Ti-poor systems such asthose of this study Nb uptake by amphibole is evenenhanced as a result of increased DNb values (Tiepoloet al 2001) Consequently in an event during which amphi-bole crystallizes continuously the remaining fluid or meltwill become progressively depleted in Nb (Ionov et al2002) resulting in formation of Nb-depleted amphibolesfurther away from the source (Zanetti et al 1996 Ionovet al 2002) such Nb-depleted amphibole is found insample Ke 785 as well as inType III-b xenoliths (eg Ke195915 Fig 9a) Amphibole signatures from xenolith suitesmodelled successfully in the context of percolative traceelement fractionation (eg Spitsbergen Ionov et al 2002)show similar ranges (eg in NbTh ratios Fig10a) to thosedocumented for our samples This further supports the

hypothesis that similar percolation mechanisms acted inthe mantle beneath Marsabit

The role of pre-existing heterogeneitiesand exotic mineralsAlthough a model based on trace element fractionationduring melt percolation is supported by textures and canaccount for many compositional features some composi-tional peculiarities remain unexplained First clinopyrox-ene and amphibole in several samples (Ke 785 19651 and196515) are characterized by extremely low HREE abun-dances more than one order of magnitude lower thanin amphiboles from the strongly metasomatizedamphibole dunite Ke 196525 (Fig 9a^c) The mainphases that crystallize during metasomatism are low-TiMg-katophorite and phlogopite However DHREE

amphmelt

Fig 10 Selected trace element ratios in amphiboles from the porphyroclastic spinel harzburgites from Marsabit compared with those ofmantle-derived amphibole from other tectonic settings (XK and XB cratonic kimberlite-hosted and off-cratonic basalt-hosted xenolithsrespectively O orogenic peridotites XS xenoliths from subduction settings) Concentrations (ratios) normalized to the primitive mantlevalues of McDonough amp Sun (1995)

17

Table 5 Representative analyses of phlogopite and apatite (oxides in wt trace elements in mgg)

Type III-b III-c III-b III-c

Mineral phlog phlog phlog phlog phlog phlog phlog apatite apatite

Sample 196515 196515 19653 785 785 785 196525 195915 19653

Texture veinlet interst in opx in opx with amph

in mp

around

chromite

incl in

amph

bleb in

glass

interst

grain

SiO2 3925 3914 4171 4190 4124 4134 4230 SiO2 016 049

TiO2 313 307 009 053 039 037 027 Al2O3 001 nd

Al2O3 1544 1583 1360 1360 1384 1377 1477 P2O5 3768 3715

Cr2O3 112 105 132 147 190 197 063 FeO(tot) 037 029

FeO(tot) 350 347 271 000 289 277 340 MnO 006 nd

MnO nd 002 008 305 004 002 005 MgO 084 091

NiO 023 027 019 023 024 028 023 CaO 4750 4237

MgO 2287 2280 2577 2591 2582 2528 2525 SrO 095 397

CaO nd 002 001 nd 004 001 002 Na2O 079 133

Na2O 215 219 282 258 264 262 208 La2O3 003 007

K2O 719 707 704 706 674 684 683 Ce2O3 088 347

Nd2O3 003 007

F 139 224

Cl 287 092

Total 9488 9493 9571 9641 9587 9535 9590 Totaly 9305 9245

Mg-no 9282 9287 9443 9380 9408 9420 9298

P 369 187 480 339 206 188 362 P 129717 139419

K 71711 72575 60747 79052 70997 69992 70189 K 110 897

Sc 099 110 131 130 088 091 160 Sc 242 079

Ti 15500 16408 890 2839 2096 1807 1736 Ti 235 5294

V 255 284 888 142 108 130 788 V 365 382

Ni 1808 1727 1744 2046 1813 1773 1905 Ni 121 397

Zn 234 221 358 382 317 321 695 Zn 119 096

Ga 111 984 na 210 190 188 235 Ga na na

Rb 185 147 289 183 133 133 342 Rb 015 049

Sr 102 116 775 325 363 350 101 Sr 7138 42233

Y 010 0086 50057 50054 0021 0006 0052 Y 988 1454

Zr 019 019 043 50088 0077 0070 073 Zr 307 101

Nb 506 628 479 177 102 146 210 Nb 024 020

Cs 175 138 261 399 216 180 798 Cs 50013 50036

Ba 1212 1096 1343 298 277 219 1124 Ba 139 126

La 010 0055 0069 50029 0020 50003 50045 La 7460 19048

Ce 0027 0011 50075 50019 014 50003 0078 Ce 6844 25945

Pr 0012 0008 50067 50024 0012 50003 50019 Pr 380 2212

Nd 0019 0035 50405 50079 0065 50009 50224 Nd 749 6530

Sm 0015 50011 50474 50090 0016 50010 50132 Sm 515 747

Eu 0022 0017 50104 50026 0013 0010 50095 Eu 147 189

Gd 50010 50011 5043 5015 50011 0012 017 Gd 314 451

Tb 50002 50002 na 50027 0002 50002 50028 Tb na na

Dy 50015 50017 045 50077 50007 50006 50077 Dy 198 281

Ho 50002 50003 50043 50020 0002 50002 50028 Ho 359 511

Er 50009 50013 5025 5011 50016 50009 5011 Er 903 119

Tm 50002 50002 50088 50013 0002 0003 50018 Tm 119 138

(continued)

18

data (eg LaTourrette et al 1995) indicate that amphibolecrystallization alone is unlikely to deplete the metasomaticagent to such a high degree in HREE The same probablyaccounts for the extreme Nb depletion in amphiboles fromKe 785 relative to the otherType III-c xenolith (Ke 196525 see also Ionov et al 2002)Further clinopyroxene phlogopite and amphibole in the

Type III-b sample Ke 196515 on one hand all showstrongly fractionated REE patterns (eg high LaCe inamphibole Fig 10c) but on the other hand are character-ized by a positiveTi anomaly decoupled from other HFSE(eg Hf and Zr Fig 9b) Trace element fractionationduring amphibole crystallization (ie formation of TypeIII-c amphibole dunites) however should lower Ti con-tents in the remaining fluid or melt (eg Moine et al 2001)Based on textures mineralogy and dominant common

compositional features the hypothesis that Type III-band -c xenoliths were metasomatized by completely inde-pendent metasomatic agents can be ruled out Thereforetwo other possibilities remain to account for the inconsis-tent observations described above (1) inference of otherphases reactants or products that are able to fractionatespecific trace elements present in the mantle beneathMarsabit but not present in the few investigated samples(2) pre-metasomatic compositional heterogeneities similarto the inherited HREE variation inType III-a clinopyrox-ene (see above)Garnet would be an important sink for HREE but

metasomatism occurred in the spinel field after decom-pression with maximum pressures of 515GPa (Kaeseret al 2006) so garnet had already broken down to sym-plectites (Fig 15) Percolation of melt through garnet-bearing websterites prior to metasomatism in Ke 19651and 196515 could be an alternativeWebsterites have been

interpreted to occur in close spatial relationship with por-phyroclastic peridotites in the shallow mantle beneathMarsabit (Kaeser et al 2006) and they occasionally con-tain rutile and ilmenite which could further account forthe strong Nb and Ta depletion in some samples (eg Ke785) The available compositional data from the webster-ites however do not indicate metasomatic modification ofwebsteritic garnet (Olker 2001 B Kaeser unpublisheddata) nor compositional similarities to Group IIIperidotitesCareful study by SEM rules out the presence of exotic

phases such as zircon or Ti-oxides described fromother xenolith suites (eg Bodinier et al 1996 Rudnicket al 1999) Nevertheless it cannot be excluded that themetasomatizing agent percolated through mantle domainscontaining these phases before infiltrating the studiedrocksThe preservation of pre-metasomatic compositional het-

erogeneities is another possibility that must be consideredThat such lsquorock signalsrsquo partly overlap with the lsquometaso-matic signalrsquo is indicated by the HREE variation in TypeIII-a clinopyroxene (see above) and also by other composi-tional parameters For example both Mg-katophorite andphlogopite growing at the expense of spinel have very highCr2O3 contents whereas euhedral Mg-katophorite inthe amphibole dunite (Ke 196525) or in phlogopiteinclusions in Opxm is comparably Cr2O3-poor (Fig 8bTables 4 and 5) Pre-existing heterogeneities would essen-tially be preserved through elements that are relativelycompatible in a given mineral (ie that are not mobilizedduring metasomatism) This would be in agreement withthe preserved HREE variations in Type III-a clinopyrox-ene high Cr2O3 in amphiboles and phlogopite and Tispikes in amphibole from Ke 196515

Table 5 Continued

Type III-b III-c III-b III-c

Mineral phlog phlog phlog phlog phlog phlog phlog apatite apatite

Sample 196515 196515 19653 785 785 785 196525 195915 19653

Texture veinlet interst in opx in opx with amph

in mp

around

chromite

incl in

amph

bleb in

glass

interst

grain

Yb 50014 50011 5047 5015 50011 50019 5017 Yb 803 724

Lu 0002 50002 50061 50014 50002 50002 50020 Lu 115 742

Hf 0019 0018 5025 50053 50012 50006 5019 Hf 50023 50120

Ta 020 022 140 0031 0012 0009 023 Ta 50015 50075

Pb 546 586 125 108 083 098 143 Pb 436 577

Th 50004 50004 50080 0043 0088 0056 50021 Th 2538 1305

U 0012 0011 0069 50018 0038 0030 0031 U 572 278

interst interstitial mp melt pocket na not analysed nd not detected

19

Further very high Mg-numbers in replacive amphibolessupport melt (fluid)^rock interaction at low melt (fluid)rock ratios where Mg and Fe contents are buffered by theambient peridotite (Kelemen et al 1995 Tiepolo et al2001) The slightly lower Mg-number of euhedralamphiboles in Ke 196525 combined with their lowerCr2O3 contents (Fig 8b) suggests a higher melt (fluid)rock ratio which is further supported by considerablylower Mg-number in associated ol-I (Fo88 Table 3)

The limited number of investigated samples does notallow us to decide which of the processes described aboveacted and to what extent However given the heterogene-ity observed in the numerous non-metasomatized xenolithsfrom Marsabit (Kaeser et al 2006) pre-metasomaticfeatures certainly affect the final product of the melt^rockreaction process to a considerable degree

The nature of the metasomatizing agent(s)Graphite stability and oxygen fugacity considerations

The Marsabit xenoliths are unusual in that they occur inan off-cratonic setting yet bear macroscopic graphite Thelatter usually either occurs in kimberlite-hosted peridotites(on-craton) or in pyroxenites from alpine-type peridotitebodies (eg Pearson et al 1994) In the phlogopite perido-tites from Finero graphite has recently been reportedas nanoscopic layers in phlogopite (Ferraris et al 2004)In xenoliths from Marsabit graphite occurs in the sametextural relationship as in kimberlite-hosted peridotitexenoliths (ie millimetre-sized flakes typically associatedwith orthopyroxene see Fig 3c^e and Pearson et al 1994)Graphite inclusions in phlogopite and Opxm couldindicate crystallization during metasomatism by alsquoreducedrsquo C-bearing fluid or melt However precipitationof graphite more probably reflects the relatively reducingconditions in the uppermost mantle prior to metasoma-tism We used the oxygen barometer of Ballhaus et al(1991) based on ferric iron contents in spinel coexistingwith olivine to constrain oxygen fugacity (fO2)conditions prior to metasomatism in the Marsabitxenoliths (Fig 16) The results show that non-metasomatized Group I and IV samples reflect similarlyreducing conditions to the cryptically metasomatizedType III-a samplesAlthough assessing Fe3thornFe2thorn ratios in spinel based

on microprobe analyses can be problematic (Wood ampVirgo 1989 Ballhaus et al 1991) all microprobeanalyses were obtained using the same operationalsettings and therefore give at least the relative fO2

variation between samples The relatively reducingconditions in the pre-metasomatic cold andhighly deformed mantle well below carbon^carbonate^silicate equilibria (Mg2Si2O6thorn 05 CaMg(CO3)2frac14Mg2SiO4thorn 05 CaMgSi2O6thornCthornO2 Eggler amp Baker1982 see Fig 16) and close to the CCO oxygen buffer arein agreement with the presence of graphite in the GroupIII harzburgites Moreover the P^T conditions at whichthe Group III xenoliths partially re-equilibrated are closeto the subsolidus decarbonation of model carbonated lher-zolite (Fig 2 see Lee amp Wyllie 2000 and referencestherein) in agreement with the observed phase assemblageincluding orthopyroxene olivine clinopyroxene andgraphite

Fig 11 Trace element patterns of (a) phlogopite and (b) apatitefrom porphyroclastic spinel harzburgites and dunites from Marsabit[concentrations normalized to the primitive mantle valuesof McDonough amp Sun (1995)] The grey field in (a) indicates thecomposition of Ti-rich phlogopite in Group I (grt)^spl lherzolites(Kaeser et al 2006)

20

Table 6 Representative analyses of silicate glass (oxides in wt trace elements in mgg)

Type III-b III-c Host

Sample 195915 196515 196515 785 196525 19653 19653 19653 195927

Remark next amph next cpx xenol margin next amph next amph melt pocket next apatite next Opxm melt pocket host basanite

SiO2 5306 5775 5161 6096 5657 5938 5974 6530 4882 4410

TiO2 017 157 106 062 030 024 020 023 378 255

Al2O3 2412 2053 2025 1779 2313 1575 1245 1355 1890 1396

Cr2O3 005 001 nd 001 001 nd nd nd 002 na

FeO(tot) 188 247 626 126 252 295 374 265 509 1283

MnO 007 000 026 001 004 005 008 007 018 021

NiO nd nd nd na nd na 006 nd 001 na

MgO 182 129 116 114 122 304 382 252 244 938

CaO 506 257 283 082 044 124 101 039 810 1030

Na2O 1107 966 969 971 1299 1388 1384 1211 673 374

K2O 154 197 465 357 138 121 142 127 300 146

Cl 031 106 038 012 014 023 061 020 015 na

P2O5 041 008 044 009 012 049 233 018 100 059

Total 9956 9896 9860 9609 9887 9845 9929 9846 9821 9912

Mg-no 6329 4816 2489 6175 4632 6474 6458 6289 4552 5658

P 1061 448 1610 230 503 1519 6738 676 2994 na

K 12563 16408 39599 32515 15872 9265 11277 10071 25900 na

Sc 179 184 126 406 151 970 136 848 691 237

Ti 665 7329 4974 2953 1965 1213 1163 1055 18199 15311

V 107 265 235 796 920 210 151 134 234 na

Ni 128 371 92 5575 983 170 5199 5298 199 191

Zn 962 103 109 110 459 440 508 309 819 na

Ga na 173 273 194 220 na na na na na

Rb 181 263 124 546 626 176 398 369 577 369

Sr 427 488 1407 188 881 1644 4997 985 1062 792

Y 543 145 224 752 262 619 158 369 301 299

Zr 466 171 353 761 660 578 365 428 242 216

Nb 416 161 137 260 136 234 164 169 837 647

Cs 014 046 186 125 007 022 102 052 063 na

Ba 238 261 1064 129 168 405 1099 641 625 563

La 698 183 848 144 889 2212 1008 106 537 462

Ce 947 256 131 308 138 333 1265 164 104 823

Pr 655 194 120 330 116 304 975 160 120 na

Nd 146 603 408 101 329 993 256 564 463 400

Sm 142 048 570 179 419 141 379 679 105 851

Eu 041 035 228 082 112 438 103 241 345 267

Gd 062 051 415 158 402 106 245 566 677 729

Tb na 0086 086 020 071 na na na na 111

Dy 090 027 359 098 434 104 171 637 732 na

Ho 023 0037 105 025 095 206 446 104 117 114

Er 045 038 195 058 255 611 153 264 305 na

Tm 009 50039 033 0081 042 085 292 5014 036 052

Yb 062 023 267 044 307 667 120 487 253 220

(continued)

21

Evidence for low-temperature SiO2^Na2O^CO2^H2O-rich melts or fluids

Potential metasomatic agents inferred from the presenceof graphite are C^O^H-rich fluids carbonate melts orCO2-bearing silicate melts The resulting phase assem-blages and geochemical signatures are the result of infiltra-tion of fluid or melt into a cold (750^8508C) shallow(515GPa Fig 2) heterogeneous and locally reduced litho-spheric mantle Metasomatism under such conditions in anopen system must result in complex geochemical signa-tures that reflect the degree of fluid^rock reaction Thismakes it particularly difficult to constrain the exactnature of the lsquooriginalrsquometasomatic agentType III-c xenolith Ke 196525 contains the highest frac-

tion of metasomatic phases and the least fractionatedamphibole trace element patterns The lsquomagmaticrsquo texturesof the Mg-katophorites in this sample suggest direct crys-tallization from a melt or a fluid rather than replacementof a pre-existing phase Therefore hypothetical liquids inequilibrium with the amphiboles in this xenolith couldyield the composition of the initial metasomatic agentsFigure 17 illustrates the trace element patterns of suchmelts and fluids calculated using sets of experimentally orempirically determined mineralmelt (fluid) partitioncoefficients including carbonate melt silicate melt andhydrous fluid (for references see caption to Fig 17)Common features for all calculated metasomatic agents

are negative Zr^Hf^Ti anomalies and LREE enrichment(Fig 17) However previous studies have shown that thesesignatures are indicative of almost all types of metasoma-tism including interaction of peridotite with carbonatiticmelt (Green amp Wallace 1988 Dautria et al 1992 Hauriet al 1993 Rudnick et al 1993 Ionov et al 1996 Yaxleyet al 1998 Coltorti et al 1999) with hydrous fluids andormelts for example originating from dehydration of altered

subducting lithosphere (Maury et al 1992 Zanetti et al1999 Grecurren goire et al 2001 Laurora et al 2001 Parkinsonet al 2003) and interaction of peridotite with volatile-and Si-rich melts that evolved through reactive porousflow (Bedini et al 1997 Ionov et al 2002 2006 Bodinieret al 2004 Rivalenti et al 2004)In the case of the Marsabit xenoliths integration of the

trace element characteristics thermobarometric inferencesand mineralogical constraints favours a Na2O^SiO2^CO2^H2O-rich silicate melt Carbonated melts can prob-ably be ruled out on major element and mineralogical con-siderations First amphiboles are Ca-poor in conflict withcarbonate melt metasomatism which is generally charac-terized by Ca enrichment (Rudnick et al 1993 Neumannet al 2002) Further orthopyroxene is a stable phase andprobably even crystallizes during metasomatism in somesamples (eg Ke 785) At near-solidus conditions coexis-tence of orthopyroxene and carbonate melt is possibleonly at pressures 20GPa (see Fig 2 eg Lee amp Wyllie2000 and references therein) which is considerably higherthan the maximum pressures indicated by the compositionof spinel (515GPa) At such low pressures metasomatismby carbonated melts will convert lherzolite and harzbur-gite immediately into wehrlite which is the major perido-tite type in all xenolith suites where carbonate meltmetasomatism is convincingly demonstrated (Rudnicket al 1993 Yaxley et al 1998 Neumann et al 2002)In Marsabit wehrlites are absent howeverAn Na2O^SiO2^CO2^H2O-rich melt or fluid on the

other hand would explain the Na-rich nature of amphiboleand phlogopite as well as the presence of orthopyroxeneGrowth of metasomatic (and Al-poor) orthopyroxene hasbeen described from several xenolith localities where peri-dotites were infiltrated either by hydrous fluids orSi-rich melts (Smith et al 1999 McInnes et al 2001Morishita et al 2003) Na-rich phlogopite is commonly

Table 6 Continued

Type III-b III-c Host

Sample 195915 196515 196515 785 196525 19653 19653 19653 195927

Remark next amph next cpx xenol margin next amph next amph melt pocket next apatite next Opxm melt pocket host basanite

Lu 014 0050 026 0041 046 080 140 058 040 031

Hf 026 052 535 014 076 110 5087 124 568 504

Ta 50080 033 638 0022 123 101 784 801 394 403

Pb 541 448 280 255 840 797 260 749 493 na

Th 194 296 160 466 703 185 247 105 534 506

U 545 069 478 099 152 447 724 216 171 126

nd not detected na not analysedAverage composition of the strongly alkaline basanites from Marsabit (Volker 1990)

22

Fig 12 Total alkalis vs SiO2 (a) Harker-type co-variation diagrams (b^e) and incompatible trace element co-variation diagram (f) showingthe compositional variability of silicate glass found in the porphyroclastic spinel harzburgites and dunites from Marsabit In (b) binary mixinglines between the typical composition of the alkali-rich glass (large open diamond) and the associated microlites are shown (each increment is10wt)They illustrate that the glass does not represent the derivate of host lava infiltration and fractional crystallization Subalkaline glass inGroup I (grt)^spl lherzolites from Marsabit is shown for comparison and is interpreted to derive from incongruent melting of Ti-pargasite(Kaeser 2006)

23

found as a reaction product between anhydrous mafic toultramafic rocks and H2O-rich evolved liquids (Costaet al 2001 and references therein) or H2O-bearing fluids(eg Odling 1994) The latter study was an experiment at20GPa simulating the effects of decreasing temperatureson the phase assemblages and their composition duringmetasomatism using a model peridotitethornC^O^H fluidThe coolest part of Odlingrsquos (1994) experiment (510758C)contained a strikingly similar phase assemblage (kato-phorite phlogopite with 2wt Na2O and Al-poororthopyroxene) to the Marsabit samples interpreted tohave formed during sub-solidus fluid^solid interactionA compositionally similar phase assemblage (highamounts of opx Si-rich hornblende Na-rich phlogopite)resulted from experiments simulating peridotite interac-tion with Si-rich melt (dacite) at lower P^T conditions(9508C at 092GPa Prouteau amp Scaillet 2003)In this study the trace element signatures of Group III-c

Mg-katophorite and the calculated metasomatic agentsfurther suggest that the metasomatizing agent was a meltrather than a fluid Hypothetical fluids in equilibrium witheuhedral Mg-katophorite from sample Ke 196525 havehigh NbLILE (eg Rb Ba) and low LILEREE ratios(Fig 17) Because experimental data predict the oppositefor fluids (LILE enriched over HFSE and REE egManning 2004 and references therein) a fluid with asignature such as that indicated in Fig 17 is unrealistic

Fig 13 Trace element patterns of silicate glass in porphyroclasticspinel harzburgites and dunites from Marsabit compared withthe trace element composition of associated clinopyroxene (Dim)micro-phenocrysts xenolith-hosting basanites from Marsabit andglass interpreted to represent xenolith^host interaction [concentra-tions normalized to the primitive mantle values of McDonough ampSun (1995)]

Table 7 Representative analyses of carbonates (oxides in

wt trace elements in mgg)

Group III-b III-b III-c III-c III-c

Mineral Dol Mg-Cc Mg-Cc Mg-Cc Mg-Cc

Sample 195915 196515 785 19653 19653

Texture in glass in glass in glass in glass in glass

SiO2 na 003 na nd 003

FeO(tot) 016 011 010 010 006

MnO 001 nd nd nd nd

MgO 1922 236 190 125 517

CaO 3189 5313 5377 5391 4853

Na2O 003 nd 004 nd 003

K2O na 002 na 001 nd

SrO 003 010 008 004 021

BaO 003 nd nd na na

CO21 4618 4444 4442 4377 4391

Total 9756 10021 10032 9909 9794

Mg-no 9954 9745 9713 9554 9931

Ca-no 5439 9418 9531 9687 8709

P 220 334 137 580 589

K 970 212 642 105 155

Sc 5065 50150 027 5027 50076

Ti 975 189 242 528 196

V 5085 0481 5042 5037 021

Ni 900 673 517 5294 593

Zn 459 5089 113 296 045

Ga na 5014 5013 na na

Rb 0084 50072 50074 50060 0046

Sr 593 860 371 251 4274

Y 012 011 130 704 015

Zr 261 024 012 097 756

Nb 144 019 0056 692 185

Cs 50063 50011 50018 50039 50008

Ba 126 230 5024 029 271

La 034 057 084 136 025

Ce 032 013 134 997 023

Pr 006 006 018 095 0026

Nd 030 013 068 228 0078

Sm 5044 50091 0074 048 50018

Eu 5012 50019 0051 020 50008

Gd 5037 50090 014 051 0026

Tb na 50010 0036 na na

Dy 5019 50039 0089 058 50038

Ho 50029 0013 0044 011 50006

Er 5012 50041 0083 033 50019

Tm 50052 50013 0016 0084 50006

Yb 5043 50063 014 027 50040

Lu 50072 50010 0011 0066 50008

(continued)

24

At the relatively low maximum pressures (515GPa) asupercritical liquid is not expected either (eg Kesselet al 2005)Finally a relatively hydrous metasomatizing agent

would be consistent with formation of Mg-katophoriteswith lowTi and presumably low Fe3thorn contents (given therelatively reducing conditions) which in turn imply high(OH)^ contents In other words partial dehydrogenationtypically observed in common (Ti-rich) mantle amphibole

(eg Popp et al 1995 Tiepolo et al 2001 and referencestherein) cannot have played an important role in thiscase as H deficiency is mainly the result of coupled substi-tutions involvingTi and Fe3thorn (Popp et al 1995) In conclu-sion the metasomatizing agent was most probably aNa2O^SiO2^CO2^H2O-rich meltComparison with experimental studies further con-

strains the low-temperature regime in which metasoma-tism occurred In ultramafic (eg Niida amp Green 1999)basaltic (eg Rapp 1995) and Si-enriched systems (egtrachybasaltic Barclay amp Carmichael 2004) amphibole(in the latter study together with phlogopite) is confinedto temperatures lower than 1000^10508C at pressures520GPa (Fig 2)Thus crystallization of the early metaso-matic phase assemblage can be explained by infiltration ofa silicate melt that rapidly cooled upon contact with theambient peridotite (750^8508C prior to metasomatismFig 2) However this low-T stage is based on Ca-in-opxthermometry (see Brey amp Kolaquo hler 1990 Kaeser et al2006 Table 1) and therefore probably underestimates theeffective temperature conditions during metasomatismThis can be explained by low diffusion rates and proximityto the closure temperature for the diopside^enstatiteexchange reaction at temperatures lower than 8508C(eg Smith 1999 and references therein) The lower tem-perature limit at which silicate melt metasomatism ispossible (ie defined by the meltrsquos solidus and viscosity)depends strongly on volatile and SiO2 contentsExperiments by Draper amp Green (1997) for exampleshowed that highly silicic melts (52^64wt SiO2) canbe in equilibrium with mantle harzburgite at temperaturesas low as 8508C (at 10^20GPa under anhydrous andC^O^H-fluid-bearing conditions) These are howeverextreme compositions for mantle melts The actual SiO2

content of the metasomatizing agent in this case is difficultto constrain from the metasomatic mineral assemblage ascompositions may have varied considerably during reac-tive percolation (eg including Si-releasing reactions suchas cpx replacement by amphibole versus Si-consumingreactions such as Opxm crystallization) Moreover it isunlikely that a melt persists at temperatures muchbelow 9008C which is the solidus of fertile lherzolite(ie primitive upper mantle composition) underH2O-saturated conditions at 15GPa (Grove et al 2006see Fig 2) Below the solidus residual volatile-enrichedfluids may percolate further for example forming the lessmetasomatized Type III-c xenoliths (eg Ke 785) as wellas Type III-b and -a peridotites (see also Bailey 1987)Relatively cold conditions would also explain the veryhigh compositional gradients developed on small scales inType III-a samples (Fig 15) Given the absence of chloritein all investigated samples the lower temperature limitof such fluid metasomatism is 8008C (eg Fumagalli ampPoli 2005 see Fig 2)

Table 7 Continued

Group III-b III-b III-c III-c III-c

Mineral Dol Mg-Cc Mg-Cc Mg-Cc Mg-Cc

Sample 195915 196515 785 19653 19653

Texture in glass in glass in glass in glass in glass

Hf 5016 50038 50036 50074 50013

Ta 0048 50012 50016 50031 0008

Pb 5024 858 5021 074 018

Th 0021 50020 50017 50028 50005

U 060 094 086 600 148

Mg-Cc Mg-calcite Dol dolomite nd not detectedna not analysed Ca-numberfrac14 100Ca(CathornMg)1Calculated from stoichiometry

Fig 14 Trace element patterns of glass-hosted carbonates in TypeIII-b and -c spinel harzburgite and dunite xenoliths from Marsabit[concentrations normalized to the primitive mantle values ofMcDonough amp Sun (1995)] The grey field outlines the compositionalrange of carbonates found in other peridotite xenoliths (Ionov ampHarmer 2002 and references therein) Carbonate material inter-preted as quenched carbonatite melt found in peridotite xenolithsfrom Kerguelen (Moine et al 2004) is shown for comparison(bold line)

25

Finally the presence of apatite even though oftentaken as indicator for carbonatite-type metasomatism(eg Yaxley et al 1998) is not in conflict with metasoma-tism by silicate melts Apatite has also been shown to crys-tallize from volatile and alkali-rich derivatives formedduring reactive porous flow of originally mafic silicatemelts (Woodland et al 1996 Bedini et al 1997 Bodinieret al 2004 Ionov et al 2006)

FORMATION OF THE LATE GLASS -BEAR ING ASSEMBLAGESTextures strongly suggest that the glass-bearing assem-blages formed later and at the expense of the early metaso-matic assemblages (Fig 4a and d) Features such as thereappearance of clinopyroxene (Na-rich Cr diopsidemicro-phenocrysts) a phase replaced by the early

Fig 15 Detailed REE systematics in primary clinopyroxene (cpx-I) in a porphyroclastic cryptically metasomatized spinel harzburgite(sample Ke 19651) showing the transition fromType III-a (Zone A and B) toType III-b (Zone C) on a scale of a few centimetres The numberson the photomicrographs (crossed polars) correspond to single laser ablation microprobe pits (for further explanation see text)

26

metasomatic assemblage indicate that melt pocket forma-tion reflects a significant change in P^T^X space in partsof the mantle beneath Marsabit after earlier metasoma-tism Based on geochemical arguments and mass-balanceconsiderations we will show that (1) the late glass-bearingassemblages are closely related to early metasomaticassemblages mainly representing breakdown productsand (2) breakdown was induced by infiltration of aCO2-rich fluid in the mantle probably followed andenhanced by heating and decompression melting duringxenolith transport in the host magma

Evidence for in situ partial meltingBSE imaging reveals that the phases becoming unstableare amphibole orthopyroxene and to minor extentsspl-I phlogopite cpx-I and apatite Compositional(inter- and intra-xenolith) heterogeneity of silicate glass istexture controlled and varies as a function of theassociated precursor phases (Fig 12a^f)TiO2 for exampleshows higher concentrations in glass from xenolith Ke196515 containing Ti-enriched amphibole (Fig 12c)whereas K2O contents are highest in phlogopite-rich sam-ples (eg Ke 785 Fig 12d) Elevated P2O5 contents arefound in glass adjacent to apatite (Fig 12e) whichmirrors at the same time the highTh and LREE contents

of apatite (Fig 12f) Significant Si enrichment is found inglass coating orthopyroxene (Fig 12d) supporting incon-gruent melting of orthopyroxene (see below) Even thoughthe melt pockets are connected along grain boundaries thestrong compositional variation (on inter- and intra-xeno-lith scale) suggests that the mobility of the melt was lim-ited or that the melting event was very short-lived (fortiming constraints see below)The co-variation plots in Fig 12 further reveal consider-

able compositional differences between all types of xeno-lith glass and the host basanite These data indicate thatthe glass cannot be the product of xenolith^host lava inter-action during ascent to the surface Also back-mixing ofmicro-phenocrysts into the melt fails to derive the glassfrom a host-related melt (Fig 12b) Glass from xenolithmargins however is compositionally closer to the bulkhost basanite (eg Fig 12b) in line with a direct derivationfrom xenolith^host lava interaction Its more evolvedmajor element composition (eg higher SiO2 lowerMg-number Fig 12b) is probably the result of fractiona-tion of basanite phenocrysts (eg olivine augite andtitanomagnetite Volker 1990) Host lava^xenolith interac-tion is further supported by the trace element signature ofglass at the margin of the xenoliths which is closely similarto that of the Marsabit basanites (Fig 13 Volker 1990)The same characteristics are observed for intra-xenolithglass in the unusual kaersutite-bearing sample Ke 195927

Fig 16 Oxygen fugacities relative to the quartz^fayalite^magnetitebuffer (QFM equation of OrsquoNeill 1987) as a function of temperatureat assumed pressure of 1GPa calculated using the composition ofcoexisting spinel and olivine (ol-I^spl-I pairs and Chrm^Olm pairsrespectively) based on the oxygen barometer of Ballhaus et al (1991)The graphite^CO2 equilibrium was calculated after Frost amp Wood(1998) and the graphite^carbonate^silicate equilibrium was calcu-lated using the equation of Eggler amp Baker (1982) (See text for furtherexplanation)

Fig 17 Calculated metasomatic agents in equilibrium with the euhe-dral Mg-katophorites from the Type III-c amphibole dunite Ke 196525 Amphibolehydrous fluid and amphibolecarbonatite melt parti-tion coefficients are from Adam et al (1997) and Adam amp Green(2001) respectively Silicate melts have been calculated using amphi-boleSi-rich melt partition coefficients from Chazot et al (1996b) andamphibolebasanite partition coefficients from LaTourrette et al(1995) The compositional range of the Marsabit basanites and alkalibasalts (Volker 1990) is shown for comparison

27

Xenolith glass unrelated to the host lava and melt pock-ets similar to those reported here in association with resi-dual phases (eg amphibole or clinopyroxene) has beenreported from many xenolith localities (eg Frey ampGreen 1974 Ionov et al 1994 1996 Chazot et al 1996a1996b Yaxley et al 1997 Laurora et al 2001 Bali et al2002) Residual phases within the melt pockets stronglyindicate in situ partial melting within the xenolithsHowever a variety of processes may trigger such in situ

melting including (1) closed-system decompression melt-ing of either anhydrous (Maale amp Printzlau 1979) orhydrous phases (Frey amp Green 1974 Francis 1976) duringxenolith ascent in the host lava (2) melting during interac-tion of the xenoliths with the host basaltic melt duringxenolith ascent (eg Zinngrebe amp Foley 1995 Shaw ampKlulaquo gel 2002 Shaw et al 2006) (3) melting in the mantleprior to xenolith entrainment induced by an infiltratingfluid (ie flux melting eg Jones et al 1983 Ionov et al1994 Demecurren ny et al 2004) However most studies agreethat that partial melting occurred very shortly beforeandor during xenolith exhumation by the host magmaThus a combination of metasomatically triggered meltingfollowed andor assisted by decompression melting duringxenolith transport may be a realistic alternative in somecases (eg Dawson 2002)Recently Ban et al (2005) have shown in a very detailed

mass-balance study on glass-bearing xenoliths from theWest Eifel that the weight proportions of glass and theassociated micro-phenocrysts match the composition ofthe preserved parent amphibole We used the sameapproach and performed simple mass-balance calculations(least-squares fitting) to constrain the conclusions drawnfrom petrography and to test which phase contributedand to what extent to the formation of the melt pocketsThe results reported in Table 8 show that in most casesamphibole-melting can account successfully for the meltpocket composition and only a minor amount (520) ofother phases (orthopyroxene andor phlogopite) needs tobe added to the reactant Such a contribution of otherphases agrees with textural observation (eg Fig 4cshowing Dim growing on resorbed Opxm) as well as withthe conclusions of Ban et al (2005) who stated that moreSi-rich glass (ie 454wt) necessitates orthopyroxenecontributing to the meltTrace element patterns of the bulk glassthornmicro-

phenocryst pockets calculated using the measured traceelement abundances of glass and micro-phenocrysts andthe respective mass-balance parameters (Table 8) arebroadly identical to the measured amphibole composition(Fig 18a and b) Higher LILE (Cs Rb Ba) concentrationsin the calculated bulk melt patches in sample Ke 785 canbe balanced by adding 7wt of phlogopite (Fig 18b)which is in line with mass balance (Table 8) The dominantcontrol of amphibole to account for the trace element

budget of the melt pockets is similar to results found inother suites of glass-bearing xenoliths (Chazot et al1996a Yaxley amp Kamenetsky 1999 Laurora et al 2001)Further the obtained phase proportions for the melt pock-ets are in good agreement with the results from image ana-lysis (Table 1) In addition they are similar to the glass^cpx^ol proportions recorded in recent experiments on the

Table 8 Results of mass-balance calculations to test melt

pocket formation by incongruent melting of the early metaso-

matic assemblages

Sample 195915 785 785 196525 196525 19653

Reactant

amph 1000 1000 0820 1000 0944 mdash

opx mdash mdash 0078 mdash 0056 0406

phl mdash mdash 0102 mdash mdash mdash

glass mdash mdash mdash mdash mdash 05941

SiO2 4484 4989 4964 5129 5167 5898

TiO2 014 039 036 025 024 018

Cr2O3 232 291 258 127 121 009

Al2O3 1368 762 769 800 757 961

FeO(tot) 336 314 333 343 357 394

MnO 006 008 008 012 012 010

NiO 009 013 014 011 011 004

MgO 1844 2029 2197 2040 2122 1625

CaO 854 783 644 966 913 078

Na2O 460 500 436 504 476 843

K2O 049 032 096 042 040 060

Product

glass 0389 0322 0335 0292 0284 07032

ol 0243 0259 0315 0266 0286 0273

cpx 0288 0373 0308 0442 0431 0024

spl 0052 0032 0028 mdash mdash mdash

carb 0028 0014 0014 mdash mdash mdash

SiO2 4477 5027 4986 5207 5181 5903

TiO2 013 041 039 015 014 017

Cr2O3 213 314 272 109 106 015

Al2O3 1364 764 770 798 776 957

FeO(tot) 374 331 351 322 332 350

MnO 007 007 007 009 009 006

NiO 011 013 015 011 012 007

MgO 1834 2024 2196 2063 2141 1635

CaO 854 759 632 952 929 080

Na2O 449 385 388 499 485 860

K2O 060 096 100 015 014 091

r2 023 200 035 084 028 033

Prefix Ke omitted1Low-Si glass 2High-Si glass (see text for explanation)

28

melting behaviour of amphibole in wehrlites at tempera-tures between 1175 and 12508C and pressures of05^10GPa (Mecurren dard et al 2006) These temperatures arein good agreement with the estimates for the formation ofolivine and chromite micro-phenocrysts in the melt

pockets (Fig 16) The heat supply required for amphibolemelting in the formerly low-T metasomatized Group IIIxenoliths (T 200^3008C) may come from local heatingshortly before or during xenolith entrainment in the hostlava or from fluid infiltration that triggered melting (seediscussion below)Finally formation of the most Si-enriched melt (adjacent

to Opxm in Group III-c samples) can be modelled assum-ing progressing interaction between infiltrating melt andorthopyroxene where the latter melts incongruently

opxthornmelt1 olthornmelt2 cpx ethseeTable 8THORN eth1THORN

To calculate reaction (1) we chose the least evolved glassobserved in melt patches in sample Ke 19653 (SiO2

5921wt Mg-number 6465) to represent the initialmelt1 whereas melt2 adjacent to opx is SiO2-rich(6530wt) Trace element abundances in glass next toorthopyroxene are shifted to lower concentrations(Table 6) which is in agreement with dilution caused byaddition of a trace element-poor orthopyroxene compo-nent The resultant phase proportions are similar to thoseobserved at orthopyroxene^melt interfaces (ie higholivine and low cpx abundances) and are in agreementwith experimental studies (Shaw et al 1998)

Partial melting triggered by fluid fluxMass-balance calculations provide an explanation for meltpocket formation by in situ partial melting Thus onemight conclude that melting is simply the result of decom-pression and heating during xenolith transport in the hostmagma However there are two points that argue thatmelting was triggered by the infiltration of a fluid phaseshortly before the xenoliths were sampled by the hostmagma (1) the presence of occasionally high amounts ofcarbonate included in the melt pockets (2) the extremelyNa-rich clinopyroxene (Dim) compositions either includedas micro-phenocrysts in the melt pocket or present asequilibrated grains around chromite in the single TypeIII-d xenolith (Fig 3f) Both arguments will be discussedin the next section

Formation of carbonates

Textures and compositions of carbonate globules can beexplained by carbonate precipitation from a CO2-bearingfluid As demonstrated by several studies highCa(CathornMg) values (Table 7) and low REE concentra-tions are strong arguments that the carbonates are cumu-late crystals precipitated from a CO2-rich melt or afluid and not quenched carbonated melts (Lee et al 2000Ionov amp Harmer 2002) Oscillatory zoning (Fig 5a and b)of individual carbonate crystals within singleglobules corroborates this interpretation The roundedcontacts toward glass (globules) are also in line with thesefindings Similar globular forms were reported fromexperimental studies and do not a priori imply the

Fig 18 Calculated trace element characteristics of reactants (in mostcases amphibolethorn orthopyroxene filled symbols) and products(melt pockets open symbols) of the lsquoamphibole meltingrsquo model[see equation (1) in text] The relative phase proportions wereobtained by mass-balance calculations and are given in Table 8 Thedashed lines in (b) show that the high LILE concentrations obtainedfor the product (bulk melt pocket) can be explained by adding about7wt of phlogopite to the reactant (amphibolethorn orthopyroxeneTable 8) Further explanation is given in the text

29

presence of immiscible carbonate and silicate melts(eg Lee amp Wyllie 1996)The high amount of carbonate in some melt pockets

however (eg Fig 4e) does need further explanation asthe incongruent melting reaction (1) does not account forthe CO2 required for carbonate precipitation duringamphibole melting Therefore amphibole breakdowneither occurred in an open system with addition of at leastCO2 (eg Laurora et al 2001 Ban et al 2005) or anotherpre-existing phase acted as a source of carbon Oxidationof graphite texturally identified as a part of the earlyassemblage (eg Fig 3d) could account for some of theCO2 necessary for carbonate formation A change in fO2

conditions is also indicated when the composition of glass-related spinel (Chrm) and Olm are used for oxygen baro-metry which yields oxygen fugacities well on the oxidizedside of the carbon^carbonate^silicate buffer (Fig 16)However increasing fO2 triggered by closed-systemdecompression-induced melting alone does not appear fea-sible Also textures such as straight contacts between gra-phite and Mg-calcite (Fig 4c) suggest that carbonategrows on graphite (ie indicating local fO2 conditions cor-responding to carbon^carbonate equilibrium) rather thanreplacing it We therefore interpret melt pocket formationto be triggered by the infiltration of a CO2-rich fluid(ie flux melting see also Ionov et al 1994) This interpre-tation also accounts for the presence of abundant vugs inthe glass that could have contained a fluid (Ionov et al1994 2006) which was presumably lost during sample pre-paration However we emphasize here that late fluid infil-tration had only subtle effects on the trace elementcomposition of Type III-b and -c xenoliths

Formation of Na-rich Cr diopside

Infiltration of CO2-rich fluidwould also explain the Na-richcomposition of Dim micro-phenocryst cores (Fig 6a and b)Ionov et al (2005) noted recently that clinopyroxenes inmeltpockets are commonly characterized by Na2O contents51wt mainly as a result of low cpxmelt partition coeffi-cients at low pressures (eg Blundy et al 1995) Thus veryhighNa2Ocontents in Dimmicro-phenocrysts (Fig6a) sug-gest replacement of amphibole by Dim and eventually Olmand Chrm during reaction with a fluid at subsolidus condi-tions An origin of micro-phenocrysts by replacementrather than crystallization from a melt is indicated by theidentical crystallographic orientation of individual Dimcrystals in singlemelt pockets (Fig4a)Once the solidus of the precursor phase(s) was reached

(ie 10508C for amphibole Niida amp Green 1999) silicatemelt was generated in situ and led to the formation of clin-opyroxene rims with lower Na contents (Fig 6b) and thedevelopment of euhedral shapes in contact with melt (egFig 4a and e) Eventual immiscibility between remainingfluid and newly formed melt (eg during xenolith ascent)would then explain the presence of carbonate globules and

vugs enclosed in glass Likewise different origins of meltand carbonate were recently proposed for similar meltpockets from Hungarian xenoliths by Demecurren ny et al(2004) who found oxygen isotope disequilibrium betweencarbonate globules and the host glass Such a two-stepmodel (fluid-induced replacement followed by melt gen-eration) can be explained by CO2-fluid infiltration occur-ring within the mantle at sub-solidus conditions ultimatelyfollowed by xenolith entrainment in the host magma andtransport to the surfaceThe host magma possibly suppliedthe additional heat necessary for melt generationFast decompression would further enhance melting andcould also account for the lower Na contents in Dimmicro-phenocryst rims as a result of decreasing cpxmeltpartition coefficients at decreasing pressures (Blundyet al 1995)Rapid replacement of amphibole ( phlogopite) by an

anhydrous phase assemblage indicates that the fluid waswater-poor (eg low H2OCO2 ratio see Ionov et al2006)This is corroborated by the fact that Mg-katophorite(presumably OH-rich see above) is occasionally rimmedby a residual kaersutitic selvage (Fig 4d) Such Ti-richmantle amphiboles are commonly OH-poor as a result ofpartial dehydrogenation (eg Tiepolo et al 2001 and refer-ences therein)Finally the presence of theType III-d xenolith contain-

ing texturally equilibrated clinopyroxene of almost identi-cal composition to Dim micro-phenocrysts in melt pockets(Table 2 Figs 6a and 7b) further strengthens the argu-ment that CO2-fluid percolation started in the mantlebefore entrainment (ie enough time elapsed to allow thisrock to anneal completely after the late metasomaticassemblage had formed) A similar conclusion can bedrawn for the clearly host-related glass found in xenolithKe 195927 (eg Fig 13b) ThisType III-c sample is charac-terized by the presence of Dim micro-phenocrysts partlyovergrown by mineral rims clearly related to the host basa-nite magma (kaersutite Ti-augite and olivine withFo885 Fig 4f Tables 3 and 4 and supplementary dataat httppetrologyoxfordjournalsorg) Kaersutite hasmajor and trace element characteristics similar to typicalbasanitic and alkali basaltic amphibole phenocrysts(Fig 9d) The special phase assemblages in this xenolithindicates that host-related basanitic melts intruded someType III-c xenoliths and partly overprinted the assemblageformed slightly earlier by in situ derived alkali-rich peralu-minous melts fully preserved in other samples (eg Ke19653 or Ke 785)

ORIGIN OF METASOMATIZ INGAGENTS T IM ING AND TECTONICSETT INGAs noted above late metasomatism was probably short-lived started at mantle depths and probably continued

30

patterns (Fig 14) Extended trace element plots exhibit avery broad range of concentrations in different samples(eg Pb Fig 14) Large positive peaks for U Pb and Srare typical In addition U and Nb are strongly enriched

Fig 6 Major element composition of clinopyroxene in porphyro-clastic spinel harzburgite and dunite xenoliths from Marsabit(a) Plot of Cr2O3 vs Na2O (wt) in micro-phenocrysts comparedwith cryptically metasomatized cpx-I in Type III-b sampleKe 195915 texturally equilibrated Na-rich clinopyroxene in TypeIII-d sample Ke 19706 and texturally equilibrated Na-rich cpxfrom fluid-metasomatized peridotite from Tok (far-east RussiaIonov et al 2005) The dashed line outlines the compositionalrange of common low-Na micro-phenocrysts in glass from otherxenolith suites (see Ionov et al 2005 and references therein)Crosses show the compositional variation of micro-phenocrysts inthe Group III-c xenolith 195927 (ie Dim overgrown by low-Naand low-Cr Ti-augite see Fig 4f) Compositional range of primarycpx-I in all Group III xenoliths (cpx-I grey field) is from Kaeseret al (2006) (b) Na2O and Cr2O3 (wt) variation across a singleDim micro-phenocryst in Type III-c xenolith Ke 19653 (electronmicroprobe profile)

Fig 7 Trace element composition of clinopyroxene from the porphyr-oclastic spinel harzburgites from Marsabit [normalized to the primi-tive mantle values of McDonough amp Sun (1995)] (a) Trace elementpatterns of primary cryptically metasomatized clinopyroxene (cpx-I)inType III-a and -b spinel harzburgites Small arrows indicate high-Na cpx-I inType III-b sample Ke 195915 (see also Fig 6a) (b) Traceelement patterns of texturally equilibrated Na-rich Cr diopside fromthe recrystallized Type III-d spinel harzburgite The shaded fieldsindicate the compositional range of glass-related clinopyroxene micro-phenocrst (Dim)

11

Table 3 Representative analyses of orthopyroxene spinel and olivine (oxides in wt trace elements in mgg)

Type III-a III-b III-b III-c III-d III-c III-d III-c III-c III-c III-d

Mineral opx-I opx-I opx-I Opxm Opxm Chrm spl-I ol-I Olm ol-I ol-I

Sample 19651 196515 195915 19653 19706 19653 19706 785 19653 196525 19706

Texture Zone C1 core core core core core core core core core core core

SiO2 5657 5642 5615 5807 5713 012 005 4093 4135 4078 4098

TiO2 004 013 006 001 005 044 016 nd 001 001 nd

Al2O3 265 269 299 046 192 1223 1880 001 001 nd 004

Cr2O3 043 056 040 032 054 5454 5043 001 029 001 005

Fe2O3 mdash mdash mdash mdash mdash 554 338 mdash mdash mdash mdash

FeO(tot) 533 568 576 538 486 1114 1207 939 583 1133 771

MnO 013 014 014 012 012 na 005 015 013 027 007

NiO na 010 009 007 009 015 009 040 024 037 039

MgO 3473 3420 3380 3561 3464 1480 1502 4900 5182 4739 5013

CaO 051 053 048 020 052 003 000 001 008 002 008

Na2O 005 008 013 009 014 na na 001 004 nd 001

Total 10044 10052 10000 10034 10002 9910 10008 9992 9980 10018 9947

Mg-no 9208 9148 9128 9218 9271 7032 6894 9029 9406 8817 9206

Na 341 423 1136 539 1120 na 438 297 262 647 230

Al 15120 14586 17785 1817 10834 na 114911 893 821 120 1201

P 156 145 175 175 248 na 198 421 292 105 244

Ca 3163 3016 3881 1487 3895 na 55348 822 579 200 658

K 172 5044 5035 981 150 na 5086 5037 5102 198 630

Sc 805 546 173 244 124 na 280 061 432 094 286

Ti 239 619 366 449 227 na 956 156 871 170 204

V 768 906 772 149 634 na 925 104 377 096 283

Ni 568 646 585 588 748 na 1010 2686 2875 2920 2649

Zn 256 301 245 606 373 na 732 630 665 146 693

Ga 181 350 na na 133 na 280 0015 na 0035 077

Sr 024 014 044 102 167 na 0089 0017 50013 0045 052

Y 0081 0073 049 069 202 na 0025 0015 016 010 019

Zr 023 0277 051 0038 088 na 124 50005 50033 50006 015

Nb 50005 0020 50007 018 0051 na 0877 50003 0025 0008 0035

Ba 0056 50018 50021 748 0035 na 50028 50016 50071 017 013

La 0019 50002 0055 0061 0045 na 0039 0021 50004 0017 0019

Ce 0020 0005 0046 0109 027 na 0052 0023 50012 0027 015

Pr 0002 50001 0005 0007 0064 na 0024 50001 50011 0009 0019

Nd 50008 0010 0043 0039 042 na 50029 50007 50052 0018 011

Sm 0014 50009 0016 50018 019 na 0022 50006 50051 50009 0032

Eu 50003 50002 50005 0012 0077 na 0018 50002 50012 50003 0011

Gd 50014 50006 0028 0021 020 na 50026 50006 50041 50016 0035

Tb 0002 50001 na na 0045 na 50004 50001 na 50001 0004

Dy 0015 0011 0086 0058 038 na 0010 50003 50043 50009 0041

Ho 0007 0003 0012 0031 0069 na 0003 50001 50006 0004 0010

Er 50012 0008 0043 0094 022 na 50015 50006 0058 0018 0027

Tm 0002 0001 0015 0014 0031 na 50003 50001 0010 0003 0004

Yb 0027 0011 019 0170 028 na 50018 0009 0064 0060 0038

Lu 0001 0002 0026 0026 0033 na 0007 0002 0040 0018 0008

(continued)

12

Table 4 Representative analyses of amphibole (oxides in wt trace elements in mgg)

Type III-b III-c

Mineral katoph katoph katoph katoph katoph katoph katoph katoph kaersut kaersut

Sample 195915 196515 196515 196515 785 785 196525 196525 195927 195927

Texture residual

in mp

interst interst lamellae

in cpx-I

residual

in mp

residual

in mp

euhedral core euhedral rim micro-xx micro-xx

SiO2 4541 4603 4625 4887 4955 4959 4952 4953 4064 4106

TiO2 015 190 203 099 030 029 022 028 612 577

Al2O3 1365 1145 1100 915 826 807 877 794 1405 1395

Cr2O3 251 123 148 144 254 263 106 115 031 008

FeO(tot) 350 293 304 278 302 294 359 346 588 615

MnO 006 003 004 002 008 002 010 008 016 010

NiO 009 014 009 010 010 006 009 012 010 010

MgO 1883 1886 1896 1997 2067 2062 2000 2024 1576 1545

CaO 851 974 960 976 805 793 863 909 1086 1075

Na2O 474 427 440 432 548 555 529 509 284 294

K2O 049 038 041 031 027 029 028 039 139 138

Total 9811 9695 9728 9771 9837 9805 9759 9741 9817 9793

Mg-no 9055 9199 9176 9275 9243 9259 9086 9124 8269 8173

P 445 222 235 na 175 196 206 120 214 266

K 4301 3916 3637 na 2393 2654 2955 3655 13289 12762

Sc 402 704 663 na 358 345 311 352 375 270

Ti 720 9867 9917 na 1732 1898 1421 1589 32154 32071

V 216 317 332 na 289 299 172 187 469 404

Ni 726 842 828 na 811 876 737 813 638 530

Zn 298 163 159 na 188 214 451 457 463 548

Ga na 981 939 na 103 114 867 872 na na

Rb 279 184 151 na 035 050 096 150 744 713

Sr 216 325 335 na 708 667 955 117 618 668

Y 101 269 315 na 111 126 390 359 310 355

Zr 539 370 331 na 292 297 114 592 933 110

Nb 339 413 447 na 102 121 192 705 265 341

Cs 0006 50001 0005 na 0006 50009 0008 0018 50019 50010

(continued)

Table 3 Continued

Type III-a III-b III-b III-c III-d III-c III-d III-c III-c III-c III-d

Mineral opx-I opx-I opx-I Opxm Opxm Chrm spl-I ol-I Olm ol-I ol-I

Sample 19651 196515 195915 19653 19706 19653 19706 785 19653 196525 19706

Texture Zone C1 core core core core core core core core core core core

Hf 0006 0010 0014 50009 0013 na 0042 50003 50017 50009 50006

Ta 50001 50001 50004 50005 50002 na 0015 0001 50018 50001 50001

Pb 50028 0041 0047 0074 50041 na 015 0036 0075 0042 0046

Th 0009 50002 0069 0020 50002 na 0010 0050 50006 0011 50001

U 0004 50001 0026 0017 0004 na 0017 0011 50006 50001 50002

1See Fig 15nd not detected na not analysed

13

compared withTh andTa respectively Hf Zr andTi showvery low concentrations Overall major and trace elementcharacteristics of the carbonates are similar to those of car-bonates found in other peridotite xenoliths (eg Ionov1998 Lee et al 2000 Laurora et al 2001 Ionov ampHarmer 2002 see Fig 14)

FORMATION OF THE EARLYASSEMBLAGESDespite large compositional variations numerous commontextural and compositional features indicate that the earlymetasomatic assemblages in all Group III samples formedduring a single metasomatic event In the discussion belowwe argue that during a first metasomatic stage the pri-mary (pre-metasomatic) phase assemblage in Group IIIspinel harzburgites was partially replaced by metasomaticminerals finally resulting in clinopyroxene-free amphi-bole- and phlogopite-bearing dunite and harzburgite(Type III-c) The initial metasomatic agent was probably

an evolved CO2^H2O-rich melt and the transitionfrom peridotites of Type III-a to -c can be explained byprogressive melt^rock reaction

Transition from cryptic (Type III-a) tomodal (Type III-b and -c) metasomatismFormation of new phases requires (1) crystallization from amelt or fluid phase (decreasing melt or fluid mass) andor(2) melt^solid or fluid^solid reactionTextural features suchas spinel being replaced by amphibole and phlogopite(eg Fig 3c) indicate melt(fluid)^rock reaction Thegeochemical signatures observed in the different peridotitetypes thus probably reflect pre-existing compositionalheterogeneities coupled with metasomatic modificationThis is illustrated best in sample Ke 19651 (Type III-a)the least metasomatized xenolith in this study It can besubdivided into three parts containing clinopyroxenewith different REE signatures (Zones A B and C inFig 15) The LREE are increasingly enriched from ZoneA to C indicating increasing cryptic metasomatism

Table 4 Continued

Type III-b III-c

Mineral katoph katoph katoph katoph katoph katoph katoph katoph kaersut kaersut

Sample 195915 196515 196515 196515 785 785 196525 196525 195927 195927

Texture residual

in mp

interst interst lamellae

in cpx-I

residual

in mp

residual

in mp

euhedral core euhedral rim micro-xx micro-xx

Ba 781 614 540 na 259 309 846 841 331 322

La 357 116 110 na 704 824 177 113 107 138

Ce 627 225 231 na 217 248 621 426 338 448

Pr 574 208 224 na 301 349 856 623 557 724

Nd 177 720 779 na 128 151 363 296 318 398

Sm 208 143 143 na 285 316 752 740 945 111

Eu 080 058 060 na 108 115 232 252 363 384

Gd 182 085 088 na 225 241 701 692 940 980

Tb 011 013 na 039 050 120 117 na na

Dy 171 057 067 na 202 252 735 698 700 819

Ho 036 010 010 na 039 050 155 144 137 147

Er 106 024 027 na 097 114 405 382 296 336

Tm 016 0030 0033 na 012 017 060 058 027 039

Yb 120 028 030 na 077 099 400 386 243 272

Lu 019 0036 0038 na 0079 0077 058 044 033 031

Hf 015 0067 0054 na 0065 0048 028 078 331 376

Ta 0033 014 014 na 0002 0004 010 107 167 198

Pb 201 360 374 na 142 173 195 222 022 039

Th 682 150 134 na 181 196 059 028 010 020

U 212 039 039 na 040 048 019 012 0018 0048

katoph Mg-katophorite kaersut kaersutite interst interstitial mp melt pocket nd not detected na notanalysed

14

HREE abundances on the other hand are very hetero-geneous in the least metasomatized Zone A moderatelydepleted in Zone B and homogeneously low in the mostmetasomatizedZoneC (containinggraphite Fig15) similarto REE patterns in clinopyroxene from the amphibole- andphlogopite-bearingType III-b sample (Ke196515 Fig7a)The decoupling of strongly (LREETh U) from moder-

ately incompatible elements (HREE) can be explained bychromatographic fractionation during percolation ofsmall amounts of melts or fluids through an initially non-metasomatized peridotite (Navon amp Stolper 1987 Godardet al 1995 Bedini et al 1997 Vernie res et al 1997 Ionovet al 2002) Upon reaction with the wall-rock a fluidor melt will become progressively enriched in incom-patible elements as a function of increasing distancefrom its source This is due to the fact that elements withhigher Dmineralmelt values (eg HREE) are selectivelyremoved from a percolating liquid when it exchanges withthe surrounding minerals The results are faster-movingchemical fronts of incompatible elements compared withthose of more compatible elements This is consistent with

the geochemical signature of cpx-I from the least metaso-matized part of sample Ke 19651 (Zone A Fig 15) whichalready started to re-equilibrate with respect to the incom-patible elements (ie addition of Th U and LREE)whereas more compatible elements still reflect pre-metasomatic heterogeneities In this case these areextreme HREE variations probably reflecting the earliergarnet breakdown (Kaeser et al 2006) On the otherhand more intensely metasomatized parts (eg Zone Cin sample Ke 19651 and sample Ke 196515) are character-ized by clinopyroxene trace element patterns wheremildly incompatible elements have also been modified(eg homogeneously depleted HREE Figs 7a and 15)Similar fractionation processes can explain the modal

and compositional differences between Type III-b and -cCompared withType III-b higher abundances of metaso-matic phases inType III-c suggests that higher amounts offluid or melt crystallized in or passed through these rocksThe amphibole-rich dunite (Ke 196525) for examplecontains low-Ti Mg-katophorite exhibiting the lowest(LaYb)N and (LaCe)N ratios and high (NbLa)N and

Fig 8 Major element characteristics of amphiboles from porphyroclastic spinel harzburgites and dunites from Marsabit compared with com-positions of mantle-derived amphiboles from other localities (XB basalt-hosted xenoliths XS xenoliths from subduction settings O orogenicperidotites) Mg-numberfrac14Mg(MgthornFetot) in molar proportions

15

(NbTh)N ratios (Fig 10a and c) indicating a moderatelyfractionated metasomatic agent Mg-katophorite fromthe phlogopite-rich harzburgite (Ke 785 Type III-c)and amphiboles from the more cpx-rich harzburgites(Type III-b) show progressively more fractionated patterns(eg Fig 10c) and decreasing abundances in lsquoamphibole-compatiblersquoelements (eg Nb Fig 9a) Thus the transitionfrom Type III-b to -c could reflect decreasing distancebetween the site at which the xenolith was sampledand the source of the lsquoinitialrsquo metasomatizing agent (egZanetti et al 1996 Ionov et al 2002 Bodinier et al 2004)

Judging from the abundance of modal metasomatic phasesthe amphibole dunite (Ke 196525) was probably closest tothe source of the metasomatizing agent The phlogopite-rich nature of Ke 785 which indicates higher K andLILE (Cs Rb Ba) contents in the remaining agent is inagreement with a more fractionated melt or fluid at some-what greater distance from the metasomatic sourceThe Nb abundances in amphibole also agree with this

model as well For example amphibole from Type III-cKe 196525 has comparatively high Nb concentrations(Fig 9c) in line with relatively high DNb

amphmelt values

Fig 9 Trace element composition of amphibole from porphyroclastic spinel harzburgites and dunites from Marsabit [normalized to the primi-tive mantle values of McDonough amp Sun (1995)] Mg-katophorite in Type III-b and -c xenoliths (a^c) and glass-related kaersutite (d) Theshaded fields indicate the compositional variation of all Mg-katophorite analyses (light grey) and the compositional range of LREE-depletedTi-pargasite from the Group I (grt)^spinel lherzolites (dark grey Kaeser et al 2006)

16

constrained by different studies (eg Ionov amp Hofmann1995 LaTourrette et al 1995) In Ti-poor systems such asthose of this study Nb uptake by amphibole is evenenhanced as a result of increased DNb values (Tiepoloet al 2001) Consequently in an event during which amphi-bole crystallizes continuously the remaining fluid or meltwill become progressively depleted in Nb (Ionov et al2002) resulting in formation of Nb-depleted amphibolesfurther away from the source (Zanetti et al 1996 Ionovet al 2002) such Nb-depleted amphibole is found insample Ke 785 as well as inType III-b xenoliths (eg Ke195915 Fig 9a) Amphibole signatures from xenolith suitesmodelled successfully in the context of percolative traceelement fractionation (eg Spitsbergen Ionov et al 2002)show similar ranges (eg in NbTh ratios Fig10a) to thosedocumented for our samples This further supports the

hypothesis that similar percolation mechanisms acted inthe mantle beneath Marsabit

The role of pre-existing heterogeneitiesand exotic mineralsAlthough a model based on trace element fractionationduring melt percolation is supported by textures and canaccount for many compositional features some composi-tional peculiarities remain unexplained First clinopyrox-ene and amphibole in several samples (Ke 785 19651 and196515) are characterized by extremely low HREE abun-dances more than one order of magnitude lower thanin amphiboles from the strongly metasomatizedamphibole dunite Ke 196525 (Fig 9a^c) The mainphases that crystallize during metasomatism are low-TiMg-katophorite and phlogopite However DHREE

amphmelt

Fig 10 Selected trace element ratios in amphiboles from the porphyroclastic spinel harzburgites from Marsabit compared with those ofmantle-derived amphibole from other tectonic settings (XK and XB cratonic kimberlite-hosted and off-cratonic basalt-hosted xenolithsrespectively O orogenic peridotites XS xenoliths from subduction settings) Concentrations (ratios) normalized to the primitive mantlevalues of McDonough amp Sun (1995)

17

Table 5 Representative analyses of phlogopite and apatite (oxides in wt trace elements in mgg)

Type III-b III-c III-b III-c

Mineral phlog phlog phlog phlog phlog phlog phlog apatite apatite

Sample 196515 196515 19653 785 785 785 196525 195915 19653

Texture veinlet interst in opx in opx with amph

in mp

around

chromite

incl in

amph

bleb in

glass

interst

grain

SiO2 3925 3914 4171 4190 4124 4134 4230 SiO2 016 049

TiO2 313 307 009 053 039 037 027 Al2O3 001 nd

Al2O3 1544 1583 1360 1360 1384 1377 1477 P2O5 3768 3715

Cr2O3 112 105 132 147 190 197 063 FeO(tot) 037 029

FeO(tot) 350 347 271 000 289 277 340 MnO 006 nd

MnO nd 002 008 305 004 002 005 MgO 084 091

NiO 023 027 019 023 024 028 023 CaO 4750 4237

MgO 2287 2280 2577 2591 2582 2528 2525 SrO 095 397

CaO nd 002 001 nd 004 001 002 Na2O 079 133

Na2O 215 219 282 258 264 262 208 La2O3 003 007

K2O 719 707 704 706 674 684 683 Ce2O3 088 347

Nd2O3 003 007

F 139 224

Cl 287 092

Total 9488 9493 9571 9641 9587 9535 9590 Totaly 9305 9245

Mg-no 9282 9287 9443 9380 9408 9420 9298

P 369 187 480 339 206 188 362 P 129717 139419

K 71711 72575 60747 79052 70997 69992 70189 K 110 897

Sc 099 110 131 130 088 091 160 Sc 242 079

Ti 15500 16408 890 2839 2096 1807 1736 Ti 235 5294

V 255 284 888 142 108 130 788 V 365 382

Ni 1808 1727 1744 2046 1813 1773 1905 Ni 121 397

Zn 234 221 358 382 317 321 695 Zn 119 096

Ga 111 984 na 210 190 188 235 Ga na na

Rb 185 147 289 183 133 133 342 Rb 015 049

Sr 102 116 775 325 363 350 101 Sr 7138 42233

Y 010 0086 50057 50054 0021 0006 0052 Y 988 1454

Zr 019 019 043 50088 0077 0070 073 Zr 307 101

Nb 506 628 479 177 102 146 210 Nb 024 020

Cs 175 138 261 399 216 180 798 Cs 50013 50036

Ba 1212 1096 1343 298 277 219 1124 Ba 139 126

La 010 0055 0069 50029 0020 50003 50045 La 7460 19048

Ce 0027 0011 50075 50019 014 50003 0078 Ce 6844 25945

Pr 0012 0008 50067 50024 0012 50003 50019 Pr 380 2212

Nd 0019 0035 50405 50079 0065 50009 50224 Nd 749 6530

Sm 0015 50011 50474 50090 0016 50010 50132 Sm 515 747

Eu 0022 0017 50104 50026 0013 0010 50095 Eu 147 189

Gd 50010 50011 5043 5015 50011 0012 017 Gd 314 451

Tb 50002 50002 na 50027 0002 50002 50028 Tb na na

Dy 50015 50017 045 50077 50007 50006 50077 Dy 198 281

Ho 50002 50003 50043 50020 0002 50002 50028 Ho 359 511

Er 50009 50013 5025 5011 50016 50009 5011 Er 903 119

Tm 50002 50002 50088 50013 0002 0003 50018 Tm 119 138

(continued)

18

data (eg LaTourrette et al 1995) indicate that amphibolecrystallization alone is unlikely to deplete the metasomaticagent to such a high degree in HREE The same probablyaccounts for the extreme Nb depletion in amphiboles fromKe 785 relative to the otherType III-c xenolith (Ke 196525 see also Ionov et al 2002)Further clinopyroxene phlogopite and amphibole in the

Type III-b sample Ke 196515 on one hand all showstrongly fractionated REE patterns (eg high LaCe inamphibole Fig 10c) but on the other hand are character-ized by a positiveTi anomaly decoupled from other HFSE(eg Hf and Zr Fig 9b) Trace element fractionationduring amphibole crystallization (ie formation of TypeIII-c amphibole dunites) however should lower Ti con-tents in the remaining fluid or melt (eg Moine et al 2001)Based on textures mineralogy and dominant common

compositional features the hypothesis that Type III-band -c xenoliths were metasomatized by completely inde-pendent metasomatic agents can be ruled out Thereforetwo other possibilities remain to account for the inconsis-tent observations described above (1) inference of otherphases reactants or products that are able to fractionatespecific trace elements present in the mantle beneathMarsabit but not present in the few investigated samples(2) pre-metasomatic compositional heterogeneities similarto the inherited HREE variation inType III-a clinopyrox-ene (see above)Garnet would be an important sink for HREE but

metasomatism occurred in the spinel field after decom-pression with maximum pressures of 515GPa (Kaeseret al 2006) so garnet had already broken down to sym-plectites (Fig 15) Percolation of melt through garnet-bearing websterites prior to metasomatism in Ke 19651and 196515 could be an alternativeWebsterites have been

interpreted to occur in close spatial relationship with por-phyroclastic peridotites in the shallow mantle beneathMarsabit (Kaeser et al 2006) and they occasionally con-tain rutile and ilmenite which could further account forthe strong Nb and Ta depletion in some samples (eg Ke785) The available compositional data from the webster-ites however do not indicate metasomatic modification ofwebsteritic garnet (Olker 2001 B Kaeser unpublisheddata) nor compositional similarities to Group IIIperidotitesCareful study by SEM rules out the presence of exotic

phases such as zircon or Ti-oxides described fromother xenolith suites (eg Bodinier et al 1996 Rudnicket al 1999) Nevertheless it cannot be excluded that themetasomatizing agent percolated through mantle domainscontaining these phases before infiltrating the studiedrocksThe preservation of pre-metasomatic compositional het-

erogeneities is another possibility that must be consideredThat such lsquorock signalsrsquo partly overlap with the lsquometaso-matic signalrsquo is indicated by the HREE variation in TypeIII-a clinopyroxene (see above) and also by other composi-tional parameters For example both Mg-katophorite andphlogopite growing at the expense of spinel have very highCr2O3 contents whereas euhedral Mg-katophorite inthe amphibole dunite (Ke 196525) or in phlogopiteinclusions in Opxm is comparably Cr2O3-poor (Fig 8bTables 4 and 5) Pre-existing heterogeneities would essen-tially be preserved through elements that are relativelycompatible in a given mineral (ie that are not mobilizedduring metasomatism) This would be in agreement withthe preserved HREE variations in Type III-a clinopyrox-ene high Cr2O3 in amphiboles and phlogopite and Tispikes in amphibole from Ke 196515

Table 5 Continued

Type III-b III-c III-b III-c

Mineral phlog phlog phlog phlog phlog phlog phlog apatite apatite

Sample 196515 196515 19653 785 785 785 196525 195915 19653

Texture veinlet interst in opx in opx with amph

in mp

around

chromite

incl in

amph

bleb in

glass

interst

grain

Yb 50014 50011 5047 5015 50011 50019 5017 Yb 803 724

Lu 0002 50002 50061 50014 50002 50002 50020 Lu 115 742

Hf 0019 0018 5025 50053 50012 50006 5019 Hf 50023 50120

Ta 020 022 140 0031 0012 0009 023 Ta 50015 50075

Pb 546 586 125 108 083 098 143 Pb 436 577

Th 50004 50004 50080 0043 0088 0056 50021 Th 2538 1305

U 0012 0011 0069 50018 0038 0030 0031 U 572 278

interst interstitial mp melt pocket na not analysed nd not detected

19

Further very high Mg-numbers in replacive amphibolessupport melt (fluid)^rock interaction at low melt (fluid)rock ratios where Mg and Fe contents are buffered by theambient peridotite (Kelemen et al 1995 Tiepolo et al2001) The slightly lower Mg-number of euhedralamphiboles in Ke 196525 combined with their lowerCr2O3 contents (Fig 8b) suggests a higher melt (fluid)rock ratio which is further supported by considerablylower Mg-number in associated ol-I (Fo88 Table 3)

The limited number of investigated samples does notallow us to decide which of the processes described aboveacted and to what extent However given the heterogene-ity observed in the numerous non-metasomatized xenolithsfrom Marsabit (Kaeser et al 2006) pre-metasomaticfeatures certainly affect the final product of the melt^rockreaction process to a considerable degree

The nature of the metasomatizing agent(s)Graphite stability and oxygen fugacity considerations

The Marsabit xenoliths are unusual in that they occur inan off-cratonic setting yet bear macroscopic graphite Thelatter usually either occurs in kimberlite-hosted peridotites(on-craton) or in pyroxenites from alpine-type peridotitebodies (eg Pearson et al 1994) In the phlogopite perido-tites from Finero graphite has recently been reportedas nanoscopic layers in phlogopite (Ferraris et al 2004)In xenoliths from Marsabit graphite occurs in the sametextural relationship as in kimberlite-hosted peridotitexenoliths (ie millimetre-sized flakes typically associatedwith orthopyroxene see Fig 3c^e and Pearson et al 1994)Graphite inclusions in phlogopite and Opxm couldindicate crystallization during metasomatism by alsquoreducedrsquo C-bearing fluid or melt However precipitationof graphite more probably reflects the relatively reducingconditions in the uppermost mantle prior to metasoma-tism We used the oxygen barometer of Ballhaus et al(1991) based on ferric iron contents in spinel coexistingwith olivine to constrain oxygen fugacity (fO2)conditions prior to metasomatism in the Marsabitxenoliths (Fig 16) The results show that non-metasomatized Group I and IV samples reflect similarlyreducing conditions to the cryptically metasomatizedType III-a samplesAlthough assessing Fe3thornFe2thorn ratios in spinel based

on microprobe analyses can be problematic (Wood ampVirgo 1989 Ballhaus et al 1991) all microprobeanalyses were obtained using the same operationalsettings and therefore give at least the relative fO2

variation between samples The relatively reducingconditions in the pre-metasomatic cold andhighly deformed mantle well below carbon^carbonate^silicate equilibria (Mg2Si2O6thorn 05 CaMg(CO3)2frac14Mg2SiO4thorn 05 CaMgSi2O6thornCthornO2 Eggler amp Baker1982 see Fig 16) and close to the CCO oxygen buffer arein agreement with the presence of graphite in the GroupIII harzburgites Moreover the P^T conditions at whichthe Group III xenoliths partially re-equilibrated are closeto the subsolidus decarbonation of model carbonated lher-zolite (Fig 2 see Lee amp Wyllie 2000 and referencestherein) in agreement with the observed phase assemblageincluding orthopyroxene olivine clinopyroxene andgraphite

Fig 11 Trace element patterns of (a) phlogopite and (b) apatitefrom porphyroclastic spinel harzburgites and dunites from Marsabit[concentrations normalized to the primitive mantle valuesof McDonough amp Sun (1995)] The grey field in (a) indicates thecomposition of Ti-rich phlogopite in Group I (grt)^spl lherzolites(Kaeser et al 2006)

20

Table 6 Representative analyses of silicate glass (oxides in wt trace elements in mgg)

Type III-b III-c Host

Sample 195915 196515 196515 785 196525 19653 19653 19653 195927

Remark next amph next cpx xenol margin next amph next amph melt pocket next apatite next Opxm melt pocket host basanite

SiO2 5306 5775 5161 6096 5657 5938 5974 6530 4882 4410

TiO2 017 157 106 062 030 024 020 023 378 255

Al2O3 2412 2053 2025 1779 2313 1575 1245 1355 1890 1396

Cr2O3 005 001 nd 001 001 nd nd nd 002 na

FeO(tot) 188 247 626 126 252 295 374 265 509 1283

MnO 007 000 026 001 004 005 008 007 018 021

NiO nd nd nd na nd na 006 nd 001 na

MgO 182 129 116 114 122 304 382 252 244 938

CaO 506 257 283 082 044 124 101 039 810 1030

Na2O 1107 966 969 971 1299 1388 1384 1211 673 374

K2O 154 197 465 357 138 121 142 127 300 146

Cl 031 106 038 012 014 023 061 020 015 na

P2O5 041 008 044 009 012 049 233 018 100 059

Total 9956 9896 9860 9609 9887 9845 9929 9846 9821 9912

Mg-no 6329 4816 2489 6175 4632 6474 6458 6289 4552 5658

P 1061 448 1610 230 503 1519 6738 676 2994 na

K 12563 16408 39599 32515 15872 9265 11277 10071 25900 na

Sc 179 184 126 406 151 970 136 848 691 237

Ti 665 7329 4974 2953 1965 1213 1163 1055 18199 15311

V 107 265 235 796 920 210 151 134 234 na

Ni 128 371 92 5575 983 170 5199 5298 199 191

Zn 962 103 109 110 459 440 508 309 819 na

Ga na 173 273 194 220 na na na na na

Rb 181 263 124 546 626 176 398 369 577 369

Sr 427 488 1407 188 881 1644 4997 985 1062 792

Y 543 145 224 752 262 619 158 369 301 299

Zr 466 171 353 761 660 578 365 428 242 216

Nb 416 161 137 260 136 234 164 169 837 647

Cs 014 046 186 125 007 022 102 052 063 na

Ba 238 261 1064 129 168 405 1099 641 625 563

La 698 183 848 144 889 2212 1008 106 537 462

Ce 947 256 131 308 138 333 1265 164 104 823

Pr 655 194 120 330 116 304 975 160 120 na

Nd 146 603 408 101 329 993 256 564 463 400

Sm 142 048 570 179 419 141 379 679 105 851

Eu 041 035 228 082 112 438 103 241 345 267

Gd 062 051 415 158 402 106 245 566 677 729

Tb na 0086 086 020 071 na na na na 111

Dy 090 027 359 098 434 104 171 637 732 na

Ho 023 0037 105 025 095 206 446 104 117 114

Er 045 038 195 058 255 611 153 264 305 na

Tm 009 50039 033 0081 042 085 292 5014 036 052

Yb 062 023 267 044 307 667 120 487 253 220

(continued)

21

Evidence for low-temperature SiO2^Na2O^CO2^H2O-rich melts or fluids

Potential metasomatic agents inferred from the presenceof graphite are C^O^H-rich fluids carbonate melts orCO2-bearing silicate melts The resulting phase assem-blages and geochemical signatures are the result of infiltra-tion of fluid or melt into a cold (750^8508C) shallow(515GPa Fig 2) heterogeneous and locally reduced litho-spheric mantle Metasomatism under such conditions in anopen system must result in complex geochemical signa-tures that reflect the degree of fluid^rock reaction Thismakes it particularly difficult to constrain the exactnature of the lsquooriginalrsquometasomatic agentType III-c xenolith Ke 196525 contains the highest frac-

tion of metasomatic phases and the least fractionatedamphibole trace element patterns The lsquomagmaticrsquo texturesof the Mg-katophorites in this sample suggest direct crys-tallization from a melt or a fluid rather than replacementof a pre-existing phase Therefore hypothetical liquids inequilibrium with the amphiboles in this xenolith couldyield the composition of the initial metasomatic agentsFigure 17 illustrates the trace element patterns of suchmelts and fluids calculated using sets of experimentally orempirically determined mineralmelt (fluid) partitioncoefficients including carbonate melt silicate melt andhydrous fluid (for references see caption to Fig 17)Common features for all calculated metasomatic agents

are negative Zr^Hf^Ti anomalies and LREE enrichment(Fig 17) However previous studies have shown that thesesignatures are indicative of almost all types of metasoma-tism including interaction of peridotite with carbonatiticmelt (Green amp Wallace 1988 Dautria et al 1992 Hauriet al 1993 Rudnick et al 1993 Ionov et al 1996 Yaxleyet al 1998 Coltorti et al 1999) with hydrous fluids andormelts for example originating from dehydration of altered

subducting lithosphere (Maury et al 1992 Zanetti et al1999 Grecurren goire et al 2001 Laurora et al 2001 Parkinsonet al 2003) and interaction of peridotite with volatile-and Si-rich melts that evolved through reactive porousflow (Bedini et al 1997 Ionov et al 2002 2006 Bodinieret al 2004 Rivalenti et al 2004)In the case of the Marsabit xenoliths integration of the

trace element characteristics thermobarometric inferencesand mineralogical constraints favours a Na2O^SiO2^CO2^H2O-rich silicate melt Carbonated melts can prob-ably be ruled out on major element and mineralogical con-siderations First amphiboles are Ca-poor in conflict withcarbonate melt metasomatism which is generally charac-terized by Ca enrichment (Rudnick et al 1993 Neumannet al 2002) Further orthopyroxene is a stable phase andprobably even crystallizes during metasomatism in somesamples (eg Ke 785) At near-solidus conditions coexis-tence of orthopyroxene and carbonate melt is possibleonly at pressures 20GPa (see Fig 2 eg Lee amp Wyllie2000 and references therein) which is considerably higherthan the maximum pressures indicated by the compositionof spinel (515GPa) At such low pressures metasomatismby carbonated melts will convert lherzolite and harzbur-gite immediately into wehrlite which is the major perido-tite type in all xenolith suites where carbonate meltmetasomatism is convincingly demonstrated (Rudnicket al 1993 Yaxley et al 1998 Neumann et al 2002)In Marsabit wehrlites are absent howeverAn Na2O^SiO2^CO2^H2O-rich melt or fluid on the

other hand would explain the Na-rich nature of amphiboleand phlogopite as well as the presence of orthopyroxeneGrowth of metasomatic (and Al-poor) orthopyroxene hasbeen described from several xenolith localities where peri-dotites were infiltrated either by hydrous fluids orSi-rich melts (Smith et al 1999 McInnes et al 2001Morishita et al 2003) Na-rich phlogopite is commonly

Table 6 Continued

Type III-b III-c Host

Sample 195915 196515 196515 785 196525 19653 19653 19653 195927

Remark next amph next cpx xenol margin next amph next amph melt pocket next apatite next Opxm melt pocket host basanite

Lu 014 0050 026 0041 046 080 140 058 040 031

Hf 026 052 535 014 076 110 5087 124 568 504

Ta 50080 033 638 0022 123 101 784 801 394 403

Pb 541 448 280 255 840 797 260 749 493 na

Th 194 296 160 466 703 185 247 105 534 506

U 545 069 478 099 152 447 724 216 171 126

nd not detected na not analysedAverage composition of the strongly alkaline basanites from Marsabit (Volker 1990)

22

Fig 12 Total alkalis vs SiO2 (a) Harker-type co-variation diagrams (b^e) and incompatible trace element co-variation diagram (f) showingthe compositional variability of silicate glass found in the porphyroclastic spinel harzburgites and dunites from Marsabit In (b) binary mixinglines between the typical composition of the alkali-rich glass (large open diamond) and the associated microlites are shown (each increment is10wt)They illustrate that the glass does not represent the derivate of host lava infiltration and fractional crystallization Subalkaline glass inGroup I (grt)^spl lherzolites from Marsabit is shown for comparison and is interpreted to derive from incongruent melting of Ti-pargasite(Kaeser 2006)

23

found as a reaction product between anhydrous mafic toultramafic rocks and H2O-rich evolved liquids (Costaet al 2001 and references therein) or H2O-bearing fluids(eg Odling 1994) The latter study was an experiment at20GPa simulating the effects of decreasing temperatureson the phase assemblages and their composition duringmetasomatism using a model peridotitethornC^O^H fluidThe coolest part of Odlingrsquos (1994) experiment (510758C)contained a strikingly similar phase assemblage (kato-phorite phlogopite with 2wt Na2O and Al-poororthopyroxene) to the Marsabit samples interpreted tohave formed during sub-solidus fluid^solid interactionA compositionally similar phase assemblage (highamounts of opx Si-rich hornblende Na-rich phlogopite)resulted from experiments simulating peridotite interac-tion with Si-rich melt (dacite) at lower P^T conditions(9508C at 092GPa Prouteau amp Scaillet 2003)In this study the trace element signatures of Group III-c

Mg-katophorite and the calculated metasomatic agentsfurther suggest that the metasomatizing agent was a meltrather than a fluid Hypothetical fluids in equilibrium witheuhedral Mg-katophorite from sample Ke 196525 havehigh NbLILE (eg Rb Ba) and low LILEREE ratios(Fig 17) Because experimental data predict the oppositefor fluids (LILE enriched over HFSE and REE egManning 2004 and references therein) a fluid with asignature such as that indicated in Fig 17 is unrealistic

Fig 13 Trace element patterns of silicate glass in porphyroclasticspinel harzburgites and dunites from Marsabit compared withthe trace element composition of associated clinopyroxene (Dim)micro-phenocrysts xenolith-hosting basanites from Marsabit andglass interpreted to represent xenolith^host interaction [concentra-tions normalized to the primitive mantle values of McDonough ampSun (1995)]

Table 7 Representative analyses of carbonates (oxides in

wt trace elements in mgg)

Group III-b III-b III-c III-c III-c

Mineral Dol Mg-Cc Mg-Cc Mg-Cc Mg-Cc

Sample 195915 196515 785 19653 19653

Texture in glass in glass in glass in glass in glass

SiO2 na 003 na nd 003

FeO(tot) 016 011 010 010 006

MnO 001 nd nd nd nd

MgO 1922 236 190 125 517

CaO 3189 5313 5377 5391 4853

Na2O 003 nd 004 nd 003

K2O na 002 na 001 nd

SrO 003 010 008 004 021

BaO 003 nd nd na na

CO21 4618 4444 4442 4377 4391

Total 9756 10021 10032 9909 9794

Mg-no 9954 9745 9713 9554 9931

Ca-no 5439 9418 9531 9687 8709

P 220 334 137 580 589

K 970 212 642 105 155

Sc 5065 50150 027 5027 50076

Ti 975 189 242 528 196

V 5085 0481 5042 5037 021

Ni 900 673 517 5294 593

Zn 459 5089 113 296 045

Ga na 5014 5013 na na

Rb 0084 50072 50074 50060 0046

Sr 593 860 371 251 4274

Y 012 011 130 704 015

Zr 261 024 012 097 756

Nb 144 019 0056 692 185

Cs 50063 50011 50018 50039 50008

Ba 126 230 5024 029 271

La 034 057 084 136 025

Ce 032 013 134 997 023

Pr 006 006 018 095 0026

Nd 030 013 068 228 0078

Sm 5044 50091 0074 048 50018

Eu 5012 50019 0051 020 50008

Gd 5037 50090 014 051 0026

Tb na 50010 0036 na na

Dy 5019 50039 0089 058 50038

Ho 50029 0013 0044 011 50006

Er 5012 50041 0083 033 50019

Tm 50052 50013 0016 0084 50006

Yb 5043 50063 014 027 50040

Lu 50072 50010 0011 0066 50008

(continued)

24

At the relatively low maximum pressures (515GPa) asupercritical liquid is not expected either (eg Kesselet al 2005)Finally a relatively hydrous metasomatizing agent

would be consistent with formation of Mg-katophoriteswith lowTi and presumably low Fe3thorn contents (given therelatively reducing conditions) which in turn imply high(OH)^ contents In other words partial dehydrogenationtypically observed in common (Ti-rich) mantle amphibole

(eg Popp et al 1995 Tiepolo et al 2001 and referencestherein) cannot have played an important role in thiscase as H deficiency is mainly the result of coupled substi-tutions involvingTi and Fe3thorn (Popp et al 1995) In conclu-sion the metasomatizing agent was most probably aNa2O^SiO2^CO2^H2O-rich meltComparison with experimental studies further con-

strains the low-temperature regime in which metasoma-tism occurred In ultramafic (eg Niida amp Green 1999)basaltic (eg Rapp 1995) and Si-enriched systems (egtrachybasaltic Barclay amp Carmichael 2004) amphibole(in the latter study together with phlogopite) is confinedto temperatures lower than 1000^10508C at pressures520GPa (Fig 2)Thus crystallization of the early metaso-matic phase assemblage can be explained by infiltration ofa silicate melt that rapidly cooled upon contact with theambient peridotite (750^8508C prior to metasomatismFig 2) However this low-T stage is based on Ca-in-opxthermometry (see Brey amp Kolaquo hler 1990 Kaeser et al2006 Table 1) and therefore probably underestimates theeffective temperature conditions during metasomatismThis can be explained by low diffusion rates and proximityto the closure temperature for the diopside^enstatiteexchange reaction at temperatures lower than 8508C(eg Smith 1999 and references therein) The lower tem-perature limit at which silicate melt metasomatism ispossible (ie defined by the meltrsquos solidus and viscosity)depends strongly on volatile and SiO2 contentsExperiments by Draper amp Green (1997) for exampleshowed that highly silicic melts (52^64wt SiO2) canbe in equilibrium with mantle harzburgite at temperaturesas low as 8508C (at 10^20GPa under anhydrous andC^O^H-fluid-bearing conditions) These are howeverextreme compositions for mantle melts The actual SiO2

content of the metasomatizing agent in this case is difficultto constrain from the metasomatic mineral assemblage ascompositions may have varied considerably during reac-tive percolation (eg including Si-releasing reactions suchas cpx replacement by amphibole versus Si-consumingreactions such as Opxm crystallization) Moreover it isunlikely that a melt persists at temperatures muchbelow 9008C which is the solidus of fertile lherzolite(ie primitive upper mantle composition) underH2O-saturated conditions at 15GPa (Grove et al 2006see Fig 2) Below the solidus residual volatile-enrichedfluids may percolate further for example forming the lessmetasomatized Type III-c xenoliths (eg Ke 785) as wellas Type III-b and -a peridotites (see also Bailey 1987)Relatively cold conditions would also explain the veryhigh compositional gradients developed on small scales inType III-a samples (Fig 15) Given the absence of chloritein all investigated samples the lower temperature limitof such fluid metasomatism is 8008C (eg Fumagalli ampPoli 2005 see Fig 2)

Table 7 Continued

Group III-b III-b III-c III-c III-c

Mineral Dol Mg-Cc Mg-Cc Mg-Cc Mg-Cc

Sample 195915 196515 785 19653 19653

Texture in glass in glass in glass in glass in glass

Hf 5016 50038 50036 50074 50013

Ta 0048 50012 50016 50031 0008

Pb 5024 858 5021 074 018

Th 0021 50020 50017 50028 50005

U 060 094 086 600 148

Mg-Cc Mg-calcite Dol dolomite nd not detectedna not analysed Ca-numberfrac14 100Ca(CathornMg)1Calculated from stoichiometry

Fig 14 Trace element patterns of glass-hosted carbonates in TypeIII-b and -c spinel harzburgite and dunite xenoliths from Marsabit[concentrations normalized to the primitive mantle values ofMcDonough amp Sun (1995)] The grey field outlines the compositionalrange of carbonates found in other peridotite xenoliths (Ionov ampHarmer 2002 and references therein) Carbonate material inter-preted as quenched carbonatite melt found in peridotite xenolithsfrom Kerguelen (Moine et al 2004) is shown for comparison(bold line)

25

Finally the presence of apatite even though oftentaken as indicator for carbonatite-type metasomatism(eg Yaxley et al 1998) is not in conflict with metasoma-tism by silicate melts Apatite has also been shown to crys-tallize from volatile and alkali-rich derivatives formedduring reactive porous flow of originally mafic silicatemelts (Woodland et al 1996 Bedini et al 1997 Bodinieret al 2004 Ionov et al 2006)

FORMATION OF THE LATE GLASS -BEAR ING ASSEMBLAGESTextures strongly suggest that the glass-bearing assem-blages formed later and at the expense of the early metaso-matic assemblages (Fig 4a and d) Features such as thereappearance of clinopyroxene (Na-rich Cr diopsidemicro-phenocrysts) a phase replaced by the early

Fig 15 Detailed REE systematics in primary clinopyroxene (cpx-I) in a porphyroclastic cryptically metasomatized spinel harzburgite(sample Ke 19651) showing the transition fromType III-a (Zone A and B) toType III-b (Zone C) on a scale of a few centimetres The numberson the photomicrographs (crossed polars) correspond to single laser ablation microprobe pits (for further explanation see text)

26

metasomatic assemblage indicate that melt pocket forma-tion reflects a significant change in P^T^X space in partsof the mantle beneath Marsabit after earlier metasoma-tism Based on geochemical arguments and mass-balanceconsiderations we will show that (1) the late glass-bearingassemblages are closely related to early metasomaticassemblages mainly representing breakdown productsand (2) breakdown was induced by infiltration of aCO2-rich fluid in the mantle probably followed andenhanced by heating and decompression melting duringxenolith transport in the host magma

Evidence for in situ partial meltingBSE imaging reveals that the phases becoming unstableare amphibole orthopyroxene and to minor extentsspl-I phlogopite cpx-I and apatite Compositional(inter- and intra-xenolith) heterogeneity of silicate glass istexture controlled and varies as a function of theassociated precursor phases (Fig 12a^f)TiO2 for exampleshows higher concentrations in glass from xenolith Ke196515 containing Ti-enriched amphibole (Fig 12c)whereas K2O contents are highest in phlogopite-rich sam-ples (eg Ke 785 Fig 12d) Elevated P2O5 contents arefound in glass adjacent to apatite (Fig 12e) whichmirrors at the same time the highTh and LREE contents

of apatite (Fig 12f) Significant Si enrichment is found inglass coating orthopyroxene (Fig 12d) supporting incon-gruent melting of orthopyroxene (see below) Even thoughthe melt pockets are connected along grain boundaries thestrong compositional variation (on inter- and intra-xeno-lith scale) suggests that the mobility of the melt was lim-ited or that the melting event was very short-lived (fortiming constraints see below)The co-variation plots in Fig 12 further reveal consider-

able compositional differences between all types of xeno-lith glass and the host basanite These data indicate thatthe glass cannot be the product of xenolith^host lava inter-action during ascent to the surface Also back-mixing ofmicro-phenocrysts into the melt fails to derive the glassfrom a host-related melt (Fig 12b) Glass from xenolithmargins however is compositionally closer to the bulkhost basanite (eg Fig 12b) in line with a direct derivationfrom xenolith^host lava interaction Its more evolvedmajor element composition (eg higher SiO2 lowerMg-number Fig 12b) is probably the result of fractiona-tion of basanite phenocrysts (eg olivine augite andtitanomagnetite Volker 1990) Host lava^xenolith interac-tion is further supported by the trace element signature ofglass at the margin of the xenoliths which is closely similarto that of the Marsabit basanites (Fig 13 Volker 1990)The same characteristics are observed for intra-xenolithglass in the unusual kaersutite-bearing sample Ke 195927

Fig 16 Oxygen fugacities relative to the quartz^fayalite^magnetitebuffer (QFM equation of OrsquoNeill 1987) as a function of temperatureat assumed pressure of 1GPa calculated using the composition ofcoexisting spinel and olivine (ol-I^spl-I pairs and Chrm^Olm pairsrespectively) based on the oxygen barometer of Ballhaus et al (1991)The graphite^CO2 equilibrium was calculated after Frost amp Wood(1998) and the graphite^carbonate^silicate equilibrium was calcu-lated using the equation of Eggler amp Baker (1982) (See text for furtherexplanation)

Fig 17 Calculated metasomatic agents in equilibrium with the euhe-dral Mg-katophorites from the Type III-c amphibole dunite Ke 196525 Amphibolehydrous fluid and amphibolecarbonatite melt parti-tion coefficients are from Adam et al (1997) and Adam amp Green(2001) respectively Silicate melts have been calculated using amphi-boleSi-rich melt partition coefficients from Chazot et al (1996b) andamphibolebasanite partition coefficients from LaTourrette et al(1995) The compositional range of the Marsabit basanites and alkalibasalts (Volker 1990) is shown for comparison

27

Xenolith glass unrelated to the host lava and melt pock-ets similar to those reported here in association with resi-dual phases (eg amphibole or clinopyroxene) has beenreported from many xenolith localities (eg Frey ampGreen 1974 Ionov et al 1994 1996 Chazot et al 1996a1996b Yaxley et al 1997 Laurora et al 2001 Bali et al2002) Residual phases within the melt pockets stronglyindicate in situ partial melting within the xenolithsHowever a variety of processes may trigger such in situ

melting including (1) closed-system decompression melt-ing of either anhydrous (Maale amp Printzlau 1979) orhydrous phases (Frey amp Green 1974 Francis 1976) duringxenolith ascent in the host lava (2) melting during interac-tion of the xenoliths with the host basaltic melt duringxenolith ascent (eg Zinngrebe amp Foley 1995 Shaw ampKlulaquo gel 2002 Shaw et al 2006) (3) melting in the mantleprior to xenolith entrainment induced by an infiltratingfluid (ie flux melting eg Jones et al 1983 Ionov et al1994 Demecurren ny et al 2004) However most studies agreethat that partial melting occurred very shortly beforeandor during xenolith exhumation by the host magmaThus a combination of metasomatically triggered meltingfollowed andor assisted by decompression melting duringxenolith transport may be a realistic alternative in somecases (eg Dawson 2002)Recently Ban et al (2005) have shown in a very detailed

mass-balance study on glass-bearing xenoliths from theWest Eifel that the weight proportions of glass and theassociated micro-phenocrysts match the composition ofthe preserved parent amphibole We used the sameapproach and performed simple mass-balance calculations(least-squares fitting) to constrain the conclusions drawnfrom petrography and to test which phase contributedand to what extent to the formation of the melt pocketsThe results reported in Table 8 show that in most casesamphibole-melting can account successfully for the meltpocket composition and only a minor amount (520) ofother phases (orthopyroxene andor phlogopite) needs tobe added to the reactant Such a contribution of otherphases agrees with textural observation (eg Fig 4cshowing Dim growing on resorbed Opxm) as well as withthe conclusions of Ban et al (2005) who stated that moreSi-rich glass (ie 454wt) necessitates orthopyroxenecontributing to the meltTrace element patterns of the bulk glassthornmicro-

phenocryst pockets calculated using the measured traceelement abundances of glass and micro-phenocrysts andthe respective mass-balance parameters (Table 8) arebroadly identical to the measured amphibole composition(Fig 18a and b) Higher LILE (Cs Rb Ba) concentrationsin the calculated bulk melt patches in sample Ke 785 canbe balanced by adding 7wt of phlogopite (Fig 18b)which is in line with mass balance (Table 8) The dominantcontrol of amphibole to account for the trace element

budget of the melt pockets is similar to results found inother suites of glass-bearing xenoliths (Chazot et al1996a Yaxley amp Kamenetsky 1999 Laurora et al 2001)Further the obtained phase proportions for the melt pock-ets are in good agreement with the results from image ana-lysis (Table 1) In addition they are similar to the glass^cpx^ol proportions recorded in recent experiments on the

Table 8 Results of mass-balance calculations to test melt

pocket formation by incongruent melting of the early metaso-

matic assemblages

Sample 195915 785 785 196525 196525 19653

Reactant

amph 1000 1000 0820 1000 0944 mdash

opx mdash mdash 0078 mdash 0056 0406

phl mdash mdash 0102 mdash mdash mdash

glass mdash mdash mdash mdash mdash 05941

SiO2 4484 4989 4964 5129 5167 5898

TiO2 014 039 036 025 024 018

Cr2O3 232 291 258 127 121 009

Al2O3 1368 762 769 800 757 961

FeO(tot) 336 314 333 343 357 394

MnO 006 008 008 012 012 010

NiO 009 013 014 011 011 004

MgO 1844 2029 2197 2040 2122 1625

CaO 854 783 644 966 913 078

Na2O 460 500 436 504 476 843

K2O 049 032 096 042 040 060

Product

glass 0389 0322 0335 0292 0284 07032

ol 0243 0259 0315 0266 0286 0273

cpx 0288 0373 0308 0442 0431 0024

spl 0052 0032 0028 mdash mdash mdash

carb 0028 0014 0014 mdash mdash mdash

SiO2 4477 5027 4986 5207 5181 5903

TiO2 013 041 039 015 014 017

Cr2O3 213 314 272 109 106 015

Al2O3 1364 764 770 798 776 957

FeO(tot) 374 331 351 322 332 350

MnO 007 007 007 009 009 006

NiO 011 013 015 011 012 007

MgO 1834 2024 2196 2063 2141 1635

CaO 854 759 632 952 929 080

Na2O 449 385 388 499 485 860

K2O 060 096 100 015 014 091

r2 023 200 035 084 028 033

Prefix Ke omitted1Low-Si glass 2High-Si glass (see text for explanation)

28

melting behaviour of amphibole in wehrlites at tempera-tures between 1175 and 12508C and pressures of05^10GPa (Mecurren dard et al 2006) These temperatures arein good agreement with the estimates for the formation ofolivine and chromite micro-phenocrysts in the melt

pockets (Fig 16) The heat supply required for amphibolemelting in the formerly low-T metasomatized Group IIIxenoliths (T 200^3008C) may come from local heatingshortly before or during xenolith entrainment in the hostlava or from fluid infiltration that triggered melting (seediscussion below)Finally formation of the most Si-enriched melt (adjacent

to Opxm in Group III-c samples) can be modelled assum-ing progressing interaction between infiltrating melt andorthopyroxene where the latter melts incongruently

opxthornmelt1 olthornmelt2 cpx ethseeTable 8THORN eth1THORN

To calculate reaction (1) we chose the least evolved glassobserved in melt patches in sample Ke 19653 (SiO2

5921wt Mg-number 6465) to represent the initialmelt1 whereas melt2 adjacent to opx is SiO2-rich(6530wt) Trace element abundances in glass next toorthopyroxene are shifted to lower concentrations(Table 6) which is in agreement with dilution caused byaddition of a trace element-poor orthopyroxene compo-nent The resultant phase proportions are similar to thoseobserved at orthopyroxene^melt interfaces (ie higholivine and low cpx abundances) and are in agreementwith experimental studies (Shaw et al 1998)

Partial melting triggered by fluid fluxMass-balance calculations provide an explanation for meltpocket formation by in situ partial melting Thus onemight conclude that melting is simply the result of decom-pression and heating during xenolith transport in the hostmagma However there are two points that argue thatmelting was triggered by the infiltration of a fluid phaseshortly before the xenoliths were sampled by the hostmagma (1) the presence of occasionally high amounts ofcarbonate included in the melt pockets (2) the extremelyNa-rich clinopyroxene (Dim) compositions either includedas micro-phenocrysts in the melt pocket or present asequilibrated grains around chromite in the single TypeIII-d xenolith (Fig 3f) Both arguments will be discussedin the next section

Formation of carbonates

Textures and compositions of carbonate globules can beexplained by carbonate precipitation from a CO2-bearingfluid As demonstrated by several studies highCa(CathornMg) values (Table 7) and low REE concentra-tions are strong arguments that the carbonates are cumu-late crystals precipitated from a CO2-rich melt or afluid and not quenched carbonated melts (Lee et al 2000Ionov amp Harmer 2002) Oscillatory zoning (Fig 5a and b)of individual carbonate crystals within singleglobules corroborates this interpretation The roundedcontacts toward glass (globules) are also in line with thesefindings Similar globular forms were reported fromexperimental studies and do not a priori imply the

Fig 18 Calculated trace element characteristics of reactants (in mostcases amphibolethorn orthopyroxene filled symbols) and products(melt pockets open symbols) of the lsquoamphibole meltingrsquo model[see equation (1) in text] The relative phase proportions wereobtained by mass-balance calculations and are given in Table 8 Thedashed lines in (b) show that the high LILE concentrations obtainedfor the product (bulk melt pocket) can be explained by adding about7wt of phlogopite to the reactant (amphibolethorn orthopyroxeneTable 8) Further explanation is given in the text

29

presence of immiscible carbonate and silicate melts(eg Lee amp Wyllie 1996)The high amount of carbonate in some melt pockets

however (eg Fig 4e) does need further explanation asthe incongruent melting reaction (1) does not account forthe CO2 required for carbonate precipitation duringamphibole melting Therefore amphibole breakdowneither occurred in an open system with addition of at leastCO2 (eg Laurora et al 2001 Ban et al 2005) or anotherpre-existing phase acted as a source of carbon Oxidationof graphite texturally identified as a part of the earlyassemblage (eg Fig 3d) could account for some of theCO2 necessary for carbonate formation A change in fO2

conditions is also indicated when the composition of glass-related spinel (Chrm) and Olm are used for oxygen baro-metry which yields oxygen fugacities well on the oxidizedside of the carbon^carbonate^silicate buffer (Fig 16)However increasing fO2 triggered by closed-systemdecompression-induced melting alone does not appear fea-sible Also textures such as straight contacts between gra-phite and Mg-calcite (Fig 4c) suggest that carbonategrows on graphite (ie indicating local fO2 conditions cor-responding to carbon^carbonate equilibrium) rather thanreplacing it We therefore interpret melt pocket formationto be triggered by the infiltration of a CO2-rich fluid(ie flux melting see also Ionov et al 1994) This interpre-tation also accounts for the presence of abundant vugs inthe glass that could have contained a fluid (Ionov et al1994 2006) which was presumably lost during sample pre-paration However we emphasize here that late fluid infil-tration had only subtle effects on the trace elementcomposition of Type III-b and -c xenoliths

Formation of Na-rich Cr diopside

Infiltration of CO2-rich fluidwould also explain the Na-richcomposition of Dim micro-phenocryst cores (Fig 6a and b)Ionov et al (2005) noted recently that clinopyroxenes inmeltpockets are commonly characterized by Na2O contents51wt mainly as a result of low cpxmelt partition coeffi-cients at low pressures (eg Blundy et al 1995) Thus veryhighNa2Ocontents in Dimmicro-phenocrysts (Fig6a) sug-gest replacement of amphibole by Dim and eventually Olmand Chrm during reaction with a fluid at subsolidus condi-tions An origin of micro-phenocrysts by replacementrather than crystallization from a melt is indicated by theidentical crystallographic orientation of individual Dimcrystals in singlemelt pockets (Fig4a)Once the solidus of the precursor phase(s) was reached

(ie 10508C for amphibole Niida amp Green 1999) silicatemelt was generated in situ and led to the formation of clin-opyroxene rims with lower Na contents (Fig 6b) and thedevelopment of euhedral shapes in contact with melt (egFig 4a and e) Eventual immiscibility between remainingfluid and newly formed melt (eg during xenolith ascent)would then explain the presence of carbonate globules and

vugs enclosed in glass Likewise different origins of meltand carbonate were recently proposed for similar meltpockets from Hungarian xenoliths by Demecurren ny et al(2004) who found oxygen isotope disequilibrium betweencarbonate globules and the host glass Such a two-stepmodel (fluid-induced replacement followed by melt gen-eration) can be explained by CO2-fluid infiltration occur-ring within the mantle at sub-solidus conditions ultimatelyfollowed by xenolith entrainment in the host magma andtransport to the surfaceThe host magma possibly suppliedthe additional heat necessary for melt generationFast decompression would further enhance melting andcould also account for the lower Na contents in Dimmicro-phenocryst rims as a result of decreasing cpxmeltpartition coefficients at decreasing pressures (Blundyet al 1995)Rapid replacement of amphibole ( phlogopite) by an

anhydrous phase assemblage indicates that the fluid waswater-poor (eg low H2OCO2 ratio see Ionov et al2006)This is corroborated by the fact that Mg-katophorite(presumably OH-rich see above) is occasionally rimmedby a residual kaersutitic selvage (Fig 4d) Such Ti-richmantle amphiboles are commonly OH-poor as a result ofpartial dehydrogenation (eg Tiepolo et al 2001 and refer-ences therein)Finally the presence of theType III-d xenolith contain-

ing texturally equilibrated clinopyroxene of almost identi-cal composition to Dim micro-phenocrysts in melt pockets(Table 2 Figs 6a and 7b) further strengthens the argu-ment that CO2-fluid percolation started in the mantlebefore entrainment (ie enough time elapsed to allow thisrock to anneal completely after the late metasomaticassemblage had formed) A similar conclusion can bedrawn for the clearly host-related glass found in xenolithKe 195927 (eg Fig 13b) ThisType III-c sample is charac-terized by the presence of Dim micro-phenocrysts partlyovergrown by mineral rims clearly related to the host basa-nite magma (kaersutite Ti-augite and olivine withFo885 Fig 4f Tables 3 and 4 and supplementary dataat httppetrologyoxfordjournalsorg) Kaersutite hasmajor and trace element characteristics similar to typicalbasanitic and alkali basaltic amphibole phenocrysts(Fig 9d) The special phase assemblages in this xenolithindicates that host-related basanitic melts intruded someType III-c xenoliths and partly overprinted the assemblageformed slightly earlier by in situ derived alkali-rich peralu-minous melts fully preserved in other samples (eg Ke19653 or Ke 785)

ORIGIN OF METASOMATIZ INGAGENTS T IM ING AND TECTONICSETT INGAs noted above late metasomatism was probably short-lived started at mantle depths and probably continued

30

Table 3 Representative analyses of orthopyroxene spinel and olivine (oxides in wt trace elements in mgg)

Type III-a III-b III-b III-c III-d III-c III-d III-c III-c III-c III-d

Mineral opx-I opx-I opx-I Opxm Opxm Chrm spl-I ol-I Olm ol-I ol-I

Sample 19651 196515 195915 19653 19706 19653 19706 785 19653 196525 19706

Texture Zone C1 core core core core core core core core core core core

SiO2 5657 5642 5615 5807 5713 012 005 4093 4135 4078 4098

TiO2 004 013 006 001 005 044 016 nd 001 001 nd

Al2O3 265 269 299 046 192 1223 1880 001 001 nd 004

Cr2O3 043 056 040 032 054 5454 5043 001 029 001 005

Fe2O3 mdash mdash mdash mdash mdash 554 338 mdash mdash mdash mdash

FeO(tot) 533 568 576 538 486 1114 1207 939 583 1133 771

MnO 013 014 014 012 012 na 005 015 013 027 007

NiO na 010 009 007 009 015 009 040 024 037 039

MgO 3473 3420 3380 3561 3464 1480 1502 4900 5182 4739 5013

CaO 051 053 048 020 052 003 000 001 008 002 008

Na2O 005 008 013 009 014 na na 001 004 nd 001

Total 10044 10052 10000 10034 10002 9910 10008 9992 9980 10018 9947

Mg-no 9208 9148 9128 9218 9271 7032 6894 9029 9406 8817 9206

Na 341 423 1136 539 1120 na 438 297 262 647 230

Al 15120 14586 17785 1817 10834 na 114911 893 821 120 1201

P 156 145 175 175 248 na 198 421 292 105 244

Ca 3163 3016 3881 1487 3895 na 55348 822 579 200 658

K 172 5044 5035 981 150 na 5086 5037 5102 198 630

Sc 805 546 173 244 124 na 280 061 432 094 286

Ti 239 619 366 449 227 na 956 156 871 170 204

V 768 906 772 149 634 na 925 104 377 096 283

Ni 568 646 585 588 748 na 1010 2686 2875 2920 2649

Zn 256 301 245 606 373 na 732 630 665 146 693

Ga 181 350 na na 133 na 280 0015 na 0035 077

Sr 024 014 044 102 167 na 0089 0017 50013 0045 052

Y 0081 0073 049 069 202 na 0025 0015 016 010 019

Zr 023 0277 051 0038 088 na 124 50005 50033 50006 015

Nb 50005 0020 50007 018 0051 na 0877 50003 0025 0008 0035

Ba 0056 50018 50021 748 0035 na 50028 50016 50071 017 013

La 0019 50002 0055 0061 0045 na 0039 0021 50004 0017 0019

Ce 0020 0005 0046 0109 027 na 0052 0023 50012 0027 015

Pr 0002 50001 0005 0007 0064 na 0024 50001 50011 0009 0019

Nd 50008 0010 0043 0039 042 na 50029 50007 50052 0018 011

Sm 0014 50009 0016 50018 019 na 0022 50006 50051 50009 0032

Eu 50003 50002 50005 0012 0077 na 0018 50002 50012 50003 0011

Gd 50014 50006 0028 0021 020 na 50026 50006 50041 50016 0035

Tb 0002 50001 na na 0045 na 50004 50001 na 50001 0004

Dy 0015 0011 0086 0058 038 na 0010 50003 50043 50009 0041

Ho 0007 0003 0012 0031 0069 na 0003 50001 50006 0004 0010

Er 50012 0008 0043 0094 022 na 50015 50006 0058 0018 0027

Tm 0002 0001 0015 0014 0031 na 50003 50001 0010 0003 0004

Yb 0027 0011 019 0170 028 na 50018 0009 0064 0060 0038

Lu 0001 0002 0026 0026 0033 na 0007 0002 0040 0018 0008

(continued)

12

Table 4 Representative analyses of amphibole (oxides in wt trace elements in mgg)

Type III-b III-c

Mineral katoph katoph katoph katoph katoph katoph katoph katoph kaersut kaersut

Sample 195915 196515 196515 196515 785 785 196525 196525 195927 195927

Texture residual

in mp

interst interst lamellae

in cpx-I

residual

in mp

residual

in mp

euhedral core euhedral rim micro-xx micro-xx

SiO2 4541 4603 4625 4887 4955 4959 4952 4953 4064 4106

TiO2 015 190 203 099 030 029 022 028 612 577

Al2O3 1365 1145 1100 915 826 807 877 794 1405 1395

Cr2O3 251 123 148 144 254 263 106 115 031 008

FeO(tot) 350 293 304 278 302 294 359 346 588 615

MnO 006 003 004 002 008 002 010 008 016 010

NiO 009 014 009 010 010 006 009 012 010 010

MgO 1883 1886 1896 1997 2067 2062 2000 2024 1576 1545

CaO 851 974 960 976 805 793 863 909 1086 1075

Na2O 474 427 440 432 548 555 529 509 284 294

K2O 049 038 041 031 027 029 028 039 139 138

Total 9811 9695 9728 9771 9837 9805 9759 9741 9817 9793

Mg-no 9055 9199 9176 9275 9243 9259 9086 9124 8269 8173

P 445 222 235 na 175 196 206 120 214 266

K 4301 3916 3637 na 2393 2654 2955 3655 13289 12762

Sc 402 704 663 na 358 345 311 352 375 270

Ti 720 9867 9917 na 1732 1898 1421 1589 32154 32071

V 216 317 332 na 289 299 172 187 469 404

Ni 726 842 828 na 811 876 737 813 638 530

Zn 298 163 159 na 188 214 451 457 463 548

Ga na 981 939 na 103 114 867 872 na na

Rb 279 184 151 na 035 050 096 150 744 713

Sr 216 325 335 na 708 667 955 117 618 668

Y 101 269 315 na 111 126 390 359 310 355

Zr 539 370 331 na 292 297 114 592 933 110

Nb 339 413 447 na 102 121 192 705 265 341

Cs 0006 50001 0005 na 0006 50009 0008 0018 50019 50010

(continued)

Table 3 Continued

Type III-a III-b III-b III-c III-d III-c III-d III-c III-c III-c III-d

Mineral opx-I opx-I opx-I Opxm Opxm Chrm spl-I ol-I Olm ol-I ol-I

Sample 19651 196515 195915 19653 19706 19653 19706 785 19653 196525 19706

Texture Zone C1 core core core core core core core core core core core

Hf 0006 0010 0014 50009 0013 na 0042 50003 50017 50009 50006

Ta 50001 50001 50004 50005 50002 na 0015 0001 50018 50001 50001

Pb 50028 0041 0047 0074 50041 na 015 0036 0075 0042 0046

Th 0009 50002 0069 0020 50002 na 0010 0050 50006 0011 50001

U 0004 50001 0026 0017 0004 na 0017 0011 50006 50001 50002

1See Fig 15nd not detected na not analysed

13

compared withTh andTa respectively Hf Zr andTi showvery low concentrations Overall major and trace elementcharacteristics of the carbonates are similar to those of car-bonates found in other peridotite xenoliths (eg Ionov1998 Lee et al 2000 Laurora et al 2001 Ionov ampHarmer 2002 see Fig 14)

FORMATION OF THE EARLYASSEMBLAGESDespite large compositional variations numerous commontextural and compositional features indicate that the earlymetasomatic assemblages in all Group III samples formedduring a single metasomatic event In the discussion belowwe argue that during a first metasomatic stage the pri-mary (pre-metasomatic) phase assemblage in Group IIIspinel harzburgites was partially replaced by metasomaticminerals finally resulting in clinopyroxene-free amphi-bole- and phlogopite-bearing dunite and harzburgite(Type III-c) The initial metasomatic agent was probably

an evolved CO2^H2O-rich melt and the transitionfrom peridotites of Type III-a to -c can be explained byprogressive melt^rock reaction

Transition from cryptic (Type III-a) tomodal (Type III-b and -c) metasomatismFormation of new phases requires (1) crystallization from amelt or fluid phase (decreasing melt or fluid mass) andor(2) melt^solid or fluid^solid reactionTextural features suchas spinel being replaced by amphibole and phlogopite(eg Fig 3c) indicate melt(fluid)^rock reaction Thegeochemical signatures observed in the different peridotitetypes thus probably reflect pre-existing compositionalheterogeneities coupled with metasomatic modificationThis is illustrated best in sample Ke 19651 (Type III-a)the least metasomatized xenolith in this study It can besubdivided into three parts containing clinopyroxenewith different REE signatures (Zones A B and C inFig 15) The LREE are increasingly enriched from ZoneA to C indicating increasing cryptic metasomatism

Table 4 Continued

Type III-b III-c

Mineral katoph katoph katoph katoph katoph katoph katoph katoph kaersut kaersut

Sample 195915 196515 196515 196515 785 785 196525 196525 195927 195927

Texture residual

in mp

interst interst lamellae

in cpx-I

residual

in mp

residual

in mp

euhedral core euhedral rim micro-xx micro-xx

Ba 781 614 540 na 259 309 846 841 331 322

La 357 116 110 na 704 824 177 113 107 138

Ce 627 225 231 na 217 248 621 426 338 448

Pr 574 208 224 na 301 349 856 623 557 724

Nd 177 720 779 na 128 151 363 296 318 398

Sm 208 143 143 na 285 316 752 740 945 111

Eu 080 058 060 na 108 115 232 252 363 384

Gd 182 085 088 na 225 241 701 692 940 980

Tb 011 013 na 039 050 120 117 na na

Dy 171 057 067 na 202 252 735 698 700 819

Ho 036 010 010 na 039 050 155 144 137 147

Er 106 024 027 na 097 114 405 382 296 336

Tm 016 0030 0033 na 012 017 060 058 027 039

Yb 120 028 030 na 077 099 400 386 243 272

Lu 019 0036 0038 na 0079 0077 058 044 033 031

Hf 015 0067 0054 na 0065 0048 028 078 331 376

Ta 0033 014 014 na 0002 0004 010 107 167 198

Pb 201 360 374 na 142 173 195 222 022 039

Th 682 150 134 na 181 196 059 028 010 020

U 212 039 039 na 040 048 019 012 0018 0048

katoph Mg-katophorite kaersut kaersutite interst interstitial mp melt pocket nd not detected na notanalysed

14

HREE abundances on the other hand are very hetero-geneous in the least metasomatized Zone A moderatelydepleted in Zone B and homogeneously low in the mostmetasomatizedZoneC (containinggraphite Fig15) similarto REE patterns in clinopyroxene from the amphibole- andphlogopite-bearingType III-b sample (Ke196515 Fig7a)The decoupling of strongly (LREETh U) from moder-

ately incompatible elements (HREE) can be explained bychromatographic fractionation during percolation ofsmall amounts of melts or fluids through an initially non-metasomatized peridotite (Navon amp Stolper 1987 Godardet al 1995 Bedini et al 1997 Vernie res et al 1997 Ionovet al 2002) Upon reaction with the wall-rock a fluidor melt will become progressively enriched in incom-patible elements as a function of increasing distancefrom its source This is due to the fact that elements withhigher Dmineralmelt values (eg HREE) are selectivelyremoved from a percolating liquid when it exchanges withthe surrounding minerals The results are faster-movingchemical fronts of incompatible elements compared withthose of more compatible elements This is consistent with

the geochemical signature of cpx-I from the least metaso-matized part of sample Ke 19651 (Zone A Fig 15) whichalready started to re-equilibrate with respect to the incom-patible elements (ie addition of Th U and LREE)whereas more compatible elements still reflect pre-metasomatic heterogeneities In this case these areextreme HREE variations probably reflecting the earliergarnet breakdown (Kaeser et al 2006) On the otherhand more intensely metasomatized parts (eg Zone Cin sample Ke 19651 and sample Ke 196515) are character-ized by clinopyroxene trace element patterns wheremildly incompatible elements have also been modified(eg homogeneously depleted HREE Figs 7a and 15)Similar fractionation processes can explain the modal

and compositional differences between Type III-b and -cCompared withType III-b higher abundances of metaso-matic phases inType III-c suggests that higher amounts offluid or melt crystallized in or passed through these rocksThe amphibole-rich dunite (Ke 196525) for examplecontains low-Ti Mg-katophorite exhibiting the lowest(LaYb)N and (LaCe)N ratios and high (NbLa)N and

Fig 8 Major element characteristics of amphiboles from porphyroclastic spinel harzburgites and dunites from Marsabit compared with com-positions of mantle-derived amphiboles from other localities (XB basalt-hosted xenoliths XS xenoliths from subduction settings O orogenicperidotites) Mg-numberfrac14Mg(MgthornFetot) in molar proportions

15

(NbTh)N ratios (Fig 10a and c) indicating a moderatelyfractionated metasomatic agent Mg-katophorite fromthe phlogopite-rich harzburgite (Ke 785 Type III-c)and amphiboles from the more cpx-rich harzburgites(Type III-b) show progressively more fractionated patterns(eg Fig 10c) and decreasing abundances in lsquoamphibole-compatiblersquoelements (eg Nb Fig 9a) Thus the transitionfrom Type III-b to -c could reflect decreasing distancebetween the site at which the xenolith was sampledand the source of the lsquoinitialrsquo metasomatizing agent (egZanetti et al 1996 Ionov et al 2002 Bodinier et al 2004)

Judging from the abundance of modal metasomatic phasesthe amphibole dunite (Ke 196525) was probably closest tothe source of the metasomatizing agent The phlogopite-rich nature of Ke 785 which indicates higher K andLILE (Cs Rb Ba) contents in the remaining agent is inagreement with a more fractionated melt or fluid at some-what greater distance from the metasomatic sourceThe Nb abundances in amphibole also agree with this

model as well For example amphibole from Type III-cKe 196525 has comparatively high Nb concentrations(Fig 9c) in line with relatively high DNb

amphmelt values

Fig 9 Trace element composition of amphibole from porphyroclastic spinel harzburgites and dunites from Marsabit [normalized to the primi-tive mantle values of McDonough amp Sun (1995)] Mg-katophorite in Type III-b and -c xenoliths (a^c) and glass-related kaersutite (d) Theshaded fields indicate the compositional variation of all Mg-katophorite analyses (light grey) and the compositional range of LREE-depletedTi-pargasite from the Group I (grt)^spinel lherzolites (dark grey Kaeser et al 2006)

16

constrained by different studies (eg Ionov amp Hofmann1995 LaTourrette et al 1995) In Ti-poor systems such asthose of this study Nb uptake by amphibole is evenenhanced as a result of increased DNb values (Tiepoloet al 2001) Consequently in an event during which amphi-bole crystallizes continuously the remaining fluid or meltwill become progressively depleted in Nb (Ionov et al2002) resulting in formation of Nb-depleted amphibolesfurther away from the source (Zanetti et al 1996 Ionovet al 2002) such Nb-depleted amphibole is found insample Ke 785 as well as inType III-b xenoliths (eg Ke195915 Fig 9a) Amphibole signatures from xenolith suitesmodelled successfully in the context of percolative traceelement fractionation (eg Spitsbergen Ionov et al 2002)show similar ranges (eg in NbTh ratios Fig10a) to thosedocumented for our samples This further supports the

hypothesis that similar percolation mechanisms acted inthe mantle beneath Marsabit

The role of pre-existing heterogeneitiesand exotic mineralsAlthough a model based on trace element fractionationduring melt percolation is supported by textures and canaccount for many compositional features some composi-tional peculiarities remain unexplained First clinopyrox-ene and amphibole in several samples (Ke 785 19651 and196515) are characterized by extremely low HREE abun-dances more than one order of magnitude lower thanin amphiboles from the strongly metasomatizedamphibole dunite Ke 196525 (Fig 9a^c) The mainphases that crystallize during metasomatism are low-TiMg-katophorite and phlogopite However DHREE

amphmelt

Fig 10 Selected trace element ratios in amphiboles from the porphyroclastic spinel harzburgites from Marsabit compared with those ofmantle-derived amphibole from other tectonic settings (XK and XB cratonic kimberlite-hosted and off-cratonic basalt-hosted xenolithsrespectively O orogenic peridotites XS xenoliths from subduction settings) Concentrations (ratios) normalized to the primitive mantlevalues of McDonough amp Sun (1995)

17

Table 5 Representative analyses of phlogopite and apatite (oxides in wt trace elements in mgg)

Type III-b III-c III-b III-c

Mineral phlog phlog phlog phlog phlog phlog phlog apatite apatite

Sample 196515 196515 19653 785 785 785 196525 195915 19653

Texture veinlet interst in opx in opx with amph

in mp

around

chromite

incl in

amph

bleb in

glass

interst

grain

SiO2 3925 3914 4171 4190 4124 4134 4230 SiO2 016 049

TiO2 313 307 009 053 039 037 027 Al2O3 001 nd

Al2O3 1544 1583 1360 1360 1384 1377 1477 P2O5 3768 3715

Cr2O3 112 105 132 147 190 197 063 FeO(tot) 037 029

FeO(tot) 350 347 271 000 289 277 340 MnO 006 nd

MnO nd 002 008 305 004 002 005 MgO 084 091

NiO 023 027 019 023 024 028 023 CaO 4750 4237

MgO 2287 2280 2577 2591 2582 2528 2525 SrO 095 397

CaO nd 002 001 nd 004 001 002 Na2O 079 133

Na2O 215 219 282 258 264 262 208 La2O3 003 007

K2O 719 707 704 706 674 684 683 Ce2O3 088 347

Nd2O3 003 007

F 139 224

Cl 287 092

Total 9488 9493 9571 9641 9587 9535 9590 Totaly 9305 9245

Mg-no 9282 9287 9443 9380 9408 9420 9298

P 369 187 480 339 206 188 362 P 129717 139419

K 71711 72575 60747 79052 70997 69992 70189 K 110 897

Sc 099 110 131 130 088 091 160 Sc 242 079

Ti 15500 16408 890 2839 2096 1807 1736 Ti 235 5294

V 255 284 888 142 108 130 788 V 365 382

Ni 1808 1727 1744 2046 1813 1773 1905 Ni 121 397

Zn 234 221 358 382 317 321 695 Zn 119 096

Ga 111 984 na 210 190 188 235 Ga na na

Rb 185 147 289 183 133 133 342 Rb 015 049

Sr 102 116 775 325 363 350 101 Sr 7138 42233

Y 010 0086 50057 50054 0021 0006 0052 Y 988 1454

Zr 019 019 043 50088 0077 0070 073 Zr 307 101

Nb 506 628 479 177 102 146 210 Nb 024 020

Cs 175 138 261 399 216 180 798 Cs 50013 50036

Ba 1212 1096 1343 298 277 219 1124 Ba 139 126

La 010 0055 0069 50029 0020 50003 50045 La 7460 19048

Ce 0027 0011 50075 50019 014 50003 0078 Ce 6844 25945

Pr 0012 0008 50067 50024 0012 50003 50019 Pr 380 2212

Nd 0019 0035 50405 50079 0065 50009 50224 Nd 749 6530

Sm 0015 50011 50474 50090 0016 50010 50132 Sm 515 747

Eu 0022 0017 50104 50026 0013 0010 50095 Eu 147 189

Gd 50010 50011 5043 5015 50011 0012 017 Gd 314 451

Tb 50002 50002 na 50027 0002 50002 50028 Tb na na

Dy 50015 50017 045 50077 50007 50006 50077 Dy 198 281

Ho 50002 50003 50043 50020 0002 50002 50028 Ho 359 511

Er 50009 50013 5025 5011 50016 50009 5011 Er 903 119

Tm 50002 50002 50088 50013 0002 0003 50018 Tm 119 138

(continued)

18

data (eg LaTourrette et al 1995) indicate that amphibolecrystallization alone is unlikely to deplete the metasomaticagent to such a high degree in HREE The same probablyaccounts for the extreme Nb depletion in amphiboles fromKe 785 relative to the otherType III-c xenolith (Ke 196525 see also Ionov et al 2002)Further clinopyroxene phlogopite and amphibole in the

Type III-b sample Ke 196515 on one hand all showstrongly fractionated REE patterns (eg high LaCe inamphibole Fig 10c) but on the other hand are character-ized by a positiveTi anomaly decoupled from other HFSE(eg Hf and Zr Fig 9b) Trace element fractionationduring amphibole crystallization (ie formation of TypeIII-c amphibole dunites) however should lower Ti con-tents in the remaining fluid or melt (eg Moine et al 2001)Based on textures mineralogy and dominant common

compositional features the hypothesis that Type III-band -c xenoliths were metasomatized by completely inde-pendent metasomatic agents can be ruled out Thereforetwo other possibilities remain to account for the inconsis-tent observations described above (1) inference of otherphases reactants or products that are able to fractionatespecific trace elements present in the mantle beneathMarsabit but not present in the few investigated samples(2) pre-metasomatic compositional heterogeneities similarto the inherited HREE variation inType III-a clinopyrox-ene (see above)Garnet would be an important sink for HREE but

metasomatism occurred in the spinel field after decom-pression with maximum pressures of 515GPa (Kaeseret al 2006) so garnet had already broken down to sym-plectites (Fig 15) Percolation of melt through garnet-bearing websterites prior to metasomatism in Ke 19651and 196515 could be an alternativeWebsterites have been

interpreted to occur in close spatial relationship with por-phyroclastic peridotites in the shallow mantle beneathMarsabit (Kaeser et al 2006) and they occasionally con-tain rutile and ilmenite which could further account forthe strong Nb and Ta depletion in some samples (eg Ke785) The available compositional data from the webster-ites however do not indicate metasomatic modification ofwebsteritic garnet (Olker 2001 B Kaeser unpublisheddata) nor compositional similarities to Group IIIperidotitesCareful study by SEM rules out the presence of exotic

phases such as zircon or Ti-oxides described fromother xenolith suites (eg Bodinier et al 1996 Rudnicket al 1999) Nevertheless it cannot be excluded that themetasomatizing agent percolated through mantle domainscontaining these phases before infiltrating the studiedrocksThe preservation of pre-metasomatic compositional het-

erogeneities is another possibility that must be consideredThat such lsquorock signalsrsquo partly overlap with the lsquometaso-matic signalrsquo is indicated by the HREE variation in TypeIII-a clinopyroxene (see above) and also by other composi-tional parameters For example both Mg-katophorite andphlogopite growing at the expense of spinel have very highCr2O3 contents whereas euhedral Mg-katophorite inthe amphibole dunite (Ke 196525) or in phlogopiteinclusions in Opxm is comparably Cr2O3-poor (Fig 8bTables 4 and 5) Pre-existing heterogeneities would essen-tially be preserved through elements that are relativelycompatible in a given mineral (ie that are not mobilizedduring metasomatism) This would be in agreement withthe preserved HREE variations in Type III-a clinopyrox-ene high Cr2O3 in amphiboles and phlogopite and Tispikes in amphibole from Ke 196515

Table 5 Continued

Type III-b III-c III-b III-c

Mineral phlog phlog phlog phlog phlog phlog phlog apatite apatite

Sample 196515 196515 19653 785 785 785 196525 195915 19653

Texture veinlet interst in opx in opx with amph

in mp

around

chromite

incl in

amph

bleb in

glass

interst

grain

Yb 50014 50011 5047 5015 50011 50019 5017 Yb 803 724

Lu 0002 50002 50061 50014 50002 50002 50020 Lu 115 742

Hf 0019 0018 5025 50053 50012 50006 5019 Hf 50023 50120

Ta 020 022 140 0031 0012 0009 023 Ta 50015 50075

Pb 546 586 125 108 083 098 143 Pb 436 577

Th 50004 50004 50080 0043 0088 0056 50021 Th 2538 1305

U 0012 0011 0069 50018 0038 0030 0031 U 572 278

interst interstitial mp melt pocket na not analysed nd not detected

19

Further very high Mg-numbers in replacive amphibolessupport melt (fluid)^rock interaction at low melt (fluid)rock ratios where Mg and Fe contents are buffered by theambient peridotite (Kelemen et al 1995 Tiepolo et al2001) The slightly lower Mg-number of euhedralamphiboles in Ke 196525 combined with their lowerCr2O3 contents (Fig 8b) suggests a higher melt (fluid)rock ratio which is further supported by considerablylower Mg-number in associated ol-I (Fo88 Table 3)

The limited number of investigated samples does notallow us to decide which of the processes described aboveacted and to what extent However given the heterogene-ity observed in the numerous non-metasomatized xenolithsfrom Marsabit (Kaeser et al 2006) pre-metasomaticfeatures certainly affect the final product of the melt^rockreaction process to a considerable degree

The nature of the metasomatizing agent(s)Graphite stability and oxygen fugacity considerations

The Marsabit xenoliths are unusual in that they occur inan off-cratonic setting yet bear macroscopic graphite Thelatter usually either occurs in kimberlite-hosted peridotites(on-craton) or in pyroxenites from alpine-type peridotitebodies (eg Pearson et al 1994) In the phlogopite perido-tites from Finero graphite has recently been reportedas nanoscopic layers in phlogopite (Ferraris et al 2004)In xenoliths from Marsabit graphite occurs in the sametextural relationship as in kimberlite-hosted peridotitexenoliths (ie millimetre-sized flakes typically associatedwith orthopyroxene see Fig 3c^e and Pearson et al 1994)Graphite inclusions in phlogopite and Opxm couldindicate crystallization during metasomatism by alsquoreducedrsquo C-bearing fluid or melt However precipitationof graphite more probably reflects the relatively reducingconditions in the uppermost mantle prior to metasoma-tism We used the oxygen barometer of Ballhaus et al(1991) based on ferric iron contents in spinel coexistingwith olivine to constrain oxygen fugacity (fO2)conditions prior to metasomatism in the Marsabitxenoliths (Fig 16) The results show that non-metasomatized Group I and IV samples reflect similarlyreducing conditions to the cryptically metasomatizedType III-a samplesAlthough assessing Fe3thornFe2thorn ratios in spinel based

on microprobe analyses can be problematic (Wood ampVirgo 1989 Ballhaus et al 1991) all microprobeanalyses were obtained using the same operationalsettings and therefore give at least the relative fO2

variation between samples The relatively reducingconditions in the pre-metasomatic cold andhighly deformed mantle well below carbon^carbonate^silicate equilibria (Mg2Si2O6thorn 05 CaMg(CO3)2frac14Mg2SiO4thorn 05 CaMgSi2O6thornCthornO2 Eggler amp Baker1982 see Fig 16) and close to the CCO oxygen buffer arein agreement with the presence of graphite in the GroupIII harzburgites Moreover the P^T conditions at whichthe Group III xenoliths partially re-equilibrated are closeto the subsolidus decarbonation of model carbonated lher-zolite (Fig 2 see Lee amp Wyllie 2000 and referencestherein) in agreement with the observed phase assemblageincluding orthopyroxene olivine clinopyroxene andgraphite

Fig 11 Trace element patterns of (a) phlogopite and (b) apatitefrom porphyroclastic spinel harzburgites and dunites from Marsabit[concentrations normalized to the primitive mantle valuesof McDonough amp Sun (1995)] The grey field in (a) indicates thecomposition of Ti-rich phlogopite in Group I (grt)^spl lherzolites(Kaeser et al 2006)

20

Table 6 Representative analyses of silicate glass (oxides in wt trace elements in mgg)

Type III-b III-c Host

Sample 195915 196515 196515 785 196525 19653 19653 19653 195927

Remark next amph next cpx xenol margin next amph next amph melt pocket next apatite next Opxm melt pocket host basanite

SiO2 5306 5775 5161 6096 5657 5938 5974 6530 4882 4410

TiO2 017 157 106 062 030 024 020 023 378 255

Al2O3 2412 2053 2025 1779 2313 1575 1245 1355 1890 1396

Cr2O3 005 001 nd 001 001 nd nd nd 002 na

FeO(tot) 188 247 626 126 252 295 374 265 509 1283

MnO 007 000 026 001 004 005 008 007 018 021

NiO nd nd nd na nd na 006 nd 001 na

MgO 182 129 116 114 122 304 382 252 244 938

CaO 506 257 283 082 044 124 101 039 810 1030

Na2O 1107 966 969 971 1299 1388 1384 1211 673 374

K2O 154 197 465 357 138 121 142 127 300 146

Cl 031 106 038 012 014 023 061 020 015 na

P2O5 041 008 044 009 012 049 233 018 100 059

Total 9956 9896 9860 9609 9887 9845 9929 9846 9821 9912

Mg-no 6329 4816 2489 6175 4632 6474 6458 6289 4552 5658

P 1061 448 1610 230 503 1519 6738 676 2994 na

K 12563 16408 39599 32515 15872 9265 11277 10071 25900 na

Sc 179 184 126 406 151 970 136 848 691 237

Ti 665 7329 4974 2953 1965 1213 1163 1055 18199 15311

V 107 265 235 796 920 210 151 134 234 na

Ni 128 371 92 5575 983 170 5199 5298 199 191

Zn 962 103 109 110 459 440 508 309 819 na

Ga na 173 273 194 220 na na na na na

Rb 181 263 124 546 626 176 398 369 577 369

Sr 427 488 1407 188 881 1644 4997 985 1062 792

Y 543 145 224 752 262 619 158 369 301 299

Zr 466 171 353 761 660 578 365 428 242 216

Nb 416 161 137 260 136 234 164 169 837 647

Cs 014 046 186 125 007 022 102 052 063 na

Ba 238 261 1064 129 168 405 1099 641 625 563

La 698 183 848 144 889 2212 1008 106 537 462

Ce 947 256 131 308 138 333 1265 164 104 823

Pr 655 194 120 330 116 304 975 160 120 na

Nd 146 603 408 101 329 993 256 564 463 400

Sm 142 048 570 179 419 141 379 679 105 851

Eu 041 035 228 082 112 438 103 241 345 267

Gd 062 051 415 158 402 106 245 566 677 729

Tb na 0086 086 020 071 na na na na 111

Dy 090 027 359 098 434 104 171 637 732 na

Ho 023 0037 105 025 095 206 446 104 117 114

Er 045 038 195 058 255 611 153 264 305 na

Tm 009 50039 033 0081 042 085 292 5014 036 052

Yb 062 023 267 044 307 667 120 487 253 220

(continued)

21

Evidence for low-temperature SiO2^Na2O^CO2^H2O-rich melts or fluids

Potential metasomatic agents inferred from the presenceof graphite are C^O^H-rich fluids carbonate melts orCO2-bearing silicate melts The resulting phase assem-blages and geochemical signatures are the result of infiltra-tion of fluid or melt into a cold (750^8508C) shallow(515GPa Fig 2) heterogeneous and locally reduced litho-spheric mantle Metasomatism under such conditions in anopen system must result in complex geochemical signa-tures that reflect the degree of fluid^rock reaction Thismakes it particularly difficult to constrain the exactnature of the lsquooriginalrsquometasomatic agentType III-c xenolith Ke 196525 contains the highest frac-

tion of metasomatic phases and the least fractionatedamphibole trace element patterns The lsquomagmaticrsquo texturesof the Mg-katophorites in this sample suggest direct crys-tallization from a melt or a fluid rather than replacementof a pre-existing phase Therefore hypothetical liquids inequilibrium with the amphiboles in this xenolith couldyield the composition of the initial metasomatic agentsFigure 17 illustrates the trace element patterns of suchmelts and fluids calculated using sets of experimentally orempirically determined mineralmelt (fluid) partitioncoefficients including carbonate melt silicate melt andhydrous fluid (for references see caption to Fig 17)Common features for all calculated metasomatic agents

are negative Zr^Hf^Ti anomalies and LREE enrichment(Fig 17) However previous studies have shown that thesesignatures are indicative of almost all types of metasoma-tism including interaction of peridotite with carbonatiticmelt (Green amp Wallace 1988 Dautria et al 1992 Hauriet al 1993 Rudnick et al 1993 Ionov et al 1996 Yaxleyet al 1998 Coltorti et al 1999) with hydrous fluids andormelts for example originating from dehydration of altered

subducting lithosphere (Maury et al 1992 Zanetti et al1999 Grecurren goire et al 2001 Laurora et al 2001 Parkinsonet al 2003) and interaction of peridotite with volatile-and Si-rich melts that evolved through reactive porousflow (Bedini et al 1997 Ionov et al 2002 2006 Bodinieret al 2004 Rivalenti et al 2004)In the case of the Marsabit xenoliths integration of the

trace element characteristics thermobarometric inferencesand mineralogical constraints favours a Na2O^SiO2^CO2^H2O-rich silicate melt Carbonated melts can prob-ably be ruled out on major element and mineralogical con-siderations First amphiboles are Ca-poor in conflict withcarbonate melt metasomatism which is generally charac-terized by Ca enrichment (Rudnick et al 1993 Neumannet al 2002) Further orthopyroxene is a stable phase andprobably even crystallizes during metasomatism in somesamples (eg Ke 785) At near-solidus conditions coexis-tence of orthopyroxene and carbonate melt is possibleonly at pressures 20GPa (see Fig 2 eg Lee amp Wyllie2000 and references therein) which is considerably higherthan the maximum pressures indicated by the compositionof spinel (515GPa) At such low pressures metasomatismby carbonated melts will convert lherzolite and harzbur-gite immediately into wehrlite which is the major perido-tite type in all xenolith suites where carbonate meltmetasomatism is convincingly demonstrated (Rudnicket al 1993 Yaxley et al 1998 Neumann et al 2002)In Marsabit wehrlites are absent howeverAn Na2O^SiO2^CO2^H2O-rich melt or fluid on the

other hand would explain the Na-rich nature of amphiboleand phlogopite as well as the presence of orthopyroxeneGrowth of metasomatic (and Al-poor) orthopyroxene hasbeen described from several xenolith localities where peri-dotites were infiltrated either by hydrous fluids orSi-rich melts (Smith et al 1999 McInnes et al 2001Morishita et al 2003) Na-rich phlogopite is commonly

Table 6 Continued

Type III-b III-c Host

Sample 195915 196515 196515 785 196525 19653 19653 19653 195927

Remark next amph next cpx xenol margin next amph next amph melt pocket next apatite next Opxm melt pocket host basanite

Lu 014 0050 026 0041 046 080 140 058 040 031

Hf 026 052 535 014 076 110 5087 124 568 504

Ta 50080 033 638 0022 123 101 784 801 394 403

Pb 541 448 280 255 840 797 260 749 493 na

Th 194 296 160 466 703 185 247 105 534 506

U 545 069 478 099 152 447 724 216 171 126

nd not detected na not analysedAverage composition of the strongly alkaline basanites from Marsabit (Volker 1990)

22

Fig 12 Total alkalis vs SiO2 (a) Harker-type co-variation diagrams (b^e) and incompatible trace element co-variation diagram (f) showingthe compositional variability of silicate glass found in the porphyroclastic spinel harzburgites and dunites from Marsabit In (b) binary mixinglines between the typical composition of the alkali-rich glass (large open diamond) and the associated microlites are shown (each increment is10wt)They illustrate that the glass does not represent the derivate of host lava infiltration and fractional crystallization Subalkaline glass inGroup I (grt)^spl lherzolites from Marsabit is shown for comparison and is interpreted to derive from incongruent melting of Ti-pargasite(Kaeser 2006)

23

found as a reaction product between anhydrous mafic toultramafic rocks and H2O-rich evolved liquids (Costaet al 2001 and references therein) or H2O-bearing fluids(eg Odling 1994) The latter study was an experiment at20GPa simulating the effects of decreasing temperatureson the phase assemblages and their composition duringmetasomatism using a model peridotitethornC^O^H fluidThe coolest part of Odlingrsquos (1994) experiment (510758C)contained a strikingly similar phase assemblage (kato-phorite phlogopite with 2wt Na2O and Al-poororthopyroxene) to the Marsabit samples interpreted tohave formed during sub-solidus fluid^solid interactionA compositionally similar phase assemblage (highamounts of opx Si-rich hornblende Na-rich phlogopite)resulted from experiments simulating peridotite interac-tion with Si-rich melt (dacite) at lower P^T conditions(9508C at 092GPa Prouteau amp Scaillet 2003)In this study the trace element signatures of Group III-c

Mg-katophorite and the calculated metasomatic agentsfurther suggest that the metasomatizing agent was a meltrather than a fluid Hypothetical fluids in equilibrium witheuhedral Mg-katophorite from sample Ke 196525 havehigh NbLILE (eg Rb Ba) and low LILEREE ratios(Fig 17) Because experimental data predict the oppositefor fluids (LILE enriched over HFSE and REE egManning 2004 and references therein) a fluid with asignature such as that indicated in Fig 17 is unrealistic

Fig 13 Trace element patterns of silicate glass in porphyroclasticspinel harzburgites and dunites from Marsabit compared withthe trace element composition of associated clinopyroxene (Dim)micro-phenocrysts xenolith-hosting basanites from Marsabit andglass interpreted to represent xenolith^host interaction [concentra-tions normalized to the primitive mantle values of McDonough ampSun (1995)]

Table 7 Representative analyses of carbonates (oxides in

wt trace elements in mgg)

Group III-b III-b III-c III-c III-c

Mineral Dol Mg-Cc Mg-Cc Mg-Cc Mg-Cc

Sample 195915 196515 785 19653 19653

Texture in glass in glass in glass in glass in glass

SiO2 na 003 na nd 003

FeO(tot) 016 011 010 010 006

MnO 001 nd nd nd nd

MgO 1922 236 190 125 517

CaO 3189 5313 5377 5391 4853

Na2O 003 nd 004 nd 003

K2O na 002 na 001 nd

SrO 003 010 008 004 021

BaO 003 nd nd na na

CO21 4618 4444 4442 4377 4391

Total 9756 10021 10032 9909 9794

Mg-no 9954 9745 9713 9554 9931

Ca-no 5439 9418 9531 9687 8709

P 220 334 137 580 589

K 970 212 642 105 155

Sc 5065 50150 027 5027 50076

Ti 975 189 242 528 196

V 5085 0481 5042 5037 021

Ni 900 673 517 5294 593

Zn 459 5089 113 296 045

Ga na 5014 5013 na na

Rb 0084 50072 50074 50060 0046

Sr 593 860 371 251 4274

Y 012 011 130 704 015

Zr 261 024 012 097 756

Nb 144 019 0056 692 185

Cs 50063 50011 50018 50039 50008

Ba 126 230 5024 029 271

La 034 057 084 136 025

Ce 032 013 134 997 023

Pr 006 006 018 095 0026

Nd 030 013 068 228 0078

Sm 5044 50091 0074 048 50018

Eu 5012 50019 0051 020 50008

Gd 5037 50090 014 051 0026

Tb na 50010 0036 na na

Dy 5019 50039 0089 058 50038

Ho 50029 0013 0044 011 50006

Er 5012 50041 0083 033 50019

Tm 50052 50013 0016 0084 50006

Yb 5043 50063 014 027 50040

Lu 50072 50010 0011 0066 50008

(continued)

24

At the relatively low maximum pressures (515GPa) asupercritical liquid is not expected either (eg Kesselet al 2005)Finally a relatively hydrous metasomatizing agent

would be consistent with formation of Mg-katophoriteswith lowTi and presumably low Fe3thorn contents (given therelatively reducing conditions) which in turn imply high(OH)^ contents In other words partial dehydrogenationtypically observed in common (Ti-rich) mantle amphibole

(eg Popp et al 1995 Tiepolo et al 2001 and referencestherein) cannot have played an important role in thiscase as H deficiency is mainly the result of coupled substi-tutions involvingTi and Fe3thorn (Popp et al 1995) In conclu-sion the metasomatizing agent was most probably aNa2O^SiO2^CO2^H2O-rich meltComparison with experimental studies further con-

strains the low-temperature regime in which metasoma-tism occurred In ultramafic (eg Niida amp Green 1999)basaltic (eg Rapp 1995) and Si-enriched systems (egtrachybasaltic Barclay amp Carmichael 2004) amphibole(in the latter study together with phlogopite) is confinedto temperatures lower than 1000^10508C at pressures520GPa (Fig 2)Thus crystallization of the early metaso-matic phase assemblage can be explained by infiltration ofa silicate melt that rapidly cooled upon contact with theambient peridotite (750^8508C prior to metasomatismFig 2) However this low-T stage is based on Ca-in-opxthermometry (see Brey amp Kolaquo hler 1990 Kaeser et al2006 Table 1) and therefore probably underestimates theeffective temperature conditions during metasomatismThis can be explained by low diffusion rates and proximityto the closure temperature for the diopside^enstatiteexchange reaction at temperatures lower than 8508C(eg Smith 1999 and references therein) The lower tem-perature limit at which silicate melt metasomatism ispossible (ie defined by the meltrsquos solidus and viscosity)depends strongly on volatile and SiO2 contentsExperiments by Draper amp Green (1997) for exampleshowed that highly silicic melts (52^64wt SiO2) canbe in equilibrium with mantle harzburgite at temperaturesas low as 8508C (at 10^20GPa under anhydrous andC^O^H-fluid-bearing conditions) These are howeverextreme compositions for mantle melts The actual SiO2

content of the metasomatizing agent in this case is difficultto constrain from the metasomatic mineral assemblage ascompositions may have varied considerably during reac-tive percolation (eg including Si-releasing reactions suchas cpx replacement by amphibole versus Si-consumingreactions such as Opxm crystallization) Moreover it isunlikely that a melt persists at temperatures muchbelow 9008C which is the solidus of fertile lherzolite(ie primitive upper mantle composition) underH2O-saturated conditions at 15GPa (Grove et al 2006see Fig 2) Below the solidus residual volatile-enrichedfluids may percolate further for example forming the lessmetasomatized Type III-c xenoliths (eg Ke 785) as wellas Type III-b and -a peridotites (see also Bailey 1987)Relatively cold conditions would also explain the veryhigh compositional gradients developed on small scales inType III-a samples (Fig 15) Given the absence of chloritein all investigated samples the lower temperature limitof such fluid metasomatism is 8008C (eg Fumagalli ampPoli 2005 see Fig 2)

Table 7 Continued

Group III-b III-b III-c III-c III-c

Mineral Dol Mg-Cc Mg-Cc Mg-Cc Mg-Cc

Sample 195915 196515 785 19653 19653

Texture in glass in glass in glass in glass in glass

Hf 5016 50038 50036 50074 50013

Ta 0048 50012 50016 50031 0008

Pb 5024 858 5021 074 018

Th 0021 50020 50017 50028 50005

U 060 094 086 600 148

Mg-Cc Mg-calcite Dol dolomite nd not detectedna not analysed Ca-numberfrac14 100Ca(CathornMg)1Calculated from stoichiometry

Fig 14 Trace element patterns of glass-hosted carbonates in TypeIII-b and -c spinel harzburgite and dunite xenoliths from Marsabit[concentrations normalized to the primitive mantle values ofMcDonough amp Sun (1995)] The grey field outlines the compositionalrange of carbonates found in other peridotite xenoliths (Ionov ampHarmer 2002 and references therein) Carbonate material inter-preted as quenched carbonatite melt found in peridotite xenolithsfrom Kerguelen (Moine et al 2004) is shown for comparison(bold line)

25

Finally the presence of apatite even though oftentaken as indicator for carbonatite-type metasomatism(eg Yaxley et al 1998) is not in conflict with metasoma-tism by silicate melts Apatite has also been shown to crys-tallize from volatile and alkali-rich derivatives formedduring reactive porous flow of originally mafic silicatemelts (Woodland et al 1996 Bedini et al 1997 Bodinieret al 2004 Ionov et al 2006)

FORMATION OF THE LATE GLASS -BEAR ING ASSEMBLAGESTextures strongly suggest that the glass-bearing assem-blages formed later and at the expense of the early metaso-matic assemblages (Fig 4a and d) Features such as thereappearance of clinopyroxene (Na-rich Cr diopsidemicro-phenocrysts) a phase replaced by the early

Fig 15 Detailed REE systematics in primary clinopyroxene (cpx-I) in a porphyroclastic cryptically metasomatized spinel harzburgite(sample Ke 19651) showing the transition fromType III-a (Zone A and B) toType III-b (Zone C) on a scale of a few centimetres The numberson the photomicrographs (crossed polars) correspond to single laser ablation microprobe pits (for further explanation see text)

26

metasomatic assemblage indicate that melt pocket forma-tion reflects a significant change in P^T^X space in partsof the mantle beneath Marsabit after earlier metasoma-tism Based on geochemical arguments and mass-balanceconsiderations we will show that (1) the late glass-bearingassemblages are closely related to early metasomaticassemblages mainly representing breakdown productsand (2) breakdown was induced by infiltration of aCO2-rich fluid in the mantle probably followed andenhanced by heating and decompression melting duringxenolith transport in the host magma

Evidence for in situ partial meltingBSE imaging reveals that the phases becoming unstableare amphibole orthopyroxene and to minor extentsspl-I phlogopite cpx-I and apatite Compositional(inter- and intra-xenolith) heterogeneity of silicate glass istexture controlled and varies as a function of theassociated precursor phases (Fig 12a^f)TiO2 for exampleshows higher concentrations in glass from xenolith Ke196515 containing Ti-enriched amphibole (Fig 12c)whereas K2O contents are highest in phlogopite-rich sam-ples (eg Ke 785 Fig 12d) Elevated P2O5 contents arefound in glass adjacent to apatite (Fig 12e) whichmirrors at the same time the highTh and LREE contents

of apatite (Fig 12f) Significant Si enrichment is found inglass coating orthopyroxene (Fig 12d) supporting incon-gruent melting of orthopyroxene (see below) Even thoughthe melt pockets are connected along grain boundaries thestrong compositional variation (on inter- and intra-xeno-lith scale) suggests that the mobility of the melt was lim-ited or that the melting event was very short-lived (fortiming constraints see below)The co-variation plots in Fig 12 further reveal consider-

able compositional differences between all types of xeno-lith glass and the host basanite These data indicate thatthe glass cannot be the product of xenolith^host lava inter-action during ascent to the surface Also back-mixing ofmicro-phenocrysts into the melt fails to derive the glassfrom a host-related melt (Fig 12b) Glass from xenolithmargins however is compositionally closer to the bulkhost basanite (eg Fig 12b) in line with a direct derivationfrom xenolith^host lava interaction Its more evolvedmajor element composition (eg higher SiO2 lowerMg-number Fig 12b) is probably the result of fractiona-tion of basanite phenocrysts (eg olivine augite andtitanomagnetite Volker 1990) Host lava^xenolith interac-tion is further supported by the trace element signature ofglass at the margin of the xenoliths which is closely similarto that of the Marsabit basanites (Fig 13 Volker 1990)The same characteristics are observed for intra-xenolithglass in the unusual kaersutite-bearing sample Ke 195927

Fig 16 Oxygen fugacities relative to the quartz^fayalite^magnetitebuffer (QFM equation of OrsquoNeill 1987) as a function of temperatureat assumed pressure of 1GPa calculated using the composition ofcoexisting spinel and olivine (ol-I^spl-I pairs and Chrm^Olm pairsrespectively) based on the oxygen barometer of Ballhaus et al (1991)The graphite^CO2 equilibrium was calculated after Frost amp Wood(1998) and the graphite^carbonate^silicate equilibrium was calcu-lated using the equation of Eggler amp Baker (1982) (See text for furtherexplanation)

Fig 17 Calculated metasomatic agents in equilibrium with the euhe-dral Mg-katophorites from the Type III-c amphibole dunite Ke 196525 Amphibolehydrous fluid and amphibolecarbonatite melt parti-tion coefficients are from Adam et al (1997) and Adam amp Green(2001) respectively Silicate melts have been calculated using amphi-boleSi-rich melt partition coefficients from Chazot et al (1996b) andamphibolebasanite partition coefficients from LaTourrette et al(1995) The compositional range of the Marsabit basanites and alkalibasalts (Volker 1990) is shown for comparison

27

Xenolith glass unrelated to the host lava and melt pock-ets similar to those reported here in association with resi-dual phases (eg amphibole or clinopyroxene) has beenreported from many xenolith localities (eg Frey ampGreen 1974 Ionov et al 1994 1996 Chazot et al 1996a1996b Yaxley et al 1997 Laurora et al 2001 Bali et al2002) Residual phases within the melt pockets stronglyindicate in situ partial melting within the xenolithsHowever a variety of processes may trigger such in situ

melting including (1) closed-system decompression melt-ing of either anhydrous (Maale amp Printzlau 1979) orhydrous phases (Frey amp Green 1974 Francis 1976) duringxenolith ascent in the host lava (2) melting during interac-tion of the xenoliths with the host basaltic melt duringxenolith ascent (eg Zinngrebe amp Foley 1995 Shaw ampKlulaquo gel 2002 Shaw et al 2006) (3) melting in the mantleprior to xenolith entrainment induced by an infiltratingfluid (ie flux melting eg Jones et al 1983 Ionov et al1994 Demecurren ny et al 2004) However most studies agreethat that partial melting occurred very shortly beforeandor during xenolith exhumation by the host magmaThus a combination of metasomatically triggered meltingfollowed andor assisted by decompression melting duringxenolith transport may be a realistic alternative in somecases (eg Dawson 2002)Recently Ban et al (2005) have shown in a very detailed

mass-balance study on glass-bearing xenoliths from theWest Eifel that the weight proportions of glass and theassociated micro-phenocrysts match the composition ofthe preserved parent amphibole We used the sameapproach and performed simple mass-balance calculations(least-squares fitting) to constrain the conclusions drawnfrom petrography and to test which phase contributedand to what extent to the formation of the melt pocketsThe results reported in Table 8 show that in most casesamphibole-melting can account successfully for the meltpocket composition and only a minor amount (520) ofother phases (orthopyroxene andor phlogopite) needs tobe added to the reactant Such a contribution of otherphases agrees with textural observation (eg Fig 4cshowing Dim growing on resorbed Opxm) as well as withthe conclusions of Ban et al (2005) who stated that moreSi-rich glass (ie 454wt) necessitates orthopyroxenecontributing to the meltTrace element patterns of the bulk glassthornmicro-

phenocryst pockets calculated using the measured traceelement abundances of glass and micro-phenocrysts andthe respective mass-balance parameters (Table 8) arebroadly identical to the measured amphibole composition(Fig 18a and b) Higher LILE (Cs Rb Ba) concentrationsin the calculated bulk melt patches in sample Ke 785 canbe balanced by adding 7wt of phlogopite (Fig 18b)which is in line with mass balance (Table 8) The dominantcontrol of amphibole to account for the trace element

budget of the melt pockets is similar to results found inother suites of glass-bearing xenoliths (Chazot et al1996a Yaxley amp Kamenetsky 1999 Laurora et al 2001)Further the obtained phase proportions for the melt pock-ets are in good agreement with the results from image ana-lysis (Table 1) In addition they are similar to the glass^cpx^ol proportions recorded in recent experiments on the

Table 8 Results of mass-balance calculations to test melt

pocket formation by incongruent melting of the early metaso-

matic assemblages

Sample 195915 785 785 196525 196525 19653

Reactant

amph 1000 1000 0820 1000 0944 mdash

opx mdash mdash 0078 mdash 0056 0406

phl mdash mdash 0102 mdash mdash mdash

glass mdash mdash mdash mdash mdash 05941

SiO2 4484 4989 4964 5129 5167 5898

TiO2 014 039 036 025 024 018

Cr2O3 232 291 258 127 121 009

Al2O3 1368 762 769 800 757 961

FeO(tot) 336 314 333 343 357 394

MnO 006 008 008 012 012 010

NiO 009 013 014 011 011 004

MgO 1844 2029 2197 2040 2122 1625

CaO 854 783 644 966 913 078

Na2O 460 500 436 504 476 843

K2O 049 032 096 042 040 060

Product

glass 0389 0322 0335 0292 0284 07032

ol 0243 0259 0315 0266 0286 0273

cpx 0288 0373 0308 0442 0431 0024

spl 0052 0032 0028 mdash mdash mdash

carb 0028 0014 0014 mdash mdash mdash

SiO2 4477 5027 4986 5207 5181 5903

TiO2 013 041 039 015 014 017

Cr2O3 213 314 272 109 106 015

Al2O3 1364 764 770 798 776 957

FeO(tot) 374 331 351 322 332 350

MnO 007 007 007 009 009 006

NiO 011 013 015 011 012 007

MgO 1834 2024 2196 2063 2141 1635

CaO 854 759 632 952 929 080

Na2O 449 385 388 499 485 860

K2O 060 096 100 015 014 091

r2 023 200 035 084 028 033

Prefix Ke omitted1Low-Si glass 2High-Si glass (see text for explanation)

28

melting behaviour of amphibole in wehrlites at tempera-tures between 1175 and 12508C and pressures of05^10GPa (Mecurren dard et al 2006) These temperatures arein good agreement with the estimates for the formation ofolivine and chromite micro-phenocrysts in the melt

pockets (Fig 16) The heat supply required for amphibolemelting in the formerly low-T metasomatized Group IIIxenoliths (T 200^3008C) may come from local heatingshortly before or during xenolith entrainment in the hostlava or from fluid infiltration that triggered melting (seediscussion below)Finally formation of the most Si-enriched melt (adjacent

to Opxm in Group III-c samples) can be modelled assum-ing progressing interaction between infiltrating melt andorthopyroxene where the latter melts incongruently

opxthornmelt1 olthornmelt2 cpx ethseeTable 8THORN eth1THORN

To calculate reaction (1) we chose the least evolved glassobserved in melt patches in sample Ke 19653 (SiO2

5921wt Mg-number 6465) to represent the initialmelt1 whereas melt2 adjacent to opx is SiO2-rich(6530wt) Trace element abundances in glass next toorthopyroxene are shifted to lower concentrations(Table 6) which is in agreement with dilution caused byaddition of a trace element-poor orthopyroxene compo-nent The resultant phase proportions are similar to thoseobserved at orthopyroxene^melt interfaces (ie higholivine and low cpx abundances) and are in agreementwith experimental studies (Shaw et al 1998)

Partial melting triggered by fluid fluxMass-balance calculations provide an explanation for meltpocket formation by in situ partial melting Thus onemight conclude that melting is simply the result of decom-pression and heating during xenolith transport in the hostmagma However there are two points that argue thatmelting was triggered by the infiltration of a fluid phaseshortly before the xenoliths were sampled by the hostmagma (1) the presence of occasionally high amounts ofcarbonate included in the melt pockets (2) the extremelyNa-rich clinopyroxene (Dim) compositions either includedas micro-phenocrysts in the melt pocket or present asequilibrated grains around chromite in the single TypeIII-d xenolith (Fig 3f) Both arguments will be discussedin the next section

Formation of carbonates

Textures and compositions of carbonate globules can beexplained by carbonate precipitation from a CO2-bearingfluid As demonstrated by several studies highCa(CathornMg) values (Table 7) and low REE concentra-tions are strong arguments that the carbonates are cumu-late crystals precipitated from a CO2-rich melt or afluid and not quenched carbonated melts (Lee et al 2000Ionov amp Harmer 2002) Oscillatory zoning (Fig 5a and b)of individual carbonate crystals within singleglobules corroborates this interpretation The roundedcontacts toward glass (globules) are also in line with thesefindings Similar globular forms were reported fromexperimental studies and do not a priori imply the

Fig 18 Calculated trace element characteristics of reactants (in mostcases amphibolethorn orthopyroxene filled symbols) and products(melt pockets open symbols) of the lsquoamphibole meltingrsquo model[see equation (1) in text] The relative phase proportions wereobtained by mass-balance calculations and are given in Table 8 Thedashed lines in (b) show that the high LILE concentrations obtainedfor the product (bulk melt pocket) can be explained by adding about7wt of phlogopite to the reactant (amphibolethorn orthopyroxeneTable 8) Further explanation is given in the text

29

presence of immiscible carbonate and silicate melts(eg Lee amp Wyllie 1996)The high amount of carbonate in some melt pockets

however (eg Fig 4e) does need further explanation asthe incongruent melting reaction (1) does not account forthe CO2 required for carbonate precipitation duringamphibole melting Therefore amphibole breakdowneither occurred in an open system with addition of at leastCO2 (eg Laurora et al 2001 Ban et al 2005) or anotherpre-existing phase acted as a source of carbon Oxidationof graphite texturally identified as a part of the earlyassemblage (eg Fig 3d) could account for some of theCO2 necessary for carbonate formation A change in fO2

conditions is also indicated when the composition of glass-related spinel (Chrm) and Olm are used for oxygen baro-metry which yields oxygen fugacities well on the oxidizedside of the carbon^carbonate^silicate buffer (Fig 16)However increasing fO2 triggered by closed-systemdecompression-induced melting alone does not appear fea-sible Also textures such as straight contacts between gra-phite and Mg-calcite (Fig 4c) suggest that carbonategrows on graphite (ie indicating local fO2 conditions cor-responding to carbon^carbonate equilibrium) rather thanreplacing it We therefore interpret melt pocket formationto be triggered by the infiltration of a CO2-rich fluid(ie flux melting see also Ionov et al 1994) This interpre-tation also accounts for the presence of abundant vugs inthe glass that could have contained a fluid (Ionov et al1994 2006) which was presumably lost during sample pre-paration However we emphasize here that late fluid infil-tration had only subtle effects on the trace elementcomposition of Type III-b and -c xenoliths

Formation of Na-rich Cr diopside

Infiltration of CO2-rich fluidwould also explain the Na-richcomposition of Dim micro-phenocryst cores (Fig 6a and b)Ionov et al (2005) noted recently that clinopyroxenes inmeltpockets are commonly characterized by Na2O contents51wt mainly as a result of low cpxmelt partition coeffi-cients at low pressures (eg Blundy et al 1995) Thus veryhighNa2Ocontents in Dimmicro-phenocrysts (Fig6a) sug-gest replacement of amphibole by Dim and eventually Olmand Chrm during reaction with a fluid at subsolidus condi-tions An origin of micro-phenocrysts by replacementrather than crystallization from a melt is indicated by theidentical crystallographic orientation of individual Dimcrystals in singlemelt pockets (Fig4a)Once the solidus of the precursor phase(s) was reached

(ie 10508C for amphibole Niida amp Green 1999) silicatemelt was generated in situ and led to the formation of clin-opyroxene rims with lower Na contents (Fig 6b) and thedevelopment of euhedral shapes in contact with melt (egFig 4a and e) Eventual immiscibility between remainingfluid and newly formed melt (eg during xenolith ascent)would then explain the presence of carbonate globules and

vugs enclosed in glass Likewise different origins of meltand carbonate were recently proposed for similar meltpockets from Hungarian xenoliths by Demecurren ny et al(2004) who found oxygen isotope disequilibrium betweencarbonate globules and the host glass Such a two-stepmodel (fluid-induced replacement followed by melt gen-eration) can be explained by CO2-fluid infiltration occur-ring within the mantle at sub-solidus conditions ultimatelyfollowed by xenolith entrainment in the host magma andtransport to the surfaceThe host magma possibly suppliedthe additional heat necessary for melt generationFast decompression would further enhance melting andcould also account for the lower Na contents in Dimmicro-phenocryst rims as a result of decreasing cpxmeltpartition coefficients at decreasing pressures (Blundyet al 1995)Rapid replacement of amphibole ( phlogopite) by an

anhydrous phase assemblage indicates that the fluid waswater-poor (eg low H2OCO2 ratio see Ionov et al2006)This is corroborated by the fact that Mg-katophorite(presumably OH-rich see above) is occasionally rimmedby a residual kaersutitic selvage (Fig 4d) Such Ti-richmantle amphiboles are commonly OH-poor as a result ofpartial dehydrogenation (eg Tiepolo et al 2001 and refer-ences therein)Finally the presence of theType III-d xenolith contain-

ing texturally equilibrated clinopyroxene of almost identi-cal composition to Dim micro-phenocrysts in melt pockets(Table 2 Figs 6a and 7b) further strengthens the argu-ment that CO2-fluid percolation started in the mantlebefore entrainment (ie enough time elapsed to allow thisrock to anneal completely after the late metasomaticassemblage had formed) A similar conclusion can bedrawn for the clearly host-related glass found in xenolithKe 195927 (eg Fig 13b) ThisType III-c sample is charac-terized by the presence of Dim micro-phenocrysts partlyovergrown by mineral rims clearly related to the host basa-nite magma (kaersutite Ti-augite and olivine withFo885 Fig 4f Tables 3 and 4 and supplementary dataat httppetrologyoxfordjournalsorg) Kaersutite hasmajor and trace element characteristics similar to typicalbasanitic and alkali basaltic amphibole phenocrysts(Fig 9d) The special phase assemblages in this xenolithindicates that host-related basanitic melts intruded someType III-c xenoliths and partly overprinted the assemblageformed slightly earlier by in situ derived alkali-rich peralu-minous melts fully preserved in other samples (eg Ke19653 or Ke 785)

ORIGIN OF METASOMATIZ INGAGENTS T IM ING AND TECTONICSETT INGAs noted above late metasomatism was probably short-lived started at mantle depths and probably continued

30

Table 4 Representative analyses of amphibole (oxides in wt trace elements in mgg)

Type III-b III-c

Mineral katoph katoph katoph katoph katoph katoph katoph katoph kaersut kaersut

Sample 195915 196515 196515 196515 785 785 196525 196525 195927 195927

Texture residual

in mp

interst interst lamellae

in cpx-I

residual

in mp

residual

in mp

euhedral core euhedral rim micro-xx micro-xx

SiO2 4541 4603 4625 4887 4955 4959 4952 4953 4064 4106

TiO2 015 190 203 099 030 029 022 028 612 577

Al2O3 1365 1145 1100 915 826 807 877 794 1405 1395

Cr2O3 251 123 148 144 254 263 106 115 031 008

FeO(tot) 350 293 304 278 302 294 359 346 588 615

MnO 006 003 004 002 008 002 010 008 016 010

NiO 009 014 009 010 010 006 009 012 010 010

MgO 1883 1886 1896 1997 2067 2062 2000 2024 1576 1545

CaO 851 974 960 976 805 793 863 909 1086 1075

Na2O 474 427 440 432 548 555 529 509 284 294

K2O 049 038 041 031 027 029 028 039 139 138

Total 9811 9695 9728 9771 9837 9805 9759 9741 9817 9793

Mg-no 9055 9199 9176 9275 9243 9259 9086 9124 8269 8173

P 445 222 235 na 175 196 206 120 214 266

K 4301 3916 3637 na 2393 2654 2955 3655 13289 12762

Sc 402 704 663 na 358 345 311 352 375 270

Ti 720 9867 9917 na 1732 1898 1421 1589 32154 32071

V 216 317 332 na 289 299 172 187 469 404

Ni 726 842 828 na 811 876 737 813 638 530

Zn 298 163 159 na 188 214 451 457 463 548

Ga na 981 939 na 103 114 867 872 na na

Rb 279 184 151 na 035 050 096 150 744 713

Sr 216 325 335 na 708 667 955 117 618 668

Y 101 269 315 na 111 126 390 359 310 355

Zr 539 370 331 na 292 297 114 592 933 110

Nb 339 413 447 na 102 121 192 705 265 341

Cs 0006 50001 0005 na 0006 50009 0008 0018 50019 50010

(continued)

Table 3 Continued

Type III-a III-b III-b III-c III-d III-c III-d III-c III-c III-c III-d

Mineral opx-I opx-I opx-I Opxm Opxm Chrm spl-I ol-I Olm ol-I ol-I

Sample 19651 196515 195915 19653 19706 19653 19706 785 19653 196525 19706

Texture Zone C1 core core core core core core core core core core core

Hf 0006 0010 0014 50009 0013 na 0042 50003 50017 50009 50006

Ta 50001 50001 50004 50005 50002 na 0015 0001 50018 50001 50001

Pb 50028 0041 0047 0074 50041 na 015 0036 0075 0042 0046

Th 0009 50002 0069 0020 50002 na 0010 0050 50006 0011 50001

U 0004 50001 0026 0017 0004 na 0017 0011 50006 50001 50002

1See Fig 15nd not detected na not analysed

13

compared withTh andTa respectively Hf Zr andTi showvery low concentrations Overall major and trace elementcharacteristics of the carbonates are similar to those of car-bonates found in other peridotite xenoliths (eg Ionov1998 Lee et al 2000 Laurora et al 2001 Ionov ampHarmer 2002 see Fig 14)

FORMATION OF THE EARLYASSEMBLAGESDespite large compositional variations numerous commontextural and compositional features indicate that the earlymetasomatic assemblages in all Group III samples formedduring a single metasomatic event In the discussion belowwe argue that during a first metasomatic stage the pri-mary (pre-metasomatic) phase assemblage in Group IIIspinel harzburgites was partially replaced by metasomaticminerals finally resulting in clinopyroxene-free amphi-bole- and phlogopite-bearing dunite and harzburgite(Type III-c) The initial metasomatic agent was probably

an evolved CO2^H2O-rich melt and the transitionfrom peridotites of Type III-a to -c can be explained byprogressive melt^rock reaction

Transition from cryptic (Type III-a) tomodal (Type III-b and -c) metasomatismFormation of new phases requires (1) crystallization from amelt or fluid phase (decreasing melt or fluid mass) andor(2) melt^solid or fluid^solid reactionTextural features suchas spinel being replaced by amphibole and phlogopite(eg Fig 3c) indicate melt(fluid)^rock reaction Thegeochemical signatures observed in the different peridotitetypes thus probably reflect pre-existing compositionalheterogeneities coupled with metasomatic modificationThis is illustrated best in sample Ke 19651 (Type III-a)the least metasomatized xenolith in this study It can besubdivided into three parts containing clinopyroxenewith different REE signatures (Zones A B and C inFig 15) The LREE are increasingly enriched from ZoneA to C indicating increasing cryptic metasomatism

Table 4 Continued

Type III-b III-c

Mineral katoph katoph katoph katoph katoph katoph katoph katoph kaersut kaersut

Sample 195915 196515 196515 196515 785 785 196525 196525 195927 195927

Texture residual

in mp

interst interst lamellae

in cpx-I

residual

in mp

residual

in mp

euhedral core euhedral rim micro-xx micro-xx

Ba 781 614 540 na 259 309 846 841 331 322

La 357 116 110 na 704 824 177 113 107 138

Ce 627 225 231 na 217 248 621 426 338 448

Pr 574 208 224 na 301 349 856 623 557 724

Nd 177 720 779 na 128 151 363 296 318 398

Sm 208 143 143 na 285 316 752 740 945 111

Eu 080 058 060 na 108 115 232 252 363 384

Gd 182 085 088 na 225 241 701 692 940 980

Tb 011 013 na 039 050 120 117 na na

Dy 171 057 067 na 202 252 735 698 700 819

Ho 036 010 010 na 039 050 155 144 137 147

Er 106 024 027 na 097 114 405 382 296 336

Tm 016 0030 0033 na 012 017 060 058 027 039

Yb 120 028 030 na 077 099 400 386 243 272

Lu 019 0036 0038 na 0079 0077 058 044 033 031

Hf 015 0067 0054 na 0065 0048 028 078 331 376

Ta 0033 014 014 na 0002 0004 010 107 167 198

Pb 201 360 374 na 142 173 195 222 022 039

Th 682 150 134 na 181 196 059 028 010 020

U 212 039 039 na 040 048 019 012 0018 0048

katoph Mg-katophorite kaersut kaersutite interst interstitial mp melt pocket nd not detected na notanalysed

14

HREE abundances on the other hand are very hetero-geneous in the least metasomatized Zone A moderatelydepleted in Zone B and homogeneously low in the mostmetasomatizedZoneC (containinggraphite Fig15) similarto REE patterns in clinopyroxene from the amphibole- andphlogopite-bearingType III-b sample (Ke196515 Fig7a)The decoupling of strongly (LREETh U) from moder-

ately incompatible elements (HREE) can be explained bychromatographic fractionation during percolation ofsmall amounts of melts or fluids through an initially non-metasomatized peridotite (Navon amp Stolper 1987 Godardet al 1995 Bedini et al 1997 Vernie res et al 1997 Ionovet al 2002) Upon reaction with the wall-rock a fluidor melt will become progressively enriched in incom-patible elements as a function of increasing distancefrom its source This is due to the fact that elements withhigher Dmineralmelt values (eg HREE) are selectivelyremoved from a percolating liquid when it exchanges withthe surrounding minerals The results are faster-movingchemical fronts of incompatible elements compared withthose of more compatible elements This is consistent with

the geochemical signature of cpx-I from the least metaso-matized part of sample Ke 19651 (Zone A Fig 15) whichalready started to re-equilibrate with respect to the incom-patible elements (ie addition of Th U and LREE)whereas more compatible elements still reflect pre-metasomatic heterogeneities In this case these areextreme HREE variations probably reflecting the earliergarnet breakdown (Kaeser et al 2006) On the otherhand more intensely metasomatized parts (eg Zone Cin sample Ke 19651 and sample Ke 196515) are character-ized by clinopyroxene trace element patterns wheremildly incompatible elements have also been modified(eg homogeneously depleted HREE Figs 7a and 15)Similar fractionation processes can explain the modal

and compositional differences between Type III-b and -cCompared withType III-b higher abundances of metaso-matic phases inType III-c suggests that higher amounts offluid or melt crystallized in or passed through these rocksThe amphibole-rich dunite (Ke 196525) for examplecontains low-Ti Mg-katophorite exhibiting the lowest(LaYb)N and (LaCe)N ratios and high (NbLa)N and

Fig 8 Major element characteristics of amphiboles from porphyroclastic spinel harzburgites and dunites from Marsabit compared with com-positions of mantle-derived amphiboles from other localities (XB basalt-hosted xenoliths XS xenoliths from subduction settings O orogenicperidotites) Mg-numberfrac14Mg(MgthornFetot) in molar proportions

15

(NbTh)N ratios (Fig 10a and c) indicating a moderatelyfractionated metasomatic agent Mg-katophorite fromthe phlogopite-rich harzburgite (Ke 785 Type III-c)and amphiboles from the more cpx-rich harzburgites(Type III-b) show progressively more fractionated patterns(eg Fig 10c) and decreasing abundances in lsquoamphibole-compatiblersquoelements (eg Nb Fig 9a) Thus the transitionfrom Type III-b to -c could reflect decreasing distancebetween the site at which the xenolith was sampledand the source of the lsquoinitialrsquo metasomatizing agent (egZanetti et al 1996 Ionov et al 2002 Bodinier et al 2004)

Judging from the abundance of modal metasomatic phasesthe amphibole dunite (Ke 196525) was probably closest tothe source of the metasomatizing agent The phlogopite-rich nature of Ke 785 which indicates higher K andLILE (Cs Rb Ba) contents in the remaining agent is inagreement with a more fractionated melt or fluid at some-what greater distance from the metasomatic sourceThe Nb abundances in amphibole also agree with this

model as well For example amphibole from Type III-cKe 196525 has comparatively high Nb concentrations(Fig 9c) in line with relatively high DNb

amphmelt values

Fig 9 Trace element composition of amphibole from porphyroclastic spinel harzburgites and dunites from Marsabit [normalized to the primi-tive mantle values of McDonough amp Sun (1995)] Mg-katophorite in Type III-b and -c xenoliths (a^c) and glass-related kaersutite (d) Theshaded fields indicate the compositional variation of all Mg-katophorite analyses (light grey) and the compositional range of LREE-depletedTi-pargasite from the Group I (grt)^spinel lherzolites (dark grey Kaeser et al 2006)

16

constrained by different studies (eg Ionov amp Hofmann1995 LaTourrette et al 1995) In Ti-poor systems such asthose of this study Nb uptake by amphibole is evenenhanced as a result of increased DNb values (Tiepoloet al 2001) Consequently in an event during which amphi-bole crystallizes continuously the remaining fluid or meltwill become progressively depleted in Nb (Ionov et al2002) resulting in formation of Nb-depleted amphibolesfurther away from the source (Zanetti et al 1996 Ionovet al 2002) such Nb-depleted amphibole is found insample Ke 785 as well as inType III-b xenoliths (eg Ke195915 Fig 9a) Amphibole signatures from xenolith suitesmodelled successfully in the context of percolative traceelement fractionation (eg Spitsbergen Ionov et al 2002)show similar ranges (eg in NbTh ratios Fig10a) to thosedocumented for our samples This further supports the

hypothesis that similar percolation mechanisms acted inthe mantle beneath Marsabit

The role of pre-existing heterogeneitiesand exotic mineralsAlthough a model based on trace element fractionationduring melt percolation is supported by textures and canaccount for many compositional features some composi-tional peculiarities remain unexplained First clinopyrox-ene and amphibole in several samples (Ke 785 19651 and196515) are characterized by extremely low HREE abun-dances more than one order of magnitude lower thanin amphiboles from the strongly metasomatizedamphibole dunite Ke 196525 (Fig 9a^c) The mainphases that crystallize during metasomatism are low-TiMg-katophorite and phlogopite However DHREE

amphmelt

Fig 10 Selected trace element ratios in amphiboles from the porphyroclastic spinel harzburgites from Marsabit compared with those ofmantle-derived amphibole from other tectonic settings (XK and XB cratonic kimberlite-hosted and off-cratonic basalt-hosted xenolithsrespectively O orogenic peridotites XS xenoliths from subduction settings) Concentrations (ratios) normalized to the primitive mantlevalues of McDonough amp Sun (1995)

17

Table 5 Representative analyses of phlogopite and apatite (oxides in wt trace elements in mgg)

Type III-b III-c III-b III-c

Mineral phlog phlog phlog phlog phlog phlog phlog apatite apatite

Sample 196515 196515 19653 785 785 785 196525 195915 19653

Texture veinlet interst in opx in opx with amph

in mp

around

chromite

incl in

amph

bleb in

glass

interst

grain

SiO2 3925 3914 4171 4190 4124 4134 4230 SiO2 016 049

TiO2 313 307 009 053 039 037 027 Al2O3 001 nd

Al2O3 1544 1583 1360 1360 1384 1377 1477 P2O5 3768 3715

Cr2O3 112 105 132 147 190 197 063 FeO(tot) 037 029

FeO(tot) 350 347 271 000 289 277 340 MnO 006 nd

MnO nd 002 008 305 004 002 005 MgO 084 091

NiO 023 027 019 023 024 028 023 CaO 4750 4237

MgO 2287 2280 2577 2591 2582 2528 2525 SrO 095 397

CaO nd 002 001 nd 004 001 002 Na2O 079 133

Na2O 215 219 282 258 264 262 208 La2O3 003 007

K2O 719 707 704 706 674 684 683 Ce2O3 088 347

Nd2O3 003 007

F 139 224

Cl 287 092

Total 9488 9493 9571 9641 9587 9535 9590 Totaly 9305 9245

Mg-no 9282 9287 9443 9380 9408 9420 9298

P 369 187 480 339 206 188 362 P 129717 139419

K 71711 72575 60747 79052 70997 69992 70189 K 110 897

Sc 099 110 131 130 088 091 160 Sc 242 079

Ti 15500 16408 890 2839 2096 1807 1736 Ti 235 5294

V 255 284 888 142 108 130 788 V 365 382

Ni 1808 1727 1744 2046 1813 1773 1905 Ni 121 397

Zn 234 221 358 382 317 321 695 Zn 119 096

Ga 111 984 na 210 190 188 235 Ga na na

Rb 185 147 289 183 133 133 342 Rb 015 049

Sr 102 116 775 325 363 350 101 Sr 7138 42233

Y 010 0086 50057 50054 0021 0006 0052 Y 988 1454

Zr 019 019 043 50088 0077 0070 073 Zr 307 101

Nb 506 628 479 177 102 146 210 Nb 024 020

Cs 175 138 261 399 216 180 798 Cs 50013 50036

Ba 1212 1096 1343 298 277 219 1124 Ba 139 126

La 010 0055 0069 50029 0020 50003 50045 La 7460 19048

Ce 0027 0011 50075 50019 014 50003 0078 Ce 6844 25945

Pr 0012 0008 50067 50024 0012 50003 50019 Pr 380 2212

Nd 0019 0035 50405 50079 0065 50009 50224 Nd 749 6530

Sm 0015 50011 50474 50090 0016 50010 50132 Sm 515 747

Eu 0022 0017 50104 50026 0013 0010 50095 Eu 147 189

Gd 50010 50011 5043 5015 50011 0012 017 Gd 314 451

Tb 50002 50002 na 50027 0002 50002 50028 Tb na na

Dy 50015 50017 045 50077 50007 50006 50077 Dy 198 281

Ho 50002 50003 50043 50020 0002 50002 50028 Ho 359 511

Er 50009 50013 5025 5011 50016 50009 5011 Er 903 119

Tm 50002 50002 50088 50013 0002 0003 50018 Tm 119 138

(continued)

18

data (eg LaTourrette et al 1995) indicate that amphibolecrystallization alone is unlikely to deplete the metasomaticagent to such a high degree in HREE The same probablyaccounts for the extreme Nb depletion in amphiboles fromKe 785 relative to the otherType III-c xenolith (Ke 196525 see also Ionov et al 2002)Further clinopyroxene phlogopite and amphibole in the

Type III-b sample Ke 196515 on one hand all showstrongly fractionated REE patterns (eg high LaCe inamphibole Fig 10c) but on the other hand are character-ized by a positiveTi anomaly decoupled from other HFSE(eg Hf and Zr Fig 9b) Trace element fractionationduring amphibole crystallization (ie formation of TypeIII-c amphibole dunites) however should lower Ti con-tents in the remaining fluid or melt (eg Moine et al 2001)Based on textures mineralogy and dominant common

compositional features the hypothesis that Type III-band -c xenoliths were metasomatized by completely inde-pendent metasomatic agents can be ruled out Thereforetwo other possibilities remain to account for the inconsis-tent observations described above (1) inference of otherphases reactants or products that are able to fractionatespecific trace elements present in the mantle beneathMarsabit but not present in the few investigated samples(2) pre-metasomatic compositional heterogeneities similarto the inherited HREE variation inType III-a clinopyrox-ene (see above)Garnet would be an important sink for HREE but

metasomatism occurred in the spinel field after decom-pression with maximum pressures of 515GPa (Kaeseret al 2006) so garnet had already broken down to sym-plectites (Fig 15) Percolation of melt through garnet-bearing websterites prior to metasomatism in Ke 19651and 196515 could be an alternativeWebsterites have been

interpreted to occur in close spatial relationship with por-phyroclastic peridotites in the shallow mantle beneathMarsabit (Kaeser et al 2006) and they occasionally con-tain rutile and ilmenite which could further account forthe strong Nb and Ta depletion in some samples (eg Ke785) The available compositional data from the webster-ites however do not indicate metasomatic modification ofwebsteritic garnet (Olker 2001 B Kaeser unpublisheddata) nor compositional similarities to Group IIIperidotitesCareful study by SEM rules out the presence of exotic

phases such as zircon or Ti-oxides described fromother xenolith suites (eg Bodinier et al 1996 Rudnicket al 1999) Nevertheless it cannot be excluded that themetasomatizing agent percolated through mantle domainscontaining these phases before infiltrating the studiedrocksThe preservation of pre-metasomatic compositional het-

erogeneities is another possibility that must be consideredThat such lsquorock signalsrsquo partly overlap with the lsquometaso-matic signalrsquo is indicated by the HREE variation in TypeIII-a clinopyroxene (see above) and also by other composi-tional parameters For example both Mg-katophorite andphlogopite growing at the expense of spinel have very highCr2O3 contents whereas euhedral Mg-katophorite inthe amphibole dunite (Ke 196525) or in phlogopiteinclusions in Opxm is comparably Cr2O3-poor (Fig 8bTables 4 and 5) Pre-existing heterogeneities would essen-tially be preserved through elements that are relativelycompatible in a given mineral (ie that are not mobilizedduring metasomatism) This would be in agreement withthe preserved HREE variations in Type III-a clinopyrox-ene high Cr2O3 in amphiboles and phlogopite and Tispikes in amphibole from Ke 196515

Table 5 Continued

Type III-b III-c III-b III-c

Mineral phlog phlog phlog phlog phlog phlog phlog apatite apatite

Sample 196515 196515 19653 785 785 785 196525 195915 19653

Texture veinlet interst in opx in opx with amph

in mp

around

chromite

incl in

amph

bleb in

glass

interst

grain

Yb 50014 50011 5047 5015 50011 50019 5017 Yb 803 724

Lu 0002 50002 50061 50014 50002 50002 50020 Lu 115 742

Hf 0019 0018 5025 50053 50012 50006 5019 Hf 50023 50120

Ta 020 022 140 0031 0012 0009 023 Ta 50015 50075

Pb 546 586 125 108 083 098 143 Pb 436 577

Th 50004 50004 50080 0043 0088 0056 50021 Th 2538 1305

U 0012 0011 0069 50018 0038 0030 0031 U 572 278

interst interstitial mp melt pocket na not analysed nd not detected

19

Further very high Mg-numbers in replacive amphibolessupport melt (fluid)^rock interaction at low melt (fluid)rock ratios where Mg and Fe contents are buffered by theambient peridotite (Kelemen et al 1995 Tiepolo et al2001) The slightly lower Mg-number of euhedralamphiboles in Ke 196525 combined with their lowerCr2O3 contents (Fig 8b) suggests a higher melt (fluid)rock ratio which is further supported by considerablylower Mg-number in associated ol-I (Fo88 Table 3)

The limited number of investigated samples does notallow us to decide which of the processes described aboveacted and to what extent However given the heterogene-ity observed in the numerous non-metasomatized xenolithsfrom Marsabit (Kaeser et al 2006) pre-metasomaticfeatures certainly affect the final product of the melt^rockreaction process to a considerable degree

The nature of the metasomatizing agent(s)Graphite stability and oxygen fugacity considerations

The Marsabit xenoliths are unusual in that they occur inan off-cratonic setting yet bear macroscopic graphite Thelatter usually either occurs in kimberlite-hosted peridotites(on-craton) or in pyroxenites from alpine-type peridotitebodies (eg Pearson et al 1994) In the phlogopite perido-tites from Finero graphite has recently been reportedas nanoscopic layers in phlogopite (Ferraris et al 2004)In xenoliths from Marsabit graphite occurs in the sametextural relationship as in kimberlite-hosted peridotitexenoliths (ie millimetre-sized flakes typically associatedwith orthopyroxene see Fig 3c^e and Pearson et al 1994)Graphite inclusions in phlogopite and Opxm couldindicate crystallization during metasomatism by alsquoreducedrsquo C-bearing fluid or melt However precipitationof graphite more probably reflects the relatively reducingconditions in the uppermost mantle prior to metasoma-tism We used the oxygen barometer of Ballhaus et al(1991) based on ferric iron contents in spinel coexistingwith olivine to constrain oxygen fugacity (fO2)conditions prior to metasomatism in the Marsabitxenoliths (Fig 16) The results show that non-metasomatized Group I and IV samples reflect similarlyreducing conditions to the cryptically metasomatizedType III-a samplesAlthough assessing Fe3thornFe2thorn ratios in spinel based

on microprobe analyses can be problematic (Wood ampVirgo 1989 Ballhaus et al 1991) all microprobeanalyses were obtained using the same operationalsettings and therefore give at least the relative fO2

variation between samples The relatively reducingconditions in the pre-metasomatic cold andhighly deformed mantle well below carbon^carbonate^silicate equilibria (Mg2Si2O6thorn 05 CaMg(CO3)2frac14Mg2SiO4thorn 05 CaMgSi2O6thornCthornO2 Eggler amp Baker1982 see Fig 16) and close to the CCO oxygen buffer arein agreement with the presence of graphite in the GroupIII harzburgites Moreover the P^T conditions at whichthe Group III xenoliths partially re-equilibrated are closeto the subsolidus decarbonation of model carbonated lher-zolite (Fig 2 see Lee amp Wyllie 2000 and referencestherein) in agreement with the observed phase assemblageincluding orthopyroxene olivine clinopyroxene andgraphite

Fig 11 Trace element patterns of (a) phlogopite and (b) apatitefrom porphyroclastic spinel harzburgites and dunites from Marsabit[concentrations normalized to the primitive mantle valuesof McDonough amp Sun (1995)] The grey field in (a) indicates thecomposition of Ti-rich phlogopite in Group I (grt)^spl lherzolites(Kaeser et al 2006)

20

Table 6 Representative analyses of silicate glass (oxides in wt trace elements in mgg)

Type III-b III-c Host

Sample 195915 196515 196515 785 196525 19653 19653 19653 195927

Remark next amph next cpx xenol margin next amph next amph melt pocket next apatite next Opxm melt pocket host basanite

SiO2 5306 5775 5161 6096 5657 5938 5974 6530 4882 4410

TiO2 017 157 106 062 030 024 020 023 378 255

Al2O3 2412 2053 2025 1779 2313 1575 1245 1355 1890 1396

Cr2O3 005 001 nd 001 001 nd nd nd 002 na

FeO(tot) 188 247 626 126 252 295 374 265 509 1283

MnO 007 000 026 001 004 005 008 007 018 021

NiO nd nd nd na nd na 006 nd 001 na

MgO 182 129 116 114 122 304 382 252 244 938

CaO 506 257 283 082 044 124 101 039 810 1030

Na2O 1107 966 969 971 1299 1388 1384 1211 673 374

K2O 154 197 465 357 138 121 142 127 300 146

Cl 031 106 038 012 014 023 061 020 015 na

P2O5 041 008 044 009 012 049 233 018 100 059

Total 9956 9896 9860 9609 9887 9845 9929 9846 9821 9912

Mg-no 6329 4816 2489 6175 4632 6474 6458 6289 4552 5658

P 1061 448 1610 230 503 1519 6738 676 2994 na

K 12563 16408 39599 32515 15872 9265 11277 10071 25900 na

Sc 179 184 126 406 151 970 136 848 691 237

Ti 665 7329 4974 2953 1965 1213 1163 1055 18199 15311

V 107 265 235 796 920 210 151 134 234 na

Ni 128 371 92 5575 983 170 5199 5298 199 191

Zn 962 103 109 110 459 440 508 309 819 na

Ga na 173 273 194 220 na na na na na

Rb 181 263 124 546 626 176 398 369 577 369

Sr 427 488 1407 188 881 1644 4997 985 1062 792

Y 543 145 224 752 262 619 158 369 301 299

Zr 466 171 353 761 660 578 365 428 242 216

Nb 416 161 137 260 136 234 164 169 837 647

Cs 014 046 186 125 007 022 102 052 063 na

Ba 238 261 1064 129 168 405 1099 641 625 563

La 698 183 848 144 889 2212 1008 106 537 462

Ce 947 256 131 308 138 333 1265 164 104 823

Pr 655 194 120 330 116 304 975 160 120 na

Nd 146 603 408 101 329 993 256 564 463 400

Sm 142 048 570 179 419 141 379 679 105 851

Eu 041 035 228 082 112 438 103 241 345 267

Gd 062 051 415 158 402 106 245 566 677 729

Tb na 0086 086 020 071 na na na na 111

Dy 090 027 359 098 434 104 171 637 732 na

Ho 023 0037 105 025 095 206 446 104 117 114

Er 045 038 195 058 255 611 153 264 305 na

Tm 009 50039 033 0081 042 085 292 5014 036 052

Yb 062 023 267 044 307 667 120 487 253 220

(continued)

21

Evidence for low-temperature SiO2^Na2O^CO2^H2O-rich melts or fluids

Potential metasomatic agents inferred from the presenceof graphite are C^O^H-rich fluids carbonate melts orCO2-bearing silicate melts The resulting phase assem-blages and geochemical signatures are the result of infiltra-tion of fluid or melt into a cold (750^8508C) shallow(515GPa Fig 2) heterogeneous and locally reduced litho-spheric mantle Metasomatism under such conditions in anopen system must result in complex geochemical signa-tures that reflect the degree of fluid^rock reaction Thismakes it particularly difficult to constrain the exactnature of the lsquooriginalrsquometasomatic agentType III-c xenolith Ke 196525 contains the highest frac-

tion of metasomatic phases and the least fractionatedamphibole trace element patterns The lsquomagmaticrsquo texturesof the Mg-katophorites in this sample suggest direct crys-tallization from a melt or a fluid rather than replacementof a pre-existing phase Therefore hypothetical liquids inequilibrium with the amphiboles in this xenolith couldyield the composition of the initial metasomatic agentsFigure 17 illustrates the trace element patterns of suchmelts and fluids calculated using sets of experimentally orempirically determined mineralmelt (fluid) partitioncoefficients including carbonate melt silicate melt andhydrous fluid (for references see caption to Fig 17)Common features for all calculated metasomatic agents

are negative Zr^Hf^Ti anomalies and LREE enrichment(Fig 17) However previous studies have shown that thesesignatures are indicative of almost all types of metasoma-tism including interaction of peridotite with carbonatiticmelt (Green amp Wallace 1988 Dautria et al 1992 Hauriet al 1993 Rudnick et al 1993 Ionov et al 1996 Yaxleyet al 1998 Coltorti et al 1999) with hydrous fluids andormelts for example originating from dehydration of altered

subducting lithosphere (Maury et al 1992 Zanetti et al1999 Grecurren goire et al 2001 Laurora et al 2001 Parkinsonet al 2003) and interaction of peridotite with volatile-and Si-rich melts that evolved through reactive porousflow (Bedini et al 1997 Ionov et al 2002 2006 Bodinieret al 2004 Rivalenti et al 2004)In the case of the Marsabit xenoliths integration of the

trace element characteristics thermobarometric inferencesand mineralogical constraints favours a Na2O^SiO2^CO2^H2O-rich silicate melt Carbonated melts can prob-ably be ruled out on major element and mineralogical con-siderations First amphiboles are Ca-poor in conflict withcarbonate melt metasomatism which is generally charac-terized by Ca enrichment (Rudnick et al 1993 Neumannet al 2002) Further orthopyroxene is a stable phase andprobably even crystallizes during metasomatism in somesamples (eg Ke 785) At near-solidus conditions coexis-tence of orthopyroxene and carbonate melt is possibleonly at pressures 20GPa (see Fig 2 eg Lee amp Wyllie2000 and references therein) which is considerably higherthan the maximum pressures indicated by the compositionof spinel (515GPa) At such low pressures metasomatismby carbonated melts will convert lherzolite and harzbur-gite immediately into wehrlite which is the major perido-tite type in all xenolith suites where carbonate meltmetasomatism is convincingly demonstrated (Rudnicket al 1993 Yaxley et al 1998 Neumann et al 2002)In Marsabit wehrlites are absent howeverAn Na2O^SiO2^CO2^H2O-rich melt or fluid on the

other hand would explain the Na-rich nature of amphiboleand phlogopite as well as the presence of orthopyroxeneGrowth of metasomatic (and Al-poor) orthopyroxene hasbeen described from several xenolith localities where peri-dotites were infiltrated either by hydrous fluids orSi-rich melts (Smith et al 1999 McInnes et al 2001Morishita et al 2003) Na-rich phlogopite is commonly

Table 6 Continued

Type III-b III-c Host

Sample 195915 196515 196515 785 196525 19653 19653 19653 195927

Remark next amph next cpx xenol margin next amph next amph melt pocket next apatite next Opxm melt pocket host basanite

Lu 014 0050 026 0041 046 080 140 058 040 031

Hf 026 052 535 014 076 110 5087 124 568 504

Ta 50080 033 638 0022 123 101 784 801 394 403

Pb 541 448 280 255 840 797 260 749 493 na

Th 194 296 160 466 703 185 247 105 534 506

U 545 069 478 099 152 447 724 216 171 126

nd not detected na not analysedAverage composition of the strongly alkaline basanites from Marsabit (Volker 1990)

22

Fig 12 Total alkalis vs SiO2 (a) Harker-type co-variation diagrams (b^e) and incompatible trace element co-variation diagram (f) showingthe compositional variability of silicate glass found in the porphyroclastic spinel harzburgites and dunites from Marsabit In (b) binary mixinglines between the typical composition of the alkali-rich glass (large open diamond) and the associated microlites are shown (each increment is10wt)They illustrate that the glass does not represent the derivate of host lava infiltration and fractional crystallization Subalkaline glass inGroup I (grt)^spl lherzolites from Marsabit is shown for comparison and is interpreted to derive from incongruent melting of Ti-pargasite(Kaeser 2006)

23

found as a reaction product between anhydrous mafic toultramafic rocks and H2O-rich evolved liquids (Costaet al 2001 and references therein) or H2O-bearing fluids(eg Odling 1994) The latter study was an experiment at20GPa simulating the effects of decreasing temperatureson the phase assemblages and their composition duringmetasomatism using a model peridotitethornC^O^H fluidThe coolest part of Odlingrsquos (1994) experiment (510758C)contained a strikingly similar phase assemblage (kato-phorite phlogopite with 2wt Na2O and Al-poororthopyroxene) to the Marsabit samples interpreted tohave formed during sub-solidus fluid^solid interactionA compositionally similar phase assemblage (highamounts of opx Si-rich hornblende Na-rich phlogopite)resulted from experiments simulating peridotite interac-tion with Si-rich melt (dacite) at lower P^T conditions(9508C at 092GPa Prouteau amp Scaillet 2003)In this study the trace element signatures of Group III-c

Mg-katophorite and the calculated metasomatic agentsfurther suggest that the metasomatizing agent was a meltrather than a fluid Hypothetical fluids in equilibrium witheuhedral Mg-katophorite from sample Ke 196525 havehigh NbLILE (eg Rb Ba) and low LILEREE ratios(Fig 17) Because experimental data predict the oppositefor fluids (LILE enriched over HFSE and REE egManning 2004 and references therein) a fluid with asignature such as that indicated in Fig 17 is unrealistic

Fig 13 Trace element patterns of silicate glass in porphyroclasticspinel harzburgites and dunites from Marsabit compared withthe trace element composition of associated clinopyroxene (Dim)micro-phenocrysts xenolith-hosting basanites from Marsabit andglass interpreted to represent xenolith^host interaction [concentra-tions normalized to the primitive mantle values of McDonough ampSun (1995)]

Table 7 Representative analyses of carbonates (oxides in

wt trace elements in mgg)

Group III-b III-b III-c III-c III-c

Mineral Dol Mg-Cc Mg-Cc Mg-Cc Mg-Cc

Sample 195915 196515 785 19653 19653

Texture in glass in glass in glass in glass in glass

SiO2 na 003 na nd 003

FeO(tot) 016 011 010 010 006

MnO 001 nd nd nd nd

MgO 1922 236 190 125 517

CaO 3189 5313 5377 5391 4853

Na2O 003 nd 004 nd 003

K2O na 002 na 001 nd

SrO 003 010 008 004 021

BaO 003 nd nd na na

CO21 4618 4444 4442 4377 4391

Total 9756 10021 10032 9909 9794

Mg-no 9954 9745 9713 9554 9931

Ca-no 5439 9418 9531 9687 8709

P 220 334 137 580 589

K 970 212 642 105 155

Sc 5065 50150 027 5027 50076

Ti 975 189 242 528 196

V 5085 0481 5042 5037 021

Ni 900 673 517 5294 593

Zn 459 5089 113 296 045

Ga na 5014 5013 na na

Rb 0084 50072 50074 50060 0046

Sr 593 860 371 251 4274

Y 012 011 130 704 015

Zr 261 024 012 097 756

Nb 144 019 0056 692 185

Cs 50063 50011 50018 50039 50008

Ba 126 230 5024 029 271

La 034 057 084 136 025

Ce 032 013 134 997 023

Pr 006 006 018 095 0026

Nd 030 013 068 228 0078

Sm 5044 50091 0074 048 50018

Eu 5012 50019 0051 020 50008

Gd 5037 50090 014 051 0026

Tb na 50010 0036 na na

Dy 5019 50039 0089 058 50038

Ho 50029 0013 0044 011 50006

Er 5012 50041 0083 033 50019

Tm 50052 50013 0016 0084 50006

Yb 5043 50063 014 027 50040

Lu 50072 50010 0011 0066 50008

(continued)

24

At the relatively low maximum pressures (515GPa) asupercritical liquid is not expected either (eg Kesselet al 2005)Finally a relatively hydrous metasomatizing agent

would be consistent with formation of Mg-katophoriteswith lowTi and presumably low Fe3thorn contents (given therelatively reducing conditions) which in turn imply high(OH)^ contents In other words partial dehydrogenationtypically observed in common (Ti-rich) mantle amphibole

(eg Popp et al 1995 Tiepolo et al 2001 and referencestherein) cannot have played an important role in thiscase as H deficiency is mainly the result of coupled substi-tutions involvingTi and Fe3thorn (Popp et al 1995) In conclu-sion the metasomatizing agent was most probably aNa2O^SiO2^CO2^H2O-rich meltComparison with experimental studies further con-

strains the low-temperature regime in which metasoma-tism occurred In ultramafic (eg Niida amp Green 1999)basaltic (eg Rapp 1995) and Si-enriched systems (egtrachybasaltic Barclay amp Carmichael 2004) amphibole(in the latter study together with phlogopite) is confinedto temperatures lower than 1000^10508C at pressures520GPa (Fig 2)Thus crystallization of the early metaso-matic phase assemblage can be explained by infiltration ofa silicate melt that rapidly cooled upon contact with theambient peridotite (750^8508C prior to metasomatismFig 2) However this low-T stage is based on Ca-in-opxthermometry (see Brey amp Kolaquo hler 1990 Kaeser et al2006 Table 1) and therefore probably underestimates theeffective temperature conditions during metasomatismThis can be explained by low diffusion rates and proximityto the closure temperature for the diopside^enstatiteexchange reaction at temperatures lower than 8508C(eg Smith 1999 and references therein) The lower tem-perature limit at which silicate melt metasomatism ispossible (ie defined by the meltrsquos solidus and viscosity)depends strongly on volatile and SiO2 contentsExperiments by Draper amp Green (1997) for exampleshowed that highly silicic melts (52^64wt SiO2) canbe in equilibrium with mantle harzburgite at temperaturesas low as 8508C (at 10^20GPa under anhydrous andC^O^H-fluid-bearing conditions) These are howeverextreme compositions for mantle melts The actual SiO2

content of the metasomatizing agent in this case is difficultto constrain from the metasomatic mineral assemblage ascompositions may have varied considerably during reac-tive percolation (eg including Si-releasing reactions suchas cpx replacement by amphibole versus Si-consumingreactions such as Opxm crystallization) Moreover it isunlikely that a melt persists at temperatures muchbelow 9008C which is the solidus of fertile lherzolite(ie primitive upper mantle composition) underH2O-saturated conditions at 15GPa (Grove et al 2006see Fig 2) Below the solidus residual volatile-enrichedfluids may percolate further for example forming the lessmetasomatized Type III-c xenoliths (eg Ke 785) as wellas Type III-b and -a peridotites (see also Bailey 1987)Relatively cold conditions would also explain the veryhigh compositional gradients developed on small scales inType III-a samples (Fig 15) Given the absence of chloritein all investigated samples the lower temperature limitof such fluid metasomatism is 8008C (eg Fumagalli ampPoli 2005 see Fig 2)

Table 7 Continued

Group III-b III-b III-c III-c III-c

Mineral Dol Mg-Cc Mg-Cc Mg-Cc Mg-Cc

Sample 195915 196515 785 19653 19653

Texture in glass in glass in glass in glass in glass

Hf 5016 50038 50036 50074 50013

Ta 0048 50012 50016 50031 0008

Pb 5024 858 5021 074 018

Th 0021 50020 50017 50028 50005

U 060 094 086 600 148

Mg-Cc Mg-calcite Dol dolomite nd not detectedna not analysed Ca-numberfrac14 100Ca(CathornMg)1Calculated from stoichiometry

Fig 14 Trace element patterns of glass-hosted carbonates in TypeIII-b and -c spinel harzburgite and dunite xenoliths from Marsabit[concentrations normalized to the primitive mantle values ofMcDonough amp Sun (1995)] The grey field outlines the compositionalrange of carbonates found in other peridotite xenoliths (Ionov ampHarmer 2002 and references therein) Carbonate material inter-preted as quenched carbonatite melt found in peridotite xenolithsfrom Kerguelen (Moine et al 2004) is shown for comparison(bold line)

25

Finally the presence of apatite even though oftentaken as indicator for carbonatite-type metasomatism(eg Yaxley et al 1998) is not in conflict with metasoma-tism by silicate melts Apatite has also been shown to crys-tallize from volatile and alkali-rich derivatives formedduring reactive porous flow of originally mafic silicatemelts (Woodland et al 1996 Bedini et al 1997 Bodinieret al 2004 Ionov et al 2006)

FORMATION OF THE LATE GLASS -BEAR ING ASSEMBLAGESTextures strongly suggest that the glass-bearing assem-blages formed later and at the expense of the early metaso-matic assemblages (Fig 4a and d) Features such as thereappearance of clinopyroxene (Na-rich Cr diopsidemicro-phenocrysts) a phase replaced by the early

Fig 15 Detailed REE systematics in primary clinopyroxene (cpx-I) in a porphyroclastic cryptically metasomatized spinel harzburgite(sample Ke 19651) showing the transition fromType III-a (Zone A and B) toType III-b (Zone C) on a scale of a few centimetres The numberson the photomicrographs (crossed polars) correspond to single laser ablation microprobe pits (for further explanation see text)

26

metasomatic assemblage indicate that melt pocket forma-tion reflects a significant change in P^T^X space in partsof the mantle beneath Marsabit after earlier metasoma-tism Based on geochemical arguments and mass-balanceconsiderations we will show that (1) the late glass-bearingassemblages are closely related to early metasomaticassemblages mainly representing breakdown productsand (2) breakdown was induced by infiltration of aCO2-rich fluid in the mantle probably followed andenhanced by heating and decompression melting duringxenolith transport in the host magma

Evidence for in situ partial meltingBSE imaging reveals that the phases becoming unstableare amphibole orthopyroxene and to minor extentsspl-I phlogopite cpx-I and apatite Compositional(inter- and intra-xenolith) heterogeneity of silicate glass istexture controlled and varies as a function of theassociated precursor phases (Fig 12a^f)TiO2 for exampleshows higher concentrations in glass from xenolith Ke196515 containing Ti-enriched amphibole (Fig 12c)whereas K2O contents are highest in phlogopite-rich sam-ples (eg Ke 785 Fig 12d) Elevated P2O5 contents arefound in glass adjacent to apatite (Fig 12e) whichmirrors at the same time the highTh and LREE contents

of apatite (Fig 12f) Significant Si enrichment is found inglass coating orthopyroxene (Fig 12d) supporting incon-gruent melting of orthopyroxene (see below) Even thoughthe melt pockets are connected along grain boundaries thestrong compositional variation (on inter- and intra-xeno-lith scale) suggests that the mobility of the melt was lim-ited or that the melting event was very short-lived (fortiming constraints see below)The co-variation plots in Fig 12 further reveal consider-

able compositional differences between all types of xeno-lith glass and the host basanite These data indicate thatthe glass cannot be the product of xenolith^host lava inter-action during ascent to the surface Also back-mixing ofmicro-phenocrysts into the melt fails to derive the glassfrom a host-related melt (Fig 12b) Glass from xenolithmargins however is compositionally closer to the bulkhost basanite (eg Fig 12b) in line with a direct derivationfrom xenolith^host lava interaction Its more evolvedmajor element composition (eg higher SiO2 lowerMg-number Fig 12b) is probably the result of fractiona-tion of basanite phenocrysts (eg olivine augite andtitanomagnetite Volker 1990) Host lava^xenolith interac-tion is further supported by the trace element signature ofglass at the margin of the xenoliths which is closely similarto that of the Marsabit basanites (Fig 13 Volker 1990)The same characteristics are observed for intra-xenolithglass in the unusual kaersutite-bearing sample Ke 195927

Fig 16 Oxygen fugacities relative to the quartz^fayalite^magnetitebuffer (QFM equation of OrsquoNeill 1987) as a function of temperatureat assumed pressure of 1GPa calculated using the composition ofcoexisting spinel and olivine (ol-I^spl-I pairs and Chrm^Olm pairsrespectively) based on the oxygen barometer of Ballhaus et al (1991)The graphite^CO2 equilibrium was calculated after Frost amp Wood(1998) and the graphite^carbonate^silicate equilibrium was calcu-lated using the equation of Eggler amp Baker (1982) (See text for furtherexplanation)

Fig 17 Calculated metasomatic agents in equilibrium with the euhe-dral Mg-katophorites from the Type III-c amphibole dunite Ke 196525 Amphibolehydrous fluid and amphibolecarbonatite melt parti-tion coefficients are from Adam et al (1997) and Adam amp Green(2001) respectively Silicate melts have been calculated using amphi-boleSi-rich melt partition coefficients from Chazot et al (1996b) andamphibolebasanite partition coefficients from LaTourrette et al(1995) The compositional range of the Marsabit basanites and alkalibasalts (Volker 1990) is shown for comparison

27

Xenolith glass unrelated to the host lava and melt pock-ets similar to those reported here in association with resi-dual phases (eg amphibole or clinopyroxene) has beenreported from many xenolith localities (eg Frey ampGreen 1974 Ionov et al 1994 1996 Chazot et al 1996a1996b Yaxley et al 1997 Laurora et al 2001 Bali et al2002) Residual phases within the melt pockets stronglyindicate in situ partial melting within the xenolithsHowever a variety of processes may trigger such in situ

melting including (1) closed-system decompression melt-ing of either anhydrous (Maale amp Printzlau 1979) orhydrous phases (Frey amp Green 1974 Francis 1976) duringxenolith ascent in the host lava (2) melting during interac-tion of the xenoliths with the host basaltic melt duringxenolith ascent (eg Zinngrebe amp Foley 1995 Shaw ampKlulaquo gel 2002 Shaw et al 2006) (3) melting in the mantleprior to xenolith entrainment induced by an infiltratingfluid (ie flux melting eg Jones et al 1983 Ionov et al1994 Demecurren ny et al 2004) However most studies agreethat that partial melting occurred very shortly beforeandor during xenolith exhumation by the host magmaThus a combination of metasomatically triggered meltingfollowed andor assisted by decompression melting duringxenolith transport may be a realistic alternative in somecases (eg Dawson 2002)Recently Ban et al (2005) have shown in a very detailed

mass-balance study on glass-bearing xenoliths from theWest Eifel that the weight proportions of glass and theassociated micro-phenocrysts match the composition ofthe preserved parent amphibole We used the sameapproach and performed simple mass-balance calculations(least-squares fitting) to constrain the conclusions drawnfrom petrography and to test which phase contributedand to what extent to the formation of the melt pocketsThe results reported in Table 8 show that in most casesamphibole-melting can account successfully for the meltpocket composition and only a minor amount (520) ofother phases (orthopyroxene andor phlogopite) needs tobe added to the reactant Such a contribution of otherphases agrees with textural observation (eg Fig 4cshowing Dim growing on resorbed Opxm) as well as withthe conclusions of Ban et al (2005) who stated that moreSi-rich glass (ie 454wt) necessitates orthopyroxenecontributing to the meltTrace element patterns of the bulk glassthornmicro-

phenocryst pockets calculated using the measured traceelement abundances of glass and micro-phenocrysts andthe respective mass-balance parameters (Table 8) arebroadly identical to the measured amphibole composition(Fig 18a and b) Higher LILE (Cs Rb Ba) concentrationsin the calculated bulk melt patches in sample Ke 785 canbe balanced by adding 7wt of phlogopite (Fig 18b)which is in line with mass balance (Table 8) The dominantcontrol of amphibole to account for the trace element

budget of the melt pockets is similar to results found inother suites of glass-bearing xenoliths (Chazot et al1996a Yaxley amp Kamenetsky 1999 Laurora et al 2001)Further the obtained phase proportions for the melt pock-ets are in good agreement with the results from image ana-lysis (Table 1) In addition they are similar to the glass^cpx^ol proportions recorded in recent experiments on the

Table 8 Results of mass-balance calculations to test melt

pocket formation by incongruent melting of the early metaso-

matic assemblages

Sample 195915 785 785 196525 196525 19653

Reactant

amph 1000 1000 0820 1000 0944 mdash

opx mdash mdash 0078 mdash 0056 0406

phl mdash mdash 0102 mdash mdash mdash

glass mdash mdash mdash mdash mdash 05941

SiO2 4484 4989 4964 5129 5167 5898

TiO2 014 039 036 025 024 018

Cr2O3 232 291 258 127 121 009

Al2O3 1368 762 769 800 757 961

FeO(tot) 336 314 333 343 357 394

MnO 006 008 008 012 012 010

NiO 009 013 014 011 011 004

MgO 1844 2029 2197 2040 2122 1625

CaO 854 783 644 966 913 078

Na2O 460 500 436 504 476 843

K2O 049 032 096 042 040 060

Product

glass 0389 0322 0335 0292 0284 07032

ol 0243 0259 0315 0266 0286 0273

cpx 0288 0373 0308 0442 0431 0024

spl 0052 0032 0028 mdash mdash mdash

carb 0028 0014 0014 mdash mdash mdash

SiO2 4477 5027 4986 5207 5181 5903

TiO2 013 041 039 015 014 017

Cr2O3 213 314 272 109 106 015

Al2O3 1364 764 770 798 776 957

FeO(tot) 374 331 351 322 332 350

MnO 007 007 007 009 009 006

NiO 011 013 015 011 012 007

MgO 1834 2024 2196 2063 2141 1635

CaO 854 759 632 952 929 080

Na2O 449 385 388 499 485 860

K2O 060 096 100 015 014 091

r2 023 200 035 084 028 033

Prefix Ke omitted1Low-Si glass 2High-Si glass (see text for explanation)

28

melting behaviour of amphibole in wehrlites at tempera-tures between 1175 and 12508C and pressures of05^10GPa (Mecurren dard et al 2006) These temperatures arein good agreement with the estimates for the formation ofolivine and chromite micro-phenocrysts in the melt

pockets (Fig 16) The heat supply required for amphibolemelting in the formerly low-T metasomatized Group IIIxenoliths (T 200^3008C) may come from local heatingshortly before or during xenolith entrainment in the hostlava or from fluid infiltration that triggered melting (seediscussion below)Finally formation of the most Si-enriched melt (adjacent

to Opxm in Group III-c samples) can be modelled assum-ing progressing interaction between infiltrating melt andorthopyroxene where the latter melts incongruently

opxthornmelt1 olthornmelt2 cpx ethseeTable 8THORN eth1THORN

To calculate reaction (1) we chose the least evolved glassobserved in melt patches in sample Ke 19653 (SiO2

5921wt Mg-number 6465) to represent the initialmelt1 whereas melt2 adjacent to opx is SiO2-rich(6530wt) Trace element abundances in glass next toorthopyroxene are shifted to lower concentrations(Table 6) which is in agreement with dilution caused byaddition of a trace element-poor orthopyroxene compo-nent The resultant phase proportions are similar to thoseobserved at orthopyroxene^melt interfaces (ie higholivine and low cpx abundances) and are in agreementwith experimental studies (Shaw et al 1998)

Partial melting triggered by fluid fluxMass-balance calculations provide an explanation for meltpocket formation by in situ partial melting Thus onemight conclude that melting is simply the result of decom-pression and heating during xenolith transport in the hostmagma However there are two points that argue thatmelting was triggered by the infiltration of a fluid phaseshortly before the xenoliths were sampled by the hostmagma (1) the presence of occasionally high amounts ofcarbonate included in the melt pockets (2) the extremelyNa-rich clinopyroxene (Dim) compositions either includedas micro-phenocrysts in the melt pocket or present asequilibrated grains around chromite in the single TypeIII-d xenolith (Fig 3f) Both arguments will be discussedin the next section

Formation of carbonates

Textures and compositions of carbonate globules can beexplained by carbonate precipitation from a CO2-bearingfluid As demonstrated by several studies highCa(CathornMg) values (Table 7) and low REE concentra-tions are strong arguments that the carbonates are cumu-late crystals precipitated from a CO2-rich melt or afluid and not quenched carbonated melts (Lee et al 2000Ionov amp Harmer 2002) Oscillatory zoning (Fig 5a and b)of individual carbonate crystals within singleglobules corroborates this interpretation The roundedcontacts toward glass (globules) are also in line with thesefindings Similar globular forms were reported fromexperimental studies and do not a priori imply the

Fig 18 Calculated trace element characteristics of reactants (in mostcases amphibolethorn orthopyroxene filled symbols) and products(melt pockets open symbols) of the lsquoamphibole meltingrsquo model[see equation (1) in text] The relative phase proportions wereobtained by mass-balance calculations and are given in Table 8 Thedashed lines in (b) show that the high LILE concentrations obtainedfor the product (bulk melt pocket) can be explained by adding about7wt of phlogopite to the reactant (amphibolethorn orthopyroxeneTable 8) Further explanation is given in the text

29

presence of immiscible carbonate and silicate melts(eg Lee amp Wyllie 1996)The high amount of carbonate in some melt pockets

however (eg Fig 4e) does need further explanation asthe incongruent melting reaction (1) does not account forthe CO2 required for carbonate precipitation duringamphibole melting Therefore amphibole breakdowneither occurred in an open system with addition of at leastCO2 (eg Laurora et al 2001 Ban et al 2005) or anotherpre-existing phase acted as a source of carbon Oxidationof graphite texturally identified as a part of the earlyassemblage (eg Fig 3d) could account for some of theCO2 necessary for carbonate formation A change in fO2

conditions is also indicated when the composition of glass-related spinel (Chrm) and Olm are used for oxygen baro-metry which yields oxygen fugacities well on the oxidizedside of the carbon^carbonate^silicate buffer (Fig 16)However increasing fO2 triggered by closed-systemdecompression-induced melting alone does not appear fea-sible Also textures such as straight contacts between gra-phite and Mg-calcite (Fig 4c) suggest that carbonategrows on graphite (ie indicating local fO2 conditions cor-responding to carbon^carbonate equilibrium) rather thanreplacing it We therefore interpret melt pocket formationto be triggered by the infiltration of a CO2-rich fluid(ie flux melting see also Ionov et al 1994) This interpre-tation also accounts for the presence of abundant vugs inthe glass that could have contained a fluid (Ionov et al1994 2006) which was presumably lost during sample pre-paration However we emphasize here that late fluid infil-tration had only subtle effects on the trace elementcomposition of Type III-b and -c xenoliths

Formation of Na-rich Cr diopside

Infiltration of CO2-rich fluidwould also explain the Na-richcomposition of Dim micro-phenocryst cores (Fig 6a and b)Ionov et al (2005) noted recently that clinopyroxenes inmeltpockets are commonly characterized by Na2O contents51wt mainly as a result of low cpxmelt partition coeffi-cients at low pressures (eg Blundy et al 1995) Thus veryhighNa2Ocontents in Dimmicro-phenocrysts (Fig6a) sug-gest replacement of amphibole by Dim and eventually Olmand Chrm during reaction with a fluid at subsolidus condi-tions An origin of micro-phenocrysts by replacementrather than crystallization from a melt is indicated by theidentical crystallographic orientation of individual Dimcrystals in singlemelt pockets (Fig4a)Once the solidus of the precursor phase(s) was reached

(ie 10508C for amphibole Niida amp Green 1999) silicatemelt was generated in situ and led to the formation of clin-opyroxene rims with lower Na contents (Fig 6b) and thedevelopment of euhedral shapes in contact with melt (egFig 4a and e) Eventual immiscibility between remainingfluid and newly formed melt (eg during xenolith ascent)would then explain the presence of carbonate globules and

vugs enclosed in glass Likewise different origins of meltand carbonate were recently proposed for similar meltpockets from Hungarian xenoliths by Demecurren ny et al(2004) who found oxygen isotope disequilibrium betweencarbonate globules and the host glass Such a two-stepmodel (fluid-induced replacement followed by melt gen-eration) can be explained by CO2-fluid infiltration occur-ring within the mantle at sub-solidus conditions ultimatelyfollowed by xenolith entrainment in the host magma andtransport to the surfaceThe host magma possibly suppliedthe additional heat necessary for melt generationFast decompression would further enhance melting andcould also account for the lower Na contents in Dimmicro-phenocryst rims as a result of decreasing cpxmeltpartition coefficients at decreasing pressures (Blundyet al 1995)Rapid replacement of amphibole ( phlogopite) by an

anhydrous phase assemblage indicates that the fluid waswater-poor (eg low H2OCO2 ratio see Ionov et al2006)This is corroborated by the fact that Mg-katophorite(presumably OH-rich see above) is occasionally rimmedby a residual kaersutitic selvage (Fig 4d) Such Ti-richmantle amphiboles are commonly OH-poor as a result ofpartial dehydrogenation (eg Tiepolo et al 2001 and refer-ences therein)Finally the presence of theType III-d xenolith contain-

ing texturally equilibrated clinopyroxene of almost identi-cal composition to Dim micro-phenocrysts in melt pockets(Table 2 Figs 6a and 7b) further strengthens the argu-ment that CO2-fluid percolation started in the mantlebefore entrainment (ie enough time elapsed to allow thisrock to anneal completely after the late metasomaticassemblage had formed) A similar conclusion can bedrawn for the clearly host-related glass found in xenolithKe 195927 (eg Fig 13b) ThisType III-c sample is charac-terized by the presence of Dim micro-phenocrysts partlyovergrown by mineral rims clearly related to the host basa-nite magma (kaersutite Ti-augite and olivine withFo885 Fig 4f Tables 3 and 4 and supplementary dataat httppetrologyoxfordjournalsorg) Kaersutite hasmajor and trace element characteristics similar to typicalbasanitic and alkali basaltic amphibole phenocrysts(Fig 9d) The special phase assemblages in this xenolithindicates that host-related basanitic melts intruded someType III-c xenoliths and partly overprinted the assemblageformed slightly earlier by in situ derived alkali-rich peralu-minous melts fully preserved in other samples (eg Ke19653 or Ke 785)

ORIGIN OF METASOMATIZ INGAGENTS T IM ING AND TECTONICSETT INGAs noted above late metasomatism was probably short-lived started at mantle depths and probably continued

30

compared withTh andTa respectively Hf Zr andTi showvery low concentrations Overall major and trace elementcharacteristics of the carbonates are similar to those of car-bonates found in other peridotite xenoliths (eg Ionov1998 Lee et al 2000 Laurora et al 2001 Ionov ampHarmer 2002 see Fig 14)

FORMATION OF THE EARLYASSEMBLAGESDespite large compositional variations numerous commontextural and compositional features indicate that the earlymetasomatic assemblages in all Group III samples formedduring a single metasomatic event In the discussion belowwe argue that during a first metasomatic stage the pri-mary (pre-metasomatic) phase assemblage in Group IIIspinel harzburgites was partially replaced by metasomaticminerals finally resulting in clinopyroxene-free amphi-bole- and phlogopite-bearing dunite and harzburgite(Type III-c) The initial metasomatic agent was probably

an evolved CO2^H2O-rich melt and the transitionfrom peridotites of Type III-a to -c can be explained byprogressive melt^rock reaction

Transition from cryptic (Type III-a) tomodal (Type III-b and -c) metasomatismFormation of new phases requires (1) crystallization from amelt or fluid phase (decreasing melt or fluid mass) andor(2) melt^solid or fluid^solid reactionTextural features suchas spinel being replaced by amphibole and phlogopite(eg Fig 3c) indicate melt(fluid)^rock reaction Thegeochemical signatures observed in the different peridotitetypes thus probably reflect pre-existing compositionalheterogeneities coupled with metasomatic modificationThis is illustrated best in sample Ke 19651 (Type III-a)the least metasomatized xenolith in this study It can besubdivided into three parts containing clinopyroxenewith different REE signatures (Zones A B and C inFig 15) The LREE are increasingly enriched from ZoneA to C indicating increasing cryptic metasomatism

Table 4 Continued

Type III-b III-c

Mineral katoph katoph katoph katoph katoph katoph katoph katoph kaersut kaersut

Sample 195915 196515 196515 196515 785 785 196525 196525 195927 195927

Texture residual

in mp

interst interst lamellae

in cpx-I

residual

in mp

residual

in mp

euhedral core euhedral rim micro-xx micro-xx

Ba 781 614 540 na 259 309 846 841 331 322

La 357 116 110 na 704 824 177 113 107 138

Ce 627 225 231 na 217 248 621 426 338 448

Pr 574 208 224 na 301 349 856 623 557 724

Nd 177 720 779 na 128 151 363 296 318 398

Sm 208 143 143 na 285 316 752 740 945 111

Eu 080 058 060 na 108 115 232 252 363 384

Gd 182 085 088 na 225 241 701 692 940 980

Tb 011 013 na 039 050 120 117 na na

Dy 171 057 067 na 202 252 735 698 700 819

Ho 036 010 010 na 039 050 155 144 137 147

Er 106 024 027 na 097 114 405 382 296 336

Tm 016 0030 0033 na 012 017 060 058 027 039

Yb 120 028 030 na 077 099 400 386 243 272

Lu 019 0036 0038 na 0079 0077 058 044 033 031

Hf 015 0067 0054 na 0065 0048 028 078 331 376

Ta 0033 014 014 na 0002 0004 010 107 167 198

Pb 201 360 374 na 142 173 195 222 022 039

Th 682 150 134 na 181 196 059 028 010 020

U 212 039 039 na 040 048 019 012 0018 0048

katoph Mg-katophorite kaersut kaersutite interst interstitial mp melt pocket nd not detected na notanalysed

14

HREE abundances on the other hand are very hetero-geneous in the least metasomatized Zone A moderatelydepleted in Zone B and homogeneously low in the mostmetasomatizedZoneC (containinggraphite Fig15) similarto REE patterns in clinopyroxene from the amphibole- andphlogopite-bearingType III-b sample (Ke196515 Fig7a)The decoupling of strongly (LREETh U) from moder-

ately incompatible elements (HREE) can be explained bychromatographic fractionation during percolation ofsmall amounts of melts or fluids through an initially non-metasomatized peridotite (Navon amp Stolper 1987 Godardet al 1995 Bedini et al 1997 Vernie res et al 1997 Ionovet al 2002) Upon reaction with the wall-rock a fluidor melt will become progressively enriched in incom-patible elements as a function of increasing distancefrom its source This is due to the fact that elements withhigher Dmineralmelt values (eg HREE) are selectivelyremoved from a percolating liquid when it exchanges withthe surrounding minerals The results are faster-movingchemical fronts of incompatible elements compared withthose of more compatible elements This is consistent with

the geochemical signature of cpx-I from the least metaso-matized part of sample Ke 19651 (Zone A Fig 15) whichalready started to re-equilibrate with respect to the incom-patible elements (ie addition of Th U and LREE)whereas more compatible elements still reflect pre-metasomatic heterogeneities In this case these areextreme HREE variations probably reflecting the earliergarnet breakdown (Kaeser et al 2006) On the otherhand more intensely metasomatized parts (eg Zone Cin sample Ke 19651 and sample Ke 196515) are character-ized by clinopyroxene trace element patterns wheremildly incompatible elements have also been modified(eg homogeneously depleted HREE Figs 7a and 15)Similar fractionation processes can explain the modal

and compositional differences between Type III-b and -cCompared withType III-b higher abundances of metaso-matic phases inType III-c suggests that higher amounts offluid or melt crystallized in or passed through these rocksThe amphibole-rich dunite (Ke 196525) for examplecontains low-Ti Mg-katophorite exhibiting the lowest(LaYb)N and (LaCe)N ratios and high (NbLa)N and

Fig 8 Major element characteristics of amphiboles from porphyroclastic spinel harzburgites and dunites from Marsabit compared with com-positions of mantle-derived amphiboles from other localities (XB basalt-hosted xenoliths XS xenoliths from subduction settings O orogenicperidotites) Mg-numberfrac14Mg(MgthornFetot) in molar proportions

15

(NbTh)N ratios (Fig 10a and c) indicating a moderatelyfractionated metasomatic agent Mg-katophorite fromthe phlogopite-rich harzburgite (Ke 785 Type III-c)and amphiboles from the more cpx-rich harzburgites(Type III-b) show progressively more fractionated patterns(eg Fig 10c) and decreasing abundances in lsquoamphibole-compatiblersquoelements (eg Nb Fig 9a) Thus the transitionfrom Type III-b to -c could reflect decreasing distancebetween the site at which the xenolith was sampledand the source of the lsquoinitialrsquo metasomatizing agent (egZanetti et al 1996 Ionov et al 2002 Bodinier et al 2004)

Judging from the abundance of modal metasomatic phasesthe amphibole dunite (Ke 196525) was probably closest tothe source of the metasomatizing agent The phlogopite-rich nature of Ke 785 which indicates higher K andLILE (Cs Rb Ba) contents in the remaining agent is inagreement with a more fractionated melt or fluid at some-what greater distance from the metasomatic sourceThe Nb abundances in amphibole also agree with this

model as well For example amphibole from Type III-cKe 196525 has comparatively high Nb concentrations(Fig 9c) in line with relatively high DNb

amphmelt values

Fig 9 Trace element composition of amphibole from porphyroclastic spinel harzburgites and dunites from Marsabit [normalized to the primi-tive mantle values of McDonough amp Sun (1995)] Mg-katophorite in Type III-b and -c xenoliths (a^c) and glass-related kaersutite (d) Theshaded fields indicate the compositional variation of all Mg-katophorite analyses (light grey) and the compositional range of LREE-depletedTi-pargasite from the Group I (grt)^spinel lherzolites (dark grey Kaeser et al 2006)

16

constrained by different studies (eg Ionov amp Hofmann1995 LaTourrette et al 1995) In Ti-poor systems such asthose of this study Nb uptake by amphibole is evenenhanced as a result of increased DNb values (Tiepoloet al 2001) Consequently in an event during which amphi-bole crystallizes continuously the remaining fluid or meltwill become progressively depleted in Nb (Ionov et al2002) resulting in formation of Nb-depleted amphibolesfurther away from the source (Zanetti et al 1996 Ionovet al 2002) such Nb-depleted amphibole is found insample Ke 785 as well as inType III-b xenoliths (eg Ke195915 Fig 9a) Amphibole signatures from xenolith suitesmodelled successfully in the context of percolative traceelement fractionation (eg Spitsbergen Ionov et al 2002)show similar ranges (eg in NbTh ratios Fig10a) to thosedocumented for our samples This further supports the

hypothesis that similar percolation mechanisms acted inthe mantle beneath Marsabit

The role of pre-existing heterogeneitiesand exotic mineralsAlthough a model based on trace element fractionationduring melt percolation is supported by textures and canaccount for many compositional features some composi-tional peculiarities remain unexplained First clinopyrox-ene and amphibole in several samples (Ke 785 19651 and196515) are characterized by extremely low HREE abun-dances more than one order of magnitude lower thanin amphiboles from the strongly metasomatizedamphibole dunite Ke 196525 (Fig 9a^c) The mainphases that crystallize during metasomatism are low-TiMg-katophorite and phlogopite However DHREE

amphmelt

Fig 10 Selected trace element ratios in amphiboles from the porphyroclastic spinel harzburgites from Marsabit compared with those ofmantle-derived amphibole from other tectonic settings (XK and XB cratonic kimberlite-hosted and off-cratonic basalt-hosted xenolithsrespectively O orogenic peridotites XS xenoliths from subduction settings) Concentrations (ratios) normalized to the primitive mantlevalues of McDonough amp Sun (1995)

17

Table 5 Representative analyses of phlogopite and apatite (oxides in wt trace elements in mgg)

Type III-b III-c III-b III-c

Mineral phlog phlog phlog phlog phlog phlog phlog apatite apatite

Sample 196515 196515 19653 785 785 785 196525 195915 19653

Texture veinlet interst in opx in opx with amph

in mp

around

chromite

incl in

amph

bleb in

glass

interst

grain

SiO2 3925 3914 4171 4190 4124 4134 4230 SiO2 016 049

TiO2 313 307 009 053 039 037 027 Al2O3 001 nd

Al2O3 1544 1583 1360 1360 1384 1377 1477 P2O5 3768 3715

Cr2O3 112 105 132 147 190 197 063 FeO(tot) 037 029

FeO(tot) 350 347 271 000 289 277 340 MnO 006 nd

MnO nd 002 008 305 004 002 005 MgO 084 091

NiO 023 027 019 023 024 028 023 CaO 4750 4237

MgO 2287 2280 2577 2591 2582 2528 2525 SrO 095 397

CaO nd 002 001 nd 004 001 002 Na2O 079 133

Na2O 215 219 282 258 264 262 208 La2O3 003 007

K2O 719 707 704 706 674 684 683 Ce2O3 088 347

Nd2O3 003 007

F 139 224

Cl 287 092

Total 9488 9493 9571 9641 9587 9535 9590 Totaly 9305 9245

Mg-no 9282 9287 9443 9380 9408 9420 9298

P 369 187 480 339 206 188 362 P 129717 139419

K 71711 72575 60747 79052 70997 69992 70189 K 110 897

Sc 099 110 131 130 088 091 160 Sc 242 079

Ti 15500 16408 890 2839 2096 1807 1736 Ti 235 5294

V 255 284 888 142 108 130 788 V 365 382

Ni 1808 1727 1744 2046 1813 1773 1905 Ni 121 397

Zn 234 221 358 382 317 321 695 Zn 119 096

Ga 111 984 na 210 190 188 235 Ga na na

Rb 185 147 289 183 133 133 342 Rb 015 049

Sr 102 116 775 325 363 350 101 Sr 7138 42233

Y 010 0086 50057 50054 0021 0006 0052 Y 988 1454

Zr 019 019 043 50088 0077 0070 073 Zr 307 101

Nb 506 628 479 177 102 146 210 Nb 024 020

Cs 175 138 261 399 216 180 798 Cs 50013 50036

Ba 1212 1096 1343 298 277 219 1124 Ba 139 126

La 010 0055 0069 50029 0020 50003 50045 La 7460 19048

Ce 0027 0011 50075 50019 014 50003 0078 Ce 6844 25945

Pr 0012 0008 50067 50024 0012 50003 50019 Pr 380 2212

Nd 0019 0035 50405 50079 0065 50009 50224 Nd 749 6530

Sm 0015 50011 50474 50090 0016 50010 50132 Sm 515 747

Eu 0022 0017 50104 50026 0013 0010 50095 Eu 147 189

Gd 50010 50011 5043 5015 50011 0012 017 Gd 314 451

Tb 50002 50002 na 50027 0002 50002 50028 Tb na na

Dy 50015 50017 045 50077 50007 50006 50077 Dy 198 281

Ho 50002 50003 50043 50020 0002 50002 50028 Ho 359 511

Er 50009 50013 5025 5011 50016 50009 5011 Er 903 119

Tm 50002 50002 50088 50013 0002 0003 50018 Tm 119 138

(continued)

18

data (eg LaTourrette et al 1995) indicate that amphibolecrystallization alone is unlikely to deplete the metasomaticagent to such a high degree in HREE The same probablyaccounts for the extreme Nb depletion in amphiboles fromKe 785 relative to the otherType III-c xenolith (Ke 196525 see also Ionov et al 2002)Further clinopyroxene phlogopite and amphibole in the

Type III-b sample Ke 196515 on one hand all showstrongly fractionated REE patterns (eg high LaCe inamphibole Fig 10c) but on the other hand are character-ized by a positiveTi anomaly decoupled from other HFSE(eg Hf and Zr Fig 9b) Trace element fractionationduring amphibole crystallization (ie formation of TypeIII-c amphibole dunites) however should lower Ti con-tents in the remaining fluid or melt (eg Moine et al 2001)Based on textures mineralogy and dominant common

compositional features the hypothesis that Type III-band -c xenoliths were metasomatized by completely inde-pendent metasomatic agents can be ruled out Thereforetwo other possibilities remain to account for the inconsis-tent observations described above (1) inference of otherphases reactants or products that are able to fractionatespecific trace elements present in the mantle beneathMarsabit but not present in the few investigated samples(2) pre-metasomatic compositional heterogeneities similarto the inherited HREE variation inType III-a clinopyrox-ene (see above)Garnet would be an important sink for HREE but

metasomatism occurred in the spinel field after decom-pression with maximum pressures of 515GPa (Kaeseret al 2006) so garnet had already broken down to sym-plectites (Fig 15) Percolation of melt through garnet-bearing websterites prior to metasomatism in Ke 19651and 196515 could be an alternativeWebsterites have been

interpreted to occur in close spatial relationship with por-phyroclastic peridotites in the shallow mantle beneathMarsabit (Kaeser et al 2006) and they occasionally con-tain rutile and ilmenite which could further account forthe strong Nb and Ta depletion in some samples (eg Ke785) The available compositional data from the webster-ites however do not indicate metasomatic modification ofwebsteritic garnet (Olker 2001 B Kaeser unpublisheddata) nor compositional similarities to Group IIIperidotitesCareful study by SEM rules out the presence of exotic

phases such as zircon or Ti-oxides described fromother xenolith suites (eg Bodinier et al 1996 Rudnicket al 1999) Nevertheless it cannot be excluded that themetasomatizing agent percolated through mantle domainscontaining these phases before infiltrating the studiedrocksThe preservation of pre-metasomatic compositional het-

erogeneities is another possibility that must be consideredThat such lsquorock signalsrsquo partly overlap with the lsquometaso-matic signalrsquo is indicated by the HREE variation in TypeIII-a clinopyroxene (see above) and also by other composi-tional parameters For example both Mg-katophorite andphlogopite growing at the expense of spinel have very highCr2O3 contents whereas euhedral Mg-katophorite inthe amphibole dunite (Ke 196525) or in phlogopiteinclusions in Opxm is comparably Cr2O3-poor (Fig 8bTables 4 and 5) Pre-existing heterogeneities would essen-tially be preserved through elements that are relativelycompatible in a given mineral (ie that are not mobilizedduring metasomatism) This would be in agreement withthe preserved HREE variations in Type III-a clinopyrox-ene high Cr2O3 in amphiboles and phlogopite and Tispikes in amphibole from Ke 196515

Table 5 Continued

Type III-b III-c III-b III-c

Mineral phlog phlog phlog phlog phlog phlog phlog apatite apatite

Sample 196515 196515 19653 785 785 785 196525 195915 19653

Texture veinlet interst in opx in opx with amph

in mp

around

chromite

incl in

amph

bleb in

glass

interst

grain

Yb 50014 50011 5047 5015 50011 50019 5017 Yb 803 724

Lu 0002 50002 50061 50014 50002 50002 50020 Lu 115 742

Hf 0019 0018 5025 50053 50012 50006 5019 Hf 50023 50120

Ta 020 022 140 0031 0012 0009 023 Ta 50015 50075

Pb 546 586 125 108 083 098 143 Pb 436 577

Th 50004 50004 50080 0043 0088 0056 50021 Th 2538 1305

U 0012 0011 0069 50018 0038 0030 0031 U 572 278

interst interstitial mp melt pocket na not analysed nd not detected

19

Further very high Mg-numbers in replacive amphibolessupport melt (fluid)^rock interaction at low melt (fluid)rock ratios where Mg and Fe contents are buffered by theambient peridotite (Kelemen et al 1995 Tiepolo et al2001) The slightly lower Mg-number of euhedralamphiboles in Ke 196525 combined with their lowerCr2O3 contents (Fig 8b) suggests a higher melt (fluid)rock ratio which is further supported by considerablylower Mg-number in associated ol-I (Fo88 Table 3)

The limited number of investigated samples does notallow us to decide which of the processes described aboveacted and to what extent However given the heterogene-ity observed in the numerous non-metasomatized xenolithsfrom Marsabit (Kaeser et al 2006) pre-metasomaticfeatures certainly affect the final product of the melt^rockreaction process to a considerable degree

The nature of the metasomatizing agent(s)Graphite stability and oxygen fugacity considerations

The Marsabit xenoliths are unusual in that they occur inan off-cratonic setting yet bear macroscopic graphite Thelatter usually either occurs in kimberlite-hosted peridotites(on-craton) or in pyroxenites from alpine-type peridotitebodies (eg Pearson et al 1994) In the phlogopite perido-tites from Finero graphite has recently been reportedas nanoscopic layers in phlogopite (Ferraris et al 2004)In xenoliths from Marsabit graphite occurs in the sametextural relationship as in kimberlite-hosted peridotitexenoliths (ie millimetre-sized flakes typically associatedwith orthopyroxene see Fig 3c^e and Pearson et al 1994)Graphite inclusions in phlogopite and Opxm couldindicate crystallization during metasomatism by alsquoreducedrsquo C-bearing fluid or melt However precipitationof graphite more probably reflects the relatively reducingconditions in the uppermost mantle prior to metasoma-tism We used the oxygen barometer of Ballhaus et al(1991) based on ferric iron contents in spinel coexistingwith olivine to constrain oxygen fugacity (fO2)conditions prior to metasomatism in the Marsabitxenoliths (Fig 16) The results show that non-metasomatized Group I and IV samples reflect similarlyreducing conditions to the cryptically metasomatizedType III-a samplesAlthough assessing Fe3thornFe2thorn ratios in spinel based

on microprobe analyses can be problematic (Wood ampVirgo 1989 Ballhaus et al 1991) all microprobeanalyses were obtained using the same operationalsettings and therefore give at least the relative fO2

variation between samples The relatively reducingconditions in the pre-metasomatic cold andhighly deformed mantle well below carbon^carbonate^silicate equilibria (Mg2Si2O6thorn 05 CaMg(CO3)2frac14Mg2SiO4thorn 05 CaMgSi2O6thornCthornO2 Eggler amp Baker1982 see Fig 16) and close to the CCO oxygen buffer arein agreement with the presence of graphite in the GroupIII harzburgites Moreover the P^T conditions at whichthe Group III xenoliths partially re-equilibrated are closeto the subsolidus decarbonation of model carbonated lher-zolite (Fig 2 see Lee amp Wyllie 2000 and referencestherein) in agreement with the observed phase assemblageincluding orthopyroxene olivine clinopyroxene andgraphite

Fig 11 Trace element patterns of (a) phlogopite and (b) apatitefrom porphyroclastic spinel harzburgites and dunites from Marsabit[concentrations normalized to the primitive mantle valuesof McDonough amp Sun (1995)] The grey field in (a) indicates thecomposition of Ti-rich phlogopite in Group I (grt)^spl lherzolites(Kaeser et al 2006)

20

Table 6 Representative analyses of silicate glass (oxides in wt trace elements in mgg)

Type III-b III-c Host

Sample 195915 196515 196515 785 196525 19653 19653 19653 195927

Remark next amph next cpx xenol margin next amph next amph melt pocket next apatite next Opxm melt pocket host basanite

SiO2 5306 5775 5161 6096 5657 5938 5974 6530 4882 4410

TiO2 017 157 106 062 030 024 020 023 378 255

Al2O3 2412 2053 2025 1779 2313 1575 1245 1355 1890 1396

Cr2O3 005 001 nd 001 001 nd nd nd 002 na

FeO(tot) 188 247 626 126 252 295 374 265 509 1283

MnO 007 000 026 001 004 005 008 007 018 021

NiO nd nd nd na nd na 006 nd 001 na

MgO 182 129 116 114 122 304 382 252 244 938

CaO 506 257 283 082 044 124 101 039 810 1030

Na2O 1107 966 969 971 1299 1388 1384 1211 673 374

K2O 154 197 465 357 138 121 142 127 300 146

Cl 031 106 038 012 014 023 061 020 015 na

P2O5 041 008 044 009 012 049 233 018 100 059

Total 9956 9896 9860 9609 9887 9845 9929 9846 9821 9912

Mg-no 6329 4816 2489 6175 4632 6474 6458 6289 4552 5658

P 1061 448 1610 230 503 1519 6738 676 2994 na

K 12563 16408 39599 32515 15872 9265 11277 10071 25900 na

Sc 179 184 126 406 151 970 136 848 691 237

Ti 665 7329 4974 2953 1965 1213 1163 1055 18199 15311

V 107 265 235 796 920 210 151 134 234 na

Ni 128 371 92 5575 983 170 5199 5298 199 191

Zn 962 103 109 110 459 440 508 309 819 na

Ga na 173 273 194 220 na na na na na

Rb 181 263 124 546 626 176 398 369 577 369

Sr 427 488 1407 188 881 1644 4997 985 1062 792

Y 543 145 224 752 262 619 158 369 301 299

Zr 466 171 353 761 660 578 365 428 242 216

Nb 416 161 137 260 136 234 164 169 837 647

Cs 014 046 186 125 007 022 102 052 063 na

Ba 238 261 1064 129 168 405 1099 641 625 563

La 698 183 848 144 889 2212 1008 106 537 462

Ce 947 256 131 308 138 333 1265 164 104 823

Pr 655 194 120 330 116 304 975 160 120 na

Nd 146 603 408 101 329 993 256 564 463 400

Sm 142 048 570 179 419 141 379 679 105 851

Eu 041 035 228 082 112 438 103 241 345 267

Gd 062 051 415 158 402 106 245 566 677 729

Tb na 0086 086 020 071 na na na na 111

Dy 090 027 359 098 434 104 171 637 732 na

Ho 023 0037 105 025 095 206 446 104 117 114

Er 045 038 195 058 255 611 153 264 305 na

Tm 009 50039 033 0081 042 085 292 5014 036 052

Yb 062 023 267 044 307 667 120 487 253 220

(continued)

21

Evidence for low-temperature SiO2^Na2O^CO2^H2O-rich melts or fluids

Potential metasomatic agents inferred from the presenceof graphite are C^O^H-rich fluids carbonate melts orCO2-bearing silicate melts The resulting phase assem-blages and geochemical signatures are the result of infiltra-tion of fluid or melt into a cold (750^8508C) shallow(515GPa Fig 2) heterogeneous and locally reduced litho-spheric mantle Metasomatism under such conditions in anopen system must result in complex geochemical signa-tures that reflect the degree of fluid^rock reaction Thismakes it particularly difficult to constrain the exactnature of the lsquooriginalrsquometasomatic agentType III-c xenolith Ke 196525 contains the highest frac-

tion of metasomatic phases and the least fractionatedamphibole trace element patterns The lsquomagmaticrsquo texturesof the Mg-katophorites in this sample suggest direct crys-tallization from a melt or a fluid rather than replacementof a pre-existing phase Therefore hypothetical liquids inequilibrium with the amphiboles in this xenolith couldyield the composition of the initial metasomatic agentsFigure 17 illustrates the trace element patterns of suchmelts and fluids calculated using sets of experimentally orempirically determined mineralmelt (fluid) partitioncoefficients including carbonate melt silicate melt andhydrous fluid (for references see caption to Fig 17)Common features for all calculated metasomatic agents

are negative Zr^Hf^Ti anomalies and LREE enrichment(Fig 17) However previous studies have shown that thesesignatures are indicative of almost all types of metasoma-tism including interaction of peridotite with carbonatiticmelt (Green amp Wallace 1988 Dautria et al 1992 Hauriet al 1993 Rudnick et al 1993 Ionov et al 1996 Yaxleyet al 1998 Coltorti et al 1999) with hydrous fluids andormelts for example originating from dehydration of altered

subducting lithosphere (Maury et al 1992 Zanetti et al1999 Grecurren goire et al 2001 Laurora et al 2001 Parkinsonet al 2003) and interaction of peridotite with volatile-and Si-rich melts that evolved through reactive porousflow (Bedini et al 1997 Ionov et al 2002 2006 Bodinieret al 2004 Rivalenti et al 2004)In the case of the Marsabit xenoliths integration of the

trace element characteristics thermobarometric inferencesand mineralogical constraints favours a Na2O^SiO2^CO2^H2O-rich silicate melt Carbonated melts can prob-ably be ruled out on major element and mineralogical con-siderations First amphiboles are Ca-poor in conflict withcarbonate melt metasomatism which is generally charac-terized by Ca enrichment (Rudnick et al 1993 Neumannet al 2002) Further orthopyroxene is a stable phase andprobably even crystallizes during metasomatism in somesamples (eg Ke 785) At near-solidus conditions coexis-tence of orthopyroxene and carbonate melt is possibleonly at pressures 20GPa (see Fig 2 eg Lee amp Wyllie2000 and references therein) which is considerably higherthan the maximum pressures indicated by the compositionof spinel (515GPa) At such low pressures metasomatismby carbonated melts will convert lherzolite and harzbur-gite immediately into wehrlite which is the major perido-tite type in all xenolith suites where carbonate meltmetasomatism is convincingly demonstrated (Rudnicket al 1993 Yaxley et al 1998 Neumann et al 2002)In Marsabit wehrlites are absent howeverAn Na2O^SiO2^CO2^H2O-rich melt or fluid on the

other hand would explain the Na-rich nature of amphiboleand phlogopite as well as the presence of orthopyroxeneGrowth of metasomatic (and Al-poor) orthopyroxene hasbeen described from several xenolith localities where peri-dotites were infiltrated either by hydrous fluids orSi-rich melts (Smith et al 1999 McInnes et al 2001Morishita et al 2003) Na-rich phlogopite is commonly

Table 6 Continued

Type III-b III-c Host

Sample 195915 196515 196515 785 196525 19653 19653 19653 195927

Remark next amph next cpx xenol margin next amph next amph melt pocket next apatite next Opxm melt pocket host basanite

Lu 014 0050 026 0041 046 080 140 058 040 031

Hf 026 052 535 014 076 110 5087 124 568 504

Ta 50080 033 638 0022 123 101 784 801 394 403

Pb 541 448 280 255 840 797 260 749 493 na

Th 194 296 160 466 703 185 247 105 534 506

U 545 069 478 099 152 447 724 216 171 126

nd not detected na not analysedAverage composition of the strongly alkaline basanites from Marsabit (Volker 1990)

22

Fig 12 Total alkalis vs SiO2 (a) Harker-type co-variation diagrams (b^e) and incompatible trace element co-variation diagram (f) showingthe compositional variability of silicate glass found in the porphyroclastic spinel harzburgites and dunites from Marsabit In (b) binary mixinglines between the typical composition of the alkali-rich glass (large open diamond) and the associated microlites are shown (each increment is10wt)They illustrate that the glass does not represent the derivate of host lava infiltration and fractional crystallization Subalkaline glass inGroup I (grt)^spl lherzolites from Marsabit is shown for comparison and is interpreted to derive from incongruent melting of Ti-pargasite(Kaeser 2006)

23

found as a reaction product between anhydrous mafic toultramafic rocks and H2O-rich evolved liquids (Costaet al 2001 and references therein) or H2O-bearing fluids(eg Odling 1994) The latter study was an experiment at20GPa simulating the effects of decreasing temperatureson the phase assemblages and their composition duringmetasomatism using a model peridotitethornC^O^H fluidThe coolest part of Odlingrsquos (1994) experiment (510758C)contained a strikingly similar phase assemblage (kato-phorite phlogopite with 2wt Na2O and Al-poororthopyroxene) to the Marsabit samples interpreted tohave formed during sub-solidus fluid^solid interactionA compositionally similar phase assemblage (highamounts of opx Si-rich hornblende Na-rich phlogopite)resulted from experiments simulating peridotite interac-tion with Si-rich melt (dacite) at lower P^T conditions(9508C at 092GPa Prouteau amp Scaillet 2003)In this study the trace element signatures of Group III-c

Mg-katophorite and the calculated metasomatic agentsfurther suggest that the metasomatizing agent was a meltrather than a fluid Hypothetical fluids in equilibrium witheuhedral Mg-katophorite from sample Ke 196525 havehigh NbLILE (eg Rb Ba) and low LILEREE ratios(Fig 17) Because experimental data predict the oppositefor fluids (LILE enriched over HFSE and REE egManning 2004 and references therein) a fluid with asignature such as that indicated in Fig 17 is unrealistic

Fig 13 Trace element patterns of silicate glass in porphyroclasticspinel harzburgites and dunites from Marsabit compared withthe trace element composition of associated clinopyroxene (Dim)micro-phenocrysts xenolith-hosting basanites from Marsabit andglass interpreted to represent xenolith^host interaction [concentra-tions normalized to the primitive mantle values of McDonough ampSun (1995)]

Table 7 Representative analyses of carbonates (oxides in

wt trace elements in mgg)

Group III-b III-b III-c III-c III-c

Mineral Dol Mg-Cc Mg-Cc Mg-Cc Mg-Cc

Sample 195915 196515 785 19653 19653

Texture in glass in glass in glass in glass in glass

SiO2 na 003 na nd 003

FeO(tot) 016 011 010 010 006

MnO 001 nd nd nd nd

MgO 1922 236 190 125 517

CaO 3189 5313 5377 5391 4853

Na2O 003 nd 004 nd 003

K2O na 002 na 001 nd

SrO 003 010 008 004 021

BaO 003 nd nd na na

CO21 4618 4444 4442 4377 4391

Total 9756 10021 10032 9909 9794

Mg-no 9954 9745 9713 9554 9931

Ca-no 5439 9418 9531 9687 8709

P 220 334 137 580 589

K 970 212 642 105 155

Sc 5065 50150 027 5027 50076

Ti 975 189 242 528 196

V 5085 0481 5042 5037 021

Ni 900 673 517 5294 593

Zn 459 5089 113 296 045

Ga na 5014 5013 na na

Rb 0084 50072 50074 50060 0046

Sr 593 860 371 251 4274

Y 012 011 130 704 015

Zr 261 024 012 097 756

Nb 144 019 0056 692 185

Cs 50063 50011 50018 50039 50008

Ba 126 230 5024 029 271

La 034 057 084 136 025

Ce 032 013 134 997 023

Pr 006 006 018 095 0026

Nd 030 013 068 228 0078

Sm 5044 50091 0074 048 50018

Eu 5012 50019 0051 020 50008

Gd 5037 50090 014 051 0026

Tb na 50010 0036 na na

Dy 5019 50039 0089 058 50038

Ho 50029 0013 0044 011 50006

Er 5012 50041 0083 033 50019

Tm 50052 50013 0016 0084 50006

Yb 5043 50063 014 027 50040

Lu 50072 50010 0011 0066 50008

(continued)

24

At the relatively low maximum pressures (515GPa) asupercritical liquid is not expected either (eg Kesselet al 2005)Finally a relatively hydrous metasomatizing agent

would be consistent with formation of Mg-katophoriteswith lowTi and presumably low Fe3thorn contents (given therelatively reducing conditions) which in turn imply high(OH)^ contents In other words partial dehydrogenationtypically observed in common (Ti-rich) mantle amphibole

(eg Popp et al 1995 Tiepolo et al 2001 and referencestherein) cannot have played an important role in thiscase as H deficiency is mainly the result of coupled substi-tutions involvingTi and Fe3thorn (Popp et al 1995) In conclu-sion the metasomatizing agent was most probably aNa2O^SiO2^CO2^H2O-rich meltComparison with experimental studies further con-

strains the low-temperature regime in which metasoma-tism occurred In ultramafic (eg Niida amp Green 1999)basaltic (eg Rapp 1995) and Si-enriched systems (egtrachybasaltic Barclay amp Carmichael 2004) amphibole(in the latter study together with phlogopite) is confinedto temperatures lower than 1000^10508C at pressures520GPa (Fig 2)Thus crystallization of the early metaso-matic phase assemblage can be explained by infiltration ofa silicate melt that rapidly cooled upon contact with theambient peridotite (750^8508C prior to metasomatismFig 2) However this low-T stage is based on Ca-in-opxthermometry (see Brey amp Kolaquo hler 1990 Kaeser et al2006 Table 1) and therefore probably underestimates theeffective temperature conditions during metasomatismThis can be explained by low diffusion rates and proximityto the closure temperature for the diopside^enstatiteexchange reaction at temperatures lower than 8508C(eg Smith 1999 and references therein) The lower tem-perature limit at which silicate melt metasomatism ispossible (ie defined by the meltrsquos solidus and viscosity)depends strongly on volatile and SiO2 contentsExperiments by Draper amp Green (1997) for exampleshowed that highly silicic melts (52^64wt SiO2) canbe in equilibrium with mantle harzburgite at temperaturesas low as 8508C (at 10^20GPa under anhydrous andC^O^H-fluid-bearing conditions) These are howeverextreme compositions for mantle melts The actual SiO2

content of the metasomatizing agent in this case is difficultto constrain from the metasomatic mineral assemblage ascompositions may have varied considerably during reac-tive percolation (eg including Si-releasing reactions suchas cpx replacement by amphibole versus Si-consumingreactions such as Opxm crystallization) Moreover it isunlikely that a melt persists at temperatures muchbelow 9008C which is the solidus of fertile lherzolite(ie primitive upper mantle composition) underH2O-saturated conditions at 15GPa (Grove et al 2006see Fig 2) Below the solidus residual volatile-enrichedfluids may percolate further for example forming the lessmetasomatized Type III-c xenoliths (eg Ke 785) as wellas Type III-b and -a peridotites (see also Bailey 1987)Relatively cold conditions would also explain the veryhigh compositional gradients developed on small scales inType III-a samples (Fig 15) Given the absence of chloritein all investigated samples the lower temperature limitof such fluid metasomatism is 8008C (eg Fumagalli ampPoli 2005 see Fig 2)

Table 7 Continued

Group III-b III-b III-c III-c III-c

Mineral Dol Mg-Cc Mg-Cc Mg-Cc Mg-Cc

Sample 195915 196515 785 19653 19653

Texture in glass in glass in glass in glass in glass

Hf 5016 50038 50036 50074 50013

Ta 0048 50012 50016 50031 0008

Pb 5024 858 5021 074 018

Th 0021 50020 50017 50028 50005

U 060 094 086 600 148

Mg-Cc Mg-calcite Dol dolomite nd not detectedna not analysed Ca-numberfrac14 100Ca(CathornMg)1Calculated from stoichiometry

Fig 14 Trace element patterns of glass-hosted carbonates in TypeIII-b and -c spinel harzburgite and dunite xenoliths from Marsabit[concentrations normalized to the primitive mantle values ofMcDonough amp Sun (1995)] The grey field outlines the compositionalrange of carbonates found in other peridotite xenoliths (Ionov ampHarmer 2002 and references therein) Carbonate material inter-preted as quenched carbonatite melt found in peridotite xenolithsfrom Kerguelen (Moine et al 2004) is shown for comparison(bold line)

25

Finally the presence of apatite even though oftentaken as indicator for carbonatite-type metasomatism(eg Yaxley et al 1998) is not in conflict with metasoma-tism by silicate melts Apatite has also been shown to crys-tallize from volatile and alkali-rich derivatives formedduring reactive porous flow of originally mafic silicatemelts (Woodland et al 1996 Bedini et al 1997 Bodinieret al 2004 Ionov et al 2006)

FORMATION OF THE LATE GLASS -BEAR ING ASSEMBLAGESTextures strongly suggest that the glass-bearing assem-blages formed later and at the expense of the early metaso-matic assemblages (Fig 4a and d) Features such as thereappearance of clinopyroxene (Na-rich Cr diopsidemicro-phenocrysts) a phase replaced by the early

Fig 15 Detailed REE systematics in primary clinopyroxene (cpx-I) in a porphyroclastic cryptically metasomatized spinel harzburgite(sample Ke 19651) showing the transition fromType III-a (Zone A and B) toType III-b (Zone C) on a scale of a few centimetres The numberson the photomicrographs (crossed polars) correspond to single laser ablation microprobe pits (for further explanation see text)

26

metasomatic assemblage indicate that melt pocket forma-tion reflects a significant change in P^T^X space in partsof the mantle beneath Marsabit after earlier metasoma-tism Based on geochemical arguments and mass-balanceconsiderations we will show that (1) the late glass-bearingassemblages are closely related to early metasomaticassemblages mainly representing breakdown productsand (2) breakdown was induced by infiltration of aCO2-rich fluid in the mantle probably followed andenhanced by heating and decompression melting duringxenolith transport in the host magma

Evidence for in situ partial meltingBSE imaging reveals that the phases becoming unstableare amphibole orthopyroxene and to minor extentsspl-I phlogopite cpx-I and apatite Compositional(inter- and intra-xenolith) heterogeneity of silicate glass istexture controlled and varies as a function of theassociated precursor phases (Fig 12a^f)TiO2 for exampleshows higher concentrations in glass from xenolith Ke196515 containing Ti-enriched amphibole (Fig 12c)whereas K2O contents are highest in phlogopite-rich sam-ples (eg Ke 785 Fig 12d) Elevated P2O5 contents arefound in glass adjacent to apatite (Fig 12e) whichmirrors at the same time the highTh and LREE contents

of apatite (Fig 12f) Significant Si enrichment is found inglass coating orthopyroxene (Fig 12d) supporting incon-gruent melting of orthopyroxene (see below) Even thoughthe melt pockets are connected along grain boundaries thestrong compositional variation (on inter- and intra-xeno-lith scale) suggests that the mobility of the melt was lim-ited or that the melting event was very short-lived (fortiming constraints see below)The co-variation plots in Fig 12 further reveal consider-

able compositional differences between all types of xeno-lith glass and the host basanite These data indicate thatthe glass cannot be the product of xenolith^host lava inter-action during ascent to the surface Also back-mixing ofmicro-phenocrysts into the melt fails to derive the glassfrom a host-related melt (Fig 12b) Glass from xenolithmargins however is compositionally closer to the bulkhost basanite (eg Fig 12b) in line with a direct derivationfrom xenolith^host lava interaction Its more evolvedmajor element composition (eg higher SiO2 lowerMg-number Fig 12b) is probably the result of fractiona-tion of basanite phenocrysts (eg olivine augite andtitanomagnetite Volker 1990) Host lava^xenolith interac-tion is further supported by the trace element signature ofglass at the margin of the xenoliths which is closely similarto that of the Marsabit basanites (Fig 13 Volker 1990)The same characteristics are observed for intra-xenolithglass in the unusual kaersutite-bearing sample Ke 195927

Fig 16 Oxygen fugacities relative to the quartz^fayalite^magnetitebuffer (QFM equation of OrsquoNeill 1987) as a function of temperatureat assumed pressure of 1GPa calculated using the composition ofcoexisting spinel and olivine (ol-I^spl-I pairs and Chrm^Olm pairsrespectively) based on the oxygen barometer of Ballhaus et al (1991)The graphite^CO2 equilibrium was calculated after Frost amp Wood(1998) and the graphite^carbonate^silicate equilibrium was calcu-lated using the equation of Eggler amp Baker (1982) (See text for furtherexplanation)

Fig 17 Calculated metasomatic agents in equilibrium with the euhe-dral Mg-katophorites from the Type III-c amphibole dunite Ke 196525 Amphibolehydrous fluid and amphibolecarbonatite melt parti-tion coefficients are from Adam et al (1997) and Adam amp Green(2001) respectively Silicate melts have been calculated using amphi-boleSi-rich melt partition coefficients from Chazot et al (1996b) andamphibolebasanite partition coefficients from LaTourrette et al(1995) The compositional range of the Marsabit basanites and alkalibasalts (Volker 1990) is shown for comparison

27

Xenolith glass unrelated to the host lava and melt pock-ets similar to those reported here in association with resi-dual phases (eg amphibole or clinopyroxene) has beenreported from many xenolith localities (eg Frey ampGreen 1974 Ionov et al 1994 1996 Chazot et al 1996a1996b Yaxley et al 1997 Laurora et al 2001 Bali et al2002) Residual phases within the melt pockets stronglyindicate in situ partial melting within the xenolithsHowever a variety of processes may trigger such in situ

melting including (1) closed-system decompression melt-ing of either anhydrous (Maale amp Printzlau 1979) orhydrous phases (Frey amp Green 1974 Francis 1976) duringxenolith ascent in the host lava (2) melting during interac-tion of the xenoliths with the host basaltic melt duringxenolith ascent (eg Zinngrebe amp Foley 1995 Shaw ampKlulaquo gel 2002 Shaw et al 2006) (3) melting in the mantleprior to xenolith entrainment induced by an infiltratingfluid (ie flux melting eg Jones et al 1983 Ionov et al1994 Demecurren ny et al 2004) However most studies agreethat that partial melting occurred very shortly beforeandor during xenolith exhumation by the host magmaThus a combination of metasomatically triggered meltingfollowed andor assisted by decompression melting duringxenolith transport may be a realistic alternative in somecases (eg Dawson 2002)Recently Ban et al (2005) have shown in a very detailed

mass-balance study on glass-bearing xenoliths from theWest Eifel that the weight proportions of glass and theassociated micro-phenocrysts match the composition ofthe preserved parent amphibole We used the sameapproach and performed simple mass-balance calculations(least-squares fitting) to constrain the conclusions drawnfrom petrography and to test which phase contributedand to what extent to the formation of the melt pocketsThe results reported in Table 8 show that in most casesamphibole-melting can account successfully for the meltpocket composition and only a minor amount (520) ofother phases (orthopyroxene andor phlogopite) needs tobe added to the reactant Such a contribution of otherphases agrees with textural observation (eg Fig 4cshowing Dim growing on resorbed Opxm) as well as withthe conclusions of Ban et al (2005) who stated that moreSi-rich glass (ie 454wt) necessitates orthopyroxenecontributing to the meltTrace element patterns of the bulk glassthornmicro-

phenocryst pockets calculated using the measured traceelement abundances of glass and micro-phenocrysts andthe respective mass-balance parameters (Table 8) arebroadly identical to the measured amphibole composition(Fig 18a and b) Higher LILE (Cs Rb Ba) concentrationsin the calculated bulk melt patches in sample Ke 785 canbe balanced by adding 7wt of phlogopite (Fig 18b)which is in line with mass balance (Table 8) The dominantcontrol of amphibole to account for the trace element

budget of the melt pockets is similar to results found inother suites of glass-bearing xenoliths (Chazot et al1996a Yaxley amp Kamenetsky 1999 Laurora et al 2001)Further the obtained phase proportions for the melt pock-ets are in good agreement with the results from image ana-lysis (Table 1) In addition they are similar to the glass^cpx^ol proportions recorded in recent experiments on the

Table 8 Results of mass-balance calculations to test melt

pocket formation by incongruent melting of the early metaso-

matic assemblages

Sample 195915 785 785 196525 196525 19653

Reactant

amph 1000 1000 0820 1000 0944 mdash

opx mdash mdash 0078 mdash 0056 0406

phl mdash mdash 0102 mdash mdash mdash

glass mdash mdash mdash mdash mdash 05941

SiO2 4484 4989 4964 5129 5167 5898

TiO2 014 039 036 025 024 018

Cr2O3 232 291 258 127 121 009

Al2O3 1368 762 769 800 757 961

FeO(tot) 336 314 333 343 357 394

MnO 006 008 008 012 012 010

NiO 009 013 014 011 011 004

MgO 1844 2029 2197 2040 2122 1625

CaO 854 783 644 966 913 078

Na2O 460 500 436 504 476 843

K2O 049 032 096 042 040 060

Product

glass 0389 0322 0335 0292 0284 07032

ol 0243 0259 0315 0266 0286 0273

cpx 0288 0373 0308 0442 0431 0024

spl 0052 0032 0028 mdash mdash mdash

carb 0028 0014 0014 mdash mdash mdash

SiO2 4477 5027 4986 5207 5181 5903

TiO2 013 041 039 015 014 017

Cr2O3 213 314 272 109 106 015

Al2O3 1364 764 770 798 776 957

FeO(tot) 374 331 351 322 332 350

MnO 007 007 007 009 009 006

NiO 011 013 015 011 012 007

MgO 1834 2024 2196 2063 2141 1635

CaO 854 759 632 952 929 080

Na2O 449 385 388 499 485 860

K2O 060 096 100 015 014 091

r2 023 200 035 084 028 033

Prefix Ke omitted1Low-Si glass 2High-Si glass (see text for explanation)

28

melting behaviour of amphibole in wehrlites at tempera-tures between 1175 and 12508C and pressures of05^10GPa (Mecurren dard et al 2006) These temperatures arein good agreement with the estimates for the formation ofolivine and chromite micro-phenocrysts in the melt

pockets (Fig 16) The heat supply required for amphibolemelting in the formerly low-T metasomatized Group IIIxenoliths (T 200^3008C) may come from local heatingshortly before or during xenolith entrainment in the hostlava or from fluid infiltration that triggered melting (seediscussion below)Finally formation of the most Si-enriched melt (adjacent

to Opxm in Group III-c samples) can be modelled assum-ing progressing interaction between infiltrating melt andorthopyroxene where the latter melts incongruently

opxthornmelt1 olthornmelt2 cpx ethseeTable 8THORN eth1THORN

To calculate reaction (1) we chose the least evolved glassobserved in melt patches in sample Ke 19653 (SiO2

5921wt Mg-number 6465) to represent the initialmelt1 whereas melt2 adjacent to opx is SiO2-rich(6530wt) Trace element abundances in glass next toorthopyroxene are shifted to lower concentrations(Table 6) which is in agreement with dilution caused byaddition of a trace element-poor orthopyroxene compo-nent The resultant phase proportions are similar to thoseobserved at orthopyroxene^melt interfaces (ie higholivine and low cpx abundances) and are in agreementwith experimental studies (Shaw et al 1998)

Partial melting triggered by fluid fluxMass-balance calculations provide an explanation for meltpocket formation by in situ partial melting Thus onemight conclude that melting is simply the result of decom-pression and heating during xenolith transport in the hostmagma However there are two points that argue thatmelting was triggered by the infiltration of a fluid phaseshortly before the xenoliths were sampled by the hostmagma (1) the presence of occasionally high amounts ofcarbonate included in the melt pockets (2) the extremelyNa-rich clinopyroxene (Dim) compositions either includedas micro-phenocrysts in the melt pocket or present asequilibrated grains around chromite in the single TypeIII-d xenolith (Fig 3f) Both arguments will be discussedin the next section

Formation of carbonates

Textures and compositions of carbonate globules can beexplained by carbonate precipitation from a CO2-bearingfluid As demonstrated by several studies highCa(CathornMg) values (Table 7) and low REE concentra-tions are strong arguments that the carbonates are cumu-late crystals precipitated from a CO2-rich melt or afluid and not quenched carbonated melts (Lee et al 2000Ionov amp Harmer 2002) Oscillatory zoning (Fig 5a and b)of individual carbonate crystals within singleglobules corroborates this interpretation The roundedcontacts toward glass (globules) are also in line with thesefindings Similar globular forms were reported fromexperimental studies and do not a priori imply the

Fig 18 Calculated trace element characteristics of reactants (in mostcases amphibolethorn orthopyroxene filled symbols) and products(melt pockets open symbols) of the lsquoamphibole meltingrsquo model[see equation (1) in text] The relative phase proportions wereobtained by mass-balance calculations and are given in Table 8 Thedashed lines in (b) show that the high LILE concentrations obtainedfor the product (bulk melt pocket) can be explained by adding about7wt of phlogopite to the reactant (amphibolethorn orthopyroxeneTable 8) Further explanation is given in the text

29

presence of immiscible carbonate and silicate melts(eg Lee amp Wyllie 1996)The high amount of carbonate in some melt pockets

however (eg Fig 4e) does need further explanation asthe incongruent melting reaction (1) does not account forthe CO2 required for carbonate precipitation duringamphibole melting Therefore amphibole breakdowneither occurred in an open system with addition of at leastCO2 (eg Laurora et al 2001 Ban et al 2005) or anotherpre-existing phase acted as a source of carbon Oxidationof graphite texturally identified as a part of the earlyassemblage (eg Fig 3d) could account for some of theCO2 necessary for carbonate formation A change in fO2

conditions is also indicated when the composition of glass-related spinel (Chrm) and Olm are used for oxygen baro-metry which yields oxygen fugacities well on the oxidizedside of the carbon^carbonate^silicate buffer (Fig 16)However increasing fO2 triggered by closed-systemdecompression-induced melting alone does not appear fea-sible Also textures such as straight contacts between gra-phite and Mg-calcite (Fig 4c) suggest that carbonategrows on graphite (ie indicating local fO2 conditions cor-responding to carbon^carbonate equilibrium) rather thanreplacing it We therefore interpret melt pocket formationto be triggered by the infiltration of a CO2-rich fluid(ie flux melting see also Ionov et al 1994) This interpre-tation also accounts for the presence of abundant vugs inthe glass that could have contained a fluid (Ionov et al1994 2006) which was presumably lost during sample pre-paration However we emphasize here that late fluid infil-tration had only subtle effects on the trace elementcomposition of Type III-b and -c xenoliths

Formation of Na-rich Cr diopside

Infiltration of CO2-rich fluidwould also explain the Na-richcomposition of Dim micro-phenocryst cores (Fig 6a and b)Ionov et al (2005) noted recently that clinopyroxenes inmeltpockets are commonly characterized by Na2O contents51wt mainly as a result of low cpxmelt partition coeffi-cients at low pressures (eg Blundy et al 1995) Thus veryhighNa2Ocontents in Dimmicro-phenocrysts (Fig6a) sug-gest replacement of amphibole by Dim and eventually Olmand Chrm during reaction with a fluid at subsolidus condi-tions An origin of micro-phenocrysts by replacementrather than crystallization from a melt is indicated by theidentical crystallographic orientation of individual Dimcrystals in singlemelt pockets (Fig4a)Once the solidus of the precursor phase(s) was reached

(ie 10508C for amphibole Niida amp Green 1999) silicatemelt was generated in situ and led to the formation of clin-opyroxene rims with lower Na contents (Fig 6b) and thedevelopment of euhedral shapes in contact with melt (egFig 4a and e) Eventual immiscibility between remainingfluid and newly formed melt (eg during xenolith ascent)would then explain the presence of carbonate globules and

vugs enclosed in glass Likewise different origins of meltand carbonate were recently proposed for similar meltpockets from Hungarian xenoliths by Demecurren ny et al(2004) who found oxygen isotope disequilibrium betweencarbonate globules and the host glass Such a two-stepmodel (fluid-induced replacement followed by melt gen-eration) can be explained by CO2-fluid infiltration occur-ring within the mantle at sub-solidus conditions ultimatelyfollowed by xenolith entrainment in the host magma andtransport to the surfaceThe host magma possibly suppliedthe additional heat necessary for melt generationFast decompression would further enhance melting andcould also account for the lower Na contents in Dimmicro-phenocryst rims as a result of decreasing cpxmeltpartition coefficients at decreasing pressures (Blundyet al 1995)Rapid replacement of amphibole ( phlogopite) by an

anhydrous phase assemblage indicates that the fluid waswater-poor (eg low H2OCO2 ratio see Ionov et al2006)This is corroborated by the fact that Mg-katophorite(presumably OH-rich see above) is occasionally rimmedby a residual kaersutitic selvage (Fig 4d) Such Ti-richmantle amphiboles are commonly OH-poor as a result ofpartial dehydrogenation (eg Tiepolo et al 2001 and refer-ences therein)Finally the presence of theType III-d xenolith contain-

ing texturally equilibrated clinopyroxene of almost identi-cal composition to Dim micro-phenocrysts in melt pockets(Table 2 Figs 6a and 7b) further strengthens the argu-ment that CO2-fluid percolation started in the mantlebefore entrainment (ie enough time elapsed to allow thisrock to anneal completely after the late metasomaticassemblage had formed) A similar conclusion can bedrawn for the clearly host-related glass found in xenolithKe 195927 (eg Fig 13b) ThisType III-c sample is charac-terized by the presence of Dim micro-phenocrysts partlyovergrown by mineral rims clearly related to the host basa-nite magma (kaersutite Ti-augite and olivine withFo885 Fig 4f Tables 3 and 4 and supplementary dataat httppetrologyoxfordjournalsorg) Kaersutite hasmajor and trace element characteristics similar to typicalbasanitic and alkali basaltic amphibole phenocrysts(Fig 9d) The special phase assemblages in this xenolithindicates that host-related basanitic melts intruded someType III-c xenoliths and partly overprinted the assemblageformed slightly earlier by in situ derived alkali-rich peralu-minous melts fully preserved in other samples (eg Ke19653 or Ke 785)

ORIGIN OF METASOMATIZ INGAGENTS T IM ING AND TECTONICSETT INGAs noted above late metasomatism was probably short-lived started at mantle depths and probably continued

30

HREE abundances on the other hand are very hetero-geneous in the least metasomatized Zone A moderatelydepleted in Zone B and homogeneously low in the mostmetasomatizedZoneC (containinggraphite Fig15) similarto REE patterns in clinopyroxene from the amphibole- andphlogopite-bearingType III-b sample (Ke196515 Fig7a)The decoupling of strongly (LREETh U) from moder-

ately incompatible elements (HREE) can be explained bychromatographic fractionation during percolation ofsmall amounts of melts or fluids through an initially non-metasomatized peridotite (Navon amp Stolper 1987 Godardet al 1995 Bedini et al 1997 Vernie res et al 1997 Ionovet al 2002) Upon reaction with the wall-rock a fluidor melt will become progressively enriched in incom-patible elements as a function of increasing distancefrom its source This is due to the fact that elements withhigher Dmineralmelt values (eg HREE) are selectivelyremoved from a percolating liquid when it exchanges withthe surrounding minerals The results are faster-movingchemical fronts of incompatible elements compared withthose of more compatible elements This is consistent with

the geochemical signature of cpx-I from the least metaso-matized part of sample Ke 19651 (Zone A Fig 15) whichalready started to re-equilibrate with respect to the incom-patible elements (ie addition of Th U and LREE)whereas more compatible elements still reflect pre-metasomatic heterogeneities In this case these areextreme HREE variations probably reflecting the earliergarnet breakdown (Kaeser et al 2006) On the otherhand more intensely metasomatized parts (eg Zone Cin sample Ke 19651 and sample Ke 196515) are character-ized by clinopyroxene trace element patterns wheremildly incompatible elements have also been modified(eg homogeneously depleted HREE Figs 7a and 15)Similar fractionation processes can explain the modal

and compositional differences between Type III-b and -cCompared withType III-b higher abundances of metaso-matic phases inType III-c suggests that higher amounts offluid or melt crystallized in or passed through these rocksThe amphibole-rich dunite (Ke 196525) for examplecontains low-Ti Mg-katophorite exhibiting the lowest(LaYb)N and (LaCe)N ratios and high (NbLa)N and

Fig 8 Major element characteristics of amphiboles from porphyroclastic spinel harzburgites and dunites from Marsabit compared with com-positions of mantle-derived amphiboles from other localities (XB basalt-hosted xenoliths XS xenoliths from subduction settings O orogenicperidotites) Mg-numberfrac14Mg(MgthornFetot) in molar proportions

15

(NbTh)N ratios (Fig 10a and c) indicating a moderatelyfractionated metasomatic agent Mg-katophorite fromthe phlogopite-rich harzburgite (Ke 785 Type III-c)and amphiboles from the more cpx-rich harzburgites(Type III-b) show progressively more fractionated patterns(eg Fig 10c) and decreasing abundances in lsquoamphibole-compatiblersquoelements (eg Nb Fig 9a) Thus the transitionfrom Type III-b to -c could reflect decreasing distancebetween the site at which the xenolith was sampledand the source of the lsquoinitialrsquo metasomatizing agent (egZanetti et al 1996 Ionov et al 2002 Bodinier et al 2004)

Judging from the abundance of modal metasomatic phasesthe amphibole dunite (Ke 196525) was probably closest tothe source of the metasomatizing agent The phlogopite-rich nature of Ke 785 which indicates higher K andLILE (Cs Rb Ba) contents in the remaining agent is inagreement with a more fractionated melt or fluid at some-what greater distance from the metasomatic sourceThe Nb abundances in amphibole also agree with this

model as well For example amphibole from Type III-cKe 196525 has comparatively high Nb concentrations(Fig 9c) in line with relatively high DNb

amphmelt values

Fig 9 Trace element composition of amphibole from porphyroclastic spinel harzburgites and dunites from Marsabit [normalized to the primi-tive mantle values of McDonough amp Sun (1995)] Mg-katophorite in Type III-b and -c xenoliths (a^c) and glass-related kaersutite (d) Theshaded fields indicate the compositional variation of all Mg-katophorite analyses (light grey) and the compositional range of LREE-depletedTi-pargasite from the Group I (grt)^spinel lherzolites (dark grey Kaeser et al 2006)

16

constrained by different studies (eg Ionov amp Hofmann1995 LaTourrette et al 1995) In Ti-poor systems such asthose of this study Nb uptake by amphibole is evenenhanced as a result of increased DNb values (Tiepoloet al 2001) Consequently in an event during which amphi-bole crystallizes continuously the remaining fluid or meltwill become progressively depleted in Nb (Ionov et al2002) resulting in formation of Nb-depleted amphibolesfurther away from the source (Zanetti et al 1996 Ionovet al 2002) such Nb-depleted amphibole is found insample Ke 785 as well as inType III-b xenoliths (eg Ke195915 Fig 9a) Amphibole signatures from xenolith suitesmodelled successfully in the context of percolative traceelement fractionation (eg Spitsbergen Ionov et al 2002)show similar ranges (eg in NbTh ratios Fig10a) to thosedocumented for our samples This further supports the

hypothesis that similar percolation mechanisms acted inthe mantle beneath Marsabit

The role of pre-existing heterogeneitiesand exotic mineralsAlthough a model based on trace element fractionationduring melt percolation is supported by textures and canaccount for many compositional features some composi-tional peculiarities remain unexplained First clinopyrox-ene and amphibole in several samples (Ke 785 19651 and196515) are characterized by extremely low HREE abun-dances more than one order of magnitude lower thanin amphiboles from the strongly metasomatizedamphibole dunite Ke 196525 (Fig 9a^c) The mainphases that crystallize during metasomatism are low-TiMg-katophorite and phlogopite However DHREE

amphmelt

Fig 10 Selected trace element ratios in amphiboles from the porphyroclastic spinel harzburgites from Marsabit compared with those ofmantle-derived amphibole from other tectonic settings (XK and XB cratonic kimberlite-hosted and off-cratonic basalt-hosted xenolithsrespectively O orogenic peridotites XS xenoliths from subduction settings) Concentrations (ratios) normalized to the primitive mantlevalues of McDonough amp Sun (1995)

17

Table 5 Representative analyses of phlogopite and apatite (oxides in wt trace elements in mgg)

Type III-b III-c III-b III-c

Mineral phlog phlog phlog phlog phlog phlog phlog apatite apatite

Sample 196515 196515 19653 785 785 785 196525 195915 19653

Texture veinlet interst in opx in opx with amph

in mp

around

chromite

incl in

amph

bleb in

glass

interst

grain

SiO2 3925 3914 4171 4190 4124 4134 4230 SiO2 016 049

TiO2 313 307 009 053 039 037 027 Al2O3 001 nd

Al2O3 1544 1583 1360 1360 1384 1377 1477 P2O5 3768 3715

Cr2O3 112 105 132 147 190 197 063 FeO(tot) 037 029

FeO(tot) 350 347 271 000 289 277 340 MnO 006 nd

MnO nd 002 008 305 004 002 005 MgO 084 091

NiO 023 027 019 023 024 028 023 CaO 4750 4237

MgO 2287 2280 2577 2591 2582 2528 2525 SrO 095 397

CaO nd 002 001 nd 004 001 002 Na2O 079 133

Na2O 215 219 282 258 264 262 208 La2O3 003 007

K2O 719 707 704 706 674 684 683 Ce2O3 088 347

Nd2O3 003 007

F 139 224

Cl 287 092

Total 9488 9493 9571 9641 9587 9535 9590 Totaly 9305 9245

Mg-no 9282 9287 9443 9380 9408 9420 9298

P 369 187 480 339 206 188 362 P 129717 139419

K 71711 72575 60747 79052 70997 69992 70189 K 110 897

Sc 099 110 131 130 088 091 160 Sc 242 079

Ti 15500 16408 890 2839 2096 1807 1736 Ti 235 5294

V 255 284 888 142 108 130 788 V 365 382

Ni 1808 1727 1744 2046 1813 1773 1905 Ni 121 397

Zn 234 221 358 382 317 321 695 Zn 119 096

Ga 111 984 na 210 190 188 235 Ga na na

Rb 185 147 289 183 133 133 342 Rb 015 049

Sr 102 116 775 325 363 350 101 Sr 7138 42233

Y 010 0086 50057 50054 0021 0006 0052 Y 988 1454

Zr 019 019 043 50088 0077 0070 073 Zr 307 101

Nb 506 628 479 177 102 146 210 Nb 024 020

Cs 175 138 261 399 216 180 798 Cs 50013 50036

Ba 1212 1096 1343 298 277 219 1124 Ba 139 126

La 010 0055 0069 50029 0020 50003 50045 La 7460 19048

Ce 0027 0011 50075 50019 014 50003 0078 Ce 6844 25945

Pr 0012 0008 50067 50024 0012 50003 50019 Pr 380 2212

Nd 0019 0035 50405 50079 0065 50009 50224 Nd 749 6530

Sm 0015 50011 50474 50090 0016 50010 50132 Sm 515 747

Eu 0022 0017 50104 50026 0013 0010 50095 Eu 147 189

Gd 50010 50011 5043 5015 50011 0012 017 Gd 314 451

Tb 50002 50002 na 50027 0002 50002 50028 Tb na na

Dy 50015 50017 045 50077 50007 50006 50077 Dy 198 281

Ho 50002 50003 50043 50020 0002 50002 50028 Ho 359 511

Er 50009 50013 5025 5011 50016 50009 5011 Er 903 119

Tm 50002 50002 50088 50013 0002 0003 50018 Tm 119 138

(continued)

18

data (eg LaTourrette et al 1995) indicate that amphibolecrystallization alone is unlikely to deplete the metasomaticagent to such a high degree in HREE The same probablyaccounts for the extreme Nb depletion in amphiboles fromKe 785 relative to the otherType III-c xenolith (Ke 196525 see also Ionov et al 2002)Further clinopyroxene phlogopite and amphibole in the

Type III-b sample Ke 196515 on one hand all showstrongly fractionated REE patterns (eg high LaCe inamphibole Fig 10c) but on the other hand are character-ized by a positiveTi anomaly decoupled from other HFSE(eg Hf and Zr Fig 9b) Trace element fractionationduring amphibole crystallization (ie formation of TypeIII-c amphibole dunites) however should lower Ti con-tents in the remaining fluid or melt (eg Moine et al 2001)Based on textures mineralogy and dominant common

compositional features the hypothesis that Type III-band -c xenoliths were metasomatized by completely inde-pendent metasomatic agents can be ruled out Thereforetwo other possibilities remain to account for the inconsis-tent observations described above (1) inference of otherphases reactants or products that are able to fractionatespecific trace elements present in the mantle beneathMarsabit but not present in the few investigated samples(2) pre-metasomatic compositional heterogeneities similarto the inherited HREE variation inType III-a clinopyrox-ene (see above)Garnet would be an important sink for HREE but

metasomatism occurred in the spinel field after decom-pression with maximum pressures of 515GPa (Kaeseret al 2006) so garnet had already broken down to sym-plectites (Fig 15) Percolation of melt through garnet-bearing websterites prior to metasomatism in Ke 19651and 196515 could be an alternativeWebsterites have been

interpreted to occur in close spatial relationship with por-phyroclastic peridotites in the shallow mantle beneathMarsabit (Kaeser et al 2006) and they occasionally con-tain rutile and ilmenite which could further account forthe strong Nb and Ta depletion in some samples (eg Ke785) The available compositional data from the webster-ites however do not indicate metasomatic modification ofwebsteritic garnet (Olker 2001 B Kaeser unpublisheddata) nor compositional similarities to Group IIIperidotitesCareful study by SEM rules out the presence of exotic

phases such as zircon or Ti-oxides described fromother xenolith suites (eg Bodinier et al 1996 Rudnicket al 1999) Nevertheless it cannot be excluded that themetasomatizing agent percolated through mantle domainscontaining these phases before infiltrating the studiedrocksThe preservation of pre-metasomatic compositional het-

erogeneities is another possibility that must be consideredThat such lsquorock signalsrsquo partly overlap with the lsquometaso-matic signalrsquo is indicated by the HREE variation in TypeIII-a clinopyroxene (see above) and also by other composi-tional parameters For example both Mg-katophorite andphlogopite growing at the expense of spinel have very highCr2O3 contents whereas euhedral Mg-katophorite inthe amphibole dunite (Ke 196525) or in phlogopiteinclusions in Opxm is comparably Cr2O3-poor (Fig 8bTables 4 and 5) Pre-existing heterogeneities would essen-tially be preserved through elements that are relativelycompatible in a given mineral (ie that are not mobilizedduring metasomatism) This would be in agreement withthe preserved HREE variations in Type III-a clinopyrox-ene high Cr2O3 in amphiboles and phlogopite and Tispikes in amphibole from Ke 196515

Table 5 Continued

Type III-b III-c III-b III-c

Mineral phlog phlog phlog phlog phlog phlog phlog apatite apatite

Sample 196515 196515 19653 785 785 785 196525 195915 19653

Texture veinlet interst in opx in opx with amph

in mp

around

chromite

incl in

amph

bleb in

glass

interst

grain

Yb 50014 50011 5047 5015 50011 50019 5017 Yb 803 724

Lu 0002 50002 50061 50014 50002 50002 50020 Lu 115 742

Hf 0019 0018 5025 50053 50012 50006 5019 Hf 50023 50120

Ta 020 022 140 0031 0012 0009 023 Ta 50015 50075

Pb 546 586 125 108 083 098 143 Pb 436 577

Th 50004 50004 50080 0043 0088 0056 50021 Th 2538 1305

U 0012 0011 0069 50018 0038 0030 0031 U 572 278

interst interstitial mp melt pocket na not analysed nd not detected

19

Further very high Mg-numbers in replacive amphibolessupport melt (fluid)^rock interaction at low melt (fluid)rock ratios where Mg and Fe contents are buffered by theambient peridotite (Kelemen et al 1995 Tiepolo et al2001) The slightly lower Mg-number of euhedralamphiboles in Ke 196525 combined with their lowerCr2O3 contents (Fig 8b) suggests a higher melt (fluid)rock ratio which is further supported by considerablylower Mg-number in associated ol-I (Fo88 Table 3)

The limited number of investigated samples does notallow us to decide which of the processes described aboveacted and to what extent However given the heterogene-ity observed in the numerous non-metasomatized xenolithsfrom Marsabit (Kaeser et al 2006) pre-metasomaticfeatures certainly affect the final product of the melt^rockreaction process to a considerable degree

The nature of the metasomatizing agent(s)Graphite stability and oxygen fugacity considerations

The Marsabit xenoliths are unusual in that they occur inan off-cratonic setting yet bear macroscopic graphite Thelatter usually either occurs in kimberlite-hosted peridotites(on-craton) or in pyroxenites from alpine-type peridotitebodies (eg Pearson et al 1994) In the phlogopite perido-tites from Finero graphite has recently been reportedas nanoscopic layers in phlogopite (Ferraris et al 2004)In xenoliths from Marsabit graphite occurs in the sametextural relationship as in kimberlite-hosted peridotitexenoliths (ie millimetre-sized flakes typically associatedwith orthopyroxene see Fig 3c^e and Pearson et al 1994)Graphite inclusions in phlogopite and Opxm couldindicate crystallization during metasomatism by alsquoreducedrsquo C-bearing fluid or melt However precipitationof graphite more probably reflects the relatively reducingconditions in the uppermost mantle prior to metasoma-tism We used the oxygen barometer of Ballhaus et al(1991) based on ferric iron contents in spinel coexistingwith olivine to constrain oxygen fugacity (fO2)conditions prior to metasomatism in the Marsabitxenoliths (Fig 16) The results show that non-metasomatized Group I and IV samples reflect similarlyreducing conditions to the cryptically metasomatizedType III-a samplesAlthough assessing Fe3thornFe2thorn ratios in spinel based

on microprobe analyses can be problematic (Wood ampVirgo 1989 Ballhaus et al 1991) all microprobeanalyses were obtained using the same operationalsettings and therefore give at least the relative fO2

variation between samples The relatively reducingconditions in the pre-metasomatic cold andhighly deformed mantle well below carbon^carbonate^silicate equilibria (Mg2Si2O6thorn 05 CaMg(CO3)2frac14Mg2SiO4thorn 05 CaMgSi2O6thornCthornO2 Eggler amp Baker1982 see Fig 16) and close to the CCO oxygen buffer arein agreement with the presence of graphite in the GroupIII harzburgites Moreover the P^T conditions at whichthe Group III xenoliths partially re-equilibrated are closeto the subsolidus decarbonation of model carbonated lher-zolite (Fig 2 see Lee amp Wyllie 2000 and referencestherein) in agreement with the observed phase assemblageincluding orthopyroxene olivine clinopyroxene andgraphite

Fig 11 Trace element patterns of (a) phlogopite and (b) apatitefrom porphyroclastic spinel harzburgites and dunites from Marsabit[concentrations normalized to the primitive mantle valuesof McDonough amp Sun (1995)] The grey field in (a) indicates thecomposition of Ti-rich phlogopite in Group I (grt)^spl lherzolites(Kaeser et al 2006)

20

Table 6 Representative analyses of silicate glass (oxides in wt trace elements in mgg)

Type III-b III-c Host

Sample 195915 196515 196515 785 196525 19653 19653 19653 195927

Remark next amph next cpx xenol margin next amph next amph melt pocket next apatite next Opxm melt pocket host basanite

SiO2 5306 5775 5161 6096 5657 5938 5974 6530 4882 4410

TiO2 017 157 106 062 030 024 020 023 378 255

Al2O3 2412 2053 2025 1779 2313 1575 1245 1355 1890 1396

Cr2O3 005 001 nd 001 001 nd nd nd 002 na

FeO(tot) 188 247 626 126 252 295 374 265 509 1283

MnO 007 000 026 001 004 005 008 007 018 021

NiO nd nd nd na nd na 006 nd 001 na

MgO 182 129 116 114 122 304 382 252 244 938

CaO 506 257 283 082 044 124 101 039 810 1030

Na2O 1107 966 969 971 1299 1388 1384 1211 673 374

K2O 154 197 465 357 138 121 142 127 300 146

Cl 031 106 038 012 014 023 061 020 015 na

P2O5 041 008 044 009 012 049 233 018 100 059

Total 9956 9896 9860 9609 9887 9845 9929 9846 9821 9912

Mg-no 6329 4816 2489 6175 4632 6474 6458 6289 4552 5658

P 1061 448 1610 230 503 1519 6738 676 2994 na

K 12563 16408 39599 32515 15872 9265 11277 10071 25900 na

Sc 179 184 126 406 151 970 136 848 691 237

Ti 665 7329 4974 2953 1965 1213 1163 1055 18199 15311

V 107 265 235 796 920 210 151 134 234 na

Ni 128 371 92 5575 983 170 5199 5298 199 191

Zn 962 103 109 110 459 440 508 309 819 na

Ga na 173 273 194 220 na na na na na

Rb 181 263 124 546 626 176 398 369 577 369

Sr 427 488 1407 188 881 1644 4997 985 1062 792

Y 543 145 224 752 262 619 158 369 301 299

Zr 466 171 353 761 660 578 365 428 242 216

Nb 416 161 137 260 136 234 164 169 837 647

Cs 014 046 186 125 007 022 102 052 063 na

Ba 238 261 1064 129 168 405 1099 641 625 563

La 698 183 848 144 889 2212 1008 106 537 462

Ce 947 256 131 308 138 333 1265 164 104 823

Pr 655 194 120 330 116 304 975 160 120 na

Nd 146 603 408 101 329 993 256 564 463 400

Sm 142 048 570 179 419 141 379 679 105 851

Eu 041 035 228 082 112 438 103 241 345 267

Gd 062 051 415 158 402 106 245 566 677 729

Tb na 0086 086 020 071 na na na na 111

Dy 090 027 359 098 434 104 171 637 732 na

Ho 023 0037 105 025 095 206 446 104 117 114

Er 045 038 195 058 255 611 153 264 305 na

Tm 009 50039 033 0081 042 085 292 5014 036 052

Yb 062 023 267 044 307 667 120 487 253 220

(continued)

21

Evidence for low-temperature SiO2^Na2O^CO2^H2O-rich melts or fluids

Potential metasomatic agents inferred from the presenceof graphite are C^O^H-rich fluids carbonate melts orCO2-bearing silicate melts The resulting phase assem-blages and geochemical signatures are the result of infiltra-tion of fluid or melt into a cold (750^8508C) shallow(515GPa Fig 2) heterogeneous and locally reduced litho-spheric mantle Metasomatism under such conditions in anopen system must result in complex geochemical signa-tures that reflect the degree of fluid^rock reaction Thismakes it particularly difficult to constrain the exactnature of the lsquooriginalrsquometasomatic agentType III-c xenolith Ke 196525 contains the highest frac-

tion of metasomatic phases and the least fractionatedamphibole trace element patterns The lsquomagmaticrsquo texturesof the Mg-katophorites in this sample suggest direct crys-tallization from a melt or a fluid rather than replacementof a pre-existing phase Therefore hypothetical liquids inequilibrium with the amphiboles in this xenolith couldyield the composition of the initial metasomatic agentsFigure 17 illustrates the trace element patterns of suchmelts and fluids calculated using sets of experimentally orempirically determined mineralmelt (fluid) partitioncoefficients including carbonate melt silicate melt andhydrous fluid (for references see caption to Fig 17)Common features for all calculated metasomatic agents

are negative Zr^Hf^Ti anomalies and LREE enrichment(Fig 17) However previous studies have shown that thesesignatures are indicative of almost all types of metasoma-tism including interaction of peridotite with carbonatiticmelt (Green amp Wallace 1988 Dautria et al 1992 Hauriet al 1993 Rudnick et al 1993 Ionov et al 1996 Yaxleyet al 1998 Coltorti et al 1999) with hydrous fluids andormelts for example originating from dehydration of altered

subducting lithosphere (Maury et al 1992 Zanetti et al1999 Grecurren goire et al 2001 Laurora et al 2001 Parkinsonet al 2003) and interaction of peridotite with volatile-and Si-rich melts that evolved through reactive porousflow (Bedini et al 1997 Ionov et al 2002 2006 Bodinieret al 2004 Rivalenti et al 2004)In the case of the Marsabit xenoliths integration of the

trace element characteristics thermobarometric inferencesand mineralogical constraints favours a Na2O^SiO2^CO2^H2O-rich silicate melt Carbonated melts can prob-ably be ruled out on major element and mineralogical con-siderations First amphiboles are Ca-poor in conflict withcarbonate melt metasomatism which is generally charac-terized by Ca enrichment (Rudnick et al 1993 Neumannet al 2002) Further orthopyroxene is a stable phase andprobably even crystallizes during metasomatism in somesamples (eg Ke 785) At near-solidus conditions coexis-tence of orthopyroxene and carbonate melt is possibleonly at pressures 20GPa (see Fig 2 eg Lee amp Wyllie2000 and references therein) which is considerably higherthan the maximum pressures indicated by the compositionof spinel (515GPa) At such low pressures metasomatismby carbonated melts will convert lherzolite and harzbur-gite immediately into wehrlite which is the major perido-tite type in all xenolith suites where carbonate meltmetasomatism is convincingly demonstrated (Rudnicket al 1993 Yaxley et al 1998 Neumann et al 2002)In Marsabit wehrlites are absent howeverAn Na2O^SiO2^CO2^H2O-rich melt or fluid on the

other hand would explain the Na-rich nature of amphiboleand phlogopite as well as the presence of orthopyroxeneGrowth of metasomatic (and Al-poor) orthopyroxene hasbeen described from several xenolith localities where peri-dotites were infiltrated either by hydrous fluids orSi-rich melts (Smith et al 1999 McInnes et al 2001Morishita et al 2003) Na-rich phlogopite is commonly

Table 6 Continued

Type III-b III-c Host

Sample 195915 196515 196515 785 196525 19653 19653 19653 195927

Remark next amph next cpx xenol margin next amph next amph melt pocket next apatite next Opxm melt pocket host basanite

Lu 014 0050 026 0041 046 080 140 058 040 031

Hf 026 052 535 014 076 110 5087 124 568 504

Ta 50080 033 638 0022 123 101 784 801 394 403

Pb 541 448 280 255 840 797 260 749 493 na

Th 194 296 160 466 703 185 247 105 534 506

U 545 069 478 099 152 447 724 216 171 126

nd not detected na not analysedAverage composition of the strongly alkaline basanites from Marsabit (Volker 1990)

22

Fig 12 Total alkalis vs SiO2 (a) Harker-type co-variation diagrams (b^e) and incompatible trace element co-variation diagram (f) showingthe compositional variability of silicate glass found in the porphyroclastic spinel harzburgites and dunites from Marsabit In (b) binary mixinglines between the typical composition of the alkali-rich glass (large open diamond) and the associated microlites are shown (each increment is10wt)They illustrate that the glass does not represent the derivate of host lava infiltration and fractional crystallization Subalkaline glass inGroup I (grt)^spl lherzolites from Marsabit is shown for comparison and is interpreted to derive from incongruent melting of Ti-pargasite(Kaeser 2006)

23

found as a reaction product between anhydrous mafic toultramafic rocks and H2O-rich evolved liquids (Costaet al 2001 and references therein) or H2O-bearing fluids(eg Odling 1994) The latter study was an experiment at20GPa simulating the effects of decreasing temperatureson the phase assemblages and their composition duringmetasomatism using a model peridotitethornC^O^H fluidThe coolest part of Odlingrsquos (1994) experiment (510758C)contained a strikingly similar phase assemblage (kato-phorite phlogopite with 2wt Na2O and Al-poororthopyroxene) to the Marsabit samples interpreted tohave formed during sub-solidus fluid^solid interactionA compositionally similar phase assemblage (highamounts of opx Si-rich hornblende Na-rich phlogopite)resulted from experiments simulating peridotite interac-tion with Si-rich melt (dacite) at lower P^T conditions(9508C at 092GPa Prouteau amp Scaillet 2003)In this study the trace element signatures of Group III-c

Mg-katophorite and the calculated metasomatic agentsfurther suggest that the metasomatizing agent was a meltrather than a fluid Hypothetical fluids in equilibrium witheuhedral Mg-katophorite from sample Ke 196525 havehigh NbLILE (eg Rb Ba) and low LILEREE ratios(Fig 17) Because experimental data predict the oppositefor fluids (LILE enriched over HFSE and REE egManning 2004 and references therein) a fluid with asignature such as that indicated in Fig 17 is unrealistic

Fig 13 Trace element patterns of silicate glass in porphyroclasticspinel harzburgites and dunites from Marsabit compared withthe trace element composition of associated clinopyroxene (Dim)micro-phenocrysts xenolith-hosting basanites from Marsabit andglass interpreted to represent xenolith^host interaction [concentra-tions normalized to the primitive mantle values of McDonough ampSun (1995)]

Table 7 Representative analyses of carbonates (oxides in

wt trace elements in mgg)

Group III-b III-b III-c III-c III-c

Mineral Dol Mg-Cc Mg-Cc Mg-Cc Mg-Cc

Sample 195915 196515 785 19653 19653

Texture in glass in glass in glass in glass in glass

SiO2 na 003 na nd 003

FeO(tot) 016 011 010 010 006

MnO 001 nd nd nd nd

MgO 1922 236 190 125 517

CaO 3189 5313 5377 5391 4853

Na2O 003 nd 004 nd 003

K2O na 002 na 001 nd

SrO 003 010 008 004 021

BaO 003 nd nd na na

CO21 4618 4444 4442 4377 4391

Total 9756 10021 10032 9909 9794

Mg-no 9954 9745 9713 9554 9931

Ca-no 5439 9418 9531 9687 8709

P 220 334 137 580 589

K 970 212 642 105 155

Sc 5065 50150 027 5027 50076

Ti 975 189 242 528 196

V 5085 0481 5042 5037 021

Ni 900 673 517 5294 593

Zn 459 5089 113 296 045

Ga na 5014 5013 na na

Rb 0084 50072 50074 50060 0046

Sr 593 860 371 251 4274

Y 012 011 130 704 015

Zr 261 024 012 097 756

Nb 144 019 0056 692 185

Cs 50063 50011 50018 50039 50008

Ba 126 230 5024 029 271

La 034 057 084 136 025

Ce 032 013 134 997 023

Pr 006 006 018 095 0026

Nd 030 013 068 228 0078

Sm 5044 50091 0074 048 50018

Eu 5012 50019 0051 020 50008

Gd 5037 50090 014 051 0026

Tb na 50010 0036 na na

Dy 5019 50039 0089 058 50038

Ho 50029 0013 0044 011 50006

Er 5012 50041 0083 033 50019

Tm 50052 50013 0016 0084 50006

Yb 5043 50063 014 027 50040

Lu 50072 50010 0011 0066 50008

(continued)

24

At the relatively low maximum pressures (515GPa) asupercritical liquid is not expected either (eg Kesselet al 2005)Finally a relatively hydrous metasomatizing agent

would be consistent with formation of Mg-katophoriteswith lowTi and presumably low Fe3thorn contents (given therelatively reducing conditions) which in turn imply high(OH)^ contents In other words partial dehydrogenationtypically observed in common (Ti-rich) mantle amphibole

(eg Popp et al 1995 Tiepolo et al 2001 and referencestherein) cannot have played an important role in thiscase as H deficiency is mainly the result of coupled substi-tutions involvingTi and Fe3thorn (Popp et al 1995) In conclu-sion the metasomatizing agent was most probably aNa2O^SiO2^CO2^H2O-rich meltComparison with experimental studies further con-

strains the low-temperature regime in which metasoma-tism occurred In ultramafic (eg Niida amp Green 1999)basaltic (eg Rapp 1995) and Si-enriched systems (egtrachybasaltic Barclay amp Carmichael 2004) amphibole(in the latter study together with phlogopite) is confinedto temperatures lower than 1000^10508C at pressures520GPa (Fig 2)Thus crystallization of the early metaso-matic phase assemblage can be explained by infiltration ofa silicate melt that rapidly cooled upon contact with theambient peridotite (750^8508C prior to metasomatismFig 2) However this low-T stage is based on Ca-in-opxthermometry (see Brey amp Kolaquo hler 1990 Kaeser et al2006 Table 1) and therefore probably underestimates theeffective temperature conditions during metasomatismThis can be explained by low diffusion rates and proximityto the closure temperature for the diopside^enstatiteexchange reaction at temperatures lower than 8508C(eg Smith 1999 and references therein) The lower tem-perature limit at which silicate melt metasomatism ispossible (ie defined by the meltrsquos solidus and viscosity)depends strongly on volatile and SiO2 contentsExperiments by Draper amp Green (1997) for exampleshowed that highly silicic melts (52^64wt SiO2) canbe in equilibrium with mantle harzburgite at temperaturesas low as 8508C (at 10^20GPa under anhydrous andC^O^H-fluid-bearing conditions) These are howeverextreme compositions for mantle melts The actual SiO2

content of the metasomatizing agent in this case is difficultto constrain from the metasomatic mineral assemblage ascompositions may have varied considerably during reac-tive percolation (eg including Si-releasing reactions suchas cpx replacement by amphibole versus Si-consumingreactions such as Opxm crystallization) Moreover it isunlikely that a melt persists at temperatures muchbelow 9008C which is the solidus of fertile lherzolite(ie primitive upper mantle composition) underH2O-saturated conditions at 15GPa (Grove et al 2006see Fig 2) Below the solidus residual volatile-enrichedfluids may percolate further for example forming the lessmetasomatized Type III-c xenoliths (eg Ke 785) as wellas Type III-b and -a peridotites (see also Bailey 1987)Relatively cold conditions would also explain the veryhigh compositional gradients developed on small scales inType III-a samples (Fig 15) Given the absence of chloritein all investigated samples the lower temperature limitof such fluid metasomatism is 8008C (eg Fumagalli ampPoli 2005 see Fig 2)

Table 7 Continued

Group III-b III-b III-c III-c III-c

Mineral Dol Mg-Cc Mg-Cc Mg-Cc Mg-Cc

Sample 195915 196515 785 19653 19653

Texture in glass in glass in glass in glass in glass

Hf 5016 50038 50036 50074 50013

Ta 0048 50012 50016 50031 0008

Pb 5024 858 5021 074 018

Th 0021 50020 50017 50028 50005

U 060 094 086 600 148

Mg-Cc Mg-calcite Dol dolomite nd not detectedna not analysed Ca-numberfrac14 100Ca(CathornMg)1Calculated from stoichiometry

Fig 14 Trace element patterns of glass-hosted carbonates in TypeIII-b and -c spinel harzburgite and dunite xenoliths from Marsabit[concentrations normalized to the primitive mantle values ofMcDonough amp Sun (1995)] The grey field outlines the compositionalrange of carbonates found in other peridotite xenoliths (Ionov ampHarmer 2002 and references therein) Carbonate material inter-preted as quenched carbonatite melt found in peridotite xenolithsfrom Kerguelen (Moine et al 2004) is shown for comparison(bold line)

25

Finally the presence of apatite even though oftentaken as indicator for carbonatite-type metasomatism(eg Yaxley et al 1998) is not in conflict with metasoma-tism by silicate melts Apatite has also been shown to crys-tallize from volatile and alkali-rich derivatives formedduring reactive porous flow of originally mafic silicatemelts (Woodland et al 1996 Bedini et al 1997 Bodinieret al 2004 Ionov et al 2006)

FORMATION OF THE LATE GLASS -BEAR ING ASSEMBLAGESTextures strongly suggest that the glass-bearing assem-blages formed later and at the expense of the early metaso-matic assemblages (Fig 4a and d) Features such as thereappearance of clinopyroxene (Na-rich Cr diopsidemicro-phenocrysts) a phase replaced by the early

Fig 15 Detailed REE systematics in primary clinopyroxene (cpx-I) in a porphyroclastic cryptically metasomatized spinel harzburgite(sample Ke 19651) showing the transition fromType III-a (Zone A and B) toType III-b (Zone C) on a scale of a few centimetres The numberson the photomicrographs (crossed polars) correspond to single laser ablation microprobe pits (for further explanation see text)

26

metasomatic assemblage indicate that melt pocket forma-tion reflects a significant change in P^T^X space in partsof the mantle beneath Marsabit after earlier metasoma-tism Based on geochemical arguments and mass-balanceconsiderations we will show that (1) the late glass-bearingassemblages are closely related to early metasomaticassemblages mainly representing breakdown productsand (2) breakdown was induced by infiltration of aCO2-rich fluid in the mantle probably followed andenhanced by heating and decompression melting duringxenolith transport in the host magma

Evidence for in situ partial meltingBSE imaging reveals that the phases becoming unstableare amphibole orthopyroxene and to minor extentsspl-I phlogopite cpx-I and apatite Compositional(inter- and intra-xenolith) heterogeneity of silicate glass istexture controlled and varies as a function of theassociated precursor phases (Fig 12a^f)TiO2 for exampleshows higher concentrations in glass from xenolith Ke196515 containing Ti-enriched amphibole (Fig 12c)whereas K2O contents are highest in phlogopite-rich sam-ples (eg Ke 785 Fig 12d) Elevated P2O5 contents arefound in glass adjacent to apatite (Fig 12e) whichmirrors at the same time the highTh and LREE contents

of apatite (Fig 12f) Significant Si enrichment is found inglass coating orthopyroxene (Fig 12d) supporting incon-gruent melting of orthopyroxene (see below) Even thoughthe melt pockets are connected along grain boundaries thestrong compositional variation (on inter- and intra-xeno-lith scale) suggests that the mobility of the melt was lim-ited or that the melting event was very short-lived (fortiming constraints see below)The co-variation plots in Fig 12 further reveal consider-

able compositional differences between all types of xeno-lith glass and the host basanite These data indicate thatthe glass cannot be the product of xenolith^host lava inter-action during ascent to the surface Also back-mixing ofmicro-phenocrysts into the melt fails to derive the glassfrom a host-related melt (Fig 12b) Glass from xenolithmargins however is compositionally closer to the bulkhost basanite (eg Fig 12b) in line with a direct derivationfrom xenolith^host lava interaction Its more evolvedmajor element composition (eg higher SiO2 lowerMg-number Fig 12b) is probably the result of fractiona-tion of basanite phenocrysts (eg olivine augite andtitanomagnetite Volker 1990) Host lava^xenolith interac-tion is further supported by the trace element signature ofglass at the margin of the xenoliths which is closely similarto that of the Marsabit basanites (Fig 13 Volker 1990)The same characteristics are observed for intra-xenolithglass in the unusual kaersutite-bearing sample Ke 195927

Fig 16 Oxygen fugacities relative to the quartz^fayalite^magnetitebuffer (QFM equation of OrsquoNeill 1987) as a function of temperatureat assumed pressure of 1GPa calculated using the composition ofcoexisting spinel and olivine (ol-I^spl-I pairs and Chrm^Olm pairsrespectively) based on the oxygen barometer of Ballhaus et al (1991)The graphite^CO2 equilibrium was calculated after Frost amp Wood(1998) and the graphite^carbonate^silicate equilibrium was calcu-lated using the equation of Eggler amp Baker (1982) (See text for furtherexplanation)

Fig 17 Calculated metasomatic agents in equilibrium with the euhe-dral Mg-katophorites from the Type III-c amphibole dunite Ke 196525 Amphibolehydrous fluid and amphibolecarbonatite melt parti-tion coefficients are from Adam et al (1997) and Adam amp Green(2001) respectively Silicate melts have been calculated using amphi-boleSi-rich melt partition coefficients from Chazot et al (1996b) andamphibolebasanite partition coefficients from LaTourrette et al(1995) The compositional range of the Marsabit basanites and alkalibasalts (Volker 1990) is shown for comparison

27

Xenolith glass unrelated to the host lava and melt pock-ets similar to those reported here in association with resi-dual phases (eg amphibole or clinopyroxene) has beenreported from many xenolith localities (eg Frey ampGreen 1974 Ionov et al 1994 1996 Chazot et al 1996a1996b Yaxley et al 1997 Laurora et al 2001 Bali et al2002) Residual phases within the melt pockets stronglyindicate in situ partial melting within the xenolithsHowever a variety of processes may trigger such in situ

melting including (1) closed-system decompression melt-ing of either anhydrous (Maale amp Printzlau 1979) orhydrous phases (Frey amp Green 1974 Francis 1976) duringxenolith ascent in the host lava (2) melting during interac-tion of the xenoliths with the host basaltic melt duringxenolith ascent (eg Zinngrebe amp Foley 1995 Shaw ampKlulaquo gel 2002 Shaw et al 2006) (3) melting in the mantleprior to xenolith entrainment induced by an infiltratingfluid (ie flux melting eg Jones et al 1983 Ionov et al1994 Demecurren ny et al 2004) However most studies agreethat that partial melting occurred very shortly beforeandor during xenolith exhumation by the host magmaThus a combination of metasomatically triggered meltingfollowed andor assisted by decompression melting duringxenolith transport may be a realistic alternative in somecases (eg Dawson 2002)Recently Ban et al (2005) have shown in a very detailed

mass-balance study on glass-bearing xenoliths from theWest Eifel that the weight proportions of glass and theassociated micro-phenocrysts match the composition ofthe preserved parent amphibole We used the sameapproach and performed simple mass-balance calculations(least-squares fitting) to constrain the conclusions drawnfrom petrography and to test which phase contributedand to what extent to the formation of the melt pocketsThe results reported in Table 8 show that in most casesamphibole-melting can account successfully for the meltpocket composition and only a minor amount (520) ofother phases (orthopyroxene andor phlogopite) needs tobe added to the reactant Such a contribution of otherphases agrees with textural observation (eg Fig 4cshowing Dim growing on resorbed Opxm) as well as withthe conclusions of Ban et al (2005) who stated that moreSi-rich glass (ie 454wt) necessitates orthopyroxenecontributing to the meltTrace element patterns of the bulk glassthornmicro-

phenocryst pockets calculated using the measured traceelement abundances of glass and micro-phenocrysts andthe respective mass-balance parameters (Table 8) arebroadly identical to the measured amphibole composition(Fig 18a and b) Higher LILE (Cs Rb Ba) concentrationsin the calculated bulk melt patches in sample Ke 785 canbe balanced by adding 7wt of phlogopite (Fig 18b)which is in line with mass balance (Table 8) The dominantcontrol of amphibole to account for the trace element

budget of the melt pockets is similar to results found inother suites of glass-bearing xenoliths (Chazot et al1996a Yaxley amp Kamenetsky 1999 Laurora et al 2001)Further the obtained phase proportions for the melt pock-ets are in good agreement with the results from image ana-lysis (Table 1) In addition they are similar to the glass^cpx^ol proportions recorded in recent experiments on the

Table 8 Results of mass-balance calculations to test melt

pocket formation by incongruent melting of the early metaso-

matic assemblages

Sample 195915 785 785 196525 196525 19653

Reactant

amph 1000 1000 0820 1000 0944 mdash

opx mdash mdash 0078 mdash 0056 0406

phl mdash mdash 0102 mdash mdash mdash

glass mdash mdash mdash mdash mdash 05941

SiO2 4484 4989 4964 5129 5167 5898

TiO2 014 039 036 025 024 018

Cr2O3 232 291 258 127 121 009

Al2O3 1368 762 769 800 757 961

FeO(tot) 336 314 333 343 357 394

MnO 006 008 008 012 012 010

NiO 009 013 014 011 011 004

MgO 1844 2029 2197 2040 2122 1625

CaO 854 783 644 966 913 078

Na2O 460 500 436 504 476 843

K2O 049 032 096 042 040 060

Product

glass 0389 0322 0335 0292 0284 07032

ol 0243 0259 0315 0266 0286 0273

cpx 0288 0373 0308 0442 0431 0024

spl 0052 0032 0028 mdash mdash mdash

carb 0028 0014 0014 mdash mdash mdash

SiO2 4477 5027 4986 5207 5181 5903

TiO2 013 041 039 015 014 017

Cr2O3 213 314 272 109 106 015

Al2O3 1364 764 770 798 776 957

FeO(tot) 374 331 351 322 332 350

MnO 007 007 007 009 009 006

NiO 011 013 015 011 012 007

MgO 1834 2024 2196 2063 2141 1635

CaO 854 759 632 952 929 080

Na2O 449 385 388 499 485 860

K2O 060 096 100 015 014 091

r2 023 200 035 084 028 033

Prefix Ke omitted1Low-Si glass 2High-Si glass (see text for explanation)

28

melting behaviour of amphibole in wehrlites at tempera-tures between 1175 and 12508C and pressures of05^10GPa (Mecurren dard et al 2006) These temperatures arein good agreement with the estimates for the formation ofolivine and chromite micro-phenocrysts in the melt

pockets (Fig 16) The heat supply required for amphibolemelting in the formerly low-T metasomatized Group IIIxenoliths (T 200^3008C) may come from local heatingshortly before or during xenolith entrainment in the hostlava or from fluid infiltration that triggered melting (seediscussion below)Finally formation of the most Si-enriched melt (adjacent

to Opxm in Group III-c samples) can be modelled assum-ing progressing interaction between infiltrating melt andorthopyroxene where the latter melts incongruently

opxthornmelt1 olthornmelt2 cpx ethseeTable 8THORN eth1THORN

To calculate reaction (1) we chose the least evolved glassobserved in melt patches in sample Ke 19653 (SiO2

5921wt Mg-number 6465) to represent the initialmelt1 whereas melt2 adjacent to opx is SiO2-rich(6530wt) Trace element abundances in glass next toorthopyroxene are shifted to lower concentrations(Table 6) which is in agreement with dilution caused byaddition of a trace element-poor orthopyroxene compo-nent The resultant phase proportions are similar to thoseobserved at orthopyroxene^melt interfaces (ie higholivine and low cpx abundances) and are in agreementwith experimental studies (Shaw et al 1998)

Partial melting triggered by fluid fluxMass-balance calculations provide an explanation for meltpocket formation by in situ partial melting Thus onemight conclude that melting is simply the result of decom-pression and heating during xenolith transport in the hostmagma However there are two points that argue thatmelting was triggered by the infiltration of a fluid phaseshortly before the xenoliths were sampled by the hostmagma (1) the presence of occasionally high amounts ofcarbonate included in the melt pockets (2) the extremelyNa-rich clinopyroxene (Dim) compositions either includedas micro-phenocrysts in the melt pocket or present asequilibrated grains around chromite in the single TypeIII-d xenolith (Fig 3f) Both arguments will be discussedin the next section

Formation of carbonates

Textures and compositions of carbonate globules can beexplained by carbonate precipitation from a CO2-bearingfluid As demonstrated by several studies highCa(CathornMg) values (Table 7) and low REE concentra-tions are strong arguments that the carbonates are cumu-late crystals precipitated from a CO2-rich melt or afluid and not quenched carbonated melts (Lee et al 2000Ionov amp Harmer 2002) Oscillatory zoning (Fig 5a and b)of individual carbonate crystals within singleglobules corroborates this interpretation The roundedcontacts toward glass (globules) are also in line with thesefindings Similar globular forms were reported fromexperimental studies and do not a priori imply the

Fig 18 Calculated trace element characteristics of reactants (in mostcases amphibolethorn orthopyroxene filled symbols) and products(melt pockets open symbols) of the lsquoamphibole meltingrsquo model[see equation (1) in text] The relative phase proportions wereobtained by mass-balance calculations and are given in Table 8 Thedashed lines in (b) show that the high LILE concentrations obtainedfor the product (bulk melt pocket) can be explained by adding about7wt of phlogopite to the reactant (amphibolethorn orthopyroxeneTable 8) Further explanation is given in the text

29

presence of immiscible carbonate and silicate melts(eg Lee amp Wyllie 1996)The high amount of carbonate in some melt pockets

however (eg Fig 4e) does need further explanation asthe incongruent melting reaction (1) does not account forthe CO2 required for carbonate precipitation duringamphibole melting Therefore amphibole breakdowneither occurred in an open system with addition of at leastCO2 (eg Laurora et al 2001 Ban et al 2005) or anotherpre-existing phase acted as a source of carbon Oxidationof graphite texturally identified as a part of the earlyassemblage (eg Fig 3d) could account for some of theCO2 necessary for carbonate formation A change in fO2

conditions is also indicated when the composition of glass-related spinel (Chrm) and Olm are used for oxygen baro-metry which yields oxygen fugacities well on the oxidizedside of the carbon^carbonate^silicate buffer (Fig 16)However increasing fO2 triggered by closed-systemdecompression-induced melting alone does not appear fea-sible Also textures such as straight contacts between gra-phite and Mg-calcite (Fig 4c) suggest that carbonategrows on graphite (ie indicating local fO2 conditions cor-responding to carbon^carbonate equilibrium) rather thanreplacing it We therefore interpret melt pocket formationto be triggered by the infiltration of a CO2-rich fluid(ie flux melting see also Ionov et al 1994) This interpre-tation also accounts for the presence of abundant vugs inthe glass that could have contained a fluid (Ionov et al1994 2006) which was presumably lost during sample pre-paration However we emphasize here that late fluid infil-tration had only subtle effects on the trace elementcomposition of Type III-b and -c xenoliths

Formation of Na-rich Cr diopside

Infiltration of CO2-rich fluidwould also explain the Na-richcomposition of Dim micro-phenocryst cores (Fig 6a and b)Ionov et al (2005) noted recently that clinopyroxenes inmeltpockets are commonly characterized by Na2O contents51wt mainly as a result of low cpxmelt partition coeffi-cients at low pressures (eg Blundy et al 1995) Thus veryhighNa2Ocontents in Dimmicro-phenocrysts (Fig6a) sug-gest replacement of amphibole by Dim and eventually Olmand Chrm during reaction with a fluid at subsolidus condi-tions An origin of micro-phenocrysts by replacementrather than crystallization from a melt is indicated by theidentical crystallographic orientation of individual Dimcrystals in singlemelt pockets (Fig4a)Once the solidus of the precursor phase(s) was reached

(ie 10508C for amphibole Niida amp Green 1999) silicatemelt was generated in situ and led to the formation of clin-opyroxene rims with lower Na contents (Fig 6b) and thedevelopment of euhedral shapes in contact with melt (egFig 4a and e) Eventual immiscibility between remainingfluid and newly formed melt (eg during xenolith ascent)would then explain the presence of carbonate globules and

vugs enclosed in glass Likewise different origins of meltand carbonate were recently proposed for similar meltpockets from Hungarian xenoliths by Demecurren ny et al(2004) who found oxygen isotope disequilibrium betweencarbonate globules and the host glass Such a two-stepmodel (fluid-induced replacement followed by melt gen-eration) can be explained by CO2-fluid infiltration occur-ring within the mantle at sub-solidus conditions ultimatelyfollowed by xenolith entrainment in the host magma andtransport to the surfaceThe host magma possibly suppliedthe additional heat necessary for melt generationFast decompression would further enhance melting andcould also account for the lower Na contents in Dimmicro-phenocryst rims as a result of decreasing cpxmeltpartition coefficients at decreasing pressures (Blundyet al 1995)Rapid replacement of amphibole ( phlogopite) by an

anhydrous phase assemblage indicates that the fluid waswater-poor (eg low H2OCO2 ratio see Ionov et al2006)This is corroborated by the fact that Mg-katophorite(presumably OH-rich see above) is occasionally rimmedby a residual kaersutitic selvage (Fig 4d) Such Ti-richmantle amphiboles are commonly OH-poor as a result ofpartial dehydrogenation (eg Tiepolo et al 2001 and refer-ences therein)Finally the presence of theType III-d xenolith contain-

ing texturally equilibrated clinopyroxene of almost identi-cal composition to Dim micro-phenocrysts in melt pockets(Table 2 Figs 6a and 7b) further strengthens the argu-ment that CO2-fluid percolation started in the mantlebefore entrainment (ie enough time elapsed to allow thisrock to anneal completely after the late metasomaticassemblage had formed) A similar conclusion can bedrawn for the clearly host-related glass found in xenolithKe 195927 (eg Fig 13b) ThisType III-c sample is charac-terized by the presence of Dim micro-phenocrysts partlyovergrown by mineral rims clearly related to the host basa-nite magma (kaersutite Ti-augite and olivine withFo885 Fig 4f Tables 3 and 4 and supplementary dataat httppetrologyoxfordjournalsorg) Kaersutite hasmajor and trace element characteristics similar to typicalbasanitic and alkali basaltic amphibole phenocrysts(Fig 9d) The special phase assemblages in this xenolithindicates that host-related basanitic melts intruded someType III-c xenoliths and partly overprinted the assemblageformed slightly earlier by in situ derived alkali-rich peralu-minous melts fully preserved in other samples (eg Ke19653 or Ke 785)

ORIGIN OF METASOMATIZ INGAGENTS T IM ING AND TECTONICSETT INGAs noted above late metasomatism was probably short-lived started at mantle depths and probably continued

30

(NbTh)N ratios (Fig 10a and c) indicating a moderatelyfractionated metasomatic agent Mg-katophorite fromthe phlogopite-rich harzburgite (Ke 785 Type III-c)and amphiboles from the more cpx-rich harzburgites(Type III-b) show progressively more fractionated patterns(eg Fig 10c) and decreasing abundances in lsquoamphibole-compatiblersquoelements (eg Nb Fig 9a) Thus the transitionfrom Type III-b to -c could reflect decreasing distancebetween the site at which the xenolith was sampledand the source of the lsquoinitialrsquo metasomatizing agent (egZanetti et al 1996 Ionov et al 2002 Bodinier et al 2004)

Judging from the abundance of modal metasomatic phasesthe amphibole dunite (Ke 196525) was probably closest tothe source of the metasomatizing agent The phlogopite-rich nature of Ke 785 which indicates higher K andLILE (Cs Rb Ba) contents in the remaining agent is inagreement with a more fractionated melt or fluid at some-what greater distance from the metasomatic sourceThe Nb abundances in amphibole also agree with this

model as well For example amphibole from Type III-cKe 196525 has comparatively high Nb concentrations(Fig 9c) in line with relatively high DNb

amphmelt values

Fig 9 Trace element composition of amphibole from porphyroclastic spinel harzburgites and dunites from Marsabit [normalized to the primi-tive mantle values of McDonough amp Sun (1995)] Mg-katophorite in Type III-b and -c xenoliths (a^c) and glass-related kaersutite (d) Theshaded fields indicate the compositional variation of all Mg-katophorite analyses (light grey) and the compositional range of LREE-depletedTi-pargasite from the Group I (grt)^spinel lherzolites (dark grey Kaeser et al 2006)

16

constrained by different studies (eg Ionov amp Hofmann1995 LaTourrette et al 1995) In Ti-poor systems such asthose of this study Nb uptake by amphibole is evenenhanced as a result of increased DNb values (Tiepoloet al 2001) Consequently in an event during which amphi-bole crystallizes continuously the remaining fluid or meltwill become progressively depleted in Nb (Ionov et al2002) resulting in formation of Nb-depleted amphibolesfurther away from the source (Zanetti et al 1996 Ionovet al 2002) such Nb-depleted amphibole is found insample Ke 785 as well as inType III-b xenoliths (eg Ke195915 Fig 9a) Amphibole signatures from xenolith suitesmodelled successfully in the context of percolative traceelement fractionation (eg Spitsbergen Ionov et al 2002)show similar ranges (eg in NbTh ratios Fig10a) to thosedocumented for our samples This further supports the

hypothesis that similar percolation mechanisms acted inthe mantle beneath Marsabit

The role of pre-existing heterogeneitiesand exotic mineralsAlthough a model based on trace element fractionationduring melt percolation is supported by textures and canaccount for many compositional features some composi-tional peculiarities remain unexplained First clinopyrox-ene and amphibole in several samples (Ke 785 19651 and196515) are characterized by extremely low HREE abun-dances more than one order of magnitude lower thanin amphiboles from the strongly metasomatizedamphibole dunite Ke 196525 (Fig 9a^c) The mainphases that crystallize during metasomatism are low-TiMg-katophorite and phlogopite However DHREE

amphmelt

Fig 10 Selected trace element ratios in amphiboles from the porphyroclastic spinel harzburgites from Marsabit compared with those ofmantle-derived amphibole from other tectonic settings (XK and XB cratonic kimberlite-hosted and off-cratonic basalt-hosted xenolithsrespectively O orogenic peridotites XS xenoliths from subduction settings) Concentrations (ratios) normalized to the primitive mantlevalues of McDonough amp Sun (1995)

17

Table 5 Representative analyses of phlogopite and apatite (oxides in wt trace elements in mgg)

Type III-b III-c III-b III-c

Mineral phlog phlog phlog phlog phlog phlog phlog apatite apatite

Sample 196515 196515 19653 785 785 785 196525 195915 19653

Texture veinlet interst in opx in opx with amph

in mp

around

chromite

incl in

amph

bleb in

glass

interst

grain

SiO2 3925 3914 4171 4190 4124 4134 4230 SiO2 016 049

TiO2 313 307 009 053 039 037 027 Al2O3 001 nd

Al2O3 1544 1583 1360 1360 1384 1377 1477 P2O5 3768 3715

Cr2O3 112 105 132 147 190 197 063 FeO(tot) 037 029

FeO(tot) 350 347 271 000 289 277 340 MnO 006 nd

MnO nd 002 008 305 004 002 005 MgO 084 091

NiO 023 027 019 023 024 028 023 CaO 4750 4237

MgO 2287 2280 2577 2591 2582 2528 2525 SrO 095 397

CaO nd 002 001 nd 004 001 002 Na2O 079 133

Na2O 215 219 282 258 264 262 208 La2O3 003 007

K2O 719 707 704 706 674 684 683 Ce2O3 088 347

Nd2O3 003 007

F 139 224

Cl 287 092

Total 9488 9493 9571 9641 9587 9535 9590 Totaly 9305 9245

Mg-no 9282 9287 9443 9380 9408 9420 9298

P 369 187 480 339 206 188 362 P 129717 139419

K 71711 72575 60747 79052 70997 69992 70189 K 110 897

Sc 099 110 131 130 088 091 160 Sc 242 079

Ti 15500 16408 890 2839 2096 1807 1736 Ti 235 5294

V 255 284 888 142 108 130 788 V 365 382

Ni 1808 1727 1744 2046 1813 1773 1905 Ni 121 397

Zn 234 221 358 382 317 321 695 Zn 119 096

Ga 111 984 na 210 190 188 235 Ga na na

Rb 185 147 289 183 133 133 342 Rb 015 049

Sr 102 116 775 325 363 350 101 Sr 7138 42233

Y 010 0086 50057 50054 0021 0006 0052 Y 988 1454

Zr 019 019 043 50088 0077 0070 073 Zr 307 101

Nb 506 628 479 177 102 146 210 Nb 024 020

Cs 175 138 261 399 216 180 798 Cs 50013 50036

Ba 1212 1096 1343 298 277 219 1124 Ba 139 126

La 010 0055 0069 50029 0020 50003 50045 La 7460 19048

Ce 0027 0011 50075 50019 014 50003 0078 Ce 6844 25945

Pr 0012 0008 50067 50024 0012 50003 50019 Pr 380 2212

Nd 0019 0035 50405 50079 0065 50009 50224 Nd 749 6530

Sm 0015 50011 50474 50090 0016 50010 50132 Sm 515 747

Eu 0022 0017 50104 50026 0013 0010 50095 Eu 147 189

Gd 50010 50011 5043 5015 50011 0012 017 Gd 314 451

Tb 50002 50002 na 50027 0002 50002 50028 Tb na na

Dy 50015 50017 045 50077 50007 50006 50077 Dy 198 281

Ho 50002 50003 50043 50020 0002 50002 50028 Ho 359 511

Er 50009 50013 5025 5011 50016 50009 5011 Er 903 119

Tm 50002 50002 50088 50013 0002 0003 50018 Tm 119 138

(continued)

18

data (eg LaTourrette et al 1995) indicate that amphibolecrystallization alone is unlikely to deplete the metasomaticagent to such a high degree in HREE The same probablyaccounts for the extreme Nb depletion in amphiboles fromKe 785 relative to the otherType III-c xenolith (Ke 196525 see also Ionov et al 2002)Further clinopyroxene phlogopite and amphibole in the

Type III-b sample Ke 196515 on one hand all showstrongly fractionated REE patterns (eg high LaCe inamphibole Fig 10c) but on the other hand are character-ized by a positiveTi anomaly decoupled from other HFSE(eg Hf and Zr Fig 9b) Trace element fractionationduring amphibole crystallization (ie formation of TypeIII-c amphibole dunites) however should lower Ti con-tents in the remaining fluid or melt (eg Moine et al 2001)Based on textures mineralogy and dominant common

compositional features the hypothesis that Type III-band -c xenoliths were metasomatized by completely inde-pendent metasomatic agents can be ruled out Thereforetwo other possibilities remain to account for the inconsis-tent observations described above (1) inference of otherphases reactants or products that are able to fractionatespecific trace elements present in the mantle beneathMarsabit but not present in the few investigated samples(2) pre-metasomatic compositional heterogeneities similarto the inherited HREE variation inType III-a clinopyrox-ene (see above)Garnet would be an important sink for HREE but

metasomatism occurred in the spinel field after decom-pression with maximum pressures of 515GPa (Kaeseret al 2006) so garnet had already broken down to sym-plectites (Fig 15) Percolation of melt through garnet-bearing websterites prior to metasomatism in Ke 19651and 196515 could be an alternativeWebsterites have been

interpreted to occur in close spatial relationship with por-phyroclastic peridotites in the shallow mantle beneathMarsabit (Kaeser et al 2006) and they occasionally con-tain rutile and ilmenite which could further account forthe strong Nb and Ta depletion in some samples (eg Ke785) The available compositional data from the webster-ites however do not indicate metasomatic modification ofwebsteritic garnet (Olker 2001 B Kaeser unpublisheddata) nor compositional similarities to Group IIIperidotitesCareful study by SEM rules out the presence of exotic

phases such as zircon or Ti-oxides described fromother xenolith suites (eg Bodinier et al 1996 Rudnicket al 1999) Nevertheless it cannot be excluded that themetasomatizing agent percolated through mantle domainscontaining these phases before infiltrating the studiedrocksThe preservation of pre-metasomatic compositional het-

erogeneities is another possibility that must be consideredThat such lsquorock signalsrsquo partly overlap with the lsquometaso-matic signalrsquo is indicated by the HREE variation in TypeIII-a clinopyroxene (see above) and also by other composi-tional parameters For example both Mg-katophorite andphlogopite growing at the expense of spinel have very highCr2O3 contents whereas euhedral Mg-katophorite inthe amphibole dunite (Ke 196525) or in phlogopiteinclusions in Opxm is comparably Cr2O3-poor (Fig 8bTables 4 and 5) Pre-existing heterogeneities would essen-tially be preserved through elements that are relativelycompatible in a given mineral (ie that are not mobilizedduring metasomatism) This would be in agreement withthe preserved HREE variations in Type III-a clinopyrox-ene high Cr2O3 in amphiboles and phlogopite and Tispikes in amphibole from Ke 196515

Table 5 Continued

Type III-b III-c III-b III-c

Mineral phlog phlog phlog phlog phlog phlog phlog apatite apatite

Sample 196515 196515 19653 785 785 785 196525 195915 19653

Texture veinlet interst in opx in opx with amph

in mp

around

chromite

incl in

amph

bleb in

glass

interst

grain

Yb 50014 50011 5047 5015 50011 50019 5017 Yb 803 724

Lu 0002 50002 50061 50014 50002 50002 50020 Lu 115 742

Hf 0019 0018 5025 50053 50012 50006 5019 Hf 50023 50120

Ta 020 022 140 0031 0012 0009 023 Ta 50015 50075

Pb 546 586 125 108 083 098 143 Pb 436 577

Th 50004 50004 50080 0043 0088 0056 50021 Th 2538 1305

U 0012 0011 0069 50018 0038 0030 0031 U 572 278

interst interstitial mp melt pocket na not analysed nd not detected

19

Further very high Mg-numbers in replacive amphibolessupport melt (fluid)^rock interaction at low melt (fluid)rock ratios where Mg and Fe contents are buffered by theambient peridotite (Kelemen et al 1995 Tiepolo et al2001) The slightly lower Mg-number of euhedralamphiboles in Ke 196525 combined with their lowerCr2O3 contents (Fig 8b) suggests a higher melt (fluid)rock ratio which is further supported by considerablylower Mg-number in associated ol-I (Fo88 Table 3)

The limited number of investigated samples does notallow us to decide which of the processes described aboveacted and to what extent However given the heterogene-ity observed in the numerous non-metasomatized xenolithsfrom Marsabit (Kaeser et al 2006) pre-metasomaticfeatures certainly affect the final product of the melt^rockreaction process to a considerable degree

The nature of the metasomatizing agent(s)Graphite stability and oxygen fugacity considerations

The Marsabit xenoliths are unusual in that they occur inan off-cratonic setting yet bear macroscopic graphite Thelatter usually either occurs in kimberlite-hosted peridotites(on-craton) or in pyroxenites from alpine-type peridotitebodies (eg Pearson et al 1994) In the phlogopite perido-tites from Finero graphite has recently been reportedas nanoscopic layers in phlogopite (Ferraris et al 2004)In xenoliths from Marsabit graphite occurs in the sametextural relationship as in kimberlite-hosted peridotitexenoliths (ie millimetre-sized flakes typically associatedwith orthopyroxene see Fig 3c^e and Pearson et al 1994)Graphite inclusions in phlogopite and Opxm couldindicate crystallization during metasomatism by alsquoreducedrsquo C-bearing fluid or melt However precipitationof graphite more probably reflects the relatively reducingconditions in the uppermost mantle prior to metasoma-tism We used the oxygen barometer of Ballhaus et al(1991) based on ferric iron contents in spinel coexistingwith olivine to constrain oxygen fugacity (fO2)conditions prior to metasomatism in the Marsabitxenoliths (Fig 16) The results show that non-metasomatized Group I and IV samples reflect similarlyreducing conditions to the cryptically metasomatizedType III-a samplesAlthough assessing Fe3thornFe2thorn ratios in spinel based

on microprobe analyses can be problematic (Wood ampVirgo 1989 Ballhaus et al 1991) all microprobeanalyses were obtained using the same operationalsettings and therefore give at least the relative fO2

variation between samples The relatively reducingconditions in the pre-metasomatic cold andhighly deformed mantle well below carbon^carbonate^silicate equilibria (Mg2Si2O6thorn 05 CaMg(CO3)2frac14Mg2SiO4thorn 05 CaMgSi2O6thornCthornO2 Eggler amp Baker1982 see Fig 16) and close to the CCO oxygen buffer arein agreement with the presence of graphite in the GroupIII harzburgites Moreover the P^T conditions at whichthe Group III xenoliths partially re-equilibrated are closeto the subsolidus decarbonation of model carbonated lher-zolite (Fig 2 see Lee amp Wyllie 2000 and referencestherein) in agreement with the observed phase assemblageincluding orthopyroxene olivine clinopyroxene andgraphite

Fig 11 Trace element patterns of (a) phlogopite and (b) apatitefrom porphyroclastic spinel harzburgites and dunites from Marsabit[concentrations normalized to the primitive mantle valuesof McDonough amp Sun (1995)] The grey field in (a) indicates thecomposition of Ti-rich phlogopite in Group I (grt)^spl lherzolites(Kaeser et al 2006)

20

Table 6 Representative analyses of silicate glass (oxides in wt trace elements in mgg)

Type III-b III-c Host

Sample 195915 196515 196515 785 196525 19653 19653 19653 195927

Remark next amph next cpx xenol margin next amph next amph melt pocket next apatite next Opxm melt pocket host basanite

SiO2 5306 5775 5161 6096 5657 5938 5974 6530 4882 4410

TiO2 017 157 106 062 030 024 020 023 378 255

Al2O3 2412 2053 2025 1779 2313 1575 1245 1355 1890 1396

Cr2O3 005 001 nd 001 001 nd nd nd 002 na

FeO(tot) 188 247 626 126 252 295 374 265 509 1283

MnO 007 000 026 001 004 005 008 007 018 021

NiO nd nd nd na nd na 006 nd 001 na

MgO 182 129 116 114 122 304 382 252 244 938

CaO 506 257 283 082 044 124 101 039 810 1030

Na2O 1107 966 969 971 1299 1388 1384 1211 673 374

K2O 154 197 465 357 138 121 142 127 300 146

Cl 031 106 038 012 014 023 061 020 015 na

P2O5 041 008 044 009 012 049 233 018 100 059

Total 9956 9896 9860 9609 9887 9845 9929 9846 9821 9912

Mg-no 6329 4816 2489 6175 4632 6474 6458 6289 4552 5658

P 1061 448 1610 230 503 1519 6738 676 2994 na

K 12563 16408 39599 32515 15872 9265 11277 10071 25900 na

Sc 179 184 126 406 151 970 136 848 691 237

Ti 665 7329 4974 2953 1965 1213 1163 1055 18199 15311

V 107 265 235 796 920 210 151 134 234 na

Ni 128 371 92 5575 983 170 5199 5298 199 191

Zn 962 103 109 110 459 440 508 309 819 na

Ga na 173 273 194 220 na na na na na

Rb 181 263 124 546 626 176 398 369 577 369

Sr 427 488 1407 188 881 1644 4997 985 1062 792

Y 543 145 224 752 262 619 158 369 301 299

Zr 466 171 353 761 660 578 365 428 242 216

Nb 416 161 137 260 136 234 164 169 837 647

Cs 014 046 186 125 007 022 102 052 063 na

Ba 238 261 1064 129 168 405 1099 641 625 563

La 698 183 848 144 889 2212 1008 106 537 462

Ce 947 256 131 308 138 333 1265 164 104 823

Pr 655 194 120 330 116 304 975 160 120 na

Nd 146 603 408 101 329 993 256 564 463 400

Sm 142 048 570 179 419 141 379 679 105 851

Eu 041 035 228 082 112 438 103 241 345 267

Gd 062 051 415 158 402 106 245 566 677 729

Tb na 0086 086 020 071 na na na na 111

Dy 090 027 359 098 434 104 171 637 732 na

Ho 023 0037 105 025 095 206 446 104 117 114

Er 045 038 195 058 255 611 153 264 305 na

Tm 009 50039 033 0081 042 085 292 5014 036 052

Yb 062 023 267 044 307 667 120 487 253 220

(continued)

21

Evidence for low-temperature SiO2^Na2O^CO2^H2O-rich melts or fluids

Potential metasomatic agents inferred from the presenceof graphite are C^O^H-rich fluids carbonate melts orCO2-bearing silicate melts The resulting phase assem-blages and geochemical signatures are the result of infiltra-tion of fluid or melt into a cold (750^8508C) shallow(515GPa Fig 2) heterogeneous and locally reduced litho-spheric mantle Metasomatism under such conditions in anopen system must result in complex geochemical signa-tures that reflect the degree of fluid^rock reaction Thismakes it particularly difficult to constrain the exactnature of the lsquooriginalrsquometasomatic agentType III-c xenolith Ke 196525 contains the highest frac-

tion of metasomatic phases and the least fractionatedamphibole trace element patterns The lsquomagmaticrsquo texturesof the Mg-katophorites in this sample suggest direct crys-tallization from a melt or a fluid rather than replacementof a pre-existing phase Therefore hypothetical liquids inequilibrium with the amphiboles in this xenolith couldyield the composition of the initial metasomatic agentsFigure 17 illustrates the trace element patterns of suchmelts and fluids calculated using sets of experimentally orempirically determined mineralmelt (fluid) partitioncoefficients including carbonate melt silicate melt andhydrous fluid (for references see caption to Fig 17)Common features for all calculated metasomatic agents

are negative Zr^Hf^Ti anomalies and LREE enrichment(Fig 17) However previous studies have shown that thesesignatures are indicative of almost all types of metasoma-tism including interaction of peridotite with carbonatiticmelt (Green amp Wallace 1988 Dautria et al 1992 Hauriet al 1993 Rudnick et al 1993 Ionov et al 1996 Yaxleyet al 1998 Coltorti et al 1999) with hydrous fluids andormelts for example originating from dehydration of altered

subducting lithosphere (Maury et al 1992 Zanetti et al1999 Grecurren goire et al 2001 Laurora et al 2001 Parkinsonet al 2003) and interaction of peridotite with volatile-and Si-rich melts that evolved through reactive porousflow (Bedini et al 1997 Ionov et al 2002 2006 Bodinieret al 2004 Rivalenti et al 2004)In the case of the Marsabit xenoliths integration of the

trace element characteristics thermobarometric inferencesand mineralogical constraints favours a Na2O^SiO2^CO2^H2O-rich silicate melt Carbonated melts can prob-ably be ruled out on major element and mineralogical con-siderations First amphiboles are Ca-poor in conflict withcarbonate melt metasomatism which is generally charac-terized by Ca enrichment (Rudnick et al 1993 Neumannet al 2002) Further orthopyroxene is a stable phase andprobably even crystallizes during metasomatism in somesamples (eg Ke 785) At near-solidus conditions coexis-tence of orthopyroxene and carbonate melt is possibleonly at pressures 20GPa (see Fig 2 eg Lee amp Wyllie2000 and references therein) which is considerably higherthan the maximum pressures indicated by the compositionof spinel (515GPa) At such low pressures metasomatismby carbonated melts will convert lherzolite and harzbur-gite immediately into wehrlite which is the major perido-tite type in all xenolith suites where carbonate meltmetasomatism is convincingly demonstrated (Rudnicket al 1993 Yaxley et al 1998 Neumann et al 2002)In Marsabit wehrlites are absent howeverAn Na2O^SiO2^CO2^H2O-rich melt or fluid on the

other hand would explain the Na-rich nature of amphiboleand phlogopite as well as the presence of orthopyroxeneGrowth of metasomatic (and Al-poor) orthopyroxene hasbeen described from several xenolith localities where peri-dotites were infiltrated either by hydrous fluids orSi-rich melts (Smith et al 1999 McInnes et al 2001Morishita et al 2003) Na-rich phlogopite is commonly

Table 6 Continued

Type III-b III-c Host

Sample 195915 196515 196515 785 196525 19653 19653 19653 195927

Remark next amph next cpx xenol margin next amph next amph melt pocket next apatite next Opxm melt pocket host basanite

Lu 014 0050 026 0041 046 080 140 058 040 031

Hf 026 052 535 014 076 110 5087 124 568 504

Ta 50080 033 638 0022 123 101 784 801 394 403

Pb 541 448 280 255 840 797 260 749 493 na

Th 194 296 160 466 703 185 247 105 534 506

U 545 069 478 099 152 447 724 216 171 126

nd not detected na not analysedAverage composition of the strongly alkaline basanites from Marsabit (Volker 1990)

22

Fig 12 Total alkalis vs SiO2 (a) Harker-type co-variation diagrams (b^e) and incompatible trace element co-variation diagram (f) showingthe compositional variability of silicate glass found in the porphyroclastic spinel harzburgites and dunites from Marsabit In (b) binary mixinglines between the typical composition of the alkali-rich glass (large open diamond) and the associated microlites are shown (each increment is10wt)They illustrate that the glass does not represent the derivate of host lava infiltration and fractional crystallization Subalkaline glass inGroup I (grt)^spl lherzolites from Marsabit is shown for comparison and is interpreted to derive from incongruent melting of Ti-pargasite(Kaeser 2006)

23

found as a reaction product between anhydrous mafic toultramafic rocks and H2O-rich evolved liquids (Costaet al 2001 and references therein) or H2O-bearing fluids(eg Odling 1994) The latter study was an experiment at20GPa simulating the effects of decreasing temperatureson the phase assemblages and their composition duringmetasomatism using a model peridotitethornC^O^H fluidThe coolest part of Odlingrsquos (1994) experiment (510758C)contained a strikingly similar phase assemblage (kato-phorite phlogopite with 2wt Na2O and Al-poororthopyroxene) to the Marsabit samples interpreted tohave formed during sub-solidus fluid^solid interactionA compositionally similar phase assemblage (highamounts of opx Si-rich hornblende Na-rich phlogopite)resulted from experiments simulating peridotite interac-tion with Si-rich melt (dacite) at lower P^T conditions(9508C at 092GPa Prouteau amp Scaillet 2003)In this study the trace element signatures of Group III-c

Mg-katophorite and the calculated metasomatic agentsfurther suggest that the metasomatizing agent was a meltrather than a fluid Hypothetical fluids in equilibrium witheuhedral Mg-katophorite from sample Ke 196525 havehigh NbLILE (eg Rb Ba) and low LILEREE ratios(Fig 17) Because experimental data predict the oppositefor fluids (LILE enriched over HFSE and REE egManning 2004 and references therein) a fluid with asignature such as that indicated in Fig 17 is unrealistic

Fig 13 Trace element patterns of silicate glass in porphyroclasticspinel harzburgites and dunites from Marsabit compared withthe trace element composition of associated clinopyroxene (Dim)micro-phenocrysts xenolith-hosting basanites from Marsabit andglass interpreted to represent xenolith^host interaction [concentra-tions normalized to the primitive mantle values of McDonough ampSun (1995)]

Table 7 Representative analyses of carbonates (oxides in

wt trace elements in mgg)

Group III-b III-b III-c III-c III-c

Mineral Dol Mg-Cc Mg-Cc Mg-Cc Mg-Cc

Sample 195915 196515 785 19653 19653

Texture in glass in glass in glass in glass in glass

SiO2 na 003 na nd 003

FeO(tot) 016 011 010 010 006

MnO 001 nd nd nd nd

MgO 1922 236 190 125 517

CaO 3189 5313 5377 5391 4853

Na2O 003 nd 004 nd 003

K2O na 002 na 001 nd

SrO 003 010 008 004 021

BaO 003 nd nd na na

CO21 4618 4444 4442 4377 4391

Total 9756 10021 10032 9909 9794

Mg-no 9954 9745 9713 9554 9931

Ca-no 5439 9418 9531 9687 8709

P 220 334 137 580 589

K 970 212 642 105 155

Sc 5065 50150 027 5027 50076

Ti 975 189 242 528 196

V 5085 0481 5042 5037 021

Ni 900 673 517 5294 593

Zn 459 5089 113 296 045

Ga na 5014 5013 na na

Rb 0084 50072 50074 50060 0046

Sr 593 860 371 251 4274

Y 012 011 130 704 015

Zr 261 024 012 097 756

Nb 144 019 0056 692 185

Cs 50063 50011 50018 50039 50008

Ba 126 230 5024 029 271

La 034 057 084 136 025

Ce 032 013 134 997 023

Pr 006 006 018 095 0026

Nd 030 013 068 228 0078

Sm 5044 50091 0074 048 50018

Eu 5012 50019 0051 020 50008

Gd 5037 50090 014 051 0026

Tb na 50010 0036 na na

Dy 5019 50039 0089 058 50038

Ho 50029 0013 0044 011 50006

Er 5012 50041 0083 033 50019

Tm 50052 50013 0016 0084 50006

Yb 5043 50063 014 027 50040

Lu 50072 50010 0011 0066 50008

(continued)

24

At the relatively low maximum pressures (515GPa) asupercritical liquid is not expected either (eg Kesselet al 2005)Finally a relatively hydrous metasomatizing agent

would be consistent with formation of Mg-katophoriteswith lowTi and presumably low Fe3thorn contents (given therelatively reducing conditions) which in turn imply high(OH)^ contents In other words partial dehydrogenationtypically observed in common (Ti-rich) mantle amphibole

(eg Popp et al 1995 Tiepolo et al 2001 and referencestherein) cannot have played an important role in thiscase as H deficiency is mainly the result of coupled substi-tutions involvingTi and Fe3thorn (Popp et al 1995) In conclu-sion the metasomatizing agent was most probably aNa2O^SiO2^CO2^H2O-rich meltComparison with experimental studies further con-

strains the low-temperature regime in which metasoma-tism occurred In ultramafic (eg Niida amp Green 1999)basaltic (eg Rapp 1995) and Si-enriched systems (egtrachybasaltic Barclay amp Carmichael 2004) amphibole(in the latter study together with phlogopite) is confinedto temperatures lower than 1000^10508C at pressures520GPa (Fig 2)Thus crystallization of the early metaso-matic phase assemblage can be explained by infiltration ofa silicate melt that rapidly cooled upon contact with theambient peridotite (750^8508C prior to metasomatismFig 2) However this low-T stage is based on Ca-in-opxthermometry (see Brey amp Kolaquo hler 1990 Kaeser et al2006 Table 1) and therefore probably underestimates theeffective temperature conditions during metasomatismThis can be explained by low diffusion rates and proximityto the closure temperature for the diopside^enstatiteexchange reaction at temperatures lower than 8508C(eg Smith 1999 and references therein) The lower tem-perature limit at which silicate melt metasomatism ispossible (ie defined by the meltrsquos solidus and viscosity)depends strongly on volatile and SiO2 contentsExperiments by Draper amp Green (1997) for exampleshowed that highly silicic melts (52^64wt SiO2) canbe in equilibrium with mantle harzburgite at temperaturesas low as 8508C (at 10^20GPa under anhydrous andC^O^H-fluid-bearing conditions) These are howeverextreme compositions for mantle melts The actual SiO2

content of the metasomatizing agent in this case is difficultto constrain from the metasomatic mineral assemblage ascompositions may have varied considerably during reac-tive percolation (eg including Si-releasing reactions suchas cpx replacement by amphibole versus Si-consumingreactions such as Opxm crystallization) Moreover it isunlikely that a melt persists at temperatures muchbelow 9008C which is the solidus of fertile lherzolite(ie primitive upper mantle composition) underH2O-saturated conditions at 15GPa (Grove et al 2006see Fig 2) Below the solidus residual volatile-enrichedfluids may percolate further for example forming the lessmetasomatized Type III-c xenoliths (eg Ke 785) as wellas Type III-b and -a peridotites (see also Bailey 1987)Relatively cold conditions would also explain the veryhigh compositional gradients developed on small scales inType III-a samples (Fig 15) Given the absence of chloritein all investigated samples the lower temperature limitof such fluid metasomatism is 8008C (eg Fumagalli ampPoli 2005 see Fig 2)

Table 7 Continued

Group III-b III-b III-c III-c III-c

Mineral Dol Mg-Cc Mg-Cc Mg-Cc Mg-Cc

Sample 195915 196515 785 19653 19653

Texture in glass in glass in glass in glass in glass

Hf 5016 50038 50036 50074 50013

Ta 0048 50012 50016 50031 0008

Pb 5024 858 5021 074 018

Th 0021 50020 50017 50028 50005

U 060 094 086 600 148

Mg-Cc Mg-calcite Dol dolomite nd not detectedna not analysed Ca-numberfrac14 100Ca(CathornMg)1Calculated from stoichiometry

Fig 14 Trace element patterns of glass-hosted carbonates in TypeIII-b and -c spinel harzburgite and dunite xenoliths from Marsabit[concentrations normalized to the primitive mantle values ofMcDonough amp Sun (1995)] The grey field outlines the compositionalrange of carbonates found in other peridotite xenoliths (Ionov ampHarmer 2002 and references therein) Carbonate material inter-preted as quenched carbonatite melt found in peridotite xenolithsfrom Kerguelen (Moine et al 2004) is shown for comparison(bold line)

25

Finally the presence of apatite even though oftentaken as indicator for carbonatite-type metasomatism(eg Yaxley et al 1998) is not in conflict with metasoma-tism by silicate melts Apatite has also been shown to crys-tallize from volatile and alkali-rich derivatives formedduring reactive porous flow of originally mafic silicatemelts (Woodland et al 1996 Bedini et al 1997 Bodinieret al 2004 Ionov et al 2006)

FORMATION OF THE LATE GLASS -BEAR ING ASSEMBLAGESTextures strongly suggest that the glass-bearing assem-blages formed later and at the expense of the early metaso-matic assemblages (Fig 4a and d) Features such as thereappearance of clinopyroxene (Na-rich Cr diopsidemicro-phenocrysts) a phase replaced by the early

Fig 15 Detailed REE systematics in primary clinopyroxene (cpx-I) in a porphyroclastic cryptically metasomatized spinel harzburgite(sample Ke 19651) showing the transition fromType III-a (Zone A and B) toType III-b (Zone C) on a scale of a few centimetres The numberson the photomicrographs (crossed polars) correspond to single laser ablation microprobe pits (for further explanation see text)

26

metasomatic assemblage indicate that melt pocket forma-tion reflects a significant change in P^T^X space in partsof the mantle beneath Marsabit after earlier metasoma-tism Based on geochemical arguments and mass-balanceconsiderations we will show that (1) the late glass-bearingassemblages are closely related to early metasomaticassemblages mainly representing breakdown productsand (2) breakdown was induced by infiltration of aCO2-rich fluid in the mantle probably followed andenhanced by heating and decompression melting duringxenolith transport in the host magma

Evidence for in situ partial meltingBSE imaging reveals that the phases becoming unstableare amphibole orthopyroxene and to minor extentsspl-I phlogopite cpx-I and apatite Compositional(inter- and intra-xenolith) heterogeneity of silicate glass istexture controlled and varies as a function of theassociated precursor phases (Fig 12a^f)TiO2 for exampleshows higher concentrations in glass from xenolith Ke196515 containing Ti-enriched amphibole (Fig 12c)whereas K2O contents are highest in phlogopite-rich sam-ples (eg Ke 785 Fig 12d) Elevated P2O5 contents arefound in glass adjacent to apatite (Fig 12e) whichmirrors at the same time the highTh and LREE contents

of apatite (Fig 12f) Significant Si enrichment is found inglass coating orthopyroxene (Fig 12d) supporting incon-gruent melting of orthopyroxene (see below) Even thoughthe melt pockets are connected along grain boundaries thestrong compositional variation (on inter- and intra-xeno-lith scale) suggests that the mobility of the melt was lim-ited or that the melting event was very short-lived (fortiming constraints see below)The co-variation plots in Fig 12 further reveal consider-

able compositional differences between all types of xeno-lith glass and the host basanite These data indicate thatthe glass cannot be the product of xenolith^host lava inter-action during ascent to the surface Also back-mixing ofmicro-phenocrysts into the melt fails to derive the glassfrom a host-related melt (Fig 12b) Glass from xenolithmargins however is compositionally closer to the bulkhost basanite (eg Fig 12b) in line with a direct derivationfrom xenolith^host lava interaction Its more evolvedmajor element composition (eg higher SiO2 lowerMg-number Fig 12b) is probably the result of fractiona-tion of basanite phenocrysts (eg olivine augite andtitanomagnetite Volker 1990) Host lava^xenolith interac-tion is further supported by the trace element signature ofglass at the margin of the xenoliths which is closely similarto that of the Marsabit basanites (Fig 13 Volker 1990)The same characteristics are observed for intra-xenolithglass in the unusual kaersutite-bearing sample Ke 195927

Fig 16 Oxygen fugacities relative to the quartz^fayalite^magnetitebuffer (QFM equation of OrsquoNeill 1987) as a function of temperatureat assumed pressure of 1GPa calculated using the composition ofcoexisting spinel and olivine (ol-I^spl-I pairs and Chrm^Olm pairsrespectively) based on the oxygen barometer of Ballhaus et al (1991)The graphite^CO2 equilibrium was calculated after Frost amp Wood(1998) and the graphite^carbonate^silicate equilibrium was calcu-lated using the equation of Eggler amp Baker (1982) (See text for furtherexplanation)

Fig 17 Calculated metasomatic agents in equilibrium with the euhe-dral Mg-katophorites from the Type III-c amphibole dunite Ke 196525 Amphibolehydrous fluid and amphibolecarbonatite melt parti-tion coefficients are from Adam et al (1997) and Adam amp Green(2001) respectively Silicate melts have been calculated using amphi-boleSi-rich melt partition coefficients from Chazot et al (1996b) andamphibolebasanite partition coefficients from LaTourrette et al(1995) The compositional range of the Marsabit basanites and alkalibasalts (Volker 1990) is shown for comparison

27

Xenolith glass unrelated to the host lava and melt pock-ets similar to those reported here in association with resi-dual phases (eg amphibole or clinopyroxene) has beenreported from many xenolith localities (eg Frey ampGreen 1974 Ionov et al 1994 1996 Chazot et al 1996a1996b Yaxley et al 1997 Laurora et al 2001 Bali et al2002) Residual phases within the melt pockets stronglyindicate in situ partial melting within the xenolithsHowever a variety of processes may trigger such in situ

melting including (1) closed-system decompression melt-ing of either anhydrous (Maale amp Printzlau 1979) orhydrous phases (Frey amp Green 1974 Francis 1976) duringxenolith ascent in the host lava (2) melting during interac-tion of the xenoliths with the host basaltic melt duringxenolith ascent (eg Zinngrebe amp Foley 1995 Shaw ampKlulaquo gel 2002 Shaw et al 2006) (3) melting in the mantleprior to xenolith entrainment induced by an infiltratingfluid (ie flux melting eg Jones et al 1983 Ionov et al1994 Demecurren ny et al 2004) However most studies agreethat that partial melting occurred very shortly beforeandor during xenolith exhumation by the host magmaThus a combination of metasomatically triggered meltingfollowed andor assisted by decompression melting duringxenolith transport may be a realistic alternative in somecases (eg Dawson 2002)Recently Ban et al (2005) have shown in a very detailed

mass-balance study on glass-bearing xenoliths from theWest Eifel that the weight proportions of glass and theassociated micro-phenocrysts match the composition ofthe preserved parent amphibole We used the sameapproach and performed simple mass-balance calculations(least-squares fitting) to constrain the conclusions drawnfrom petrography and to test which phase contributedand to what extent to the formation of the melt pocketsThe results reported in Table 8 show that in most casesamphibole-melting can account successfully for the meltpocket composition and only a minor amount (520) ofother phases (orthopyroxene andor phlogopite) needs tobe added to the reactant Such a contribution of otherphases agrees with textural observation (eg Fig 4cshowing Dim growing on resorbed Opxm) as well as withthe conclusions of Ban et al (2005) who stated that moreSi-rich glass (ie 454wt) necessitates orthopyroxenecontributing to the meltTrace element patterns of the bulk glassthornmicro-

phenocryst pockets calculated using the measured traceelement abundances of glass and micro-phenocrysts andthe respective mass-balance parameters (Table 8) arebroadly identical to the measured amphibole composition(Fig 18a and b) Higher LILE (Cs Rb Ba) concentrationsin the calculated bulk melt patches in sample Ke 785 canbe balanced by adding 7wt of phlogopite (Fig 18b)which is in line with mass balance (Table 8) The dominantcontrol of amphibole to account for the trace element

budget of the melt pockets is similar to results found inother suites of glass-bearing xenoliths (Chazot et al1996a Yaxley amp Kamenetsky 1999 Laurora et al 2001)Further the obtained phase proportions for the melt pock-ets are in good agreement with the results from image ana-lysis (Table 1) In addition they are similar to the glass^cpx^ol proportions recorded in recent experiments on the

Table 8 Results of mass-balance calculations to test melt

pocket formation by incongruent melting of the early metaso-

matic assemblages

Sample 195915 785 785 196525 196525 19653

Reactant

amph 1000 1000 0820 1000 0944 mdash

opx mdash mdash 0078 mdash 0056 0406

phl mdash mdash 0102 mdash mdash mdash

glass mdash mdash mdash mdash mdash 05941

SiO2 4484 4989 4964 5129 5167 5898

TiO2 014 039 036 025 024 018

Cr2O3 232 291 258 127 121 009

Al2O3 1368 762 769 800 757 961

FeO(tot) 336 314 333 343 357 394

MnO 006 008 008 012 012 010

NiO 009 013 014 011 011 004

MgO 1844 2029 2197 2040 2122 1625

CaO 854 783 644 966 913 078

Na2O 460 500 436 504 476 843

K2O 049 032 096 042 040 060

Product

glass 0389 0322 0335 0292 0284 07032

ol 0243 0259 0315 0266 0286 0273

cpx 0288 0373 0308 0442 0431 0024

spl 0052 0032 0028 mdash mdash mdash

carb 0028 0014 0014 mdash mdash mdash

SiO2 4477 5027 4986 5207 5181 5903

TiO2 013 041 039 015 014 017

Cr2O3 213 314 272 109 106 015

Al2O3 1364 764 770 798 776 957

FeO(tot) 374 331 351 322 332 350

MnO 007 007 007 009 009 006

NiO 011 013 015 011 012 007

MgO 1834 2024 2196 2063 2141 1635

CaO 854 759 632 952 929 080

Na2O 449 385 388 499 485 860

K2O 060 096 100 015 014 091

r2 023 200 035 084 028 033

Prefix Ke omitted1Low-Si glass 2High-Si glass (see text for explanation)

28

melting behaviour of amphibole in wehrlites at tempera-tures between 1175 and 12508C and pressures of05^10GPa (Mecurren dard et al 2006) These temperatures arein good agreement with the estimates for the formation ofolivine and chromite micro-phenocrysts in the melt

pockets (Fig 16) The heat supply required for amphibolemelting in the formerly low-T metasomatized Group IIIxenoliths (T 200^3008C) may come from local heatingshortly before or during xenolith entrainment in the hostlava or from fluid infiltration that triggered melting (seediscussion below)Finally formation of the most Si-enriched melt (adjacent

to Opxm in Group III-c samples) can be modelled assum-ing progressing interaction between infiltrating melt andorthopyroxene where the latter melts incongruently

opxthornmelt1 olthornmelt2 cpx ethseeTable 8THORN eth1THORN

To calculate reaction (1) we chose the least evolved glassobserved in melt patches in sample Ke 19653 (SiO2

5921wt Mg-number 6465) to represent the initialmelt1 whereas melt2 adjacent to opx is SiO2-rich(6530wt) Trace element abundances in glass next toorthopyroxene are shifted to lower concentrations(Table 6) which is in agreement with dilution caused byaddition of a trace element-poor orthopyroxene compo-nent The resultant phase proportions are similar to thoseobserved at orthopyroxene^melt interfaces (ie higholivine and low cpx abundances) and are in agreementwith experimental studies (Shaw et al 1998)

Partial melting triggered by fluid fluxMass-balance calculations provide an explanation for meltpocket formation by in situ partial melting Thus onemight conclude that melting is simply the result of decom-pression and heating during xenolith transport in the hostmagma However there are two points that argue thatmelting was triggered by the infiltration of a fluid phaseshortly before the xenoliths were sampled by the hostmagma (1) the presence of occasionally high amounts ofcarbonate included in the melt pockets (2) the extremelyNa-rich clinopyroxene (Dim) compositions either includedas micro-phenocrysts in the melt pocket or present asequilibrated grains around chromite in the single TypeIII-d xenolith (Fig 3f) Both arguments will be discussedin the next section

Formation of carbonates

Textures and compositions of carbonate globules can beexplained by carbonate precipitation from a CO2-bearingfluid As demonstrated by several studies highCa(CathornMg) values (Table 7) and low REE concentra-tions are strong arguments that the carbonates are cumu-late crystals precipitated from a CO2-rich melt or afluid and not quenched carbonated melts (Lee et al 2000Ionov amp Harmer 2002) Oscillatory zoning (Fig 5a and b)of individual carbonate crystals within singleglobules corroborates this interpretation The roundedcontacts toward glass (globules) are also in line with thesefindings Similar globular forms were reported fromexperimental studies and do not a priori imply the

Fig 18 Calculated trace element characteristics of reactants (in mostcases amphibolethorn orthopyroxene filled symbols) and products(melt pockets open symbols) of the lsquoamphibole meltingrsquo model[see equation (1) in text] The relative phase proportions wereobtained by mass-balance calculations and are given in Table 8 Thedashed lines in (b) show that the high LILE concentrations obtainedfor the product (bulk melt pocket) can be explained by adding about7wt of phlogopite to the reactant (amphibolethorn orthopyroxeneTable 8) Further explanation is given in the text

29

presence of immiscible carbonate and silicate melts(eg Lee amp Wyllie 1996)The high amount of carbonate in some melt pockets

however (eg Fig 4e) does need further explanation asthe incongruent melting reaction (1) does not account forthe CO2 required for carbonate precipitation duringamphibole melting Therefore amphibole breakdowneither occurred in an open system with addition of at leastCO2 (eg Laurora et al 2001 Ban et al 2005) or anotherpre-existing phase acted as a source of carbon Oxidationof graphite texturally identified as a part of the earlyassemblage (eg Fig 3d) could account for some of theCO2 necessary for carbonate formation A change in fO2

conditions is also indicated when the composition of glass-related spinel (Chrm) and Olm are used for oxygen baro-metry which yields oxygen fugacities well on the oxidizedside of the carbon^carbonate^silicate buffer (Fig 16)However increasing fO2 triggered by closed-systemdecompression-induced melting alone does not appear fea-sible Also textures such as straight contacts between gra-phite and Mg-calcite (Fig 4c) suggest that carbonategrows on graphite (ie indicating local fO2 conditions cor-responding to carbon^carbonate equilibrium) rather thanreplacing it We therefore interpret melt pocket formationto be triggered by the infiltration of a CO2-rich fluid(ie flux melting see also Ionov et al 1994) This interpre-tation also accounts for the presence of abundant vugs inthe glass that could have contained a fluid (Ionov et al1994 2006) which was presumably lost during sample pre-paration However we emphasize here that late fluid infil-tration had only subtle effects on the trace elementcomposition of Type III-b and -c xenoliths

Formation of Na-rich Cr diopside

Infiltration of CO2-rich fluidwould also explain the Na-richcomposition of Dim micro-phenocryst cores (Fig 6a and b)Ionov et al (2005) noted recently that clinopyroxenes inmeltpockets are commonly characterized by Na2O contents51wt mainly as a result of low cpxmelt partition coeffi-cients at low pressures (eg Blundy et al 1995) Thus veryhighNa2Ocontents in Dimmicro-phenocrysts (Fig6a) sug-gest replacement of amphibole by Dim and eventually Olmand Chrm during reaction with a fluid at subsolidus condi-tions An origin of micro-phenocrysts by replacementrather than crystallization from a melt is indicated by theidentical crystallographic orientation of individual Dimcrystals in singlemelt pockets (Fig4a)Once the solidus of the precursor phase(s) was reached

(ie 10508C for amphibole Niida amp Green 1999) silicatemelt was generated in situ and led to the formation of clin-opyroxene rims with lower Na contents (Fig 6b) and thedevelopment of euhedral shapes in contact with melt (egFig 4a and e) Eventual immiscibility between remainingfluid and newly formed melt (eg during xenolith ascent)would then explain the presence of carbonate globules and

vugs enclosed in glass Likewise different origins of meltand carbonate were recently proposed for similar meltpockets from Hungarian xenoliths by Demecurren ny et al(2004) who found oxygen isotope disequilibrium betweencarbonate globules and the host glass Such a two-stepmodel (fluid-induced replacement followed by melt gen-eration) can be explained by CO2-fluid infiltration occur-ring within the mantle at sub-solidus conditions ultimatelyfollowed by xenolith entrainment in the host magma andtransport to the surfaceThe host magma possibly suppliedthe additional heat necessary for melt generationFast decompression would further enhance melting andcould also account for the lower Na contents in Dimmicro-phenocryst rims as a result of decreasing cpxmeltpartition coefficients at decreasing pressures (Blundyet al 1995)Rapid replacement of amphibole ( phlogopite) by an

anhydrous phase assemblage indicates that the fluid waswater-poor (eg low H2OCO2 ratio see Ionov et al2006)This is corroborated by the fact that Mg-katophorite(presumably OH-rich see above) is occasionally rimmedby a residual kaersutitic selvage (Fig 4d) Such Ti-richmantle amphiboles are commonly OH-poor as a result ofpartial dehydrogenation (eg Tiepolo et al 2001 and refer-ences therein)Finally the presence of theType III-d xenolith contain-

ing texturally equilibrated clinopyroxene of almost identi-cal composition to Dim micro-phenocrysts in melt pockets(Table 2 Figs 6a and 7b) further strengthens the argu-ment that CO2-fluid percolation started in the mantlebefore entrainment (ie enough time elapsed to allow thisrock to anneal completely after the late metasomaticassemblage had formed) A similar conclusion can bedrawn for the clearly host-related glass found in xenolithKe 195927 (eg Fig 13b) ThisType III-c sample is charac-terized by the presence of Dim micro-phenocrysts partlyovergrown by mineral rims clearly related to the host basa-nite magma (kaersutite Ti-augite and olivine withFo885 Fig 4f Tables 3 and 4 and supplementary dataat httppetrologyoxfordjournalsorg) Kaersutite hasmajor and trace element characteristics similar to typicalbasanitic and alkali basaltic amphibole phenocrysts(Fig 9d) The special phase assemblages in this xenolithindicates that host-related basanitic melts intruded someType III-c xenoliths and partly overprinted the assemblageformed slightly earlier by in situ derived alkali-rich peralu-minous melts fully preserved in other samples (eg Ke19653 or Ke 785)

ORIGIN OF METASOMATIZ INGAGENTS T IM ING AND TECTONICSETT INGAs noted above late metasomatism was probably short-lived started at mantle depths and probably continued

30

constrained by different studies (eg Ionov amp Hofmann1995 LaTourrette et al 1995) In Ti-poor systems such asthose of this study Nb uptake by amphibole is evenenhanced as a result of increased DNb values (Tiepoloet al 2001) Consequently in an event during which amphi-bole crystallizes continuously the remaining fluid or meltwill become progressively depleted in Nb (Ionov et al2002) resulting in formation of Nb-depleted amphibolesfurther away from the source (Zanetti et al 1996 Ionovet al 2002) such Nb-depleted amphibole is found insample Ke 785 as well as inType III-b xenoliths (eg Ke195915 Fig 9a) Amphibole signatures from xenolith suitesmodelled successfully in the context of percolative traceelement fractionation (eg Spitsbergen Ionov et al 2002)show similar ranges (eg in NbTh ratios Fig10a) to thosedocumented for our samples This further supports the

hypothesis that similar percolation mechanisms acted inthe mantle beneath Marsabit

The role of pre-existing heterogeneitiesand exotic mineralsAlthough a model based on trace element fractionationduring melt percolation is supported by textures and canaccount for many compositional features some composi-tional peculiarities remain unexplained First clinopyrox-ene and amphibole in several samples (Ke 785 19651 and196515) are characterized by extremely low HREE abun-dances more than one order of magnitude lower thanin amphiboles from the strongly metasomatizedamphibole dunite Ke 196525 (Fig 9a^c) The mainphases that crystallize during metasomatism are low-TiMg-katophorite and phlogopite However DHREE

amphmelt

Fig 10 Selected trace element ratios in amphiboles from the porphyroclastic spinel harzburgites from Marsabit compared with those ofmantle-derived amphibole from other tectonic settings (XK and XB cratonic kimberlite-hosted and off-cratonic basalt-hosted xenolithsrespectively O orogenic peridotites XS xenoliths from subduction settings) Concentrations (ratios) normalized to the primitive mantlevalues of McDonough amp Sun (1995)

17

Table 5 Representative analyses of phlogopite and apatite (oxides in wt trace elements in mgg)

Type III-b III-c III-b III-c

Mineral phlog phlog phlog phlog phlog phlog phlog apatite apatite

Sample 196515 196515 19653 785 785 785 196525 195915 19653

Texture veinlet interst in opx in opx with amph

in mp

around

chromite

incl in

amph

bleb in

glass

interst

grain

SiO2 3925 3914 4171 4190 4124 4134 4230 SiO2 016 049

TiO2 313 307 009 053 039 037 027 Al2O3 001 nd

Al2O3 1544 1583 1360 1360 1384 1377 1477 P2O5 3768 3715

Cr2O3 112 105 132 147 190 197 063 FeO(tot) 037 029

FeO(tot) 350 347 271 000 289 277 340 MnO 006 nd

MnO nd 002 008 305 004 002 005 MgO 084 091

NiO 023 027 019 023 024 028 023 CaO 4750 4237

MgO 2287 2280 2577 2591 2582 2528 2525 SrO 095 397

CaO nd 002 001 nd 004 001 002 Na2O 079 133

Na2O 215 219 282 258 264 262 208 La2O3 003 007

K2O 719 707 704 706 674 684 683 Ce2O3 088 347

Nd2O3 003 007

F 139 224

Cl 287 092

Total 9488 9493 9571 9641 9587 9535 9590 Totaly 9305 9245

Mg-no 9282 9287 9443 9380 9408 9420 9298

P 369 187 480 339 206 188 362 P 129717 139419

K 71711 72575 60747 79052 70997 69992 70189 K 110 897

Sc 099 110 131 130 088 091 160 Sc 242 079

Ti 15500 16408 890 2839 2096 1807 1736 Ti 235 5294

V 255 284 888 142 108 130 788 V 365 382

Ni 1808 1727 1744 2046 1813 1773 1905 Ni 121 397

Zn 234 221 358 382 317 321 695 Zn 119 096

Ga 111 984 na 210 190 188 235 Ga na na

Rb 185 147 289 183 133 133 342 Rb 015 049

Sr 102 116 775 325 363 350 101 Sr 7138 42233

Y 010 0086 50057 50054 0021 0006 0052 Y 988 1454

Zr 019 019 043 50088 0077 0070 073 Zr 307 101

Nb 506 628 479 177 102 146 210 Nb 024 020

Cs 175 138 261 399 216 180 798 Cs 50013 50036

Ba 1212 1096 1343 298 277 219 1124 Ba 139 126

La 010 0055 0069 50029 0020 50003 50045 La 7460 19048

Ce 0027 0011 50075 50019 014 50003 0078 Ce 6844 25945

Pr 0012 0008 50067 50024 0012 50003 50019 Pr 380 2212

Nd 0019 0035 50405 50079 0065 50009 50224 Nd 749 6530

Sm 0015 50011 50474 50090 0016 50010 50132 Sm 515 747

Eu 0022 0017 50104 50026 0013 0010 50095 Eu 147 189

Gd 50010 50011 5043 5015 50011 0012 017 Gd 314 451

Tb 50002 50002 na 50027 0002 50002 50028 Tb na na

Dy 50015 50017 045 50077 50007 50006 50077 Dy 198 281

Ho 50002 50003 50043 50020 0002 50002 50028 Ho 359 511

Er 50009 50013 5025 5011 50016 50009 5011 Er 903 119

Tm 50002 50002 50088 50013 0002 0003 50018 Tm 119 138

(continued)

18

data (eg LaTourrette et al 1995) indicate that amphibolecrystallization alone is unlikely to deplete the metasomaticagent to such a high degree in HREE The same probablyaccounts for the extreme Nb depletion in amphiboles fromKe 785 relative to the otherType III-c xenolith (Ke 196525 see also Ionov et al 2002)Further clinopyroxene phlogopite and amphibole in the

Type III-b sample Ke 196515 on one hand all showstrongly fractionated REE patterns (eg high LaCe inamphibole Fig 10c) but on the other hand are character-ized by a positiveTi anomaly decoupled from other HFSE(eg Hf and Zr Fig 9b) Trace element fractionationduring amphibole crystallization (ie formation of TypeIII-c amphibole dunites) however should lower Ti con-tents in the remaining fluid or melt (eg Moine et al 2001)Based on textures mineralogy and dominant common

compositional features the hypothesis that Type III-band -c xenoliths were metasomatized by completely inde-pendent metasomatic agents can be ruled out Thereforetwo other possibilities remain to account for the inconsis-tent observations described above (1) inference of otherphases reactants or products that are able to fractionatespecific trace elements present in the mantle beneathMarsabit but not present in the few investigated samples(2) pre-metasomatic compositional heterogeneities similarto the inherited HREE variation inType III-a clinopyrox-ene (see above)Garnet would be an important sink for HREE but

metasomatism occurred in the spinel field after decom-pression with maximum pressures of 515GPa (Kaeseret al 2006) so garnet had already broken down to sym-plectites (Fig 15) Percolation of melt through garnet-bearing websterites prior to metasomatism in Ke 19651and 196515 could be an alternativeWebsterites have been

interpreted to occur in close spatial relationship with por-phyroclastic peridotites in the shallow mantle beneathMarsabit (Kaeser et al 2006) and they occasionally con-tain rutile and ilmenite which could further account forthe strong Nb and Ta depletion in some samples (eg Ke785) The available compositional data from the webster-ites however do not indicate metasomatic modification ofwebsteritic garnet (Olker 2001 B Kaeser unpublisheddata) nor compositional similarities to Group IIIperidotitesCareful study by SEM rules out the presence of exotic

phases such as zircon or Ti-oxides described fromother xenolith suites (eg Bodinier et al 1996 Rudnicket al 1999) Nevertheless it cannot be excluded that themetasomatizing agent percolated through mantle domainscontaining these phases before infiltrating the studiedrocksThe preservation of pre-metasomatic compositional het-

erogeneities is another possibility that must be consideredThat such lsquorock signalsrsquo partly overlap with the lsquometaso-matic signalrsquo is indicated by the HREE variation in TypeIII-a clinopyroxene (see above) and also by other composi-tional parameters For example both Mg-katophorite andphlogopite growing at the expense of spinel have very highCr2O3 contents whereas euhedral Mg-katophorite inthe amphibole dunite (Ke 196525) or in phlogopiteinclusions in Opxm is comparably Cr2O3-poor (Fig 8bTables 4 and 5) Pre-existing heterogeneities would essen-tially be preserved through elements that are relativelycompatible in a given mineral (ie that are not mobilizedduring metasomatism) This would be in agreement withthe preserved HREE variations in Type III-a clinopyrox-ene high Cr2O3 in amphiboles and phlogopite and Tispikes in amphibole from Ke 196515

Table 5 Continued

Type III-b III-c III-b III-c

Mineral phlog phlog phlog phlog phlog phlog phlog apatite apatite

Sample 196515 196515 19653 785 785 785 196525 195915 19653

Texture veinlet interst in opx in opx with amph

in mp

around

chromite

incl in

amph

bleb in

glass

interst

grain

Yb 50014 50011 5047 5015 50011 50019 5017 Yb 803 724

Lu 0002 50002 50061 50014 50002 50002 50020 Lu 115 742

Hf 0019 0018 5025 50053 50012 50006 5019 Hf 50023 50120

Ta 020 022 140 0031 0012 0009 023 Ta 50015 50075

Pb 546 586 125 108 083 098 143 Pb 436 577

Th 50004 50004 50080 0043 0088 0056 50021 Th 2538 1305

U 0012 0011 0069 50018 0038 0030 0031 U 572 278

interst interstitial mp melt pocket na not analysed nd not detected

19

Further very high Mg-numbers in replacive amphibolessupport melt (fluid)^rock interaction at low melt (fluid)rock ratios where Mg and Fe contents are buffered by theambient peridotite (Kelemen et al 1995 Tiepolo et al2001) The slightly lower Mg-number of euhedralamphiboles in Ke 196525 combined with their lowerCr2O3 contents (Fig 8b) suggests a higher melt (fluid)rock ratio which is further supported by considerablylower Mg-number in associated ol-I (Fo88 Table 3)

The limited number of investigated samples does notallow us to decide which of the processes described aboveacted and to what extent However given the heterogene-ity observed in the numerous non-metasomatized xenolithsfrom Marsabit (Kaeser et al 2006) pre-metasomaticfeatures certainly affect the final product of the melt^rockreaction process to a considerable degree

The nature of the metasomatizing agent(s)Graphite stability and oxygen fugacity considerations

The Marsabit xenoliths are unusual in that they occur inan off-cratonic setting yet bear macroscopic graphite Thelatter usually either occurs in kimberlite-hosted peridotites(on-craton) or in pyroxenites from alpine-type peridotitebodies (eg Pearson et al 1994) In the phlogopite perido-tites from Finero graphite has recently been reportedas nanoscopic layers in phlogopite (Ferraris et al 2004)In xenoliths from Marsabit graphite occurs in the sametextural relationship as in kimberlite-hosted peridotitexenoliths (ie millimetre-sized flakes typically associatedwith orthopyroxene see Fig 3c^e and Pearson et al 1994)Graphite inclusions in phlogopite and Opxm couldindicate crystallization during metasomatism by alsquoreducedrsquo C-bearing fluid or melt However precipitationof graphite more probably reflects the relatively reducingconditions in the uppermost mantle prior to metasoma-tism We used the oxygen barometer of Ballhaus et al(1991) based on ferric iron contents in spinel coexistingwith olivine to constrain oxygen fugacity (fO2)conditions prior to metasomatism in the Marsabitxenoliths (Fig 16) The results show that non-metasomatized Group I and IV samples reflect similarlyreducing conditions to the cryptically metasomatizedType III-a samplesAlthough assessing Fe3thornFe2thorn ratios in spinel based

on microprobe analyses can be problematic (Wood ampVirgo 1989 Ballhaus et al 1991) all microprobeanalyses were obtained using the same operationalsettings and therefore give at least the relative fO2

variation between samples The relatively reducingconditions in the pre-metasomatic cold andhighly deformed mantle well below carbon^carbonate^silicate equilibria (Mg2Si2O6thorn 05 CaMg(CO3)2frac14Mg2SiO4thorn 05 CaMgSi2O6thornCthornO2 Eggler amp Baker1982 see Fig 16) and close to the CCO oxygen buffer arein agreement with the presence of graphite in the GroupIII harzburgites Moreover the P^T conditions at whichthe Group III xenoliths partially re-equilibrated are closeto the subsolidus decarbonation of model carbonated lher-zolite (Fig 2 see Lee amp Wyllie 2000 and referencestherein) in agreement with the observed phase assemblageincluding orthopyroxene olivine clinopyroxene andgraphite

Fig 11 Trace element patterns of (a) phlogopite and (b) apatitefrom porphyroclastic spinel harzburgites and dunites from Marsabit[concentrations normalized to the primitive mantle valuesof McDonough amp Sun (1995)] The grey field in (a) indicates thecomposition of Ti-rich phlogopite in Group I (grt)^spl lherzolites(Kaeser et al 2006)

20

Table 6 Representative analyses of silicate glass (oxides in wt trace elements in mgg)

Type III-b III-c Host

Sample 195915 196515 196515 785 196525 19653 19653 19653 195927

Remark next amph next cpx xenol margin next amph next amph melt pocket next apatite next Opxm melt pocket host basanite

SiO2 5306 5775 5161 6096 5657 5938 5974 6530 4882 4410

TiO2 017 157 106 062 030 024 020 023 378 255

Al2O3 2412 2053 2025 1779 2313 1575 1245 1355 1890 1396

Cr2O3 005 001 nd 001 001 nd nd nd 002 na

FeO(tot) 188 247 626 126 252 295 374 265 509 1283

MnO 007 000 026 001 004 005 008 007 018 021

NiO nd nd nd na nd na 006 nd 001 na

MgO 182 129 116 114 122 304 382 252 244 938

CaO 506 257 283 082 044 124 101 039 810 1030

Na2O 1107 966 969 971 1299 1388 1384 1211 673 374

K2O 154 197 465 357 138 121 142 127 300 146

Cl 031 106 038 012 014 023 061 020 015 na

P2O5 041 008 044 009 012 049 233 018 100 059

Total 9956 9896 9860 9609 9887 9845 9929 9846 9821 9912

Mg-no 6329 4816 2489 6175 4632 6474 6458 6289 4552 5658

P 1061 448 1610 230 503 1519 6738 676 2994 na

K 12563 16408 39599 32515 15872 9265 11277 10071 25900 na

Sc 179 184 126 406 151 970 136 848 691 237

Ti 665 7329 4974 2953 1965 1213 1163 1055 18199 15311

V 107 265 235 796 920 210 151 134 234 na

Ni 128 371 92 5575 983 170 5199 5298 199 191

Zn 962 103 109 110 459 440 508 309 819 na

Ga na 173 273 194 220 na na na na na

Rb 181 263 124 546 626 176 398 369 577 369

Sr 427 488 1407 188 881 1644 4997 985 1062 792

Y 543 145 224 752 262 619 158 369 301 299

Zr 466 171 353 761 660 578 365 428 242 216

Nb 416 161 137 260 136 234 164 169 837 647

Cs 014 046 186 125 007 022 102 052 063 na

Ba 238 261 1064 129 168 405 1099 641 625 563

La 698 183 848 144 889 2212 1008 106 537 462

Ce 947 256 131 308 138 333 1265 164 104 823

Pr 655 194 120 330 116 304 975 160 120 na

Nd 146 603 408 101 329 993 256 564 463 400

Sm 142 048 570 179 419 141 379 679 105 851

Eu 041 035 228 082 112 438 103 241 345 267

Gd 062 051 415 158 402 106 245 566 677 729

Tb na 0086 086 020 071 na na na na 111

Dy 090 027 359 098 434 104 171 637 732 na

Ho 023 0037 105 025 095 206 446 104 117 114

Er 045 038 195 058 255 611 153 264 305 na

Tm 009 50039 033 0081 042 085 292 5014 036 052

Yb 062 023 267 044 307 667 120 487 253 220

(continued)

21

Evidence for low-temperature SiO2^Na2O^CO2^H2O-rich melts or fluids

Potential metasomatic agents inferred from the presenceof graphite are C^O^H-rich fluids carbonate melts orCO2-bearing silicate melts The resulting phase assem-blages and geochemical signatures are the result of infiltra-tion of fluid or melt into a cold (750^8508C) shallow(515GPa Fig 2) heterogeneous and locally reduced litho-spheric mantle Metasomatism under such conditions in anopen system must result in complex geochemical signa-tures that reflect the degree of fluid^rock reaction Thismakes it particularly difficult to constrain the exactnature of the lsquooriginalrsquometasomatic agentType III-c xenolith Ke 196525 contains the highest frac-

tion of metasomatic phases and the least fractionatedamphibole trace element patterns The lsquomagmaticrsquo texturesof the Mg-katophorites in this sample suggest direct crys-tallization from a melt or a fluid rather than replacementof a pre-existing phase Therefore hypothetical liquids inequilibrium with the amphiboles in this xenolith couldyield the composition of the initial metasomatic agentsFigure 17 illustrates the trace element patterns of suchmelts and fluids calculated using sets of experimentally orempirically determined mineralmelt (fluid) partitioncoefficients including carbonate melt silicate melt andhydrous fluid (for references see caption to Fig 17)Common features for all calculated metasomatic agents

are negative Zr^Hf^Ti anomalies and LREE enrichment(Fig 17) However previous studies have shown that thesesignatures are indicative of almost all types of metasoma-tism including interaction of peridotite with carbonatiticmelt (Green amp Wallace 1988 Dautria et al 1992 Hauriet al 1993 Rudnick et al 1993 Ionov et al 1996 Yaxleyet al 1998 Coltorti et al 1999) with hydrous fluids andormelts for example originating from dehydration of altered

subducting lithosphere (Maury et al 1992 Zanetti et al1999 Grecurren goire et al 2001 Laurora et al 2001 Parkinsonet al 2003) and interaction of peridotite with volatile-and Si-rich melts that evolved through reactive porousflow (Bedini et al 1997 Ionov et al 2002 2006 Bodinieret al 2004 Rivalenti et al 2004)In the case of the Marsabit xenoliths integration of the

trace element characteristics thermobarometric inferencesand mineralogical constraints favours a Na2O^SiO2^CO2^H2O-rich silicate melt Carbonated melts can prob-ably be ruled out on major element and mineralogical con-siderations First amphiboles are Ca-poor in conflict withcarbonate melt metasomatism which is generally charac-terized by Ca enrichment (Rudnick et al 1993 Neumannet al 2002) Further orthopyroxene is a stable phase andprobably even crystallizes during metasomatism in somesamples (eg Ke 785) At near-solidus conditions coexis-tence of orthopyroxene and carbonate melt is possibleonly at pressures 20GPa (see Fig 2 eg Lee amp Wyllie2000 and references therein) which is considerably higherthan the maximum pressures indicated by the compositionof spinel (515GPa) At such low pressures metasomatismby carbonated melts will convert lherzolite and harzbur-gite immediately into wehrlite which is the major perido-tite type in all xenolith suites where carbonate meltmetasomatism is convincingly demonstrated (Rudnicket al 1993 Yaxley et al 1998 Neumann et al 2002)In Marsabit wehrlites are absent howeverAn Na2O^SiO2^CO2^H2O-rich melt or fluid on the

other hand would explain the Na-rich nature of amphiboleand phlogopite as well as the presence of orthopyroxeneGrowth of metasomatic (and Al-poor) orthopyroxene hasbeen described from several xenolith localities where peri-dotites were infiltrated either by hydrous fluids orSi-rich melts (Smith et al 1999 McInnes et al 2001Morishita et al 2003) Na-rich phlogopite is commonly

Table 6 Continued

Type III-b III-c Host

Sample 195915 196515 196515 785 196525 19653 19653 19653 195927

Remark next amph next cpx xenol margin next amph next amph melt pocket next apatite next Opxm melt pocket host basanite

Lu 014 0050 026 0041 046 080 140 058 040 031

Hf 026 052 535 014 076 110 5087 124 568 504

Ta 50080 033 638 0022 123 101 784 801 394 403

Pb 541 448 280 255 840 797 260 749 493 na

Th 194 296 160 466 703 185 247 105 534 506

U 545 069 478 099 152 447 724 216 171 126

nd not detected na not analysedAverage composition of the strongly alkaline basanites from Marsabit (Volker 1990)

22

Fig 12 Total alkalis vs SiO2 (a) Harker-type co-variation diagrams (b^e) and incompatible trace element co-variation diagram (f) showingthe compositional variability of silicate glass found in the porphyroclastic spinel harzburgites and dunites from Marsabit In (b) binary mixinglines between the typical composition of the alkali-rich glass (large open diamond) and the associated microlites are shown (each increment is10wt)They illustrate that the glass does not represent the derivate of host lava infiltration and fractional crystallization Subalkaline glass inGroup I (grt)^spl lherzolites from Marsabit is shown for comparison and is interpreted to derive from incongruent melting of Ti-pargasite(Kaeser 2006)

23

found as a reaction product between anhydrous mafic toultramafic rocks and H2O-rich evolved liquids (Costaet al 2001 and references therein) or H2O-bearing fluids(eg Odling 1994) The latter study was an experiment at20GPa simulating the effects of decreasing temperatureson the phase assemblages and their composition duringmetasomatism using a model peridotitethornC^O^H fluidThe coolest part of Odlingrsquos (1994) experiment (510758C)contained a strikingly similar phase assemblage (kato-phorite phlogopite with 2wt Na2O and Al-poororthopyroxene) to the Marsabit samples interpreted tohave formed during sub-solidus fluid^solid interactionA compositionally similar phase assemblage (highamounts of opx Si-rich hornblende Na-rich phlogopite)resulted from experiments simulating peridotite interac-tion with Si-rich melt (dacite) at lower P^T conditions(9508C at 092GPa Prouteau amp Scaillet 2003)In this study the trace element signatures of Group III-c

Mg-katophorite and the calculated metasomatic agentsfurther suggest that the metasomatizing agent was a meltrather than a fluid Hypothetical fluids in equilibrium witheuhedral Mg-katophorite from sample Ke 196525 havehigh NbLILE (eg Rb Ba) and low LILEREE ratios(Fig 17) Because experimental data predict the oppositefor fluids (LILE enriched over HFSE and REE egManning 2004 and references therein) a fluid with asignature such as that indicated in Fig 17 is unrealistic

Fig 13 Trace element patterns of silicate glass in porphyroclasticspinel harzburgites and dunites from Marsabit compared withthe trace element composition of associated clinopyroxene (Dim)micro-phenocrysts xenolith-hosting basanites from Marsabit andglass interpreted to represent xenolith^host interaction [concentra-tions normalized to the primitive mantle values of McDonough ampSun (1995)]

Table 7 Representative analyses of carbonates (oxides in

wt trace elements in mgg)

Group III-b III-b III-c III-c III-c

Mineral Dol Mg-Cc Mg-Cc Mg-Cc Mg-Cc

Sample 195915 196515 785 19653 19653

Texture in glass in glass in glass in glass in glass

SiO2 na 003 na nd 003

FeO(tot) 016 011 010 010 006

MnO 001 nd nd nd nd

MgO 1922 236 190 125 517

CaO 3189 5313 5377 5391 4853

Na2O 003 nd 004 nd 003

K2O na 002 na 001 nd

SrO 003 010 008 004 021

BaO 003 nd nd na na

CO21 4618 4444 4442 4377 4391

Total 9756 10021 10032 9909 9794

Mg-no 9954 9745 9713 9554 9931

Ca-no 5439 9418 9531 9687 8709

P 220 334 137 580 589

K 970 212 642 105 155

Sc 5065 50150 027 5027 50076

Ti 975 189 242 528 196

V 5085 0481 5042 5037 021

Ni 900 673 517 5294 593

Zn 459 5089 113 296 045

Ga na 5014 5013 na na

Rb 0084 50072 50074 50060 0046

Sr 593 860 371 251 4274

Y 012 011 130 704 015

Zr 261 024 012 097 756

Nb 144 019 0056 692 185

Cs 50063 50011 50018 50039 50008

Ba 126 230 5024 029 271

La 034 057 084 136 025

Ce 032 013 134 997 023

Pr 006 006 018 095 0026

Nd 030 013 068 228 0078

Sm 5044 50091 0074 048 50018

Eu 5012 50019 0051 020 50008

Gd 5037 50090 014 051 0026

Tb na 50010 0036 na na

Dy 5019 50039 0089 058 50038

Ho 50029 0013 0044 011 50006

Er 5012 50041 0083 033 50019

Tm 50052 50013 0016 0084 50006

Yb 5043 50063 014 027 50040

Lu 50072 50010 0011 0066 50008

(continued)

24

At the relatively low maximum pressures (515GPa) asupercritical liquid is not expected either (eg Kesselet al 2005)Finally a relatively hydrous metasomatizing agent

would be consistent with formation of Mg-katophoriteswith lowTi and presumably low Fe3thorn contents (given therelatively reducing conditions) which in turn imply high(OH)^ contents In other words partial dehydrogenationtypically observed in common (Ti-rich) mantle amphibole

(eg Popp et al 1995 Tiepolo et al 2001 and referencestherein) cannot have played an important role in thiscase as H deficiency is mainly the result of coupled substi-tutions involvingTi and Fe3thorn (Popp et al 1995) In conclu-sion the metasomatizing agent was most probably aNa2O^SiO2^CO2^H2O-rich meltComparison with experimental studies further con-

strains the low-temperature regime in which metasoma-tism occurred In ultramafic (eg Niida amp Green 1999)basaltic (eg Rapp 1995) and Si-enriched systems (egtrachybasaltic Barclay amp Carmichael 2004) amphibole(in the latter study together with phlogopite) is confinedto temperatures lower than 1000^10508C at pressures520GPa (Fig 2)Thus crystallization of the early metaso-matic phase assemblage can be explained by infiltration ofa silicate melt that rapidly cooled upon contact with theambient peridotite (750^8508C prior to metasomatismFig 2) However this low-T stage is based on Ca-in-opxthermometry (see Brey amp Kolaquo hler 1990 Kaeser et al2006 Table 1) and therefore probably underestimates theeffective temperature conditions during metasomatismThis can be explained by low diffusion rates and proximityto the closure temperature for the diopside^enstatiteexchange reaction at temperatures lower than 8508C(eg Smith 1999 and references therein) The lower tem-perature limit at which silicate melt metasomatism ispossible (ie defined by the meltrsquos solidus and viscosity)depends strongly on volatile and SiO2 contentsExperiments by Draper amp Green (1997) for exampleshowed that highly silicic melts (52^64wt SiO2) canbe in equilibrium with mantle harzburgite at temperaturesas low as 8508C (at 10^20GPa under anhydrous andC^O^H-fluid-bearing conditions) These are howeverextreme compositions for mantle melts The actual SiO2

content of the metasomatizing agent in this case is difficultto constrain from the metasomatic mineral assemblage ascompositions may have varied considerably during reac-tive percolation (eg including Si-releasing reactions suchas cpx replacement by amphibole versus Si-consumingreactions such as Opxm crystallization) Moreover it isunlikely that a melt persists at temperatures muchbelow 9008C which is the solidus of fertile lherzolite(ie primitive upper mantle composition) underH2O-saturated conditions at 15GPa (Grove et al 2006see Fig 2) Below the solidus residual volatile-enrichedfluids may percolate further for example forming the lessmetasomatized Type III-c xenoliths (eg Ke 785) as wellas Type III-b and -a peridotites (see also Bailey 1987)Relatively cold conditions would also explain the veryhigh compositional gradients developed on small scales inType III-a samples (Fig 15) Given the absence of chloritein all investigated samples the lower temperature limitof such fluid metasomatism is 8008C (eg Fumagalli ampPoli 2005 see Fig 2)

Table 7 Continued

Group III-b III-b III-c III-c III-c

Mineral Dol Mg-Cc Mg-Cc Mg-Cc Mg-Cc

Sample 195915 196515 785 19653 19653

Texture in glass in glass in glass in glass in glass

Hf 5016 50038 50036 50074 50013

Ta 0048 50012 50016 50031 0008

Pb 5024 858 5021 074 018

Th 0021 50020 50017 50028 50005

U 060 094 086 600 148

Mg-Cc Mg-calcite Dol dolomite nd not detectedna not analysed Ca-numberfrac14 100Ca(CathornMg)1Calculated from stoichiometry

Fig 14 Trace element patterns of glass-hosted carbonates in TypeIII-b and -c spinel harzburgite and dunite xenoliths from Marsabit[concentrations normalized to the primitive mantle values ofMcDonough amp Sun (1995)] The grey field outlines the compositionalrange of carbonates found in other peridotite xenoliths (Ionov ampHarmer 2002 and references therein) Carbonate material inter-preted as quenched carbonatite melt found in peridotite xenolithsfrom Kerguelen (Moine et al 2004) is shown for comparison(bold line)

25

Finally the presence of apatite even though oftentaken as indicator for carbonatite-type metasomatism(eg Yaxley et al 1998) is not in conflict with metasoma-tism by silicate melts Apatite has also been shown to crys-tallize from volatile and alkali-rich derivatives formedduring reactive porous flow of originally mafic silicatemelts (Woodland et al 1996 Bedini et al 1997 Bodinieret al 2004 Ionov et al 2006)

FORMATION OF THE LATE GLASS -BEAR ING ASSEMBLAGESTextures strongly suggest that the glass-bearing assem-blages formed later and at the expense of the early metaso-matic assemblages (Fig 4a and d) Features such as thereappearance of clinopyroxene (Na-rich Cr diopsidemicro-phenocrysts) a phase replaced by the early

Fig 15 Detailed REE systematics in primary clinopyroxene (cpx-I) in a porphyroclastic cryptically metasomatized spinel harzburgite(sample Ke 19651) showing the transition fromType III-a (Zone A and B) toType III-b (Zone C) on a scale of a few centimetres The numberson the photomicrographs (crossed polars) correspond to single laser ablation microprobe pits (for further explanation see text)

26

metasomatic assemblage indicate that melt pocket forma-tion reflects a significant change in P^T^X space in partsof the mantle beneath Marsabit after earlier metasoma-tism Based on geochemical arguments and mass-balanceconsiderations we will show that (1) the late glass-bearingassemblages are closely related to early metasomaticassemblages mainly representing breakdown productsand (2) breakdown was induced by infiltration of aCO2-rich fluid in the mantle probably followed andenhanced by heating and decompression melting duringxenolith transport in the host magma

Evidence for in situ partial meltingBSE imaging reveals that the phases becoming unstableare amphibole orthopyroxene and to minor extentsspl-I phlogopite cpx-I and apatite Compositional(inter- and intra-xenolith) heterogeneity of silicate glass istexture controlled and varies as a function of theassociated precursor phases (Fig 12a^f)TiO2 for exampleshows higher concentrations in glass from xenolith Ke196515 containing Ti-enriched amphibole (Fig 12c)whereas K2O contents are highest in phlogopite-rich sam-ples (eg Ke 785 Fig 12d) Elevated P2O5 contents arefound in glass adjacent to apatite (Fig 12e) whichmirrors at the same time the highTh and LREE contents

of apatite (Fig 12f) Significant Si enrichment is found inglass coating orthopyroxene (Fig 12d) supporting incon-gruent melting of orthopyroxene (see below) Even thoughthe melt pockets are connected along grain boundaries thestrong compositional variation (on inter- and intra-xeno-lith scale) suggests that the mobility of the melt was lim-ited or that the melting event was very short-lived (fortiming constraints see below)The co-variation plots in Fig 12 further reveal consider-

able compositional differences between all types of xeno-lith glass and the host basanite These data indicate thatthe glass cannot be the product of xenolith^host lava inter-action during ascent to the surface Also back-mixing ofmicro-phenocrysts into the melt fails to derive the glassfrom a host-related melt (Fig 12b) Glass from xenolithmargins however is compositionally closer to the bulkhost basanite (eg Fig 12b) in line with a direct derivationfrom xenolith^host lava interaction Its more evolvedmajor element composition (eg higher SiO2 lowerMg-number Fig 12b) is probably the result of fractiona-tion of basanite phenocrysts (eg olivine augite andtitanomagnetite Volker 1990) Host lava^xenolith interac-tion is further supported by the trace element signature ofglass at the margin of the xenoliths which is closely similarto that of the Marsabit basanites (Fig 13 Volker 1990)The same characteristics are observed for intra-xenolithglass in the unusual kaersutite-bearing sample Ke 195927

Fig 16 Oxygen fugacities relative to the quartz^fayalite^magnetitebuffer (QFM equation of OrsquoNeill 1987) as a function of temperatureat assumed pressure of 1GPa calculated using the composition ofcoexisting spinel and olivine (ol-I^spl-I pairs and Chrm^Olm pairsrespectively) based on the oxygen barometer of Ballhaus et al (1991)The graphite^CO2 equilibrium was calculated after Frost amp Wood(1998) and the graphite^carbonate^silicate equilibrium was calcu-lated using the equation of Eggler amp Baker (1982) (See text for furtherexplanation)

Fig 17 Calculated metasomatic agents in equilibrium with the euhe-dral Mg-katophorites from the Type III-c amphibole dunite Ke 196525 Amphibolehydrous fluid and amphibolecarbonatite melt parti-tion coefficients are from Adam et al (1997) and Adam amp Green(2001) respectively Silicate melts have been calculated using amphi-boleSi-rich melt partition coefficients from Chazot et al (1996b) andamphibolebasanite partition coefficients from LaTourrette et al(1995) The compositional range of the Marsabit basanites and alkalibasalts (Volker 1990) is shown for comparison

27

Xenolith glass unrelated to the host lava and melt pock-ets similar to those reported here in association with resi-dual phases (eg amphibole or clinopyroxene) has beenreported from many xenolith localities (eg Frey ampGreen 1974 Ionov et al 1994 1996 Chazot et al 1996a1996b Yaxley et al 1997 Laurora et al 2001 Bali et al2002) Residual phases within the melt pockets stronglyindicate in situ partial melting within the xenolithsHowever a variety of processes may trigger such in situ

melting including (1) closed-system decompression melt-ing of either anhydrous (Maale amp Printzlau 1979) orhydrous phases (Frey amp Green 1974 Francis 1976) duringxenolith ascent in the host lava (2) melting during interac-tion of the xenoliths with the host basaltic melt duringxenolith ascent (eg Zinngrebe amp Foley 1995 Shaw ampKlulaquo gel 2002 Shaw et al 2006) (3) melting in the mantleprior to xenolith entrainment induced by an infiltratingfluid (ie flux melting eg Jones et al 1983 Ionov et al1994 Demecurren ny et al 2004) However most studies agreethat that partial melting occurred very shortly beforeandor during xenolith exhumation by the host magmaThus a combination of metasomatically triggered meltingfollowed andor assisted by decompression melting duringxenolith transport may be a realistic alternative in somecases (eg Dawson 2002)Recently Ban et al (2005) have shown in a very detailed

mass-balance study on glass-bearing xenoliths from theWest Eifel that the weight proportions of glass and theassociated micro-phenocrysts match the composition ofthe preserved parent amphibole We used the sameapproach and performed simple mass-balance calculations(least-squares fitting) to constrain the conclusions drawnfrom petrography and to test which phase contributedand to what extent to the formation of the melt pocketsThe results reported in Table 8 show that in most casesamphibole-melting can account successfully for the meltpocket composition and only a minor amount (520) ofother phases (orthopyroxene andor phlogopite) needs tobe added to the reactant Such a contribution of otherphases agrees with textural observation (eg Fig 4cshowing Dim growing on resorbed Opxm) as well as withthe conclusions of Ban et al (2005) who stated that moreSi-rich glass (ie 454wt) necessitates orthopyroxenecontributing to the meltTrace element patterns of the bulk glassthornmicro-

phenocryst pockets calculated using the measured traceelement abundances of glass and micro-phenocrysts andthe respective mass-balance parameters (Table 8) arebroadly identical to the measured amphibole composition(Fig 18a and b) Higher LILE (Cs Rb Ba) concentrationsin the calculated bulk melt patches in sample Ke 785 canbe balanced by adding 7wt of phlogopite (Fig 18b)which is in line with mass balance (Table 8) The dominantcontrol of amphibole to account for the trace element

budget of the melt pockets is similar to results found inother suites of glass-bearing xenoliths (Chazot et al1996a Yaxley amp Kamenetsky 1999 Laurora et al 2001)Further the obtained phase proportions for the melt pock-ets are in good agreement with the results from image ana-lysis (Table 1) In addition they are similar to the glass^cpx^ol proportions recorded in recent experiments on the

Table 8 Results of mass-balance calculations to test melt

pocket formation by incongruent melting of the early metaso-

matic assemblages

Sample 195915 785 785 196525 196525 19653

Reactant

amph 1000 1000 0820 1000 0944 mdash

opx mdash mdash 0078 mdash 0056 0406

phl mdash mdash 0102 mdash mdash mdash

glass mdash mdash mdash mdash mdash 05941

SiO2 4484 4989 4964 5129 5167 5898

TiO2 014 039 036 025 024 018

Cr2O3 232 291 258 127 121 009

Al2O3 1368 762 769 800 757 961

FeO(tot) 336 314 333 343 357 394

MnO 006 008 008 012 012 010

NiO 009 013 014 011 011 004

MgO 1844 2029 2197 2040 2122 1625

CaO 854 783 644 966 913 078

Na2O 460 500 436 504 476 843

K2O 049 032 096 042 040 060

Product

glass 0389 0322 0335 0292 0284 07032

ol 0243 0259 0315 0266 0286 0273

cpx 0288 0373 0308 0442 0431 0024

spl 0052 0032 0028 mdash mdash mdash

carb 0028 0014 0014 mdash mdash mdash

SiO2 4477 5027 4986 5207 5181 5903

TiO2 013 041 039 015 014 017

Cr2O3 213 314 272 109 106 015

Al2O3 1364 764 770 798 776 957

FeO(tot) 374 331 351 322 332 350

MnO 007 007 007 009 009 006

NiO 011 013 015 011 012 007

MgO 1834 2024 2196 2063 2141 1635

CaO 854 759 632 952 929 080

Na2O 449 385 388 499 485 860

K2O 060 096 100 015 014 091

r2 023 200 035 084 028 033

Prefix Ke omitted1Low-Si glass 2High-Si glass (see text for explanation)

28

melting behaviour of amphibole in wehrlites at tempera-tures between 1175 and 12508C and pressures of05^10GPa (Mecurren dard et al 2006) These temperatures arein good agreement with the estimates for the formation ofolivine and chromite micro-phenocrysts in the melt

pockets (Fig 16) The heat supply required for amphibolemelting in the formerly low-T metasomatized Group IIIxenoliths (T 200^3008C) may come from local heatingshortly before or during xenolith entrainment in the hostlava or from fluid infiltration that triggered melting (seediscussion below)Finally formation of the most Si-enriched melt (adjacent

to Opxm in Group III-c samples) can be modelled assum-ing progressing interaction between infiltrating melt andorthopyroxene where the latter melts incongruently

opxthornmelt1 olthornmelt2 cpx ethseeTable 8THORN eth1THORN

To calculate reaction (1) we chose the least evolved glassobserved in melt patches in sample Ke 19653 (SiO2

5921wt Mg-number 6465) to represent the initialmelt1 whereas melt2 adjacent to opx is SiO2-rich(6530wt) Trace element abundances in glass next toorthopyroxene are shifted to lower concentrations(Table 6) which is in agreement with dilution caused byaddition of a trace element-poor orthopyroxene compo-nent The resultant phase proportions are similar to thoseobserved at orthopyroxene^melt interfaces (ie higholivine and low cpx abundances) and are in agreementwith experimental studies (Shaw et al 1998)

Partial melting triggered by fluid fluxMass-balance calculations provide an explanation for meltpocket formation by in situ partial melting Thus onemight conclude that melting is simply the result of decom-pression and heating during xenolith transport in the hostmagma However there are two points that argue thatmelting was triggered by the infiltration of a fluid phaseshortly before the xenoliths were sampled by the hostmagma (1) the presence of occasionally high amounts ofcarbonate included in the melt pockets (2) the extremelyNa-rich clinopyroxene (Dim) compositions either includedas micro-phenocrysts in the melt pocket or present asequilibrated grains around chromite in the single TypeIII-d xenolith (Fig 3f) Both arguments will be discussedin the next section

Formation of carbonates

Textures and compositions of carbonate globules can beexplained by carbonate precipitation from a CO2-bearingfluid As demonstrated by several studies highCa(CathornMg) values (Table 7) and low REE concentra-tions are strong arguments that the carbonates are cumu-late crystals precipitated from a CO2-rich melt or afluid and not quenched carbonated melts (Lee et al 2000Ionov amp Harmer 2002) Oscillatory zoning (Fig 5a and b)of individual carbonate crystals within singleglobules corroborates this interpretation The roundedcontacts toward glass (globules) are also in line with thesefindings Similar globular forms were reported fromexperimental studies and do not a priori imply the

Fig 18 Calculated trace element characteristics of reactants (in mostcases amphibolethorn orthopyroxene filled symbols) and products(melt pockets open symbols) of the lsquoamphibole meltingrsquo model[see equation (1) in text] The relative phase proportions wereobtained by mass-balance calculations and are given in Table 8 Thedashed lines in (b) show that the high LILE concentrations obtainedfor the product (bulk melt pocket) can be explained by adding about7wt of phlogopite to the reactant (amphibolethorn orthopyroxeneTable 8) Further explanation is given in the text

29

presence of immiscible carbonate and silicate melts(eg Lee amp Wyllie 1996)The high amount of carbonate in some melt pockets

however (eg Fig 4e) does need further explanation asthe incongruent melting reaction (1) does not account forthe CO2 required for carbonate precipitation duringamphibole melting Therefore amphibole breakdowneither occurred in an open system with addition of at leastCO2 (eg Laurora et al 2001 Ban et al 2005) or anotherpre-existing phase acted as a source of carbon Oxidationof graphite texturally identified as a part of the earlyassemblage (eg Fig 3d) could account for some of theCO2 necessary for carbonate formation A change in fO2

conditions is also indicated when the composition of glass-related spinel (Chrm) and Olm are used for oxygen baro-metry which yields oxygen fugacities well on the oxidizedside of the carbon^carbonate^silicate buffer (Fig 16)However increasing fO2 triggered by closed-systemdecompression-induced melting alone does not appear fea-sible Also textures such as straight contacts between gra-phite and Mg-calcite (Fig 4c) suggest that carbonategrows on graphite (ie indicating local fO2 conditions cor-responding to carbon^carbonate equilibrium) rather thanreplacing it We therefore interpret melt pocket formationto be triggered by the infiltration of a CO2-rich fluid(ie flux melting see also Ionov et al 1994) This interpre-tation also accounts for the presence of abundant vugs inthe glass that could have contained a fluid (Ionov et al1994 2006) which was presumably lost during sample pre-paration However we emphasize here that late fluid infil-tration had only subtle effects on the trace elementcomposition of Type III-b and -c xenoliths

Formation of Na-rich Cr diopside

Infiltration of CO2-rich fluidwould also explain the Na-richcomposition of Dim micro-phenocryst cores (Fig 6a and b)Ionov et al (2005) noted recently that clinopyroxenes inmeltpockets are commonly characterized by Na2O contents51wt mainly as a result of low cpxmelt partition coeffi-cients at low pressures (eg Blundy et al 1995) Thus veryhighNa2Ocontents in Dimmicro-phenocrysts (Fig6a) sug-gest replacement of amphibole by Dim and eventually Olmand Chrm during reaction with a fluid at subsolidus condi-tions An origin of micro-phenocrysts by replacementrather than crystallization from a melt is indicated by theidentical crystallographic orientation of individual Dimcrystals in singlemelt pockets (Fig4a)Once the solidus of the precursor phase(s) was reached

(ie 10508C for amphibole Niida amp Green 1999) silicatemelt was generated in situ and led to the formation of clin-opyroxene rims with lower Na contents (Fig 6b) and thedevelopment of euhedral shapes in contact with melt (egFig 4a and e) Eventual immiscibility between remainingfluid and newly formed melt (eg during xenolith ascent)would then explain the presence of carbonate globules and

vugs enclosed in glass Likewise different origins of meltand carbonate were recently proposed for similar meltpockets from Hungarian xenoliths by Demecurren ny et al(2004) who found oxygen isotope disequilibrium betweencarbonate globules and the host glass Such a two-stepmodel (fluid-induced replacement followed by melt gen-eration) can be explained by CO2-fluid infiltration occur-ring within the mantle at sub-solidus conditions ultimatelyfollowed by xenolith entrainment in the host magma andtransport to the surfaceThe host magma possibly suppliedthe additional heat necessary for melt generationFast decompression would further enhance melting andcould also account for the lower Na contents in Dimmicro-phenocryst rims as a result of decreasing cpxmeltpartition coefficients at decreasing pressures (Blundyet al 1995)Rapid replacement of amphibole ( phlogopite) by an

anhydrous phase assemblage indicates that the fluid waswater-poor (eg low H2OCO2 ratio see Ionov et al2006)This is corroborated by the fact that Mg-katophorite(presumably OH-rich see above) is occasionally rimmedby a residual kaersutitic selvage (Fig 4d) Such Ti-richmantle amphiboles are commonly OH-poor as a result ofpartial dehydrogenation (eg Tiepolo et al 2001 and refer-ences therein)Finally the presence of theType III-d xenolith contain-

ing texturally equilibrated clinopyroxene of almost identi-cal composition to Dim micro-phenocrysts in melt pockets(Table 2 Figs 6a and 7b) further strengthens the argu-ment that CO2-fluid percolation started in the mantlebefore entrainment (ie enough time elapsed to allow thisrock to anneal completely after the late metasomaticassemblage had formed) A similar conclusion can bedrawn for the clearly host-related glass found in xenolithKe 195927 (eg Fig 13b) ThisType III-c sample is charac-terized by the presence of Dim micro-phenocrysts partlyovergrown by mineral rims clearly related to the host basa-nite magma (kaersutite Ti-augite and olivine withFo885 Fig 4f Tables 3 and 4 and supplementary dataat httppetrologyoxfordjournalsorg) Kaersutite hasmajor and trace element characteristics similar to typicalbasanitic and alkali basaltic amphibole phenocrysts(Fig 9d) The special phase assemblages in this xenolithindicates that host-related basanitic melts intruded someType III-c xenoliths and partly overprinted the assemblageformed slightly earlier by in situ derived alkali-rich peralu-minous melts fully preserved in other samples (eg Ke19653 or Ke 785)

ORIGIN OF METASOMATIZ INGAGENTS T IM ING AND TECTONICSETT INGAs noted above late metasomatism was probably short-lived started at mantle depths and probably continued

30

Table 5 Representative analyses of phlogopite and apatite (oxides in wt trace elements in mgg)

Type III-b III-c III-b III-c

Mineral phlog phlog phlog phlog phlog phlog phlog apatite apatite

Sample 196515 196515 19653 785 785 785 196525 195915 19653

Texture veinlet interst in opx in opx with amph

in mp

around

chromite

incl in

amph

bleb in

glass

interst

grain

SiO2 3925 3914 4171 4190 4124 4134 4230 SiO2 016 049

TiO2 313 307 009 053 039 037 027 Al2O3 001 nd

Al2O3 1544 1583 1360 1360 1384 1377 1477 P2O5 3768 3715

Cr2O3 112 105 132 147 190 197 063 FeO(tot) 037 029

FeO(tot) 350 347 271 000 289 277 340 MnO 006 nd

MnO nd 002 008 305 004 002 005 MgO 084 091

NiO 023 027 019 023 024 028 023 CaO 4750 4237

MgO 2287 2280 2577 2591 2582 2528 2525 SrO 095 397

CaO nd 002 001 nd 004 001 002 Na2O 079 133

Na2O 215 219 282 258 264 262 208 La2O3 003 007

K2O 719 707 704 706 674 684 683 Ce2O3 088 347

Nd2O3 003 007

F 139 224

Cl 287 092

Total 9488 9493 9571 9641 9587 9535 9590 Totaly 9305 9245

Mg-no 9282 9287 9443 9380 9408 9420 9298

P 369 187 480 339 206 188 362 P 129717 139419

K 71711 72575 60747 79052 70997 69992 70189 K 110 897

Sc 099 110 131 130 088 091 160 Sc 242 079

Ti 15500 16408 890 2839 2096 1807 1736 Ti 235 5294

V 255 284 888 142 108 130 788 V 365 382

Ni 1808 1727 1744 2046 1813 1773 1905 Ni 121 397

Zn 234 221 358 382 317 321 695 Zn 119 096

Ga 111 984 na 210 190 188 235 Ga na na

Rb 185 147 289 183 133 133 342 Rb 015 049

Sr 102 116 775 325 363 350 101 Sr 7138 42233

Y 010 0086 50057 50054 0021 0006 0052 Y 988 1454

Zr 019 019 043 50088 0077 0070 073 Zr 307 101

Nb 506 628 479 177 102 146 210 Nb 024 020

Cs 175 138 261 399 216 180 798 Cs 50013 50036

Ba 1212 1096 1343 298 277 219 1124 Ba 139 126

La 010 0055 0069 50029 0020 50003 50045 La 7460 19048

Ce 0027 0011 50075 50019 014 50003 0078 Ce 6844 25945

Pr 0012 0008 50067 50024 0012 50003 50019 Pr 380 2212

Nd 0019 0035 50405 50079 0065 50009 50224 Nd 749 6530

Sm 0015 50011 50474 50090 0016 50010 50132 Sm 515 747

Eu 0022 0017 50104 50026 0013 0010 50095 Eu 147 189

Gd 50010 50011 5043 5015 50011 0012 017 Gd 314 451

Tb 50002 50002 na 50027 0002 50002 50028 Tb na na

Dy 50015 50017 045 50077 50007 50006 50077 Dy 198 281

Ho 50002 50003 50043 50020 0002 50002 50028 Ho 359 511

Er 50009 50013 5025 5011 50016 50009 5011 Er 903 119

Tm 50002 50002 50088 50013 0002 0003 50018 Tm 119 138

(continued)

18

data (eg LaTourrette et al 1995) indicate that amphibolecrystallization alone is unlikely to deplete the metasomaticagent to such a high degree in HREE The same probablyaccounts for the extreme Nb depletion in amphiboles fromKe 785 relative to the otherType III-c xenolith (Ke 196525 see also Ionov et al 2002)Further clinopyroxene phlogopite and amphibole in the

Type III-b sample Ke 196515 on one hand all showstrongly fractionated REE patterns (eg high LaCe inamphibole Fig 10c) but on the other hand are character-ized by a positiveTi anomaly decoupled from other HFSE(eg Hf and Zr Fig 9b) Trace element fractionationduring amphibole crystallization (ie formation of TypeIII-c amphibole dunites) however should lower Ti con-tents in the remaining fluid or melt (eg Moine et al 2001)Based on textures mineralogy and dominant common

compositional features the hypothesis that Type III-band -c xenoliths were metasomatized by completely inde-pendent metasomatic agents can be ruled out Thereforetwo other possibilities remain to account for the inconsis-tent observations described above (1) inference of otherphases reactants or products that are able to fractionatespecific trace elements present in the mantle beneathMarsabit but not present in the few investigated samples(2) pre-metasomatic compositional heterogeneities similarto the inherited HREE variation inType III-a clinopyrox-ene (see above)Garnet would be an important sink for HREE but

metasomatism occurred in the spinel field after decom-pression with maximum pressures of 515GPa (Kaeseret al 2006) so garnet had already broken down to sym-plectites (Fig 15) Percolation of melt through garnet-bearing websterites prior to metasomatism in Ke 19651and 196515 could be an alternativeWebsterites have been

interpreted to occur in close spatial relationship with por-phyroclastic peridotites in the shallow mantle beneathMarsabit (Kaeser et al 2006) and they occasionally con-tain rutile and ilmenite which could further account forthe strong Nb and Ta depletion in some samples (eg Ke785) The available compositional data from the webster-ites however do not indicate metasomatic modification ofwebsteritic garnet (Olker 2001 B Kaeser unpublisheddata) nor compositional similarities to Group IIIperidotitesCareful study by SEM rules out the presence of exotic

phases such as zircon or Ti-oxides described fromother xenolith suites (eg Bodinier et al 1996 Rudnicket al 1999) Nevertheless it cannot be excluded that themetasomatizing agent percolated through mantle domainscontaining these phases before infiltrating the studiedrocksThe preservation of pre-metasomatic compositional het-

erogeneities is another possibility that must be consideredThat such lsquorock signalsrsquo partly overlap with the lsquometaso-matic signalrsquo is indicated by the HREE variation in TypeIII-a clinopyroxene (see above) and also by other composi-tional parameters For example both Mg-katophorite andphlogopite growing at the expense of spinel have very highCr2O3 contents whereas euhedral Mg-katophorite inthe amphibole dunite (Ke 196525) or in phlogopiteinclusions in Opxm is comparably Cr2O3-poor (Fig 8bTables 4 and 5) Pre-existing heterogeneities would essen-tially be preserved through elements that are relativelycompatible in a given mineral (ie that are not mobilizedduring metasomatism) This would be in agreement withthe preserved HREE variations in Type III-a clinopyrox-ene high Cr2O3 in amphiboles and phlogopite and Tispikes in amphibole from Ke 196515

Table 5 Continued

Type III-b III-c III-b III-c

Mineral phlog phlog phlog phlog phlog phlog phlog apatite apatite

Sample 196515 196515 19653 785 785 785 196525 195915 19653

Texture veinlet interst in opx in opx with amph

in mp

around

chromite

incl in

amph

bleb in

glass

interst

grain

Yb 50014 50011 5047 5015 50011 50019 5017 Yb 803 724

Lu 0002 50002 50061 50014 50002 50002 50020 Lu 115 742

Hf 0019 0018 5025 50053 50012 50006 5019 Hf 50023 50120

Ta 020 022 140 0031 0012 0009 023 Ta 50015 50075

Pb 546 586 125 108 083 098 143 Pb 436 577

Th 50004 50004 50080 0043 0088 0056 50021 Th 2538 1305

U 0012 0011 0069 50018 0038 0030 0031 U 572 278

interst interstitial mp melt pocket na not analysed nd not detected

19

Further very high Mg-numbers in replacive amphibolessupport melt (fluid)^rock interaction at low melt (fluid)rock ratios where Mg and Fe contents are buffered by theambient peridotite (Kelemen et al 1995 Tiepolo et al2001) The slightly lower Mg-number of euhedralamphiboles in Ke 196525 combined with their lowerCr2O3 contents (Fig 8b) suggests a higher melt (fluid)rock ratio which is further supported by considerablylower Mg-number in associated ol-I (Fo88 Table 3)

The limited number of investigated samples does notallow us to decide which of the processes described aboveacted and to what extent However given the heterogene-ity observed in the numerous non-metasomatized xenolithsfrom Marsabit (Kaeser et al 2006) pre-metasomaticfeatures certainly affect the final product of the melt^rockreaction process to a considerable degree

The nature of the metasomatizing agent(s)Graphite stability and oxygen fugacity considerations

The Marsabit xenoliths are unusual in that they occur inan off-cratonic setting yet bear macroscopic graphite Thelatter usually either occurs in kimberlite-hosted peridotites(on-craton) or in pyroxenites from alpine-type peridotitebodies (eg Pearson et al 1994) In the phlogopite perido-tites from Finero graphite has recently been reportedas nanoscopic layers in phlogopite (Ferraris et al 2004)In xenoliths from Marsabit graphite occurs in the sametextural relationship as in kimberlite-hosted peridotitexenoliths (ie millimetre-sized flakes typically associatedwith orthopyroxene see Fig 3c^e and Pearson et al 1994)Graphite inclusions in phlogopite and Opxm couldindicate crystallization during metasomatism by alsquoreducedrsquo C-bearing fluid or melt However precipitationof graphite more probably reflects the relatively reducingconditions in the uppermost mantle prior to metasoma-tism We used the oxygen barometer of Ballhaus et al(1991) based on ferric iron contents in spinel coexistingwith olivine to constrain oxygen fugacity (fO2)conditions prior to metasomatism in the Marsabitxenoliths (Fig 16) The results show that non-metasomatized Group I and IV samples reflect similarlyreducing conditions to the cryptically metasomatizedType III-a samplesAlthough assessing Fe3thornFe2thorn ratios in spinel based

on microprobe analyses can be problematic (Wood ampVirgo 1989 Ballhaus et al 1991) all microprobeanalyses were obtained using the same operationalsettings and therefore give at least the relative fO2

variation between samples The relatively reducingconditions in the pre-metasomatic cold andhighly deformed mantle well below carbon^carbonate^silicate equilibria (Mg2Si2O6thorn 05 CaMg(CO3)2frac14Mg2SiO4thorn 05 CaMgSi2O6thornCthornO2 Eggler amp Baker1982 see Fig 16) and close to the CCO oxygen buffer arein agreement with the presence of graphite in the GroupIII harzburgites Moreover the P^T conditions at whichthe Group III xenoliths partially re-equilibrated are closeto the subsolidus decarbonation of model carbonated lher-zolite (Fig 2 see Lee amp Wyllie 2000 and referencestherein) in agreement with the observed phase assemblageincluding orthopyroxene olivine clinopyroxene andgraphite

Fig 11 Trace element patterns of (a) phlogopite and (b) apatitefrom porphyroclastic spinel harzburgites and dunites from Marsabit[concentrations normalized to the primitive mantle valuesof McDonough amp Sun (1995)] The grey field in (a) indicates thecomposition of Ti-rich phlogopite in Group I (grt)^spl lherzolites(Kaeser et al 2006)

20

Table 6 Representative analyses of silicate glass (oxides in wt trace elements in mgg)

Type III-b III-c Host

Sample 195915 196515 196515 785 196525 19653 19653 19653 195927

Remark next amph next cpx xenol margin next amph next amph melt pocket next apatite next Opxm melt pocket host basanite

SiO2 5306 5775 5161 6096 5657 5938 5974 6530 4882 4410

TiO2 017 157 106 062 030 024 020 023 378 255

Al2O3 2412 2053 2025 1779 2313 1575 1245 1355 1890 1396

Cr2O3 005 001 nd 001 001 nd nd nd 002 na

FeO(tot) 188 247 626 126 252 295 374 265 509 1283

MnO 007 000 026 001 004 005 008 007 018 021

NiO nd nd nd na nd na 006 nd 001 na

MgO 182 129 116 114 122 304 382 252 244 938

CaO 506 257 283 082 044 124 101 039 810 1030

Na2O 1107 966 969 971 1299 1388 1384 1211 673 374

K2O 154 197 465 357 138 121 142 127 300 146

Cl 031 106 038 012 014 023 061 020 015 na

P2O5 041 008 044 009 012 049 233 018 100 059

Total 9956 9896 9860 9609 9887 9845 9929 9846 9821 9912

Mg-no 6329 4816 2489 6175 4632 6474 6458 6289 4552 5658

P 1061 448 1610 230 503 1519 6738 676 2994 na

K 12563 16408 39599 32515 15872 9265 11277 10071 25900 na

Sc 179 184 126 406 151 970 136 848 691 237

Ti 665 7329 4974 2953 1965 1213 1163 1055 18199 15311

V 107 265 235 796 920 210 151 134 234 na

Ni 128 371 92 5575 983 170 5199 5298 199 191

Zn 962 103 109 110 459 440 508 309 819 na

Ga na 173 273 194 220 na na na na na

Rb 181 263 124 546 626 176 398 369 577 369

Sr 427 488 1407 188 881 1644 4997 985 1062 792

Y 543 145 224 752 262 619 158 369 301 299

Zr 466 171 353 761 660 578 365 428 242 216

Nb 416 161 137 260 136 234 164 169 837 647

Cs 014 046 186 125 007 022 102 052 063 na

Ba 238 261 1064 129 168 405 1099 641 625 563

La 698 183 848 144 889 2212 1008 106 537 462

Ce 947 256 131 308 138 333 1265 164 104 823

Pr 655 194 120 330 116 304 975 160 120 na

Nd 146 603 408 101 329 993 256 564 463 400

Sm 142 048 570 179 419 141 379 679 105 851

Eu 041 035 228 082 112 438 103 241 345 267

Gd 062 051 415 158 402 106 245 566 677 729

Tb na 0086 086 020 071 na na na na 111

Dy 090 027 359 098 434 104 171 637 732 na

Ho 023 0037 105 025 095 206 446 104 117 114

Er 045 038 195 058 255 611 153 264 305 na

Tm 009 50039 033 0081 042 085 292 5014 036 052

Yb 062 023 267 044 307 667 120 487 253 220

(continued)

21

Evidence for low-temperature SiO2^Na2O^CO2^H2O-rich melts or fluids

Potential metasomatic agents inferred from the presenceof graphite are C^O^H-rich fluids carbonate melts orCO2-bearing silicate melts The resulting phase assem-blages and geochemical signatures are the result of infiltra-tion of fluid or melt into a cold (750^8508C) shallow(515GPa Fig 2) heterogeneous and locally reduced litho-spheric mantle Metasomatism under such conditions in anopen system must result in complex geochemical signa-tures that reflect the degree of fluid^rock reaction Thismakes it particularly difficult to constrain the exactnature of the lsquooriginalrsquometasomatic agentType III-c xenolith Ke 196525 contains the highest frac-

tion of metasomatic phases and the least fractionatedamphibole trace element patterns The lsquomagmaticrsquo texturesof the Mg-katophorites in this sample suggest direct crys-tallization from a melt or a fluid rather than replacementof a pre-existing phase Therefore hypothetical liquids inequilibrium with the amphiboles in this xenolith couldyield the composition of the initial metasomatic agentsFigure 17 illustrates the trace element patterns of suchmelts and fluids calculated using sets of experimentally orempirically determined mineralmelt (fluid) partitioncoefficients including carbonate melt silicate melt andhydrous fluid (for references see caption to Fig 17)Common features for all calculated metasomatic agents

are negative Zr^Hf^Ti anomalies and LREE enrichment(Fig 17) However previous studies have shown that thesesignatures are indicative of almost all types of metasoma-tism including interaction of peridotite with carbonatiticmelt (Green amp Wallace 1988 Dautria et al 1992 Hauriet al 1993 Rudnick et al 1993 Ionov et al 1996 Yaxleyet al 1998 Coltorti et al 1999) with hydrous fluids andormelts for example originating from dehydration of altered

subducting lithosphere (Maury et al 1992 Zanetti et al1999 Grecurren goire et al 2001 Laurora et al 2001 Parkinsonet al 2003) and interaction of peridotite with volatile-and Si-rich melts that evolved through reactive porousflow (Bedini et al 1997 Ionov et al 2002 2006 Bodinieret al 2004 Rivalenti et al 2004)In the case of the Marsabit xenoliths integration of the

trace element characteristics thermobarometric inferencesand mineralogical constraints favours a Na2O^SiO2^CO2^H2O-rich silicate melt Carbonated melts can prob-ably be ruled out on major element and mineralogical con-siderations First amphiboles are Ca-poor in conflict withcarbonate melt metasomatism which is generally charac-terized by Ca enrichment (Rudnick et al 1993 Neumannet al 2002) Further orthopyroxene is a stable phase andprobably even crystallizes during metasomatism in somesamples (eg Ke 785) At near-solidus conditions coexis-tence of orthopyroxene and carbonate melt is possibleonly at pressures 20GPa (see Fig 2 eg Lee amp Wyllie2000 and references therein) which is considerably higherthan the maximum pressures indicated by the compositionof spinel (515GPa) At such low pressures metasomatismby carbonated melts will convert lherzolite and harzbur-gite immediately into wehrlite which is the major perido-tite type in all xenolith suites where carbonate meltmetasomatism is convincingly demonstrated (Rudnicket al 1993 Yaxley et al 1998 Neumann et al 2002)In Marsabit wehrlites are absent howeverAn Na2O^SiO2^CO2^H2O-rich melt or fluid on the

other hand would explain the Na-rich nature of amphiboleand phlogopite as well as the presence of orthopyroxeneGrowth of metasomatic (and Al-poor) orthopyroxene hasbeen described from several xenolith localities where peri-dotites were infiltrated either by hydrous fluids orSi-rich melts (Smith et al 1999 McInnes et al 2001Morishita et al 2003) Na-rich phlogopite is commonly

Table 6 Continued

Type III-b III-c Host

Sample 195915 196515 196515 785 196525 19653 19653 19653 195927

Remark next amph next cpx xenol margin next amph next amph melt pocket next apatite next Opxm melt pocket host basanite

Lu 014 0050 026 0041 046 080 140 058 040 031

Hf 026 052 535 014 076 110 5087 124 568 504

Ta 50080 033 638 0022 123 101 784 801 394 403

Pb 541 448 280 255 840 797 260 749 493 na

Th 194 296 160 466 703 185 247 105 534 506

U 545 069 478 099 152 447 724 216 171 126

nd not detected na not analysedAverage composition of the strongly alkaline basanites from Marsabit (Volker 1990)

22

Fig 12 Total alkalis vs SiO2 (a) Harker-type co-variation diagrams (b^e) and incompatible trace element co-variation diagram (f) showingthe compositional variability of silicate glass found in the porphyroclastic spinel harzburgites and dunites from Marsabit In (b) binary mixinglines between the typical composition of the alkali-rich glass (large open diamond) and the associated microlites are shown (each increment is10wt)They illustrate that the glass does not represent the derivate of host lava infiltration and fractional crystallization Subalkaline glass inGroup I (grt)^spl lherzolites from Marsabit is shown for comparison and is interpreted to derive from incongruent melting of Ti-pargasite(Kaeser 2006)

23

found as a reaction product between anhydrous mafic toultramafic rocks and H2O-rich evolved liquids (Costaet al 2001 and references therein) or H2O-bearing fluids(eg Odling 1994) The latter study was an experiment at20GPa simulating the effects of decreasing temperatureson the phase assemblages and their composition duringmetasomatism using a model peridotitethornC^O^H fluidThe coolest part of Odlingrsquos (1994) experiment (510758C)contained a strikingly similar phase assemblage (kato-phorite phlogopite with 2wt Na2O and Al-poororthopyroxene) to the Marsabit samples interpreted tohave formed during sub-solidus fluid^solid interactionA compositionally similar phase assemblage (highamounts of opx Si-rich hornblende Na-rich phlogopite)resulted from experiments simulating peridotite interac-tion with Si-rich melt (dacite) at lower P^T conditions(9508C at 092GPa Prouteau amp Scaillet 2003)In this study the trace element signatures of Group III-c

Mg-katophorite and the calculated metasomatic agentsfurther suggest that the metasomatizing agent was a meltrather than a fluid Hypothetical fluids in equilibrium witheuhedral Mg-katophorite from sample Ke 196525 havehigh NbLILE (eg Rb Ba) and low LILEREE ratios(Fig 17) Because experimental data predict the oppositefor fluids (LILE enriched over HFSE and REE egManning 2004 and references therein) a fluid with asignature such as that indicated in Fig 17 is unrealistic

Fig 13 Trace element patterns of silicate glass in porphyroclasticspinel harzburgites and dunites from Marsabit compared withthe trace element composition of associated clinopyroxene (Dim)micro-phenocrysts xenolith-hosting basanites from Marsabit andglass interpreted to represent xenolith^host interaction [concentra-tions normalized to the primitive mantle values of McDonough ampSun (1995)]

Table 7 Representative analyses of carbonates (oxides in

wt trace elements in mgg)

Group III-b III-b III-c III-c III-c

Mineral Dol Mg-Cc Mg-Cc Mg-Cc Mg-Cc

Sample 195915 196515 785 19653 19653

Texture in glass in glass in glass in glass in glass

SiO2 na 003 na nd 003

FeO(tot) 016 011 010 010 006

MnO 001 nd nd nd nd

MgO 1922 236 190 125 517

CaO 3189 5313 5377 5391 4853

Na2O 003 nd 004 nd 003

K2O na 002 na 001 nd

SrO 003 010 008 004 021

BaO 003 nd nd na na

CO21 4618 4444 4442 4377 4391

Total 9756 10021 10032 9909 9794

Mg-no 9954 9745 9713 9554 9931

Ca-no 5439 9418 9531 9687 8709

P 220 334 137 580 589

K 970 212 642 105 155

Sc 5065 50150 027 5027 50076

Ti 975 189 242 528 196

V 5085 0481 5042 5037 021

Ni 900 673 517 5294 593

Zn 459 5089 113 296 045

Ga na 5014 5013 na na

Rb 0084 50072 50074 50060 0046

Sr 593 860 371 251 4274

Y 012 011 130 704 015

Zr 261 024 012 097 756

Nb 144 019 0056 692 185

Cs 50063 50011 50018 50039 50008

Ba 126 230 5024 029 271

La 034 057 084 136 025

Ce 032 013 134 997 023

Pr 006 006 018 095 0026

Nd 030 013 068 228 0078

Sm 5044 50091 0074 048 50018

Eu 5012 50019 0051 020 50008

Gd 5037 50090 014 051 0026

Tb na 50010 0036 na na

Dy 5019 50039 0089 058 50038

Ho 50029 0013 0044 011 50006

Er 5012 50041 0083 033 50019

Tm 50052 50013 0016 0084 50006

Yb 5043 50063 014 027 50040

Lu 50072 50010 0011 0066 50008

(continued)

24

At the relatively low maximum pressures (515GPa) asupercritical liquid is not expected either (eg Kesselet al 2005)Finally a relatively hydrous metasomatizing agent

would be consistent with formation of Mg-katophoriteswith lowTi and presumably low Fe3thorn contents (given therelatively reducing conditions) which in turn imply high(OH)^ contents In other words partial dehydrogenationtypically observed in common (Ti-rich) mantle amphibole

(eg Popp et al 1995 Tiepolo et al 2001 and referencestherein) cannot have played an important role in thiscase as H deficiency is mainly the result of coupled substi-tutions involvingTi and Fe3thorn (Popp et al 1995) In conclu-sion the metasomatizing agent was most probably aNa2O^SiO2^CO2^H2O-rich meltComparison with experimental studies further con-

strains the low-temperature regime in which metasoma-tism occurred In ultramafic (eg Niida amp Green 1999)basaltic (eg Rapp 1995) and Si-enriched systems (egtrachybasaltic Barclay amp Carmichael 2004) amphibole(in the latter study together with phlogopite) is confinedto temperatures lower than 1000^10508C at pressures520GPa (Fig 2)Thus crystallization of the early metaso-matic phase assemblage can be explained by infiltration ofa silicate melt that rapidly cooled upon contact with theambient peridotite (750^8508C prior to metasomatismFig 2) However this low-T stage is based on Ca-in-opxthermometry (see Brey amp Kolaquo hler 1990 Kaeser et al2006 Table 1) and therefore probably underestimates theeffective temperature conditions during metasomatismThis can be explained by low diffusion rates and proximityto the closure temperature for the diopside^enstatiteexchange reaction at temperatures lower than 8508C(eg Smith 1999 and references therein) The lower tem-perature limit at which silicate melt metasomatism ispossible (ie defined by the meltrsquos solidus and viscosity)depends strongly on volatile and SiO2 contentsExperiments by Draper amp Green (1997) for exampleshowed that highly silicic melts (52^64wt SiO2) canbe in equilibrium with mantle harzburgite at temperaturesas low as 8508C (at 10^20GPa under anhydrous andC^O^H-fluid-bearing conditions) These are howeverextreme compositions for mantle melts The actual SiO2

content of the metasomatizing agent in this case is difficultto constrain from the metasomatic mineral assemblage ascompositions may have varied considerably during reac-tive percolation (eg including Si-releasing reactions suchas cpx replacement by amphibole versus Si-consumingreactions such as Opxm crystallization) Moreover it isunlikely that a melt persists at temperatures muchbelow 9008C which is the solidus of fertile lherzolite(ie primitive upper mantle composition) underH2O-saturated conditions at 15GPa (Grove et al 2006see Fig 2) Below the solidus residual volatile-enrichedfluids may percolate further for example forming the lessmetasomatized Type III-c xenoliths (eg Ke 785) as wellas Type III-b and -a peridotites (see also Bailey 1987)Relatively cold conditions would also explain the veryhigh compositional gradients developed on small scales inType III-a samples (Fig 15) Given the absence of chloritein all investigated samples the lower temperature limitof such fluid metasomatism is 8008C (eg Fumagalli ampPoli 2005 see Fig 2)

Table 7 Continued

Group III-b III-b III-c III-c III-c

Mineral Dol Mg-Cc Mg-Cc Mg-Cc Mg-Cc

Sample 195915 196515 785 19653 19653

Texture in glass in glass in glass in glass in glass

Hf 5016 50038 50036 50074 50013

Ta 0048 50012 50016 50031 0008

Pb 5024 858 5021 074 018

Th 0021 50020 50017 50028 50005

U 060 094 086 600 148

Mg-Cc Mg-calcite Dol dolomite nd not detectedna not analysed Ca-numberfrac14 100Ca(CathornMg)1Calculated from stoichiometry

Fig 14 Trace element patterns of glass-hosted carbonates in TypeIII-b and -c spinel harzburgite and dunite xenoliths from Marsabit[concentrations normalized to the primitive mantle values ofMcDonough amp Sun (1995)] The grey field outlines the compositionalrange of carbonates found in other peridotite xenoliths (Ionov ampHarmer 2002 and references therein) Carbonate material inter-preted as quenched carbonatite melt found in peridotite xenolithsfrom Kerguelen (Moine et al 2004) is shown for comparison(bold line)

25

Finally the presence of apatite even though oftentaken as indicator for carbonatite-type metasomatism(eg Yaxley et al 1998) is not in conflict with metasoma-tism by silicate melts Apatite has also been shown to crys-tallize from volatile and alkali-rich derivatives formedduring reactive porous flow of originally mafic silicatemelts (Woodland et al 1996 Bedini et al 1997 Bodinieret al 2004 Ionov et al 2006)

FORMATION OF THE LATE GLASS -BEAR ING ASSEMBLAGESTextures strongly suggest that the glass-bearing assem-blages formed later and at the expense of the early metaso-matic assemblages (Fig 4a and d) Features such as thereappearance of clinopyroxene (Na-rich Cr diopsidemicro-phenocrysts) a phase replaced by the early

Fig 15 Detailed REE systematics in primary clinopyroxene (cpx-I) in a porphyroclastic cryptically metasomatized spinel harzburgite(sample Ke 19651) showing the transition fromType III-a (Zone A and B) toType III-b (Zone C) on a scale of a few centimetres The numberson the photomicrographs (crossed polars) correspond to single laser ablation microprobe pits (for further explanation see text)

26

metasomatic assemblage indicate that melt pocket forma-tion reflects a significant change in P^T^X space in partsof the mantle beneath Marsabit after earlier metasoma-tism Based on geochemical arguments and mass-balanceconsiderations we will show that (1) the late glass-bearingassemblages are closely related to early metasomaticassemblages mainly representing breakdown productsand (2) breakdown was induced by infiltration of aCO2-rich fluid in the mantle probably followed andenhanced by heating and decompression melting duringxenolith transport in the host magma

Evidence for in situ partial meltingBSE imaging reveals that the phases becoming unstableare amphibole orthopyroxene and to minor extentsspl-I phlogopite cpx-I and apatite Compositional(inter- and intra-xenolith) heterogeneity of silicate glass istexture controlled and varies as a function of theassociated precursor phases (Fig 12a^f)TiO2 for exampleshows higher concentrations in glass from xenolith Ke196515 containing Ti-enriched amphibole (Fig 12c)whereas K2O contents are highest in phlogopite-rich sam-ples (eg Ke 785 Fig 12d) Elevated P2O5 contents arefound in glass adjacent to apatite (Fig 12e) whichmirrors at the same time the highTh and LREE contents

of apatite (Fig 12f) Significant Si enrichment is found inglass coating orthopyroxene (Fig 12d) supporting incon-gruent melting of orthopyroxene (see below) Even thoughthe melt pockets are connected along grain boundaries thestrong compositional variation (on inter- and intra-xeno-lith scale) suggests that the mobility of the melt was lim-ited or that the melting event was very short-lived (fortiming constraints see below)The co-variation plots in Fig 12 further reveal consider-

able compositional differences between all types of xeno-lith glass and the host basanite These data indicate thatthe glass cannot be the product of xenolith^host lava inter-action during ascent to the surface Also back-mixing ofmicro-phenocrysts into the melt fails to derive the glassfrom a host-related melt (Fig 12b) Glass from xenolithmargins however is compositionally closer to the bulkhost basanite (eg Fig 12b) in line with a direct derivationfrom xenolith^host lava interaction Its more evolvedmajor element composition (eg higher SiO2 lowerMg-number Fig 12b) is probably the result of fractiona-tion of basanite phenocrysts (eg olivine augite andtitanomagnetite Volker 1990) Host lava^xenolith interac-tion is further supported by the trace element signature ofglass at the margin of the xenoliths which is closely similarto that of the Marsabit basanites (Fig 13 Volker 1990)The same characteristics are observed for intra-xenolithglass in the unusual kaersutite-bearing sample Ke 195927

Fig 16 Oxygen fugacities relative to the quartz^fayalite^magnetitebuffer (QFM equation of OrsquoNeill 1987) as a function of temperatureat assumed pressure of 1GPa calculated using the composition ofcoexisting spinel and olivine (ol-I^spl-I pairs and Chrm^Olm pairsrespectively) based on the oxygen barometer of Ballhaus et al (1991)The graphite^CO2 equilibrium was calculated after Frost amp Wood(1998) and the graphite^carbonate^silicate equilibrium was calcu-lated using the equation of Eggler amp Baker (1982) (See text for furtherexplanation)

Fig 17 Calculated metasomatic agents in equilibrium with the euhe-dral Mg-katophorites from the Type III-c amphibole dunite Ke 196525 Amphibolehydrous fluid and amphibolecarbonatite melt parti-tion coefficients are from Adam et al (1997) and Adam amp Green(2001) respectively Silicate melts have been calculated using amphi-boleSi-rich melt partition coefficients from Chazot et al (1996b) andamphibolebasanite partition coefficients from LaTourrette et al(1995) The compositional range of the Marsabit basanites and alkalibasalts (Volker 1990) is shown for comparison

27

Xenolith glass unrelated to the host lava and melt pock-ets similar to those reported here in association with resi-dual phases (eg amphibole or clinopyroxene) has beenreported from many xenolith localities (eg Frey ampGreen 1974 Ionov et al 1994 1996 Chazot et al 1996a1996b Yaxley et al 1997 Laurora et al 2001 Bali et al2002) Residual phases within the melt pockets stronglyindicate in situ partial melting within the xenolithsHowever a variety of processes may trigger such in situ

melting including (1) closed-system decompression melt-ing of either anhydrous (Maale amp Printzlau 1979) orhydrous phases (Frey amp Green 1974 Francis 1976) duringxenolith ascent in the host lava (2) melting during interac-tion of the xenoliths with the host basaltic melt duringxenolith ascent (eg Zinngrebe amp Foley 1995 Shaw ampKlulaquo gel 2002 Shaw et al 2006) (3) melting in the mantleprior to xenolith entrainment induced by an infiltratingfluid (ie flux melting eg Jones et al 1983 Ionov et al1994 Demecurren ny et al 2004) However most studies agreethat that partial melting occurred very shortly beforeandor during xenolith exhumation by the host magmaThus a combination of metasomatically triggered meltingfollowed andor assisted by decompression melting duringxenolith transport may be a realistic alternative in somecases (eg Dawson 2002)Recently Ban et al (2005) have shown in a very detailed

mass-balance study on glass-bearing xenoliths from theWest Eifel that the weight proportions of glass and theassociated micro-phenocrysts match the composition ofthe preserved parent amphibole We used the sameapproach and performed simple mass-balance calculations(least-squares fitting) to constrain the conclusions drawnfrom petrography and to test which phase contributedand to what extent to the formation of the melt pocketsThe results reported in Table 8 show that in most casesamphibole-melting can account successfully for the meltpocket composition and only a minor amount (520) ofother phases (orthopyroxene andor phlogopite) needs tobe added to the reactant Such a contribution of otherphases agrees with textural observation (eg Fig 4cshowing Dim growing on resorbed Opxm) as well as withthe conclusions of Ban et al (2005) who stated that moreSi-rich glass (ie 454wt) necessitates orthopyroxenecontributing to the meltTrace element patterns of the bulk glassthornmicro-

phenocryst pockets calculated using the measured traceelement abundances of glass and micro-phenocrysts andthe respective mass-balance parameters (Table 8) arebroadly identical to the measured amphibole composition(Fig 18a and b) Higher LILE (Cs Rb Ba) concentrationsin the calculated bulk melt patches in sample Ke 785 canbe balanced by adding 7wt of phlogopite (Fig 18b)which is in line with mass balance (Table 8) The dominantcontrol of amphibole to account for the trace element

budget of the melt pockets is similar to results found inother suites of glass-bearing xenoliths (Chazot et al1996a Yaxley amp Kamenetsky 1999 Laurora et al 2001)Further the obtained phase proportions for the melt pock-ets are in good agreement with the results from image ana-lysis (Table 1) In addition they are similar to the glass^cpx^ol proportions recorded in recent experiments on the

Table 8 Results of mass-balance calculations to test melt

pocket formation by incongruent melting of the early metaso-

matic assemblages

Sample 195915 785 785 196525 196525 19653

Reactant

amph 1000 1000 0820 1000 0944 mdash

opx mdash mdash 0078 mdash 0056 0406

phl mdash mdash 0102 mdash mdash mdash

glass mdash mdash mdash mdash mdash 05941

SiO2 4484 4989 4964 5129 5167 5898

TiO2 014 039 036 025 024 018

Cr2O3 232 291 258 127 121 009

Al2O3 1368 762 769 800 757 961

FeO(tot) 336 314 333 343 357 394

MnO 006 008 008 012 012 010

NiO 009 013 014 011 011 004

MgO 1844 2029 2197 2040 2122 1625

CaO 854 783 644 966 913 078

Na2O 460 500 436 504 476 843

K2O 049 032 096 042 040 060

Product

glass 0389 0322 0335 0292 0284 07032

ol 0243 0259 0315 0266 0286 0273

cpx 0288 0373 0308 0442 0431 0024

spl 0052 0032 0028 mdash mdash mdash

carb 0028 0014 0014 mdash mdash mdash

SiO2 4477 5027 4986 5207 5181 5903

TiO2 013 041 039 015 014 017

Cr2O3 213 314 272 109 106 015

Al2O3 1364 764 770 798 776 957

FeO(tot) 374 331 351 322 332 350

MnO 007 007 007 009 009 006

NiO 011 013 015 011 012 007

MgO 1834 2024 2196 2063 2141 1635

CaO 854 759 632 952 929 080

Na2O 449 385 388 499 485 860

K2O 060 096 100 015 014 091

r2 023 200 035 084 028 033

Prefix Ke omitted1Low-Si glass 2High-Si glass (see text for explanation)

28

melting behaviour of amphibole in wehrlites at tempera-tures between 1175 and 12508C and pressures of05^10GPa (Mecurren dard et al 2006) These temperatures arein good agreement with the estimates for the formation ofolivine and chromite micro-phenocrysts in the melt

pockets (Fig 16) The heat supply required for amphibolemelting in the formerly low-T metasomatized Group IIIxenoliths (T 200^3008C) may come from local heatingshortly before or during xenolith entrainment in the hostlava or from fluid infiltration that triggered melting (seediscussion below)Finally formation of the most Si-enriched melt (adjacent

to Opxm in Group III-c samples) can be modelled assum-ing progressing interaction between infiltrating melt andorthopyroxene where the latter melts incongruently

opxthornmelt1 olthornmelt2 cpx ethseeTable 8THORN eth1THORN

To calculate reaction (1) we chose the least evolved glassobserved in melt patches in sample Ke 19653 (SiO2

5921wt Mg-number 6465) to represent the initialmelt1 whereas melt2 adjacent to opx is SiO2-rich(6530wt) Trace element abundances in glass next toorthopyroxene are shifted to lower concentrations(Table 6) which is in agreement with dilution caused byaddition of a trace element-poor orthopyroxene compo-nent The resultant phase proportions are similar to thoseobserved at orthopyroxene^melt interfaces (ie higholivine and low cpx abundances) and are in agreementwith experimental studies (Shaw et al 1998)

Partial melting triggered by fluid fluxMass-balance calculations provide an explanation for meltpocket formation by in situ partial melting Thus onemight conclude that melting is simply the result of decom-pression and heating during xenolith transport in the hostmagma However there are two points that argue thatmelting was triggered by the infiltration of a fluid phaseshortly before the xenoliths were sampled by the hostmagma (1) the presence of occasionally high amounts ofcarbonate included in the melt pockets (2) the extremelyNa-rich clinopyroxene (Dim) compositions either includedas micro-phenocrysts in the melt pocket or present asequilibrated grains around chromite in the single TypeIII-d xenolith (Fig 3f) Both arguments will be discussedin the next section

Formation of carbonates

Textures and compositions of carbonate globules can beexplained by carbonate precipitation from a CO2-bearingfluid As demonstrated by several studies highCa(CathornMg) values (Table 7) and low REE concentra-tions are strong arguments that the carbonates are cumu-late crystals precipitated from a CO2-rich melt or afluid and not quenched carbonated melts (Lee et al 2000Ionov amp Harmer 2002) Oscillatory zoning (Fig 5a and b)of individual carbonate crystals within singleglobules corroborates this interpretation The roundedcontacts toward glass (globules) are also in line with thesefindings Similar globular forms were reported fromexperimental studies and do not a priori imply the

Fig 18 Calculated trace element characteristics of reactants (in mostcases amphibolethorn orthopyroxene filled symbols) and products(melt pockets open symbols) of the lsquoamphibole meltingrsquo model[see equation (1) in text] The relative phase proportions wereobtained by mass-balance calculations and are given in Table 8 Thedashed lines in (b) show that the high LILE concentrations obtainedfor the product (bulk melt pocket) can be explained by adding about7wt of phlogopite to the reactant (amphibolethorn orthopyroxeneTable 8) Further explanation is given in the text

29

presence of immiscible carbonate and silicate melts(eg Lee amp Wyllie 1996)The high amount of carbonate in some melt pockets

however (eg Fig 4e) does need further explanation asthe incongruent melting reaction (1) does not account forthe CO2 required for carbonate precipitation duringamphibole melting Therefore amphibole breakdowneither occurred in an open system with addition of at leastCO2 (eg Laurora et al 2001 Ban et al 2005) or anotherpre-existing phase acted as a source of carbon Oxidationof graphite texturally identified as a part of the earlyassemblage (eg Fig 3d) could account for some of theCO2 necessary for carbonate formation A change in fO2

conditions is also indicated when the composition of glass-related spinel (Chrm) and Olm are used for oxygen baro-metry which yields oxygen fugacities well on the oxidizedside of the carbon^carbonate^silicate buffer (Fig 16)However increasing fO2 triggered by closed-systemdecompression-induced melting alone does not appear fea-sible Also textures such as straight contacts between gra-phite and Mg-calcite (Fig 4c) suggest that carbonategrows on graphite (ie indicating local fO2 conditions cor-responding to carbon^carbonate equilibrium) rather thanreplacing it We therefore interpret melt pocket formationto be triggered by the infiltration of a CO2-rich fluid(ie flux melting see also Ionov et al 1994) This interpre-tation also accounts for the presence of abundant vugs inthe glass that could have contained a fluid (Ionov et al1994 2006) which was presumably lost during sample pre-paration However we emphasize here that late fluid infil-tration had only subtle effects on the trace elementcomposition of Type III-b and -c xenoliths

Formation of Na-rich Cr diopside

Infiltration of CO2-rich fluidwould also explain the Na-richcomposition of Dim micro-phenocryst cores (Fig 6a and b)Ionov et al (2005) noted recently that clinopyroxenes inmeltpockets are commonly characterized by Na2O contents51wt mainly as a result of low cpxmelt partition coeffi-cients at low pressures (eg Blundy et al 1995) Thus veryhighNa2Ocontents in Dimmicro-phenocrysts (Fig6a) sug-gest replacement of amphibole by Dim and eventually Olmand Chrm during reaction with a fluid at subsolidus condi-tions An origin of micro-phenocrysts by replacementrather than crystallization from a melt is indicated by theidentical crystallographic orientation of individual Dimcrystals in singlemelt pockets (Fig4a)Once the solidus of the precursor phase(s) was reached

(ie 10508C for amphibole Niida amp Green 1999) silicatemelt was generated in situ and led to the formation of clin-opyroxene rims with lower Na contents (Fig 6b) and thedevelopment of euhedral shapes in contact with melt (egFig 4a and e) Eventual immiscibility between remainingfluid and newly formed melt (eg during xenolith ascent)would then explain the presence of carbonate globules and

vugs enclosed in glass Likewise different origins of meltand carbonate were recently proposed for similar meltpockets from Hungarian xenoliths by Demecurren ny et al(2004) who found oxygen isotope disequilibrium betweencarbonate globules and the host glass Such a two-stepmodel (fluid-induced replacement followed by melt gen-eration) can be explained by CO2-fluid infiltration occur-ring within the mantle at sub-solidus conditions ultimatelyfollowed by xenolith entrainment in the host magma andtransport to the surfaceThe host magma possibly suppliedthe additional heat necessary for melt generationFast decompression would further enhance melting andcould also account for the lower Na contents in Dimmicro-phenocryst rims as a result of decreasing cpxmeltpartition coefficients at decreasing pressures (Blundyet al 1995)Rapid replacement of amphibole ( phlogopite) by an

anhydrous phase assemblage indicates that the fluid waswater-poor (eg low H2OCO2 ratio see Ionov et al2006)This is corroborated by the fact that Mg-katophorite(presumably OH-rich see above) is occasionally rimmedby a residual kaersutitic selvage (Fig 4d) Such Ti-richmantle amphiboles are commonly OH-poor as a result ofpartial dehydrogenation (eg Tiepolo et al 2001 and refer-ences therein)Finally the presence of theType III-d xenolith contain-

ing texturally equilibrated clinopyroxene of almost identi-cal composition to Dim micro-phenocrysts in melt pockets(Table 2 Figs 6a and 7b) further strengthens the argu-ment that CO2-fluid percolation started in the mantlebefore entrainment (ie enough time elapsed to allow thisrock to anneal completely after the late metasomaticassemblage had formed) A similar conclusion can bedrawn for the clearly host-related glass found in xenolithKe 195927 (eg Fig 13b) ThisType III-c sample is charac-terized by the presence of Dim micro-phenocrysts partlyovergrown by mineral rims clearly related to the host basa-nite magma (kaersutite Ti-augite and olivine withFo885 Fig 4f Tables 3 and 4 and supplementary dataat httppetrologyoxfordjournalsorg) Kaersutite hasmajor and trace element characteristics similar to typicalbasanitic and alkali basaltic amphibole phenocrysts(Fig 9d) The special phase assemblages in this xenolithindicates that host-related basanitic melts intruded someType III-c xenoliths and partly overprinted the assemblageformed slightly earlier by in situ derived alkali-rich peralu-minous melts fully preserved in other samples (eg Ke19653 or Ke 785)

ORIGIN OF METASOMATIZ INGAGENTS T IM ING AND TECTONICSETT INGAs noted above late metasomatism was probably short-lived started at mantle depths and probably continued

30

data (eg LaTourrette et al 1995) indicate that amphibolecrystallization alone is unlikely to deplete the metasomaticagent to such a high degree in HREE The same probablyaccounts for the extreme Nb depletion in amphiboles fromKe 785 relative to the otherType III-c xenolith (Ke 196525 see also Ionov et al 2002)Further clinopyroxene phlogopite and amphibole in the

Type III-b sample Ke 196515 on one hand all showstrongly fractionated REE patterns (eg high LaCe inamphibole Fig 10c) but on the other hand are character-ized by a positiveTi anomaly decoupled from other HFSE(eg Hf and Zr Fig 9b) Trace element fractionationduring amphibole crystallization (ie formation of TypeIII-c amphibole dunites) however should lower Ti con-tents in the remaining fluid or melt (eg Moine et al 2001)Based on textures mineralogy and dominant common

compositional features the hypothesis that Type III-band -c xenoliths were metasomatized by completely inde-pendent metasomatic agents can be ruled out Thereforetwo other possibilities remain to account for the inconsis-tent observations described above (1) inference of otherphases reactants or products that are able to fractionatespecific trace elements present in the mantle beneathMarsabit but not present in the few investigated samples(2) pre-metasomatic compositional heterogeneities similarto the inherited HREE variation inType III-a clinopyrox-ene (see above)Garnet would be an important sink for HREE but

metasomatism occurred in the spinel field after decom-pression with maximum pressures of 515GPa (Kaeseret al 2006) so garnet had already broken down to sym-plectites (Fig 15) Percolation of melt through garnet-bearing websterites prior to metasomatism in Ke 19651and 196515 could be an alternativeWebsterites have been

interpreted to occur in close spatial relationship with por-phyroclastic peridotites in the shallow mantle beneathMarsabit (Kaeser et al 2006) and they occasionally con-tain rutile and ilmenite which could further account forthe strong Nb and Ta depletion in some samples (eg Ke785) The available compositional data from the webster-ites however do not indicate metasomatic modification ofwebsteritic garnet (Olker 2001 B Kaeser unpublisheddata) nor compositional similarities to Group IIIperidotitesCareful study by SEM rules out the presence of exotic

phases such as zircon or Ti-oxides described fromother xenolith suites (eg Bodinier et al 1996 Rudnicket al 1999) Nevertheless it cannot be excluded that themetasomatizing agent percolated through mantle domainscontaining these phases before infiltrating the studiedrocksThe preservation of pre-metasomatic compositional het-

erogeneities is another possibility that must be consideredThat such lsquorock signalsrsquo partly overlap with the lsquometaso-matic signalrsquo is indicated by the HREE variation in TypeIII-a clinopyroxene (see above) and also by other composi-tional parameters For example both Mg-katophorite andphlogopite growing at the expense of spinel have very highCr2O3 contents whereas euhedral Mg-katophorite inthe amphibole dunite (Ke 196525) or in phlogopiteinclusions in Opxm is comparably Cr2O3-poor (Fig 8bTables 4 and 5) Pre-existing heterogeneities would essen-tially be preserved through elements that are relativelycompatible in a given mineral (ie that are not mobilizedduring metasomatism) This would be in agreement withthe preserved HREE variations in Type III-a clinopyrox-ene high Cr2O3 in amphiboles and phlogopite and Tispikes in amphibole from Ke 196515

Table 5 Continued

Type III-b III-c III-b III-c

Mineral phlog phlog phlog phlog phlog phlog phlog apatite apatite

Sample 196515 196515 19653 785 785 785 196525 195915 19653

Texture veinlet interst in opx in opx with amph

in mp

around

chromite

incl in

amph

bleb in

glass

interst

grain

Yb 50014 50011 5047 5015 50011 50019 5017 Yb 803 724

Lu 0002 50002 50061 50014 50002 50002 50020 Lu 115 742

Hf 0019 0018 5025 50053 50012 50006 5019 Hf 50023 50120

Ta 020 022 140 0031 0012 0009 023 Ta 50015 50075

Pb 546 586 125 108 083 098 143 Pb 436 577

Th 50004 50004 50080 0043 0088 0056 50021 Th 2538 1305

U 0012 0011 0069 50018 0038 0030 0031 U 572 278

interst interstitial mp melt pocket na not analysed nd not detected

19

Further very high Mg-numbers in replacive amphibolessupport melt (fluid)^rock interaction at low melt (fluid)rock ratios where Mg and Fe contents are buffered by theambient peridotite (Kelemen et al 1995 Tiepolo et al2001) The slightly lower Mg-number of euhedralamphiboles in Ke 196525 combined with their lowerCr2O3 contents (Fig 8b) suggests a higher melt (fluid)rock ratio which is further supported by considerablylower Mg-number in associated ol-I (Fo88 Table 3)

The limited number of investigated samples does notallow us to decide which of the processes described aboveacted and to what extent However given the heterogene-ity observed in the numerous non-metasomatized xenolithsfrom Marsabit (Kaeser et al 2006) pre-metasomaticfeatures certainly affect the final product of the melt^rockreaction process to a considerable degree

The nature of the metasomatizing agent(s)Graphite stability and oxygen fugacity considerations

The Marsabit xenoliths are unusual in that they occur inan off-cratonic setting yet bear macroscopic graphite Thelatter usually either occurs in kimberlite-hosted peridotites(on-craton) or in pyroxenites from alpine-type peridotitebodies (eg Pearson et al 1994) In the phlogopite perido-tites from Finero graphite has recently been reportedas nanoscopic layers in phlogopite (Ferraris et al 2004)In xenoliths from Marsabit graphite occurs in the sametextural relationship as in kimberlite-hosted peridotitexenoliths (ie millimetre-sized flakes typically associatedwith orthopyroxene see Fig 3c^e and Pearson et al 1994)Graphite inclusions in phlogopite and Opxm couldindicate crystallization during metasomatism by alsquoreducedrsquo C-bearing fluid or melt However precipitationof graphite more probably reflects the relatively reducingconditions in the uppermost mantle prior to metasoma-tism We used the oxygen barometer of Ballhaus et al(1991) based on ferric iron contents in spinel coexistingwith olivine to constrain oxygen fugacity (fO2)conditions prior to metasomatism in the Marsabitxenoliths (Fig 16) The results show that non-metasomatized Group I and IV samples reflect similarlyreducing conditions to the cryptically metasomatizedType III-a samplesAlthough assessing Fe3thornFe2thorn ratios in spinel based

on microprobe analyses can be problematic (Wood ampVirgo 1989 Ballhaus et al 1991) all microprobeanalyses were obtained using the same operationalsettings and therefore give at least the relative fO2

variation between samples The relatively reducingconditions in the pre-metasomatic cold andhighly deformed mantle well below carbon^carbonate^silicate equilibria (Mg2Si2O6thorn 05 CaMg(CO3)2frac14Mg2SiO4thorn 05 CaMgSi2O6thornCthornO2 Eggler amp Baker1982 see Fig 16) and close to the CCO oxygen buffer arein agreement with the presence of graphite in the GroupIII harzburgites Moreover the P^T conditions at whichthe Group III xenoliths partially re-equilibrated are closeto the subsolidus decarbonation of model carbonated lher-zolite (Fig 2 see Lee amp Wyllie 2000 and referencestherein) in agreement with the observed phase assemblageincluding orthopyroxene olivine clinopyroxene andgraphite

Fig 11 Trace element patterns of (a) phlogopite and (b) apatitefrom porphyroclastic spinel harzburgites and dunites from Marsabit[concentrations normalized to the primitive mantle valuesof McDonough amp Sun (1995)] The grey field in (a) indicates thecomposition of Ti-rich phlogopite in Group I (grt)^spl lherzolites(Kaeser et al 2006)

20

Table 6 Representative analyses of silicate glass (oxides in wt trace elements in mgg)

Type III-b III-c Host

Sample 195915 196515 196515 785 196525 19653 19653 19653 195927

Remark next amph next cpx xenol margin next amph next amph melt pocket next apatite next Opxm melt pocket host basanite

SiO2 5306 5775 5161 6096 5657 5938 5974 6530 4882 4410

TiO2 017 157 106 062 030 024 020 023 378 255

Al2O3 2412 2053 2025 1779 2313 1575 1245 1355 1890 1396

Cr2O3 005 001 nd 001 001 nd nd nd 002 na

FeO(tot) 188 247 626 126 252 295 374 265 509 1283

MnO 007 000 026 001 004 005 008 007 018 021

NiO nd nd nd na nd na 006 nd 001 na

MgO 182 129 116 114 122 304 382 252 244 938

CaO 506 257 283 082 044 124 101 039 810 1030

Na2O 1107 966 969 971 1299 1388 1384 1211 673 374

K2O 154 197 465 357 138 121 142 127 300 146

Cl 031 106 038 012 014 023 061 020 015 na

P2O5 041 008 044 009 012 049 233 018 100 059

Total 9956 9896 9860 9609 9887 9845 9929 9846 9821 9912

Mg-no 6329 4816 2489 6175 4632 6474 6458 6289 4552 5658

P 1061 448 1610 230 503 1519 6738 676 2994 na

K 12563 16408 39599 32515 15872 9265 11277 10071 25900 na

Sc 179 184 126 406 151 970 136 848 691 237

Ti 665 7329 4974 2953 1965 1213 1163 1055 18199 15311

V 107 265 235 796 920 210 151 134 234 na

Ni 128 371 92 5575 983 170 5199 5298 199 191

Zn 962 103 109 110 459 440 508 309 819 na

Ga na 173 273 194 220 na na na na na

Rb 181 263 124 546 626 176 398 369 577 369

Sr 427 488 1407 188 881 1644 4997 985 1062 792

Y 543 145 224 752 262 619 158 369 301 299

Zr 466 171 353 761 660 578 365 428 242 216

Nb 416 161 137 260 136 234 164 169 837 647

Cs 014 046 186 125 007 022 102 052 063 na

Ba 238 261 1064 129 168 405 1099 641 625 563

La 698 183 848 144 889 2212 1008 106 537 462

Ce 947 256 131 308 138 333 1265 164 104 823

Pr 655 194 120 330 116 304 975 160 120 na

Nd 146 603 408 101 329 993 256 564 463 400

Sm 142 048 570 179 419 141 379 679 105 851

Eu 041 035 228 082 112 438 103 241 345 267

Gd 062 051 415 158 402 106 245 566 677 729

Tb na 0086 086 020 071 na na na na 111

Dy 090 027 359 098 434 104 171 637 732 na

Ho 023 0037 105 025 095 206 446 104 117 114

Er 045 038 195 058 255 611 153 264 305 na

Tm 009 50039 033 0081 042 085 292 5014 036 052

Yb 062 023 267 044 307 667 120 487 253 220

(continued)

21

Evidence for low-temperature SiO2^Na2O^CO2^H2O-rich melts or fluids

Potential metasomatic agents inferred from the presenceof graphite are C^O^H-rich fluids carbonate melts orCO2-bearing silicate melts The resulting phase assem-blages and geochemical signatures are the result of infiltra-tion of fluid or melt into a cold (750^8508C) shallow(515GPa Fig 2) heterogeneous and locally reduced litho-spheric mantle Metasomatism under such conditions in anopen system must result in complex geochemical signa-tures that reflect the degree of fluid^rock reaction Thismakes it particularly difficult to constrain the exactnature of the lsquooriginalrsquometasomatic agentType III-c xenolith Ke 196525 contains the highest frac-

tion of metasomatic phases and the least fractionatedamphibole trace element patterns The lsquomagmaticrsquo texturesof the Mg-katophorites in this sample suggest direct crys-tallization from a melt or a fluid rather than replacementof a pre-existing phase Therefore hypothetical liquids inequilibrium with the amphiboles in this xenolith couldyield the composition of the initial metasomatic agentsFigure 17 illustrates the trace element patterns of suchmelts and fluids calculated using sets of experimentally orempirically determined mineralmelt (fluid) partitioncoefficients including carbonate melt silicate melt andhydrous fluid (for references see caption to Fig 17)Common features for all calculated metasomatic agents

are negative Zr^Hf^Ti anomalies and LREE enrichment(Fig 17) However previous studies have shown that thesesignatures are indicative of almost all types of metasoma-tism including interaction of peridotite with carbonatiticmelt (Green amp Wallace 1988 Dautria et al 1992 Hauriet al 1993 Rudnick et al 1993 Ionov et al 1996 Yaxleyet al 1998 Coltorti et al 1999) with hydrous fluids andormelts for example originating from dehydration of altered

subducting lithosphere (Maury et al 1992 Zanetti et al1999 Grecurren goire et al 2001 Laurora et al 2001 Parkinsonet al 2003) and interaction of peridotite with volatile-and Si-rich melts that evolved through reactive porousflow (Bedini et al 1997 Ionov et al 2002 2006 Bodinieret al 2004 Rivalenti et al 2004)In the case of the Marsabit xenoliths integration of the

trace element characteristics thermobarometric inferencesand mineralogical constraints favours a Na2O^SiO2^CO2^H2O-rich silicate melt Carbonated melts can prob-ably be ruled out on major element and mineralogical con-siderations First amphiboles are Ca-poor in conflict withcarbonate melt metasomatism which is generally charac-terized by Ca enrichment (Rudnick et al 1993 Neumannet al 2002) Further orthopyroxene is a stable phase andprobably even crystallizes during metasomatism in somesamples (eg Ke 785) At near-solidus conditions coexis-tence of orthopyroxene and carbonate melt is possibleonly at pressures 20GPa (see Fig 2 eg Lee amp Wyllie2000 and references therein) which is considerably higherthan the maximum pressures indicated by the compositionof spinel (515GPa) At such low pressures metasomatismby carbonated melts will convert lherzolite and harzbur-gite immediately into wehrlite which is the major perido-tite type in all xenolith suites where carbonate meltmetasomatism is convincingly demonstrated (Rudnicket al 1993 Yaxley et al 1998 Neumann et al 2002)In Marsabit wehrlites are absent howeverAn Na2O^SiO2^CO2^H2O-rich melt or fluid on the

other hand would explain the Na-rich nature of amphiboleand phlogopite as well as the presence of orthopyroxeneGrowth of metasomatic (and Al-poor) orthopyroxene hasbeen described from several xenolith localities where peri-dotites were infiltrated either by hydrous fluids orSi-rich melts (Smith et al 1999 McInnes et al 2001Morishita et al 2003) Na-rich phlogopite is commonly

Table 6 Continued

Type III-b III-c Host

Sample 195915 196515 196515 785 196525 19653 19653 19653 195927

Remark next amph next cpx xenol margin next amph next amph melt pocket next apatite next Opxm melt pocket host basanite

Lu 014 0050 026 0041 046 080 140 058 040 031

Hf 026 052 535 014 076 110 5087 124 568 504

Ta 50080 033 638 0022 123 101 784 801 394 403

Pb 541 448 280 255 840 797 260 749 493 na

Th 194 296 160 466 703 185 247 105 534 506

U 545 069 478 099 152 447 724 216 171 126

nd not detected na not analysedAverage composition of the strongly alkaline basanites from Marsabit (Volker 1990)

22

Fig 12 Total alkalis vs SiO2 (a) Harker-type co-variation diagrams (b^e) and incompatible trace element co-variation diagram (f) showingthe compositional variability of silicate glass found in the porphyroclastic spinel harzburgites and dunites from Marsabit In (b) binary mixinglines between the typical composition of the alkali-rich glass (large open diamond) and the associated microlites are shown (each increment is10wt)They illustrate that the glass does not represent the derivate of host lava infiltration and fractional crystallization Subalkaline glass inGroup I (grt)^spl lherzolites from Marsabit is shown for comparison and is interpreted to derive from incongruent melting of Ti-pargasite(Kaeser 2006)

23

found as a reaction product between anhydrous mafic toultramafic rocks and H2O-rich evolved liquids (Costaet al 2001 and references therein) or H2O-bearing fluids(eg Odling 1994) The latter study was an experiment at20GPa simulating the effects of decreasing temperatureson the phase assemblages and their composition duringmetasomatism using a model peridotitethornC^O^H fluidThe coolest part of Odlingrsquos (1994) experiment (510758C)contained a strikingly similar phase assemblage (kato-phorite phlogopite with 2wt Na2O and Al-poororthopyroxene) to the Marsabit samples interpreted tohave formed during sub-solidus fluid^solid interactionA compositionally similar phase assemblage (highamounts of opx Si-rich hornblende Na-rich phlogopite)resulted from experiments simulating peridotite interac-tion with Si-rich melt (dacite) at lower P^T conditions(9508C at 092GPa Prouteau amp Scaillet 2003)In this study the trace element signatures of Group III-c

Mg-katophorite and the calculated metasomatic agentsfurther suggest that the metasomatizing agent was a meltrather than a fluid Hypothetical fluids in equilibrium witheuhedral Mg-katophorite from sample Ke 196525 havehigh NbLILE (eg Rb Ba) and low LILEREE ratios(Fig 17) Because experimental data predict the oppositefor fluids (LILE enriched over HFSE and REE egManning 2004 and references therein) a fluid with asignature such as that indicated in Fig 17 is unrealistic

Fig 13 Trace element patterns of silicate glass in porphyroclasticspinel harzburgites and dunites from Marsabit compared withthe trace element composition of associated clinopyroxene (Dim)micro-phenocrysts xenolith-hosting basanites from Marsabit andglass interpreted to represent xenolith^host interaction [concentra-tions normalized to the primitive mantle values of McDonough ampSun (1995)]

Table 7 Representative analyses of carbonates (oxides in

wt trace elements in mgg)

Group III-b III-b III-c III-c III-c

Mineral Dol Mg-Cc Mg-Cc Mg-Cc Mg-Cc

Sample 195915 196515 785 19653 19653

Texture in glass in glass in glass in glass in glass

SiO2 na 003 na nd 003

FeO(tot) 016 011 010 010 006

MnO 001 nd nd nd nd

MgO 1922 236 190 125 517

CaO 3189 5313 5377 5391 4853

Na2O 003 nd 004 nd 003

K2O na 002 na 001 nd

SrO 003 010 008 004 021

BaO 003 nd nd na na

CO21 4618 4444 4442 4377 4391

Total 9756 10021 10032 9909 9794

Mg-no 9954 9745 9713 9554 9931

Ca-no 5439 9418 9531 9687 8709

P 220 334 137 580 589

K 970 212 642 105 155

Sc 5065 50150 027 5027 50076

Ti 975 189 242 528 196

V 5085 0481 5042 5037 021

Ni 900 673 517 5294 593

Zn 459 5089 113 296 045

Ga na 5014 5013 na na

Rb 0084 50072 50074 50060 0046

Sr 593 860 371 251 4274

Y 012 011 130 704 015

Zr 261 024 012 097 756

Nb 144 019 0056 692 185

Cs 50063 50011 50018 50039 50008

Ba 126 230 5024 029 271

La 034 057 084 136 025

Ce 032 013 134 997 023

Pr 006 006 018 095 0026

Nd 030 013 068 228 0078

Sm 5044 50091 0074 048 50018

Eu 5012 50019 0051 020 50008

Gd 5037 50090 014 051 0026

Tb na 50010 0036 na na

Dy 5019 50039 0089 058 50038

Ho 50029 0013 0044 011 50006

Er 5012 50041 0083 033 50019

Tm 50052 50013 0016 0084 50006

Yb 5043 50063 014 027 50040

Lu 50072 50010 0011 0066 50008

(continued)

24

At the relatively low maximum pressures (515GPa) asupercritical liquid is not expected either (eg Kesselet al 2005)Finally a relatively hydrous metasomatizing agent

would be consistent with formation of Mg-katophoriteswith lowTi and presumably low Fe3thorn contents (given therelatively reducing conditions) which in turn imply high(OH)^ contents In other words partial dehydrogenationtypically observed in common (Ti-rich) mantle amphibole

(eg Popp et al 1995 Tiepolo et al 2001 and referencestherein) cannot have played an important role in thiscase as H deficiency is mainly the result of coupled substi-tutions involvingTi and Fe3thorn (Popp et al 1995) In conclu-sion the metasomatizing agent was most probably aNa2O^SiO2^CO2^H2O-rich meltComparison with experimental studies further con-

strains the low-temperature regime in which metasoma-tism occurred In ultramafic (eg Niida amp Green 1999)basaltic (eg Rapp 1995) and Si-enriched systems (egtrachybasaltic Barclay amp Carmichael 2004) amphibole(in the latter study together with phlogopite) is confinedto temperatures lower than 1000^10508C at pressures520GPa (Fig 2)Thus crystallization of the early metaso-matic phase assemblage can be explained by infiltration ofa silicate melt that rapidly cooled upon contact with theambient peridotite (750^8508C prior to metasomatismFig 2) However this low-T stage is based on Ca-in-opxthermometry (see Brey amp Kolaquo hler 1990 Kaeser et al2006 Table 1) and therefore probably underestimates theeffective temperature conditions during metasomatismThis can be explained by low diffusion rates and proximityto the closure temperature for the diopside^enstatiteexchange reaction at temperatures lower than 8508C(eg Smith 1999 and references therein) The lower tem-perature limit at which silicate melt metasomatism ispossible (ie defined by the meltrsquos solidus and viscosity)depends strongly on volatile and SiO2 contentsExperiments by Draper amp Green (1997) for exampleshowed that highly silicic melts (52^64wt SiO2) canbe in equilibrium with mantle harzburgite at temperaturesas low as 8508C (at 10^20GPa under anhydrous andC^O^H-fluid-bearing conditions) These are howeverextreme compositions for mantle melts The actual SiO2

content of the metasomatizing agent in this case is difficultto constrain from the metasomatic mineral assemblage ascompositions may have varied considerably during reac-tive percolation (eg including Si-releasing reactions suchas cpx replacement by amphibole versus Si-consumingreactions such as Opxm crystallization) Moreover it isunlikely that a melt persists at temperatures muchbelow 9008C which is the solidus of fertile lherzolite(ie primitive upper mantle composition) underH2O-saturated conditions at 15GPa (Grove et al 2006see Fig 2) Below the solidus residual volatile-enrichedfluids may percolate further for example forming the lessmetasomatized Type III-c xenoliths (eg Ke 785) as wellas Type III-b and -a peridotites (see also Bailey 1987)Relatively cold conditions would also explain the veryhigh compositional gradients developed on small scales inType III-a samples (Fig 15) Given the absence of chloritein all investigated samples the lower temperature limitof such fluid metasomatism is 8008C (eg Fumagalli ampPoli 2005 see Fig 2)

Table 7 Continued

Group III-b III-b III-c III-c III-c

Mineral Dol Mg-Cc Mg-Cc Mg-Cc Mg-Cc

Sample 195915 196515 785 19653 19653

Texture in glass in glass in glass in glass in glass

Hf 5016 50038 50036 50074 50013

Ta 0048 50012 50016 50031 0008

Pb 5024 858 5021 074 018

Th 0021 50020 50017 50028 50005

U 060 094 086 600 148

Mg-Cc Mg-calcite Dol dolomite nd not detectedna not analysed Ca-numberfrac14 100Ca(CathornMg)1Calculated from stoichiometry

Fig 14 Trace element patterns of glass-hosted carbonates in TypeIII-b and -c spinel harzburgite and dunite xenoliths from Marsabit[concentrations normalized to the primitive mantle values ofMcDonough amp Sun (1995)] The grey field outlines the compositionalrange of carbonates found in other peridotite xenoliths (Ionov ampHarmer 2002 and references therein) Carbonate material inter-preted as quenched carbonatite melt found in peridotite xenolithsfrom Kerguelen (Moine et al 2004) is shown for comparison(bold line)

25

Finally the presence of apatite even though oftentaken as indicator for carbonatite-type metasomatism(eg Yaxley et al 1998) is not in conflict with metasoma-tism by silicate melts Apatite has also been shown to crys-tallize from volatile and alkali-rich derivatives formedduring reactive porous flow of originally mafic silicatemelts (Woodland et al 1996 Bedini et al 1997 Bodinieret al 2004 Ionov et al 2006)

FORMATION OF THE LATE GLASS -BEAR ING ASSEMBLAGESTextures strongly suggest that the glass-bearing assem-blages formed later and at the expense of the early metaso-matic assemblages (Fig 4a and d) Features such as thereappearance of clinopyroxene (Na-rich Cr diopsidemicro-phenocrysts) a phase replaced by the early

Fig 15 Detailed REE systematics in primary clinopyroxene (cpx-I) in a porphyroclastic cryptically metasomatized spinel harzburgite(sample Ke 19651) showing the transition fromType III-a (Zone A and B) toType III-b (Zone C) on a scale of a few centimetres The numberson the photomicrographs (crossed polars) correspond to single laser ablation microprobe pits (for further explanation see text)

26

metasomatic assemblage indicate that melt pocket forma-tion reflects a significant change in P^T^X space in partsof the mantle beneath Marsabit after earlier metasoma-tism Based on geochemical arguments and mass-balanceconsiderations we will show that (1) the late glass-bearingassemblages are closely related to early metasomaticassemblages mainly representing breakdown productsand (2) breakdown was induced by infiltration of aCO2-rich fluid in the mantle probably followed andenhanced by heating and decompression melting duringxenolith transport in the host magma

Evidence for in situ partial meltingBSE imaging reveals that the phases becoming unstableare amphibole orthopyroxene and to minor extentsspl-I phlogopite cpx-I and apatite Compositional(inter- and intra-xenolith) heterogeneity of silicate glass istexture controlled and varies as a function of theassociated precursor phases (Fig 12a^f)TiO2 for exampleshows higher concentrations in glass from xenolith Ke196515 containing Ti-enriched amphibole (Fig 12c)whereas K2O contents are highest in phlogopite-rich sam-ples (eg Ke 785 Fig 12d) Elevated P2O5 contents arefound in glass adjacent to apatite (Fig 12e) whichmirrors at the same time the highTh and LREE contents

of apatite (Fig 12f) Significant Si enrichment is found inglass coating orthopyroxene (Fig 12d) supporting incon-gruent melting of orthopyroxene (see below) Even thoughthe melt pockets are connected along grain boundaries thestrong compositional variation (on inter- and intra-xeno-lith scale) suggests that the mobility of the melt was lim-ited or that the melting event was very short-lived (fortiming constraints see below)The co-variation plots in Fig 12 further reveal consider-

able compositional differences between all types of xeno-lith glass and the host basanite These data indicate thatthe glass cannot be the product of xenolith^host lava inter-action during ascent to the surface Also back-mixing ofmicro-phenocrysts into the melt fails to derive the glassfrom a host-related melt (Fig 12b) Glass from xenolithmargins however is compositionally closer to the bulkhost basanite (eg Fig 12b) in line with a direct derivationfrom xenolith^host lava interaction Its more evolvedmajor element composition (eg higher SiO2 lowerMg-number Fig 12b) is probably the result of fractiona-tion of basanite phenocrysts (eg olivine augite andtitanomagnetite Volker 1990) Host lava^xenolith interac-tion is further supported by the trace element signature ofglass at the margin of the xenoliths which is closely similarto that of the Marsabit basanites (Fig 13 Volker 1990)The same characteristics are observed for intra-xenolithglass in the unusual kaersutite-bearing sample Ke 195927

Fig 16 Oxygen fugacities relative to the quartz^fayalite^magnetitebuffer (QFM equation of OrsquoNeill 1987) as a function of temperatureat assumed pressure of 1GPa calculated using the composition ofcoexisting spinel and olivine (ol-I^spl-I pairs and Chrm^Olm pairsrespectively) based on the oxygen barometer of Ballhaus et al (1991)The graphite^CO2 equilibrium was calculated after Frost amp Wood(1998) and the graphite^carbonate^silicate equilibrium was calcu-lated using the equation of Eggler amp Baker (1982) (See text for furtherexplanation)

Fig 17 Calculated metasomatic agents in equilibrium with the euhe-dral Mg-katophorites from the Type III-c amphibole dunite Ke 196525 Amphibolehydrous fluid and amphibolecarbonatite melt parti-tion coefficients are from Adam et al (1997) and Adam amp Green(2001) respectively Silicate melts have been calculated using amphi-boleSi-rich melt partition coefficients from Chazot et al (1996b) andamphibolebasanite partition coefficients from LaTourrette et al(1995) The compositional range of the Marsabit basanites and alkalibasalts (Volker 1990) is shown for comparison

27

Xenolith glass unrelated to the host lava and melt pock-ets similar to those reported here in association with resi-dual phases (eg amphibole or clinopyroxene) has beenreported from many xenolith localities (eg Frey ampGreen 1974 Ionov et al 1994 1996 Chazot et al 1996a1996b Yaxley et al 1997 Laurora et al 2001 Bali et al2002) Residual phases within the melt pockets stronglyindicate in situ partial melting within the xenolithsHowever a variety of processes may trigger such in situ

melting including (1) closed-system decompression melt-ing of either anhydrous (Maale amp Printzlau 1979) orhydrous phases (Frey amp Green 1974 Francis 1976) duringxenolith ascent in the host lava (2) melting during interac-tion of the xenoliths with the host basaltic melt duringxenolith ascent (eg Zinngrebe amp Foley 1995 Shaw ampKlulaquo gel 2002 Shaw et al 2006) (3) melting in the mantleprior to xenolith entrainment induced by an infiltratingfluid (ie flux melting eg Jones et al 1983 Ionov et al1994 Demecurren ny et al 2004) However most studies agreethat that partial melting occurred very shortly beforeandor during xenolith exhumation by the host magmaThus a combination of metasomatically triggered meltingfollowed andor assisted by decompression melting duringxenolith transport may be a realistic alternative in somecases (eg Dawson 2002)Recently Ban et al (2005) have shown in a very detailed

mass-balance study on glass-bearing xenoliths from theWest Eifel that the weight proportions of glass and theassociated micro-phenocrysts match the composition ofthe preserved parent amphibole We used the sameapproach and performed simple mass-balance calculations(least-squares fitting) to constrain the conclusions drawnfrom petrography and to test which phase contributedand to what extent to the formation of the melt pocketsThe results reported in Table 8 show that in most casesamphibole-melting can account successfully for the meltpocket composition and only a minor amount (520) ofother phases (orthopyroxene andor phlogopite) needs tobe added to the reactant Such a contribution of otherphases agrees with textural observation (eg Fig 4cshowing Dim growing on resorbed Opxm) as well as withthe conclusions of Ban et al (2005) who stated that moreSi-rich glass (ie 454wt) necessitates orthopyroxenecontributing to the meltTrace element patterns of the bulk glassthornmicro-

phenocryst pockets calculated using the measured traceelement abundances of glass and micro-phenocrysts andthe respective mass-balance parameters (Table 8) arebroadly identical to the measured amphibole composition(Fig 18a and b) Higher LILE (Cs Rb Ba) concentrationsin the calculated bulk melt patches in sample Ke 785 canbe balanced by adding 7wt of phlogopite (Fig 18b)which is in line with mass balance (Table 8) The dominantcontrol of amphibole to account for the trace element

budget of the melt pockets is similar to results found inother suites of glass-bearing xenoliths (Chazot et al1996a Yaxley amp Kamenetsky 1999 Laurora et al 2001)Further the obtained phase proportions for the melt pock-ets are in good agreement with the results from image ana-lysis (Table 1) In addition they are similar to the glass^cpx^ol proportions recorded in recent experiments on the

Table 8 Results of mass-balance calculations to test melt

pocket formation by incongruent melting of the early metaso-

matic assemblages

Sample 195915 785 785 196525 196525 19653

Reactant

amph 1000 1000 0820 1000 0944 mdash

opx mdash mdash 0078 mdash 0056 0406

phl mdash mdash 0102 mdash mdash mdash

glass mdash mdash mdash mdash mdash 05941

SiO2 4484 4989 4964 5129 5167 5898

TiO2 014 039 036 025 024 018

Cr2O3 232 291 258 127 121 009

Al2O3 1368 762 769 800 757 961

FeO(tot) 336 314 333 343 357 394

MnO 006 008 008 012 012 010

NiO 009 013 014 011 011 004

MgO 1844 2029 2197 2040 2122 1625

CaO 854 783 644 966 913 078

Na2O 460 500 436 504 476 843

K2O 049 032 096 042 040 060

Product

glass 0389 0322 0335 0292 0284 07032

ol 0243 0259 0315 0266 0286 0273

cpx 0288 0373 0308 0442 0431 0024

spl 0052 0032 0028 mdash mdash mdash

carb 0028 0014 0014 mdash mdash mdash

SiO2 4477 5027 4986 5207 5181 5903

TiO2 013 041 039 015 014 017

Cr2O3 213 314 272 109 106 015

Al2O3 1364 764 770 798 776 957

FeO(tot) 374 331 351 322 332 350

MnO 007 007 007 009 009 006

NiO 011 013 015 011 012 007

MgO 1834 2024 2196 2063 2141 1635

CaO 854 759 632 952 929 080

Na2O 449 385 388 499 485 860

K2O 060 096 100 015 014 091

r2 023 200 035 084 028 033

Prefix Ke omitted1Low-Si glass 2High-Si glass (see text for explanation)

28

melting behaviour of amphibole in wehrlites at tempera-tures between 1175 and 12508C and pressures of05^10GPa (Mecurren dard et al 2006) These temperatures arein good agreement with the estimates for the formation ofolivine and chromite micro-phenocrysts in the melt

pockets (Fig 16) The heat supply required for amphibolemelting in the formerly low-T metasomatized Group IIIxenoliths (T 200^3008C) may come from local heatingshortly before or during xenolith entrainment in the hostlava or from fluid infiltration that triggered melting (seediscussion below)Finally formation of the most Si-enriched melt (adjacent

to Opxm in Group III-c samples) can be modelled assum-ing progressing interaction between infiltrating melt andorthopyroxene where the latter melts incongruently

opxthornmelt1 olthornmelt2 cpx ethseeTable 8THORN eth1THORN

To calculate reaction (1) we chose the least evolved glassobserved in melt patches in sample Ke 19653 (SiO2

5921wt Mg-number 6465) to represent the initialmelt1 whereas melt2 adjacent to opx is SiO2-rich(6530wt) Trace element abundances in glass next toorthopyroxene are shifted to lower concentrations(Table 6) which is in agreement with dilution caused byaddition of a trace element-poor orthopyroxene compo-nent The resultant phase proportions are similar to thoseobserved at orthopyroxene^melt interfaces (ie higholivine and low cpx abundances) and are in agreementwith experimental studies (Shaw et al 1998)

Partial melting triggered by fluid fluxMass-balance calculations provide an explanation for meltpocket formation by in situ partial melting Thus onemight conclude that melting is simply the result of decom-pression and heating during xenolith transport in the hostmagma However there are two points that argue thatmelting was triggered by the infiltration of a fluid phaseshortly before the xenoliths were sampled by the hostmagma (1) the presence of occasionally high amounts ofcarbonate included in the melt pockets (2) the extremelyNa-rich clinopyroxene (Dim) compositions either includedas micro-phenocrysts in the melt pocket or present asequilibrated grains around chromite in the single TypeIII-d xenolith (Fig 3f) Both arguments will be discussedin the next section

Formation of carbonates

Textures and compositions of carbonate globules can beexplained by carbonate precipitation from a CO2-bearingfluid As demonstrated by several studies highCa(CathornMg) values (Table 7) and low REE concentra-tions are strong arguments that the carbonates are cumu-late crystals precipitated from a CO2-rich melt or afluid and not quenched carbonated melts (Lee et al 2000Ionov amp Harmer 2002) Oscillatory zoning (Fig 5a and b)of individual carbonate crystals within singleglobules corroborates this interpretation The roundedcontacts toward glass (globules) are also in line with thesefindings Similar globular forms were reported fromexperimental studies and do not a priori imply the

Fig 18 Calculated trace element characteristics of reactants (in mostcases amphibolethorn orthopyroxene filled symbols) and products(melt pockets open symbols) of the lsquoamphibole meltingrsquo model[see equation (1) in text] The relative phase proportions wereobtained by mass-balance calculations and are given in Table 8 Thedashed lines in (b) show that the high LILE concentrations obtainedfor the product (bulk melt pocket) can be explained by adding about7wt of phlogopite to the reactant (amphibolethorn orthopyroxeneTable 8) Further explanation is given in the text

29

presence of immiscible carbonate and silicate melts(eg Lee amp Wyllie 1996)The high amount of carbonate in some melt pockets

however (eg Fig 4e) does need further explanation asthe incongruent melting reaction (1) does not account forthe CO2 required for carbonate precipitation duringamphibole melting Therefore amphibole breakdowneither occurred in an open system with addition of at leastCO2 (eg Laurora et al 2001 Ban et al 2005) or anotherpre-existing phase acted as a source of carbon Oxidationof graphite texturally identified as a part of the earlyassemblage (eg Fig 3d) could account for some of theCO2 necessary for carbonate formation A change in fO2

conditions is also indicated when the composition of glass-related spinel (Chrm) and Olm are used for oxygen baro-metry which yields oxygen fugacities well on the oxidizedside of the carbon^carbonate^silicate buffer (Fig 16)However increasing fO2 triggered by closed-systemdecompression-induced melting alone does not appear fea-sible Also textures such as straight contacts between gra-phite and Mg-calcite (Fig 4c) suggest that carbonategrows on graphite (ie indicating local fO2 conditions cor-responding to carbon^carbonate equilibrium) rather thanreplacing it We therefore interpret melt pocket formationto be triggered by the infiltration of a CO2-rich fluid(ie flux melting see also Ionov et al 1994) This interpre-tation also accounts for the presence of abundant vugs inthe glass that could have contained a fluid (Ionov et al1994 2006) which was presumably lost during sample pre-paration However we emphasize here that late fluid infil-tration had only subtle effects on the trace elementcomposition of Type III-b and -c xenoliths

Formation of Na-rich Cr diopside

Infiltration of CO2-rich fluidwould also explain the Na-richcomposition of Dim micro-phenocryst cores (Fig 6a and b)Ionov et al (2005) noted recently that clinopyroxenes inmeltpockets are commonly characterized by Na2O contents51wt mainly as a result of low cpxmelt partition coeffi-cients at low pressures (eg Blundy et al 1995) Thus veryhighNa2Ocontents in Dimmicro-phenocrysts (Fig6a) sug-gest replacement of amphibole by Dim and eventually Olmand Chrm during reaction with a fluid at subsolidus condi-tions An origin of micro-phenocrysts by replacementrather than crystallization from a melt is indicated by theidentical crystallographic orientation of individual Dimcrystals in singlemelt pockets (Fig4a)Once the solidus of the precursor phase(s) was reached

(ie 10508C for amphibole Niida amp Green 1999) silicatemelt was generated in situ and led to the formation of clin-opyroxene rims with lower Na contents (Fig 6b) and thedevelopment of euhedral shapes in contact with melt (egFig 4a and e) Eventual immiscibility between remainingfluid and newly formed melt (eg during xenolith ascent)would then explain the presence of carbonate globules and

vugs enclosed in glass Likewise different origins of meltand carbonate were recently proposed for similar meltpockets from Hungarian xenoliths by Demecurren ny et al(2004) who found oxygen isotope disequilibrium betweencarbonate globules and the host glass Such a two-stepmodel (fluid-induced replacement followed by melt gen-eration) can be explained by CO2-fluid infiltration occur-ring within the mantle at sub-solidus conditions ultimatelyfollowed by xenolith entrainment in the host magma andtransport to the surfaceThe host magma possibly suppliedthe additional heat necessary for melt generationFast decompression would further enhance melting andcould also account for the lower Na contents in Dimmicro-phenocryst rims as a result of decreasing cpxmeltpartition coefficients at decreasing pressures (Blundyet al 1995)Rapid replacement of amphibole ( phlogopite) by an

anhydrous phase assemblage indicates that the fluid waswater-poor (eg low H2OCO2 ratio see Ionov et al2006)This is corroborated by the fact that Mg-katophorite(presumably OH-rich see above) is occasionally rimmedby a residual kaersutitic selvage (Fig 4d) Such Ti-richmantle amphiboles are commonly OH-poor as a result ofpartial dehydrogenation (eg Tiepolo et al 2001 and refer-ences therein)Finally the presence of theType III-d xenolith contain-

ing texturally equilibrated clinopyroxene of almost identi-cal composition to Dim micro-phenocrysts in melt pockets(Table 2 Figs 6a and 7b) further strengthens the argu-ment that CO2-fluid percolation started in the mantlebefore entrainment (ie enough time elapsed to allow thisrock to anneal completely after the late metasomaticassemblage had formed) A similar conclusion can bedrawn for the clearly host-related glass found in xenolithKe 195927 (eg Fig 13b) ThisType III-c sample is charac-terized by the presence of Dim micro-phenocrysts partlyovergrown by mineral rims clearly related to the host basa-nite magma (kaersutite Ti-augite and olivine withFo885 Fig 4f Tables 3 and 4 and supplementary dataat httppetrologyoxfordjournalsorg) Kaersutite hasmajor and trace element characteristics similar to typicalbasanitic and alkali basaltic amphibole phenocrysts(Fig 9d) The special phase assemblages in this xenolithindicates that host-related basanitic melts intruded someType III-c xenoliths and partly overprinted the assemblageformed slightly earlier by in situ derived alkali-rich peralu-minous melts fully preserved in other samples (eg Ke19653 or Ke 785)

ORIGIN OF METASOMATIZ INGAGENTS T IM ING AND TECTONICSETT INGAs noted above late metasomatism was probably short-lived started at mantle depths and probably continued

30

Further very high Mg-numbers in replacive amphibolessupport melt (fluid)^rock interaction at low melt (fluid)rock ratios where Mg and Fe contents are buffered by theambient peridotite (Kelemen et al 1995 Tiepolo et al2001) The slightly lower Mg-number of euhedralamphiboles in Ke 196525 combined with their lowerCr2O3 contents (Fig 8b) suggests a higher melt (fluid)rock ratio which is further supported by considerablylower Mg-number in associated ol-I (Fo88 Table 3)

The limited number of investigated samples does notallow us to decide which of the processes described aboveacted and to what extent However given the heterogene-ity observed in the numerous non-metasomatized xenolithsfrom Marsabit (Kaeser et al 2006) pre-metasomaticfeatures certainly affect the final product of the melt^rockreaction process to a considerable degree

The nature of the metasomatizing agent(s)Graphite stability and oxygen fugacity considerations

The Marsabit xenoliths are unusual in that they occur inan off-cratonic setting yet bear macroscopic graphite Thelatter usually either occurs in kimberlite-hosted peridotites(on-craton) or in pyroxenites from alpine-type peridotitebodies (eg Pearson et al 1994) In the phlogopite perido-tites from Finero graphite has recently been reportedas nanoscopic layers in phlogopite (Ferraris et al 2004)In xenoliths from Marsabit graphite occurs in the sametextural relationship as in kimberlite-hosted peridotitexenoliths (ie millimetre-sized flakes typically associatedwith orthopyroxene see Fig 3c^e and Pearson et al 1994)Graphite inclusions in phlogopite and Opxm couldindicate crystallization during metasomatism by alsquoreducedrsquo C-bearing fluid or melt However precipitationof graphite more probably reflects the relatively reducingconditions in the uppermost mantle prior to metasoma-tism We used the oxygen barometer of Ballhaus et al(1991) based on ferric iron contents in spinel coexistingwith olivine to constrain oxygen fugacity (fO2)conditions prior to metasomatism in the Marsabitxenoliths (Fig 16) The results show that non-metasomatized Group I and IV samples reflect similarlyreducing conditions to the cryptically metasomatizedType III-a samplesAlthough assessing Fe3thornFe2thorn ratios in spinel based

on microprobe analyses can be problematic (Wood ampVirgo 1989 Ballhaus et al 1991) all microprobeanalyses were obtained using the same operationalsettings and therefore give at least the relative fO2

variation between samples The relatively reducingconditions in the pre-metasomatic cold andhighly deformed mantle well below carbon^carbonate^silicate equilibria (Mg2Si2O6thorn 05 CaMg(CO3)2frac14Mg2SiO4thorn 05 CaMgSi2O6thornCthornO2 Eggler amp Baker1982 see Fig 16) and close to the CCO oxygen buffer arein agreement with the presence of graphite in the GroupIII harzburgites Moreover the P^T conditions at whichthe Group III xenoliths partially re-equilibrated are closeto the subsolidus decarbonation of model carbonated lher-zolite (Fig 2 see Lee amp Wyllie 2000 and referencestherein) in agreement with the observed phase assemblageincluding orthopyroxene olivine clinopyroxene andgraphite

Fig 11 Trace element patterns of (a) phlogopite and (b) apatitefrom porphyroclastic spinel harzburgites and dunites from Marsabit[concentrations normalized to the primitive mantle valuesof McDonough amp Sun (1995)] The grey field in (a) indicates thecomposition of Ti-rich phlogopite in Group I (grt)^spl lherzolites(Kaeser et al 2006)

20

Table 6 Representative analyses of silicate glass (oxides in wt trace elements in mgg)

Type III-b III-c Host

Sample 195915 196515 196515 785 196525 19653 19653 19653 195927

Remark next amph next cpx xenol margin next amph next amph melt pocket next apatite next Opxm melt pocket host basanite

SiO2 5306 5775 5161 6096 5657 5938 5974 6530 4882 4410

TiO2 017 157 106 062 030 024 020 023 378 255

Al2O3 2412 2053 2025 1779 2313 1575 1245 1355 1890 1396

Cr2O3 005 001 nd 001 001 nd nd nd 002 na

FeO(tot) 188 247 626 126 252 295 374 265 509 1283

MnO 007 000 026 001 004 005 008 007 018 021

NiO nd nd nd na nd na 006 nd 001 na

MgO 182 129 116 114 122 304 382 252 244 938

CaO 506 257 283 082 044 124 101 039 810 1030

Na2O 1107 966 969 971 1299 1388 1384 1211 673 374

K2O 154 197 465 357 138 121 142 127 300 146

Cl 031 106 038 012 014 023 061 020 015 na

P2O5 041 008 044 009 012 049 233 018 100 059

Total 9956 9896 9860 9609 9887 9845 9929 9846 9821 9912

Mg-no 6329 4816 2489 6175 4632 6474 6458 6289 4552 5658

P 1061 448 1610 230 503 1519 6738 676 2994 na

K 12563 16408 39599 32515 15872 9265 11277 10071 25900 na

Sc 179 184 126 406 151 970 136 848 691 237

Ti 665 7329 4974 2953 1965 1213 1163 1055 18199 15311

V 107 265 235 796 920 210 151 134 234 na

Ni 128 371 92 5575 983 170 5199 5298 199 191

Zn 962 103 109 110 459 440 508 309 819 na

Ga na 173 273 194 220 na na na na na

Rb 181 263 124 546 626 176 398 369 577 369

Sr 427 488 1407 188 881 1644 4997 985 1062 792

Y 543 145 224 752 262 619 158 369 301 299

Zr 466 171 353 761 660 578 365 428 242 216

Nb 416 161 137 260 136 234 164 169 837 647

Cs 014 046 186 125 007 022 102 052 063 na

Ba 238 261 1064 129 168 405 1099 641 625 563

La 698 183 848 144 889 2212 1008 106 537 462

Ce 947 256 131 308 138 333 1265 164 104 823

Pr 655 194 120 330 116 304 975 160 120 na

Nd 146 603 408 101 329 993 256 564 463 400

Sm 142 048 570 179 419 141 379 679 105 851

Eu 041 035 228 082 112 438 103 241 345 267

Gd 062 051 415 158 402 106 245 566 677 729

Tb na 0086 086 020 071 na na na na 111

Dy 090 027 359 098 434 104 171 637 732 na

Ho 023 0037 105 025 095 206 446 104 117 114

Er 045 038 195 058 255 611 153 264 305 na

Tm 009 50039 033 0081 042 085 292 5014 036 052

Yb 062 023 267 044 307 667 120 487 253 220

(continued)

21

Evidence for low-temperature SiO2^Na2O^CO2^H2O-rich melts or fluids

Potential metasomatic agents inferred from the presenceof graphite are C^O^H-rich fluids carbonate melts orCO2-bearing silicate melts The resulting phase assem-blages and geochemical signatures are the result of infiltra-tion of fluid or melt into a cold (750^8508C) shallow(515GPa Fig 2) heterogeneous and locally reduced litho-spheric mantle Metasomatism under such conditions in anopen system must result in complex geochemical signa-tures that reflect the degree of fluid^rock reaction Thismakes it particularly difficult to constrain the exactnature of the lsquooriginalrsquometasomatic agentType III-c xenolith Ke 196525 contains the highest frac-

tion of metasomatic phases and the least fractionatedamphibole trace element patterns The lsquomagmaticrsquo texturesof the Mg-katophorites in this sample suggest direct crys-tallization from a melt or a fluid rather than replacementof a pre-existing phase Therefore hypothetical liquids inequilibrium with the amphiboles in this xenolith couldyield the composition of the initial metasomatic agentsFigure 17 illustrates the trace element patterns of suchmelts and fluids calculated using sets of experimentally orempirically determined mineralmelt (fluid) partitioncoefficients including carbonate melt silicate melt andhydrous fluid (for references see caption to Fig 17)Common features for all calculated metasomatic agents

are negative Zr^Hf^Ti anomalies and LREE enrichment(Fig 17) However previous studies have shown that thesesignatures are indicative of almost all types of metasoma-tism including interaction of peridotite with carbonatiticmelt (Green amp Wallace 1988 Dautria et al 1992 Hauriet al 1993 Rudnick et al 1993 Ionov et al 1996 Yaxleyet al 1998 Coltorti et al 1999) with hydrous fluids andormelts for example originating from dehydration of altered

subducting lithosphere (Maury et al 1992 Zanetti et al1999 Grecurren goire et al 2001 Laurora et al 2001 Parkinsonet al 2003) and interaction of peridotite with volatile-and Si-rich melts that evolved through reactive porousflow (Bedini et al 1997 Ionov et al 2002 2006 Bodinieret al 2004 Rivalenti et al 2004)In the case of the Marsabit xenoliths integration of the

trace element characteristics thermobarometric inferencesand mineralogical constraints favours a Na2O^SiO2^CO2^H2O-rich silicate melt Carbonated melts can prob-ably be ruled out on major element and mineralogical con-siderations First amphiboles are Ca-poor in conflict withcarbonate melt metasomatism which is generally charac-terized by Ca enrichment (Rudnick et al 1993 Neumannet al 2002) Further orthopyroxene is a stable phase andprobably even crystallizes during metasomatism in somesamples (eg Ke 785) At near-solidus conditions coexis-tence of orthopyroxene and carbonate melt is possibleonly at pressures 20GPa (see Fig 2 eg Lee amp Wyllie2000 and references therein) which is considerably higherthan the maximum pressures indicated by the compositionof spinel (515GPa) At such low pressures metasomatismby carbonated melts will convert lherzolite and harzbur-gite immediately into wehrlite which is the major perido-tite type in all xenolith suites where carbonate meltmetasomatism is convincingly demonstrated (Rudnicket al 1993 Yaxley et al 1998 Neumann et al 2002)In Marsabit wehrlites are absent howeverAn Na2O^SiO2^CO2^H2O-rich melt or fluid on the

other hand would explain the Na-rich nature of amphiboleand phlogopite as well as the presence of orthopyroxeneGrowth of metasomatic (and Al-poor) orthopyroxene hasbeen described from several xenolith localities where peri-dotites were infiltrated either by hydrous fluids orSi-rich melts (Smith et al 1999 McInnes et al 2001Morishita et al 2003) Na-rich phlogopite is commonly

Table 6 Continued

Type III-b III-c Host

Sample 195915 196515 196515 785 196525 19653 19653 19653 195927

Remark next amph next cpx xenol margin next amph next amph melt pocket next apatite next Opxm melt pocket host basanite

Lu 014 0050 026 0041 046 080 140 058 040 031

Hf 026 052 535 014 076 110 5087 124 568 504

Ta 50080 033 638 0022 123 101 784 801 394 403

Pb 541 448 280 255 840 797 260 749 493 na

Th 194 296 160 466 703 185 247 105 534 506

U 545 069 478 099 152 447 724 216 171 126

nd not detected na not analysedAverage composition of the strongly alkaline basanites from Marsabit (Volker 1990)

22

Fig 12 Total alkalis vs SiO2 (a) Harker-type co-variation diagrams (b^e) and incompatible trace element co-variation diagram (f) showingthe compositional variability of silicate glass found in the porphyroclastic spinel harzburgites and dunites from Marsabit In (b) binary mixinglines between the typical composition of the alkali-rich glass (large open diamond) and the associated microlites are shown (each increment is10wt)They illustrate that the glass does not represent the derivate of host lava infiltration and fractional crystallization Subalkaline glass inGroup I (grt)^spl lherzolites from Marsabit is shown for comparison and is interpreted to derive from incongruent melting of Ti-pargasite(Kaeser 2006)

23

found as a reaction product between anhydrous mafic toultramafic rocks and H2O-rich evolved liquids (Costaet al 2001 and references therein) or H2O-bearing fluids(eg Odling 1994) The latter study was an experiment at20GPa simulating the effects of decreasing temperatureson the phase assemblages and their composition duringmetasomatism using a model peridotitethornC^O^H fluidThe coolest part of Odlingrsquos (1994) experiment (510758C)contained a strikingly similar phase assemblage (kato-phorite phlogopite with 2wt Na2O and Al-poororthopyroxene) to the Marsabit samples interpreted tohave formed during sub-solidus fluid^solid interactionA compositionally similar phase assemblage (highamounts of opx Si-rich hornblende Na-rich phlogopite)resulted from experiments simulating peridotite interac-tion with Si-rich melt (dacite) at lower P^T conditions(9508C at 092GPa Prouteau amp Scaillet 2003)In this study the trace element signatures of Group III-c

Mg-katophorite and the calculated metasomatic agentsfurther suggest that the metasomatizing agent was a meltrather than a fluid Hypothetical fluids in equilibrium witheuhedral Mg-katophorite from sample Ke 196525 havehigh NbLILE (eg Rb Ba) and low LILEREE ratios(Fig 17) Because experimental data predict the oppositefor fluids (LILE enriched over HFSE and REE egManning 2004 and references therein) a fluid with asignature such as that indicated in Fig 17 is unrealistic

Fig 13 Trace element patterns of silicate glass in porphyroclasticspinel harzburgites and dunites from Marsabit compared withthe trace element composition of associated clinopyroxene (Dim)micro-phenocrysts xenolith-hosting basanites from Marsabit andglass interpreted to represent xenolith^host interaction [concentra-tions normalized to the primitive mantle values of McDonough ampSun (1995)]

Table 7 Representative analyses of carbonates (oxides in

wt trace elements in mgg)

Group III-b III-b III-c III-c III-c

Mineral Dol Mg-Cc Mg-Cc Mg-Cc Mg-Cc

Sample 195915 196515 785 19653 19653

Texture in glass in glass in glass in glass in glass

SiO2 na 003 na nd 003

FeO(tot) 016 011 010 010 006

MnO 001 nd nd nd nd

MgO 1922 236 190 125 517

CaO 3189 5313 5377 5391 4853

Na2O 003 nd 004 nd 003

K2O na 002 na 001 nd

SrO 003 010 008 004 021

BaO 003 nd nd na na

CO21 4618 4444 4442 4377 4391

Total 9756 10021 10032 9909 9794

Mg-no 9954 9745 9713 9554 9931

Ca-no 5439 9418 9531 9687 8709

P 220 334 137 580 589

K 970 212 642 105 155

Sc 5065 50150 027 5027 50076

Ti 975 189 242 528 196

V 5085 0481 5042 5037 021

Ni 900 673 517 5294 593

Zn 459 5089 113 296 045

Ga na 5014 5013 na na

Rb 0084 50072 50074 50060 0046

Sr 593 860 371 251 4274

Y 012 011 130 704 015

Zr 261 024 012 097 756

Nb 144 019 0056 692 185

Cs 50063 50011 50018 50039 50008

Ba 126 230 5024 029 271

La 034 057 084 136 025

Ce 032 013 134 997 023

Pr 006 006 018 095 0026

Nd 030 013 068 228 0078

Sm 5044 50091 0074 048 50018

Eu 5012 50019 0051 020 50008

Gd 5037 50090 014 051 0026

Tb na 50010 0036 na na

Dy 5019 50039 0089 058 50038

Ho 50029 0013 0044 011 50006

Er 5012 50041 0083 033 50019

Tm 50052 50013 0016 0084 50006

Yb 5043 50063 014 027 50040

Lu 50072 50010 0011 0066 50008

(continued)

24

At the relatively low maximum pressures (515GPa) asupercritical liquid is not expected either (eg Kesselet al 2005)Finally a relatively hydrous metasomatizing agent

would be consistent with formation of Mg-katophoriteswith lowTi and presumably low Fe3thorn contents (given therelatively reducing conditions) which in turn imply high(OH)^ contents In other words partial dehydrogenationtypically observed in common (Ti-rich) mantle amphibole

(eg Popp et al 1995 Tiepolo et al 2001 and referencestherein) cannot have played an important role in thiscase as H deficiency is mainly the result of coupled substi-tutions involvingTi and Fe3thorn (Popp et al 1995) In conclu-sion the metasomatizing agent was most probably aNa2O^SiO2^CO2^H2O-rich meltComparison with experimental studies further con-

strains the low-temperature regime in which metasoma-tism occurred In ultramafic (eg Niida amp Green 1999)basaltic (eg Rapp 1995) and Si-enriched systems (egtrachybasaltic Barclay amp Carmichael 2004) amphibole(in the latter study together with phlogopite) is confinedto temperatures lower than 1000^10508C at pressures520GPa (Fig 2)Thus crystallization of the early metaso-matic phase assemblage can be explained by infiltration ofa silicate melt that rapidly cooled upon contact with theambient peridotite (750^8508C prior to metasomatismFig 2) However this low-T stage is based on Ca-in-opxthermometry (see Brey amp Kolaquo hler 1990 Kaeser et al2006 Table 1) and therefore probably underestimates theeffective temperature conditions during metasomatismThis can be explained by low diffusion rates and proximityto the closure temperature for the diopside^enstatiteexchange reaction at temperatures lower than 8508C(eg Smith 1999 and references therein) The lower tem-perature limit at which silicate melt metasomatism ispossible (ie defined by the meltrsquos solidus and viscosity)depends strongly on volatile and SiO2 contentsExperiments by Draper amp Green (1997) for exampleshowed that highly silicic melts (52^64wt SiO2) canbe in equilibrium with mantle harzburgite at temperaturesas low as 8508C (at 10^20GPa under anhydrous andC^O^H-fluid-bearing conditions) These are howeverextreme compositions for mantle melts The actual SiO2

content of the metasomatizing agent in this case is difficultto constrain from the metasomatic mineral assemblage ascompositions may have varied considerably during reac-tive percolation (eg including Si-releasing reactions suchas cpx replacement by amphibole versus Si-consumingreactions such as Opxm crystallization) Moreover it isunlikely that a melt persists at temperatures muchbelow 9008C which is the solidus of fertile lherzolite(ie primitive upper mantle composition) underH2O-saturated conditions at 15GPa (Grove et al 2006see Fig 2) Below the solidus residual volatile-enrichedfluids may percolate further for example forming the lessmetasomatized Type III-c xenoliths (eg Ke 785) as wellas Type III-b and -a peridotites (see also Bailey 1987)Relatively cold conditions would also explain the veryhigh compositional gradients developed on small scales inType III-a samples (Fig 15) Given the absence of chloritein all investigated samples the lower temperature limitof such fluid metasomatism is 8008C (eg Fumagalli ampPoli 2005 see Fig 2)

Table 7 Continued

Group III-b III-b III-c III-c III-c

Mineral Dol Mg-Cc Mg-Cc Mg-Cc Mg-Cc

Sample 195915 196515 785 19653 19653

Texture in glass in glass in glass in glass in glass

Hf 5016 50038 50036 50074 50013

Ta 0048 50012 50016 50031 0008

Pb 5024 858 5021 074 018

Th 0021 50020 50017 50028 50005

U 060 094 086 600 148

Mg-Cc Mg-calcite Dol dolomite nd not detectedna not analysed Ca-numberfrac14 100Ca(CathornMg)1Calculated from stoichiometry

Fig 14 Trace element patterns of glass-hosted carbonates in TypeIII-b and -c spinel harzburgite and dunite xenoliths from Marsabit[concentrations normalized to the primitive mantle values ofMcDonough amp Sun (1995)] The grey field outlines the compositionalrange of carbonates found in other peridotite xenoliths (Ionov ampHarmer 2002 and references therein) Carbonate material inter-preted as quenched carbonatite melt found in peridotite xenolithsfrom Kerguelen (Moine et al 2004) is shown for comparison(bold line)

25

Finally the presence of apatite even though oftentaken as indicator for carbonatite-type metasomatism(eg Yaxley et al 1998) is not in conflict with metasoma-tism by silicate melts Apatite has also been shown to crys-tallize from volatile and alkali-rich derivatives formedduring reactive porous flow of originally mafic silicatemelts (Woodland et al 1996 Bedini et al 1997 Bodinieret al 2004 Ionov et al 2006)

FORMATION OF THE LATE GLASS -BEAR ING ASSEMBLAGESTextures strongly suggest that the glass-bearing assem-blages formed later and at the expense of the early metaso-matic assemblages (Fig 4a and d) Features such as thereappearance of clinopyroxene (Na-rich Cr diopsidemicro-phenocrysts) a phase replaced by the early

Fig 15 Detailed REE systematics in primary clinopyroxene (cpx-I) in a porphyroclastic cryptically metasomatized spinel harzburgite(sample Ke 19651) showing the transition fromType III-a (Zone A and B) toType III-b (Zone C) on a scale of a few centimetres The numberson the photomicrographs (crossed polars) correspond to single laser ablation microprobe pits (for further explanation see text)

26

metasomatic assemblage indicate that melt pocket forma-tion reflects a significant change in P^T^X space in partsof the mantle beneath Marsabit after earlier metasoma-tism Based on geochemical arguments and mass-balanceconsiderations we will show that (1) the late glass-bearingassemblages are closely related to early metasomaticassemblages mainly representing breakdown productsand (2) breakdown was induced by infiltration of aCO2-rich fluid in the mantle probably followed andenhanced by heating and decompression melting duringxenolith transport in the host magma

Evidence for in situ partial meltingBSE imaging reveals that the phases becoming unstableare amphibole orthopyroxene and to minor extentsspl-I phlogopite cpx-I and apatite Compositional(inter- and intra-xenolith) heterogeneity of silicate glass istexture controlled and varies as a function of theassociated precursor phases (Fig 12a^f)TiO2 for exampleshows higher concentrations in glass from xenolith Ke196515 containing Ti-enriched amphibole (Fig 12c)whereas K2O contents are highest in phlogopite-rich sam-ples (eg Ke 785 Fig 12d) Elevated P2O5 contents arefound in glass adjacent to apatite (Fig 12e) whichmirrors at the same time the highTh and LREE contents

of apatite (Fig 12f) Significant Si enrichment is found inglass coating orthopyroxene (Fig 12d) supporting incon-gruent melting of orthopyroxene (see below) Even thoughthe melt pockets are connected along grain boundaries thestrong compositional variation (on inter- and intra-xeno-lith scale) suggests that the mobility of the melt was lim-ited or that the melting event was very short-lived (fortiming constraints see below)The co-variation plots in Fig 12 further reveal consider-

able compositional differences between all types of xeno-lith glass and the host basanite These data indicate thatthe glass cannot be the product of xenolith^host lava inter-action during ascent to the surface Also back-mixing ofmicro-phenocrysts into the melt fails to derive the glassfrom a host-related melt (Fig 12b) Glass from xenolithmargins however is compositionally closer to the bulkhost basanite (eg Fig 12b) in line with a direct derivationfrom xenolith^host lava interaction Its more evolvedmajor element composition (eg higher SiO2 lowerMg-number Fig 12b) is probably the result of fractiona-tion of basanite phenocrysts (eg olivine augite andtitanomagnetite Volker 1990) Host lava^xenolith interac-tion is further supported by the trace element signature ofglass at the margin of the xenoliths which is closely similarto that of the Marsabit basanites (Fig 13 Volker 1990)The same characteristics are observed for intra-xenolithglass in the unusual kaersutite-bearing sample Ke 195927

Fig 16 Oxygen fugacities relative to the quartz^fayalite^magnetitebuffer (QFM equation of OrsquoNeill 1987) as a function of temperatureat assumed pressure of 1GPa calculated using the composition ofcoexisting spinel and olivine (ol-I^spl-I pairs and Chrm^Olm pairsrespectively) based on the oxygen barometer of Ballhaus et al (1991)The graphite^CO2 equilibrium was calculated after Frost amp Wood(1998) and the graphite^carbonate^silicate equilibrium was calcu-lated using the equation of Eggler amp Baker (1982) (See text for furtherexplanation)

Fig 17 Calculated metasomatic agents in equilibrium with the euhe-dral Mg-katophorites from the Type III-c amphibole dunite Ke 196525 Amphibolehydrous fluid and amphibolecarbonatite melt parti-tion coefficients are from Adam et al (1997) and Adam amp Green(2001) respectively Silicate melts have been calculated using amphi-boleSi-rich melt partition coefficients from Chazot et al (1996b) andamphibolebasanite partition coefficients from LaTourrette et al(1995) The compositional range of the Marsabit basanites and alkalibasalts (Volker 1990) is shown for comparison

27

Xenolith glass unrelated to the host lava and melt pock-ets similar to those reported here in association with resi-dual phases (eg amphibole or clinopyroxene) has beenreported from many xenolith localities (eg Frey ampGreen 1974 Ionov et al 1994 1996 Chazot et al 1996a1996b Yaxley et al 1997 Laurora et al 2001 Bali et al2002) Residual phases within the melt pockets stronglyindicate in situ partial melting within the xenolithsHowever a variety of processes may trigger such in situ

melting including (1) closed-system decompression melt-ing of either anhydrous (Maale amp Printzlau 1979) orhydrous phases (Frey amp Green 1974 Francis 1976) duringxenolith ascent in the host lava (2) melting during interac-tion of the xenoliths with the host basaltic melt duringxenolith ascent (eg Zinngrebe amp Foley 1995 Shaw ampKlulaquo gel 2002 Shaw et al 2006) (3) melting in the mantleprior to xenolith entrainment induced by an infiltratingfluid (ie flux melting eg Jones et al 1983 Ionov et al1994 Demecurren ny et al 2004) However most studies agreethat that partial melting occurred very shortly beforeandor during xenolith exhumation by the host magmaThus a combination of metasomatically triggered meltingfollowed andor assisted by decompression melting duringxenolith transport may be a realistic alternative in somecases (eg Dawson 2002)Recently Ban et al (2005) have shown in a very detailed

mass-balance study on glass-bearing xenoliths from theWest Eifel that the weight proportions of glass and theassociated micro-phenocrysts match the composition ofthe preserved parent amphibole We used the sameapproach and performed simple mass-balance calculations(least-squares fitting) to constrain the conclusions drawnfrom petrography and to test which phase contributedand to what extent to the formation of the melt pocketsThe results reported in Table 8 show that in most casesamphibole-melting can account successfully for the meltpocket composition and only a minor amount (520) ofother phases (orthopyroxene andor phlogopite) needs tobe added to the reactant Such a contribution of otherphases agrees with textural observation (eg Fig 4cshowing Dim growing on resorbed Opxm) as well as withthe conclusions of Ban et al (2005) who stated that moreSi-rich glass (ie 454wt) necessitates orthopyroxenecontributing to the meltTrace element patterns of the bulk glassthornmicro-

phenocryst pockets calculated using the measured traceelement abundances of glass and micro-phenocrysts andthe respective mass-balance parameters (Table 8) arebroadly identical to the measured amphibole composition(Fig 18a and b) Higher LILE (Cs Rb Ba) concentrationsin the calculated bulk melt patches in sample Ke 785 canbe balanced by adding 7wt of phlogopite (Fig 18b)which is in line with mass balance (Table 8) The dominantcontrol of amphibole to account for the trace element

budget of the melt pockets is similar to results found inother suites of glass-bearing xenoliths (Chazot et al1996a Yaxley amp Kamenetsky 1999 Laurora et al 2001)Further the obtained phase proportions for the melt pock-ets are in good agreement with the results from image ana-lysis (Table 1) In addition they are similar to the glass^cpx^ol proportions recorded in recent experiments on the

Table 8 Results of mass-balance calculations to test melt

pocket formation by incongruent melting of the early metaso-

matic assemblages

Sample 195915 785 785 196525 196525 19653

Reactant

amph 1000 1000 0820 1000 0944 mdash

opx mdash mdash 0078 mdash 0056 0406

phl mdash mdash 0102 mdash mdash mdash

glass mdash mdash mdash mdash mdash 05941

SiO2 4484 4989 4964 5129 5167 5898

TiO2 014 039 036 025 024 018

Cr2O3 232 291 258 127 121 009

Al2O3 1368 762 769 800 757 961

FeO(tot) 336 314 333 343 357 394

MnO 006 008 008 012 012 010

NiO 009 013 014 011 011 004

MgO 1844 2029 2197 2040 2122 1625

CaO 854 783 644 966 913 078

Na2O 460 500 436 504 476 843

K2O 049 032 096 042 040 060

Product

glass 0389 0322 0335 0292 0284 07032

ol 0243 0259 0315 0266 0286 0273

cpx 0288 0373 0308 0442 0431 0024

spl 0052 0032 0028 mdash mdash mdash

carb 0028 0014 0014 mdash mdash mdash

SiO2 4477 5027 4986 5207 5181 5903

TiO2 013 041 039 015 014 017

Cr2O3 213 314 272 109 106 015

Al2O3 1364 764 770 798 776 957

FeO(tot) 374 331 351 322 332 350

MnO 007 007 007 009 009 006

NiO 011 013 015 011 012 007

MgO 1834 2024 2196 2063 2141 1635

CaO 854 759 632 952 929 080

Na2O 449 385 388 499 485 860

K2O 060 096 100 015 014 091

r2 023 200 035 084 028 033

Prefix Ke omitted1Low-Si glass 2High-Si glass (see text for explanation)

28

melting behaviour of amphibole in wehrlites at tempera-tures between 1175 and 12508C and pressures of05^10GPa (Mecurren dard et al 2006) These temperatures arein good agreement with the estimates for the formation ofolivine and chromite micro-phenocrysts in the melt

pockets (Fig 16) The heat supply required for amphibolemelting in the formerly low-T metasomatized Group IIIxenoliths (T 200^3008C) may come from local heatingshortly before or during xenolith entrainment in the hostlava or from fluid infiltration that triggered melting (seediscussion below)Finally formation of the most Si-enriched melt (adjacent

to Opxm in Group III-c samples) can be modelled assum-ing progressing interaction between infiltrating melt andorthopyroxene where the latter melts incongruently

opxthornmelt1 olthornmelt2 cpx ethseeTable 8THORN eth1THORN

To calculate reaction (1) we chose the least evolved glassobserved in melt patches in sample Ke 19653 (SiO2

5921wt Mg-number 6465) to represent the initialmelt1 whereas melt2 adjacent to opx is SiO2-rich(6530wt) Trace element abundances in glass next toorthopyroxene are shifted to lower concentrations(Table 6) which is in agreement with dilution caused byaddition of a trace element-poor orthopyroxene compo-nent The resultant phase proportions are similar to thoseobserved at orthopyroxene^melt interfaces (ie higholivine and low cpx abundances) and are in agreementwith experimental studies (Shaw et al 1998)

Partial melting triggered by fluid fluxMass-balance calculations provide an explanation for meltpocket formation by in situ partial melting Thus onemight conclude that melting is simply the result of decom-pression and heating during xenolith transport in the hostmagma However there are two points that argue thatmelting was triggered by the infiltration of a fluid phaseshortly before the xenoliths were sampled by the hostmagma (1) the presence of occasionally high amounts ofcarbonate included in the melt pockets (2) the extremelyNa-rich clinopyroxene (Dim) compositions either includedas micro-phenocrysts in the melt pocket or present asequilibrated grains around chromite in the single TypeIII-d xenolith (Fig 3f) Both arguments will be discussedin the next section

Formation of carbonates

Textures and compositions of carbonate globules can beexplained by carbonate precipitation from a CO2-bearingfluid As demonstrated by several studies highCa(CathornMg) values (Table 7) and low REE concentra-tions are strong arguments that the carbonates are cumu-late crystals precipitated from a CO2-rich melt or afluid and not quenched carbonated melts (Lee et al 2000Ionov amp Harmer 2002) Oscillatory zoning (Fig 5a and b)of individual carbonate crystals within singleglobules corroborates this interpretation The roundedcontacts toward glass (globules) are also in line with thesefindings Similar globular forms were reported fromexperimental studies and do not a priori imply the

Fig 18 Calculated trace element characteristics of reactants (in mostcases amphibolethorn orthopyroxene filled symbols) and products(melt pockets open symbols) of the lsquoamphibole meltingrsquo model[see equation (1) in text] The relative phase proportions wereobtained by mass-balance calculations and are given in Table 8 Thedashed lines in (b) show that the high LILE concentrations obtainedfor the product (bulk melt pocket) can be explained by adding about7wt of phlogopite to the reactant (amphibolethorn orthopyroxeneTable 8) Further explanation is given in the text

29

presence of immiscible carbonate and silicate melts(eg Lee amp Wyllie 1996)The high amount of carbonate in some melt pockets

however (eg Fig 4e) does need further explanation asthe incongruent melting reaction (1) does not account forthe CO2 required for carbonate precipitation duringamphibole melting Therefore amphibole breakdowneither occurred in an open system with addition of at leastCO2 (eg Laurora et al 2001 Ban et al 2005) or anotherpre-existing phase acted as a source of carbon Oxidationof graphite texturally identified as a part of the earlyassemblage (eg Fig 3d) could account for some of theCO2 necessary for carbonate formation A change in fO2

conditions is also indicated when the composition of glass-related spinel (Chrm) and Olm are used for oxygen baro-metry which yields oxygen fugacities well on the oxidizedside of the carbon^carbonate^silicate buffer (Fig 16)However increasing fO2 triggered by closed-systemdecompression-induced melting alone does not appear fea-sible Also textures such as straight contacts between gra-phite and Mg-calcite (Fig 4c) suggest that carbonategrows on graphite (ie indicating local fO2 conditions cor-responding to carbon^carbonate equilibrium) rather thanreplacing it We therefore interpret melt pocket formationto be triggered by the infiltration of a CO2-rich fluid(ie flux melting see also Ionov et al 1994) This interpre-tation also accounts for the presence of abundant vugs inthe glass that could have contained a fluid (Ionov et al1994 2006) which was presumably lost during sample pre-paration However we emphasize here that late fluid infil-tration had only subtle effects on the trace elementcomposition of Type III-b and -c xenoliths

Formation of Na-rich Cr diopside

Infiltration of CO2-rich fluidwould also explain the Na-richcomposition of Dim micro-phenocryst cores (Fig 6a and b)Ionov et al (2005) noted recently that clinopyroxenes inmeltpockets are commonly characterized by Na2O contents51wt mainly as a result of low cpxmelt partition coeffi-cients at low pressures (eg Blundy et al 1995) Thus veryhighNa2Ocontents in Dimmicro-phenocrysts (Fig6a) sug-gest replacement of amphibole by Dim and eventually Olmand Chrm during reaction with a fluid at subsolidus condi-tions An origin of micro-phenocrysts by replacementrather than crystallization from a melt is indicated by theidentical crystallographic orientation of individual Dimcrystals in singlemelt pockets (Fig4a)Once the solidus of the precursor phase(s) was reached

(ie 10508C for amphibole Niida amp Green 1999) silicatemelt was generated in situ and led to the formation of clin-opyroxene rims with lower Na contents (Fig 6b) and thedevelopment of euhedral shapes in contact with melt (egFig 4a and e) Eventual immiscibility between remainingfluid and newly formed melt (eg during xenolith ascent)would then explain the presence of carbonate globules and

vugs enclosed in glass Likewise different origins of meltand carbonate were recently proposed for similar meltpockets from Hungarian xenoliths by Demecurren ny et al(2004) who found oxygen isotope disequilibrium betweencarbonate globules and the host glass Such a two-stepmodel (fluid-induced replacement followed by melt gen-eration) can be explained by CO2-fluid infiltration occur-ring within the mantle at sub-solidus conditions ultimatelyfollowed by xenolith entrainment in the host magma andtransport to the surfaceThe host magma possibly suppliedthe additional heat necessary for melt generationFast decompression would further enhance melting andcould also account for the lower Na contents in Dimmicro-phenocryst rims as a result of decreasing cpxmeltpartition coefficients at decreasing pressures (Blundyet al 1995)Rapid replacement of amphibole ( phlogopite) by an

anhydrous phase assemblage indicates that the fluid waswater-poor (eg low H2OCO2 ratio see Ionov et al2006)This is corroborated by the fact that Mg-katophorite(presumably OH-rich see above) is occasionally rimmedby a residual kaersutitic selvage (Fig 4d) Such Ti-richmantle amphiboles are commonly OH-poor as a result ofpartial dehydrogenation (eg Tiepolo et al 2001 and refer-ences therein)Finally the presence of theType III-d xenolith contain-

ing texturally equilibrated clinopyroxene of almost identi-cal composition to Dim micro-phenocrysts in melt pockets(Table 2 Figs 6a and 7b) further strengthens the argu-ment that CO2-fluid percolation started in the mantlebefore entrainment (ie enough time elapsed to allow thisrock to anneal completely after the late metasomaticassemblage had formed) A similar conclusion can bedrawn for the clearly host-related glass found in xenolithKe 195927 (eg Fig 13b) ThisType III-c sample is charac-terized by the presence of Dim micro-phenocrysts partlyovergrown by mineral rims clearly related to the host basa-nite magma (kaersutite Ti-augite and olivine withFo885 Fig 4f Tables 3 and 4 and supplementary dataat httppetrologyoxfordjournalsorg) Kaersutite hasmajor and trace element characteristics similar to typicalbasanitic and alkali basaltic amphibole phenocrysts(Fig 9d) The special phase assemblages in this xenolithindicates that host-related basanitic melts intruded someType III-c xenoliths and partly overprinted the assemblageformed slightly earlier by in situ derived alkali-rich peralu-minous melts fully preserved in other samples (eg Ke19653 or Ke 785)

ORIGIN OF METASOMATIZ INGAGENTS T IM ING AND TECTONICSETT INGAs noted above late metasomatism was probably short-lived started at mantle depths and probably continued

30

Table 6 Representative analyses of silicate glass (oxides in wt trace elements in mgg)

Type III-b III-c Host

Sample 195915 196515 196515 785 196525 19653 19653 19653 195927

Remark next amph next cpx xenol margin next amph next amph melt pocket next apatite next Opxm melt pocket host basanite

SiO2 5306 5775 5161 6096 5657 5938 5974 6530 4882 4410

TiO2 017 157 106 062 030 024 020 023 378 255

Al2O3 2412 2053 2025 1779 2313 1575 1245 1355 1890 1396

Cr2O3 005 001 nd 001 001 nd nd nd 002 na

FeO(tot) 188 247 626 126 252 295 374 265 509 1283

MnO 007 000 026 001 004 005 008 007 018 021

NiO nd nd nd na nd na 006 nd 001 na

MgO 182 129 116 114 122 304 382 252 244 938

CaO 506 257 283 082 044 124 101 039 810 1030

Na2O 1107 966 969 971 1299 1388 1384 1211 673 374

K2O 154 197 465 357 138 121 142 127 300 146

Cl 031 106 038 012 014 023 061 020 015 na

P2O5 041 008 044 009 012 049 233 018 100 059

Total 9956 9896 9860 9609 9887 9845 9929 9846 9821 9912

Mg-no 6329 4816 2489 6175 4632 6474 6458 6289 4552 5658

P 1061 448 1610 230 503 1519 6738 676 2994 na

K 12563 16408 39599 32515 15872 9265 11277 10071 25900 na

Sc 179 184 126 406 151 970 136 848 691 237

Ti 665 7329 4974 2953 1965 1213 1163 1055 18199 15311

V 107 265 235 796 920 210 151 134 234 na

Ni 128 371 92 5575 983 170 5199 5298 199 191

Zn 962 103 109 110 459 440 508 309 819 na

Ga na 173 273 194 220 na na na na na

Rb 181 263 124 546 626 176 398 369 577 369

Sr 427 488 1407 188 881 1644 4997 985 1062 792

Y 543 145 224 752 262 619 158 369 301 299

Zr 466 171 353 761 660 578 365 428 242 216

Nb 416 161 137 260 136 234 164 169 837 647

Cs 014 046 186 125 007 022 102 052 063 na

Ba 238 261 1064 129 168 405 1099 641 625 563

La 698 183 848 144 889 2212 1008 106 537 462

Ce 947 256 131 308 138 333 1265 164 104 823

Pr 655 194 120 330 116 304 975 160 120 na

Nd 146 603 408 101 329 993 256 564 463 400

Sm 142 048 570 179 419 141 379 679 105 851

Eu 041 035 228 082 112 438 103 241 345 267

Gd 062 051 415 158 402 106 245 566 677 729

Tb na 0086 086 020 071 na na na na 111

Dy 090 027 359 098 434 104 171 637 732 na

Ho 023 0037 105 025 095 206 446 104 117 114

Er 045 038 195 058 255 611 153 264 305 na

Tm 009 50039 033 0081 042 085 292 5014 036 052

Yb 062 023 267 044 307 667 120 487 253 220

(continued)

21

Evidence for low-temperature SiO2^Na2O^CO2^H2O-rich melts or fluids

Potential metasomatic agents inferred from the presenceof graphite are C^O^H-rich fluids carbonate melts orCO2-bearing silicate melts The resulting phase assem-blages and geochemical signatures are the result of infiltra-tion of fluid or melt into a cold (750^8508C) shallow(515GPa Fig 2) heterogeneous and locally reduced litho-spheric mantle Metasomatism under such conditions in anopen system must result in complex geochemical signa-tures that reflect the degree of fluid^rock reaction Thismakes it particularly difficult to constrain the exactnature of the lsquooriginalrsquometasomatic agentType III-c xenolith Ke 196525 contains the highest frac-

tion of metasomatic phases and the least fractionatedamphibole trace element patterns The lsquomagmaticrsquo texturesof the Mg-katophorites in this sample suggest direct crys-tallization from a melt or a fluid rather than replacementof a pre-existing phase Therefore hypothetical liquids inequilibrium with the amphiboles in this xenolith couldyield the composition of the initial metasomatic agentsFigure 17 illustrates the trace element patterns of suchmelts and fluids calculated using sets of experimentally orempirically determined mineralmelt (fluid) partitioncoefficients including carbonate melt silicate melt andhydrous fluid (for references see caption to Fig 17)Common features for all calculated metasomatic agents

are negative Zr^Hf^Ti anomalies and LREE enrichment(Fig 17) However previous studies have shown that thesesignatures are indicative of almost all types of metasoma-tism including interaction of peridotite with carbonatiticmelt (Green amp Wallace 1988 Dautria et al 1992 Hauriet al 1993 Rudnick et al 1993 Ionov et al 1996 Yaxleyet al 1998 Coltorti et al 1999) with hydrous fluids andormelts for example originating from dehydration of altered

subducting lithosphere (Maury et al 1992 Zanetti et al1999 Grecurren goire et al 2001 Laurora et al 2001 Parkinsonet al 2003) and interaction of peridotite with volatile-and Si-rich melts that evolved through reactive porousflow (Bedini et al 1997 Ionov et al 2002 2006 Bodinieret al 2004 Rivalenti et al 2004)In the case of the Marsabit xenoliths integration of the

trace element characteristics thermobarometric inferencesand mineralogical constraints favours a Na2O^SiO2^CO2^H2O-rich silicate melt Carbonated melts can prob-ably be ruled out on major element and mineralogical con-siderations First amphiboles are Ca-poor in conflict withcarbonate melt metasomatism which is generally charac-terized by Ca enrichment (Rudnick et al 1993 Neumannet al 2002) Further orthopyroxene is a stable phase andprobably even crystallizes during metasomatism in somesamples (eg Ke 785) At near-solidus conditions coexis-tence of orthopyroxene and carbonate melt is possibleonly at pressures 20GPa (see Fig 2 eg Lee amp Wyllie2000 and references therein) which is considerably higherthan the maximum pressures indicated by the compositionof spinel (515GPa) At such low pressures metasomatismby carbonated melts will convert lherzolite and harzbur-gite immediately into wehrlite which is the major perido-tite type in all xenolith suites where carbonate meltmetasomatism is convincingly demonstrated (Rudnicket al 1993 Yaxley et al 1998 Neumann et al 2002)In Marsabit wehrlites are absent howeverAn Na2O^SiO2^CO2^H2O-rich melt or fluid on the

other hand would explain the Na-rich nature of amphiboleand phlogopite as well as the presence of orthopyroxeneGrowth of metasomatic (and Al-poor) orthopyroxene hasbeen described from several xenolith localities where peri-dotites were infiltrated either by hydrous fluids orSi-rich melts (Smith et al 1999 McInnes et al 2001Morishita et al 2003) Na-rich phlogopite is commonly

Table 6 Continued

Type III-b III-c Host

Sample 195915 196515 196515 785 196525 19653 19653 19653 195927

Remark next amph next cpx xenol margin next amph next amph melt pocket next apatite next Opxm melt pocket host basanite

Lu 014 0050 026 0041 046 080 140 058 040 031

Hf 026 052 535 014 076 110 5087 124 568 504

Ta 50080 033 638 0022 123 101 784 801 394 403

Pb 541 448 280 255 840 797 260 749 493 na

Th 194 296 160 466 703 185 247 105 534 506

U 545 069 478 099 152 447 724 216 171 126

nd not detected na not analysedAverage composition of the strongly alkaline basanites from Marsabit (Volker 1990)

22

Fig 12 Total alkalis vs SiO2 (a) Harker-type co-variation diagrams (b^e) and incompatible trace element co-variation diagram (f) showingthe compositional variability of silicate glass found in the porphyroclastic spinel harzburgites and dunites from Marsabit In (b) binary mixinglines between the typical composition of the alkali-rich glass (large open diamond) and the associated microlites are shown (each increment is10wt)They illustrate that the glass does not represent the derivate of host lava infiltration and fractional crystallization Subalkaline glass inGroup I (grt)^spl lherzolites from Marsabit is shown for comparison and is interpreted to derive from incongruent melting of Ti-pargasite(Kaeser 2006)

23

found as a reaction product between anhydrous mafic toultramafic rocks and H2O-rich evolved liquids (Costaet al 2001 and references therein) or H2O-bearing fluids(eg Odling 1994) The latter study was an experiment at20GPa simulating the effects of decreasing temperatureson the phase assemblages and their composition duringmetasomatism using a model peridotitethornC^O^H fluidThe coolest part of Odlingrsquos (1994) experiment (510758C)contained a strikingly similar phase assemblage (kato-phorite phlogopite with 2wt Na2O and Al-poororthopyroxene) to the Marsabit samples interpreted tohave formed during sub-solidus fluid^solid interactionA compositionally similar phase assemblage (highamounts of opx Si-rich hornblende Na-rich phlogopite)resulted from experiments simulating peridotite interac-tion with Si-rich melt (dacite) at lower P^T conditions(9508C at 092GPa Prouteau amp Scaillet 2003)In this study the trace element signatures of Group III-c

Mg-katophorite and the calculated metasomatic agentsfurther suggest that the metasomatizing agent was a meltrather than a fluid Hypothetical fluids in equilibrium witheuhedral Mg-katophorite from sample Ke 196525 havehigh NbLILE (eg Rb Ba) and low LILEREE ratios(Fig 17) Because experimental data predict the oppositefor fluids (LILE enriched over HFSE and REE egManning 2004 and references therein) a fluid with asignature such as that indicated in Fig 17 is unrealistic

Fig 13 Trace element patterns of silicate glass in porphyroclasticspinel harzburgites and dunites from Marsabit compared withthe trace element composition of associated clinopyroxene (Dim)micro-phenocrysts xenolith-hosting basanites from Marsabit andglass interpreted to represent xenolith^host interaction [concentra-tions normalized to the primitive mantle values of McDonough ampSun (1995)]

Table 7 Representative analyses of carbonates (oxides in

wt trace elements in mgg)

Group III-b III-b III-c III-c III-c

Mineral Dol Mg-Cc Mg-Cc Mg-Cc Mg-Cc

Sample 195915 196515 785 19653 19653

Texture in glass in glass in glass in glass in glass

SiO2 na 003 na nd 003

FeO(tot) 016 011 010 010 006

MnO 001 nd nd nd nd

MgO 1922 236 190 125 517

CaO 3189 5313 5377 5391 4853

Na2O 003 nd 004 nd 003

K2O na 002 na 001 nd

SrO 003 010 008 004 021

BaO 003 nd nd na na

CO21 4618 4444 4442 4377 4391

Total 9756 10021 10032 9909 9794

Mg-no 9954 9745 9713 9554 9931

Ca-no 5439 9418 9531 9687 8709

P 220 334 137 580 589

K 970 212 642 105 155

Sc 5065 50150 027 5027 50076

Ti 975 189 242 528 196

V 5085 0481 5042 5037 021

Ni 900 673 517 5294 593

Zn 459 5089 113 296 045

Ga na 5014 5013 na na

Rb 0084 50072 50074 50060 0046

Sr 593 860 371 251 4274

Y 012 011 130 704 015

Zr 261 024 012 097 756

Nb 144 019 0056 692 185

Cs 50063 50011 50018 50039 50008

Ba 126 230 5024 029 271

La 034 057 084 136 025

Ce 032 013 134 997 023

Pr 006 006 018 095 0026

Nd 030 013 068 228 0078

Sm 5044 50091 0074 048 50018

Eu 5012 50019 0051 020 50008

Gd 5037 50090 014 051 0026

Tb na 50010 0036 na na

Dy 5019 50039 0089 058 50038

Ho 50029 0013 0044 011 50006

Er 5012 50041 0083 033 50019

Tm 50052 50013 0016 0084 50006

Yb 5043 50063 014 027 50040

Lu 50072 50010 0011 0066 50008

(continued)

24

At the relatively low maximum pressures (515GPa) asupercritical liquid is not expected either (eg Kesselet al 2005)Finally a relatively hydrous metasomatizing agent

would be consistent with formation of Mg-katophoriteswith lowTi and presumably low Fe3thorn contents (given therelatively reducing conditions) which in turn imply high(OH)^ contents In other words partial dehydrogenationtypically observed in common (Ti-rich) mantle amphibole

(eg Popp et al 1995 Tiepolo et al 2001 and referencestherein) cannot have played an important role in thiscase as H deficiency is mainly the result of coupled substi-tutions involvingTi and Fe3thorn (Popp et al 1995) In conclu-sion the metasomatizing agent was most probably aNa2O^SiO2^CO2^H2O-rich meltComparison with experimental studies further con-

strains the low-temperature regime in which metasoma-tism occurred In ultramafic (eg Niida amp Green 1999)basaltic (eg Rapp 1995) and Si-enriched systems (egtrachybasaltic Barclay amp Carmichael 2004) amphibole(in the latter study together with phlogopite) is confinedto temperatures lower than 1000^10508C at pressures520GPa (Fig 2)Thus crystallization of the early metaso-matic phase assemblage can be explained by infiltration ofa silicate melt that rapidly cooled upon contact with theambient peridotite (750^8508C prior to metasomatismFig 2) However this low-T stage is based on Ca-in-opxthermometry (see Brey amp Kolaquo hler 1990 Kaeser et al2006 Table 1) and therefore probably underestimates theeffective temperature conditions during metasomatismThis can be explained by low diffusion rates and proximityto the closure temperature for the diopside^enstatiteexchange reaction at temperatures lower than 8508C(eg Smith 1999 and references therein) The lower tem-perature limit at which silicate melt metasomatism ispossible (ie defined by the meltrsquos solidus and viscosity)depends strongly on volatile and SiO2 contentsExperiments by Draper amp Green (1997) for exampleshowed that highly silicic melts (52^64wt SiO2) canbe in equilibrium with mantle harzburgite at temperaturesas low as 8508C (at 10^20GPa under anhydrous andC^O^H-fluid-bearing conditions) These are howeverextreme compositions for mantle melts The actual SiO2

content of the metasomatizing agent in this case is difficultto constrain from the metasomatic mineral assemblage ascompositions may have varied considerably during reac-tive percolation (eg including Si-releasing reactions suchas cpx replacement by amphibole versus Si-consumingreactions such as Opxm crystallization) Moreover it isunlikely that a melt persists at temperatures muchbelow 9008C which is the solidus of fertile lherzolite(ie primitive upper mantle composition) underH2O-saturated conditions at 15GPa (Grove et al 2006see Fig 2) Below the solidus residual volatile-enrichedfluids may percolate further for example forming the lessmetasomatized Type III-c xenoliths (eg Ke 785) as wellas Type III-b and -a peridotites (see also Bailey 1987)Relatively cold conditions would also explain the veryhigh compositional gradients developed on small scales inType III-a samples (Fig 15) Given the absence of chloritein all investigated samples the lower temperature limitof such fluid metasomatism is 8008C (eg Fumagalli ampPoli 2005 see Fig 2)

Table 7 Continued

Group III-b III-b III-c III-c III-c

Mineral Dol Mg-Cc Mg-Cc Mg-Cc Mg-Cc

Sample 195915 196515 785 19653 19653

Texture in glass in glass in glass in glass in glass

Hf 5016 50038 50036 50074 50013

Ta 0048 50012 50016 50031 0008

Pb 5024 858 5021 074 018

Th 0021 50020 50017 50028 50005

U 060 094 086 600 148

Mg-Cc Mg-calcite Dol dolomite nd not detectedna not analysed Ca-numberfrac14 100Ca(CathornMg)1Calculated from stoichiometry

Fig 14 Trace element patterns of glass-hosted carbonates in TypeIII-b and -c spinel harzburgite and dunite xenoliths from Marsabit[concentrations normalized to the primitive mantle values ofMcDonough amp Sun (1995)] The grey field outlines the compositionalrange of carbonates found in other peridotite xenoliths (Ionov ampHarmer 2002 and references therein) Carbonate material inter-preted as quenched carbonatite melt found in peridotite xenolithsfrom Kerguelen (Moine et al 2004) is shown for comparison(bold line)

25

Finally the presence of apatite even though oftentaken as indicator for carbonatite-type metasomatism(eg Yaxley et al 1998) is not in conflict with metasoma-tism by silicate melts Apatite has also been shown to crys-tallize from volatile and alkali-rich derivatives formedduring reactive porous flow of originally mafic silicatemelts (Woodland et al 1996 Bedini et al 1997 Bodinieret al 2004 Ionov et al 2006)

FORMATION OF THE LATE GLASS -BEAR ING ASSEMBLAGESTextures strongly suggest that the glass-bearing assem-blages formed later and at the expense of the early metaso-matic assemblages (Fig 4a and d) Features such as thereappearance of clinopyroxene (Na-rich Cr diopsidemicro-phenocrysts) a phase replaced by the early

Fig 15 Detailed REE systematics in primary clinopyroxene (cpx-I) in a porphyroclastic cryptically metasomatized spinel harzburgite(sample Ke 19651) showing the transition fromType III-a (Zone A and B) toType III-b (Zone C) on a scale of a few centimetres The numberson the photomicrographs (crossed polars) correspond to single laser ablation microprobe pits (for further explanation see text)

26

metasomatic assemblage indicate that melt pocket forma-tion reflects a significant change in P^T^X space in partsof the mantle beneath Marsabit after earlier metasoma-tism Based on geochemical arguments and mass-balanceconsiderations we will show that (1) the late glass-bearingassemblages are closely related to early metasomaticassemblages mainly representing breakdown productsand (2) breakdown was induced by infiltration of aCO2-rich fluid in the mantle probably followed andenhanced by heating and decompression melting duringxenolith transport in the host magma

Evidence for in situ partial meltingBSE imaging reveals that the phases becoming unstableare amphibole orthopyroxene and to minor extentsspl-I phlogopite cpx-I and apatite Compositional(inter- and intra-xenolith) heterogeneity of silicate glass istexture controlled and varies as a function of theassociated precursor phases (Fig 12a^f)TiO2 for exampleshows higher concentrations in glass from xenolith Ke196515 containing Ti-enriched amphibole (Fig 12c)whereas K2O contents are highest in phlogopite-rich sam-ples (eg Ke 785 Fig 12d) Elevated P2O5 contents arefound in glass adjacent to apatite (Fig 12e) whichmirrors at the same time the highTh and LREE contents

of apatite (Fig 12f) Significant Si enrichment is found inglass coating orthopyroxene (Fig 12d) supporting incon-gruent melting of orthopyroxene (see below) Even thoughthe melt pockets are connected along grain boundaries thestrong compositional variation (on inter- and intra-xeno-lith scale) suggests that the mobility of the melt was lim-ited or that the melting event was very short-lived (fortiming constraints see below)The co-variation plots in Fig 12 further reveal consider-

able compositional differences between all types of xeno-lith glass and the host basanite These data indicate thatthe glass cannot be the product of xenolith^host lava inter-action during ascent to the surface Also back-mixing ofmicro-phenocrysts into the melt fails to derive the glassfrom a host-related melt (Fig 12b) Glass from xenolithmargins however is compositionally closer to the bulkhost basanite (eg Fig 12b) in line with a direct derivationfrom xenolith^host lava interaction Its more evolvedmajor element composition (eg higher SiO2 lowerMg-number Fig 12b) is probably the result of fractiona-tion of basanite phenocrysts (eg olivine augite andtitanomagnetite Volker 1990) Host lava^xenolith interac-tion is further supported by the trace element signature ofglass at the margin of the xenoliths which is closely similarto that of the Marsabit basanites (Fig 13 Volker 1990)The same characteristics are observed for intra-xenolithglass in the unusual kaersutite-bearing sample Ke 195927

Fig 16 Oxygen fugacities relative to the quartz^fayalite^magnetitebuffer (QFM equation of OrsquoNeill 1987) as a function of temperatureat assumed pressure of 1GPa calculated using the composition ofcoexisting spinel and olivine (ol-I^spl-I pairs and Chrm^Olm pairsrespectively) based on the oxygen barometer of Ballhaus et al (1991)The graphite^CO2 equilibrium was calculated after Frost amp Wood(1998) and the graphite^carbonate^silicate equilibrium was calcu-lated using the equation of Eggler amp Baker (1982) (See text for furtherexplanation)

Fig 17 Calculated metasomatic agents in equilibrium with the euhe-dral Mg-katophorites from the Type III-c amphibole dunite Ke 196525 Amphibolehydrous fluid and amphibolecarbonatite melt parti-tion coefficients are from Adam et al (1997) and Adam amp Green(2001) respectively Silicate melts have been calculated using amphi-boleSi-rich melt partition coefficients from Chazot et al (1996b) andamphibolebasanite partition coefficients from LaTourrette et al(1995) The compositional range of the Marsabit basanites and alkalibasalts (Volker 1990) is shown for comparison

27

Xenolith glass unrelated to the host lava and melt pock-ets similar to those reported here in association with resi-dual phases (eg amphibole or clinopyroxene) has beenreported from many xenolith localities (eg Frey ampGreen 1974 Ionov et al 1994 1996 Chazot et al 1996a1996b Yaxley et al 1997 Laurora et al 2001 Bali et al2002) Residual phases within the melt pockets stronglyindicate in situ partial melting within the xenolithsHowever a variety of processes may trigger such in situ

melting including (1) closed-system decompression melt-ing of either anhydrous (Maale amp Printzlau 1979) orhydrous phases (Frey amp Green 1974 Francis 1976) duringxenolith ascent in the host lava (2) melting during interac-tion of the xenoliths with the host basaltic melt duringxenolith ascent (eg Zinngrebe amp Foley 1995 Shaw ampKlulaquo gel 2002 Shaw et al 2006) (3) melting in the mantleprior to xenolith entrainment induced by an infiltratingfluid (ie flux melting eg Jones et al 1983 Ionov et al1994 Demecurren ny et al 2004) However most studies agreethat that partial melting occurred very shortly beforeandor during xenolith exhumation by the host magmaThus a combination of metasomatically triggered meltingfollowed andor assisted by decompression melting duringxenolith transport may be a realistic alternative in somecases (eg Dawson 2002)Recently Ban et al (2005) have shown in a very detailed

mass-balance study on glass-bearing xenoliths from theWest Eifel that the weight proportions of glass and theassociated micro-phenocrysts match the composition ofthe preserved parent amphibole We used the sameapproach and performed simple mass-balance calculations(least-squares fitting) to constrain the conclusions drawnfrom petrography and to test which phase contributedand to what extent to the formation of the melt pocketsThe results reported in Table 8 show that in most casesamphibole-melting can account successfully for the meltpocket composition and only a minor amount (520) ofother phases (orthopyroxene andor phlogopite) needs tobe added to the reactant Such a contribution of otherphases agrees with textural observation (eg Fig 4cshowing Dim growing on resorbed Opxm) as well as withthe conclusions of Ban et al (2005) who stated that moreSi-rich glass (ie 454wt) necessitates orthopyroxenecontributing to the meltTrace element patterns of the bulk glassthornmicro-

phenocryst pockets calculated using the measured traceelement abundances of glass and micro-phenocrysts andthe respective mass-balance parameters (Table 8) arebroadly identical to the measured amphibole composition(Fig 18a and b) Higher LILE (Cs Rb Ba) concentrationsin the calculated bulk melt patches in sample Ke 785 canbe balanced by adding 7wt of phlogopite (Fig 18b)which is in line with mass balance (Table 8) The dominantcontrol of amphibole to account for the trace element

budget of the melt pockets is similar to results found inother suites of glass-bearing xenoliths (Chazot et al1996a Yaxley amp Kamenetsky 1999 Laurora et al 2001)Further the obtained phase proportions for the melt pock-ets are in good agreement with the results from image ana-lysis (Table 1) In addition they are similar to the glass^cpx^ol proportions recorded in recent experiments on the

Table 8 Results of mass-balance calculations to test melt

pocket formation by incongruent melting of the early metaso-

matic assemblages

Sample 195915 785 785 196525 196525 19653

Reactant

amph 1000 1000 0820 1000 0944 mdash

opx mdash mdash 0078 mdash 0056 0406

phl mdash mdash 0102 mdash mdash mdash

glass mdash mdash mdash mdash mdash 05941

SiO2 4484 4989 4964 5129 5167 5898

TiO2 014 039 036 025 024 018

Cr2O3 232 291 258 127 121 009

Al2O3 1368 762 769 800 757 961

FeO(tot) 336 314 333 343 357 394

MnO 006 008 008 012 012 010

NiO 009 013 014 011 011 004

MgO 1844 2029 2197 2040 2122 1625

CaO 854 783 644 966 913 078

Na2O 460 500 436 504 476 843

K2O 049 032 096 042 040 060

Product

glass 0389 0322 0335 0292 0284 07032

ol 0243 0259 0315 0266 0286 0273

cpx 0288 0373 0308 0442 0431 0024

spl 0052 0032 0028 mdash mdash mdash

carb 0028 0014 0014 mdash mdash mdash

SiO2 4477 5027 4986 5207 5181 5903

TiO2 013 041 039 015 014 017

Cr2O3 213 314 272 109 106 015

Al2O3 1364 764 770 798 776 957

FeO(tot) 374 331 351 322 332 350

MnO 007 007 007 009 009 006

NiO 011 013 015 011 012 007

MgO 1834 2024 2196 2063 2141 1635

CaO 854 759 632 952 929 080

Na2O 449 385 388 499 485 860

K2O 060 096 100 015 014 091

r2 023 200 035 084 028 033

Prefix Ke omitted1Low-Si glass 2High-Si glass (see text for explanation)

28

melting behaviour of amphibole in wehrlites at tempera-tures between 1175 and 12508C and pressures of05^10GPa (Mecurren dard et al 2006) These temperatures arein good agreement with the estimates for the formation ofolivine and chromite micro-phenocrysts in the melt

pockets (Fig 16) The heat supply required for amphibolemelting in the formerly low-T metasomatized Group IIIxenoliths (T 200^3008C) may come from local heatingshortly before or during xenolith entrainment in the hostlava or from fluid infiltration that triggered melting (seediscussion below)Finally formation of the most Si-enriched melt (adjacent

to Opxm in Group III-c samples) can be modelled assum-ing progressing interaction between infiltrating melt andorthopyroxene where the latter melts incongruently

opxthornmelt1 olthornmelt2 cpx ethseeTable 8THORN eth1THORN

To calculate reaction (1) we chose the least evolved glassobserved in melt patches in sample Ke 19653 (SiO2

5921wt Mg-number 6465) to represent the initialmelt1 whereas melt2 adjacent to opx is SiO2-rich(6530wt) Trace element abundances in glass next toorthopyroxene are shifted to lower concentrations(Table 6) which is in agreement with dilution caused byaddition of a trace element-poor orthopyroxene compo-nent The resultant phase proportions are similar to thoseobserved at orthopyroxene^melt interfaces (ie higholivine and low cpx abundances) and are in agreementwith experimental studies (Shaw et al 1998)

Partial melting triggered by fluid fluxMass-balance calculations provide an explanation for meltpocket formation by in situ partial melting Thus onemight conclude that melting is simply the result of decom-pression and heating during xenolith transport in the hostmagma However there are two points that argue thatmelting was triggered by the infiltration of a fluid phaseshortly before the xenoliths were sampled by the hostmagma (1) the presence of occasionally high amounts ofcarbonate included in the melt pockets (2) the extremelyNa-rich clinopyroxene (Dim) compositions either includedas micro-phenocrysts in the melt pocket or present asequilibrated grains around chromite in the single TypeIII-d xenolith (Fig 3f) Both arguments will be discussedin the next section

Formation of carbonates

Textures and compositions of carbonate globules can beexplained by carbonate precipitation from a CO2-bearingfluid As demonstrated by several studies highCa(CathornMg) values (Table 7) and low REE concentra-tions are strong arguments that the carbonates are cumu-late crystals precipitated from a CO2-rich melt or afluid and not quenched carbonated melts (Lee et al 2000Ionov amp Harmer 2002) Oscillatory zoning (Fig 5a and b)of individual carbonate crystals within singleglobules corroborates this interpretation The roundedcontacts toward glass (globules) are also in line with thesefindings Similar globular forms were reported fromexperimental studies and do not a priori imply the

Fig 18 Calculated trace element characteristics of reactants (in mostcases amphibolethorn orthopyroxene filled symbols) and products(melt pockets open symbols) of the lsquoamphibole meltingrsquo model[see equation (1) in text] The relative phase proportions wereobtained by mass-balance calculations and are given in Table 8 Thedashed lines in (b) show that the high LILE concentrations obtainedfor the product (bulk melt pocket) can be explained by adding about7wt of phlogopite to the reactant (amphibolethorn orthopyroxeneTable 8) Further explanation is given in the text

29

presence of immiscible carbonate and silicate melts(eg Lee amp Wyllie 1996)The high amount of carbonate in some melt pockets

however (eg Fig 4e) does need further explanation asthe incongruent melting reaction (1) does not account forthe CO2 required for carbonate precipitation duringamphibole melting Therefore amphibole breakdowneither occurred in an open system with addition of at leastCO2 (eg Laurora et al 2001 Ban et al 2005) or anotherpre-existing phase acted as a source of carbon Oxidationof graphite texturally identified as a part of the earlyassemblage (eg Fig 3d) could account for some of theCO2 necessary for carbonate formation A change in fO2

conditions is also indicated when the composition of glass-related spinel (Chrm) and Olm are used for oxygen baro-metry which yields oxygen fugacities well on the oxidizedside of the carbon^carbonate^silicate buffer (Fig 16)However increasing fO2 triggered by closed-systemdecompression-induced melting alone does not appear fea-sible Also textures such as straight contacts between gra-phite and Mg-calcite (Fig 4c) suggest that carbonategrows on graphite (ie indicating local fO2 conditions cor-responding to carbon^carbonate equilibrium) rather thanreplacing it We therefore interpret melt pocket formationto be triggered by the infiltration of a CO2-rich fluid(ie flux melting see also Ionov et al 1994) This interpre-tation also accounts for the presence of abundant vugs inthe glass that could have contained a fluid (Ionov et al1994 2006) which was presumably lost during sample pre-paration However we emphasize here that late fluid infil-tration had only subtle effects on the trace elementcomposition of Type III-b and -c xenoliths

Formation of Na-rich Cr diopside

Infiltration of CO2-rich fluidwould also explain the Na-richcomposition of Dim micro-phenocryst cores (Fig 6a and b)Ionov et al (2005) noted recently that clinopyroxenes inmeltpockets are commonly characterized by Na2O contents51wt mainly as a result of low cpxmelt partition coeffi-cients at low pressures (eg Blundy et al 1995) Thus veryhighNa2Ocontents in Dimmicro-phenocrysts (Fig6a) sug-gest replacement of amphibole by Dim and eventually Olmand Chrm during reaction with a fluid at subsolidus condi-tions An origin of micro-phenocrysts by replacementrather than crystallization from a melt is indicated by theidentical crystallographic orientation of individual Dimcrystals in singlemelt pockets (Fig4a)Once the solidus of the precursor phase(s) was reached

(ie 10508C for amphibole Niida amp Green 1999) silicatemelt was generated in situ and led to the formation of clin-opyroxene rims with lower Na contents (Fig 6b) and thedevelopment of euhedral shapes in contact with melt (egFig 4a and e) Eventual immiscibility between remainingfluid and newly formed melt (eg during xenolith ascent)would then explain the presence of carbonate globules and

vugs enclosed in glass Likewise different origins of meltand carbonate were recently proposed for similar meltpockets from Hungarian xenoliths by Demecurren ny et al(2004) who found oxygen isotope disequilibrium betweencarbonate globules and the host glass Such a two-stepmodel (fluid-induced replacement followed by melt gen-eration) can be explained by CO2-fluid infiltration occur-ring within the mantle at sub-solidus conditions ultimatelyfollowed by xenolith entrainment in the host magma andtransport to the surfaceThe host magma possibly suppliedthe additional heat necessary for melt generationFast decompression would further enhance melting andcould also account for the lower Na contents in Dimmicro-phenocryst rims as a result of decreasing cpxmeltpartition coefficients at decreasing pressures (Blundyet al 1995)Rapid replacement of amphibole ( phlogopite) by an

anhydrous phase assemblage indicates that the fluid waswater-poor (eg low H2OCO2 ratio see Ionov et al2006)This is corroborated by the fact that Mg-katophorite(presumably OH-rich see above) is occasionally rimmedby a residual kaersutitic selvage (Fig 4d) Such Ti-richmantle amphiboles are commonly OH-poor as a result ofpartial dehydrogenation (eg Tiepolo et al 2001 and refer-ences therein)Finally the presence of theType III-d xenolith contain-

ing texturally equilibrated clinopyroxene of almost identi-cal composition to Dim micro-phenocrysts in melt pockets(Table 2 Figs 6a and 7b) further strengthens the argu-ment that CO2-fluid percolation started in the mantlebefore entrainment (ie enough time elapsed to allow thisrock to anneal completely after the late metasomaticassemblage had formed) A similar conclusion can bedrawn for the clearly host-related glass found in xenolithKe 195927 (eg Fig 13b) ThisType III-c sample is charac-terized by the presence of Dim micro-phenocrysts partlyovergrown by mineral rims clearly related to the host basa-nite magma (kaersutite Ti-augite and olivine withFo885 Fig 4f Tables 3 and 4 and supplementary dataat httppetrologyoxfordjournalsorg) Kaersutite hasmajor and trace element characteristics similar to typicalbasanitic and alkali basaltic amphibole phenocrysts(Fig 9d) The special phase assemblages in this xenolithindicates that host-related basanitic melts intruded someType III-c xenoliths and partly overprinted the assemblageformed slightly earlier by in situ derived alkali-rich peralu-minous melts fully preserved in other samples (eg Ke19653 or Ke 785)

ORIGIN OF METASOMATIZ INGAGENTS T IM ING AND TECTONICSETT INGAs noted above late metasomatism was probably short-lived started at mantle depths and probably continued

30

Evidence for low-temperature SiO2^Na2O^CO2^H2O-rich melts or fluids

Potential metasomatic agents inferred from the presenceof graphite are C^O^H-rich fluids carbonate melts orCO2-bearing silicate melts The resulting phase assem-blages and geochemical signatures are the result of infiltra-tion of fluid or melt into a cold (750^8508C) shallow(515GPa Fig 2) heterogeneous and locally reduced litho-spheric mantle Metasomatism under such conditions in anopen system must result in complex geochemical signa-tures that reflect the degree of fluid^rock reaction Thismakes it particularly difficult to constrain the exactnature of the lsquooriginalrsquometasomatic agentType III-c xenolith Ke 196525 contains the highest frac-

tion of metasomatic phases and the least fractionatedamphibole trace element patterns The lsquomagmaticrsquo texturesof the Mg-katophorites in this sample suggest direct crys-tallization from a melt or a fluid rather than replacementof a pre-existing phase Therefore hypothetical liquids inequilibrium with the amphiboles in this xenolith couldyield the composition of the initial metasomatic agentsFigure 17 illustrates the trace element patterns of suchmelts and fluids calculated using sets of experimentally orempirically determined mineralmelt (fluid) partitioncoefficients including carbonate melt silicate melt andhydrous fluid (for references see caption to Fig 17)Common features for all calculated metasomatic agents

are negative Zr^Hf^Ti anomalies and LREE enrichment(Fig 17) However previous studies have shown that thesesignatures are indicative of almost all types of metasoma-tism including interaction of peridotite with carbonatiticmelt (Green amp Wallace 1988 Dautria et al 1992 Hauriet al 1993 Rudnick et al 1993 Ionov et al 1996 Yaxleyet al 1998 Coltorti et al 1999) with hydrous fluids andormelts for example originating from dehydration of altered

subducting lithosphere (Maury et al 1992 Zanetti et al1999 Grecurren goire et al 2001 Laurora et al 2001 Parkinsonet al 2003) and interaction of peridotite with volatile-and Si-rich melts that evolved through reactive porousflow (Bedini et al 1997 Ionov et al 2002 2006 Bodinieret al 2004 Rivalenti et al 2004)In the case of the Marsabit xenoliths integration of the

trace element characteristics thermobarometric inferencesand mineralogical constraints favours a Na2O^SiO2^CO2^H2O-rich silicate melt Carbonated melts can prob-ably be ruled out on major element and mineralogical con-siderations First amphiboles are Ca-poor in conflict withcarbonate melt metasomatism which is generally charac-terized by Ca enrichment (Rudnick et al 1993 Neumannet al 2002) Further orthopyroxene is a stable phase andprobably even crystallizes during metasomatism in somesamples (eg Ke 785) At near-solidus conditions coexis-tence of orthopyroxene and carbonate melt is possibleonly at pressures 20GPa (see Fig 2 eg Lee amp Wyllie2000 and references therein) which is considerably higherthan the maximum pressures indicated by the compositionof spinel (515GPa) At such low pressures metasomatismby carbonated melts will convert lherzolite and harzbur-gite immediately into wehrlite which is the major perido-tite type in all xenolith suites where carbonate meltmetasomatism is convincingly demonstrated (Rudnicket al 1993 Yaxley et al 1998 Neumann et al 2002)In Marsabit wehrlites are absent howeverAn Na2O^SiO2^CO2^H2O-rich melt or fluid on the

other hand would explain the Na-rich nature of amphiboleand phlogopite as well as the presence of orthopyroxeneGrowth of metasomatic (and Al-poor) orthopyroxene hasbeen described from several xenolith localities where peri-dotites were infiltrated either by hydrous fluids orSi-rich melts (Smith et al 1999 McInnes et al 2001Morishita et al 2003) Na-rich phlogopite is commonly

Table 6 Continued

Type III-b III-c Host

Sample 195915 196515 196515 785 196525 19653 19653 19653 195927

Remark next amph next cpx xenol margin next amph next amph melt pocket next apatite next Opxm melt pocket host basanite

Lu 014 0050 026 0041 046 080 140 058 040 031

Hf 026 052 535 014 076 110 5087 124 568 504

Ta 50080 033 638 0022 123 101 784 801 394 403

Pb 541 448 280 255 840 797 260 749 493 na

Th 194 296 160 466 703 185 247 105 534 506

U 545 069 478 099 152 447 724 216 171 126

nd not detected na not analysedAverage composition of the strongly alkaline basanites from Marsabit (Volker 1990)

22

Fig 12 Total alkalis vs SiO2 (a) Harker-type co-variation diagrams (b^e) and incompatible trace element co-variation diagram (f) showingthe compositional variability of silicate glass found in the porphyroclastic spinel harzburgites and dunites from Marsabit In (b) binary mixinglines between the typical composition of the alkali-rich glass (large open diamond) and the associated microlites are shown (each increment is10wt)They illustrate that the glass does not represent the derivate of host lava infiltration and fractional crystallization Subalkaline glass inGroup I (grt)^spl lherzolites from Marsabit is shown for comparison and is interpreted to derive from incongruent melting of Ti-pargasite(Kaeser 2006)

23

found as a reaction product between anhydrous mafic toultramafic rocks and H2O-rich evolved liquids (Costaet al 2001 and references therein) or H2O-bearing fluids(eg Odling 1994) The latter study was an experiment at20GPa simulating the effects of decreasing temperatureson the phase assemblages and their composition duringmetasomatism using a model peridotitethornC^O^H fluidThe coolest part of Odlingrsquos (1994) experiment (510758C)contained a strikingly similar phase assemblage (kato-phorite phlogopite with 2wt Na2O and Al-poororthopyroxene) to the Marsabit samples interpreted tohave formed during sub-solidus fluid^solid interactionA compositionally similar phase assemblage (highamounts of opx Si-rich hornblende Na-rich phlogopite)resulted from experiments simulating peridotite interac-tion with Si-rich melt (dacite) at lower P^T conditions(9508C at 092GPa Prouteau amp Scaillet 2003)In this study the trace element signatures of Group III-c

Mg-katophorite and the calculated metasomatic agentsfurther suggest that the metasomatizing agent was a meltrather than a fluid Hypothetical fluids in equilibrium witheuhedral Mg-katophorite from sample Ke 196525 havehigh NbLILE (eg Rb Ba) and low LILEREE ratios(Fig 17) Because experimental data predict the oppositefor fluids (LILE enriched over HFSE and REE egManning 2004 and references therein) a fluid with asignature such as that indicated in Fig 17 is unrealistic

Fig 13 Trace element patterns of silicate glass in porphyroclasticspinel harzburgites and dunites from Marsabit compared withthe trace element composition of associated clinopyroxene (Dim)micro-phenocrysts xenolith-hosting basanites from Marsabit andglass interpreted to represent xenolith^host interaction [concentra-tions normalized to the primitive mantle values of McDonough ampSun (1995)]

Table 7 Representative analyses of carbonates (oxides in

wt trace elements in mgg)

Group III-b III-b III-c III-c III-c

Mineral Dol Mg-Cc Mg-Cc Mg-Cc Mg-Cc

Sample 195915 196515 785 19653 19653

Texture in glass in glass in glass in glass in glass

SiO2 na 003 na nd 003

FeO(tot) 016 011 010 010 006

MnO 001 nd nd nd nd

MgO 1922 236 190 125 517

CaO 3189 5313 5377 5391 4853

Na2O 003 nd 004 nd 003

K2O na 002 na 001 nd

SrO 003 010 008 004 021

BaO 003 nd nd na na

CO21 4618 4444 4442 4377 4391

Total 9756 10021 10032 9909 9794

Mg-no 9954 9745 9713 9554 9931

Ca-no 5439 9418 9531 9687 8709

P 220 334 137 580 589

K 970 212 642 105 155

Sc 5065 50150 027 5027 50076

Ti 975 189 242 528 196

V 5085 0481 5042 5037 021

Ni 900 673 517 5294 593

Zn 459 5089 113 296 045

Ga na 5014 5013 na na

Rb 0084 50072 50074 50060 0046

Sr 593 860 371 251 4274

Y 012 011 130 704 015

Zr 261 024 012 097 756

Nb 144 019 0056 692 185

Cs 50063 50011 50018 50039 50008

Ba 126 230 5024 029 271

La 034 057 084 136 025

Ce 032 013 134 997 023

Pr 006 006 018 095 0026

Nd 030 013 068 228 0078

Sm 5044 50091 0074 048 50018

Eu 5012 50019 0051 020 50008

Gd 5037 50090 014 051 0026

Tb na 50010 0036 na na

Dy 5019 50039 0089 058 50038

Ho 50029 0013 0044 011 50006

Er 5012 50041 0083 033 50019

Tm 50052 50013 0016 0084 50006

Yb 5043 50063 014 027 50040

Lu 50072 50010 0011 0066 50008

(continued)

24

At the relatively low maximum pressures (515GPa) asupercritical liquid is not expected either (eg Kesselet al 2005)Finally a relatively hydrous metasomatizing agent

would be consistent with formation of Mg-katophoriteswith lowTi and presumably low Fe3thorn contents (given therelatively reducing conditions) which in turn imply high(OH)^ contents In other words partial dehydrogenationtypically observed in common (Ti-rich) mantle amphibole

(eg Popp et al 1995 Tiepolo et al 2001 and referencestherein) cannot have played an important role in thiscase as H deficiency is mainly the result of coupled substi-tutions involvingTi and Fe3thorn (Popp et al 1995) In conclu-sion the metasomatizing agent was most probably aNa2O^SiO2^CO2^H2O-rich meltComparison with experimental studies further con-

strains the low-temperature regime in which metasoma-tism occurred In ultramafic (eg Niida amp Green 1999)basaltic (eg Rapp 1995) and Si-enriched systems (egtrachybasaltic Barclay amp Carmichael 2004) amphibole(in the latter study together with phlogopite) is confinedto temperatures lower than 1000^10508C at pressures520GPa (Fig 2)Thus crystallization of the early metaso-matic phase assemblage can be explained by infiltration ofa silicate melt that rapidly cooled upon contact with theambient peridotite (750^8508C prior to metasomatismFig 2) However this low-T stage is based on Ca-in-opxthermometry (see Brey amp Kolaquo hler 1990 Kaeser et al2006 Table 1) and therefore probably underestimates theeffective temperature conditions during metasomatismThis can be explained by low diffusion rates and proximityto the closure temperature for the diopside^enstatiteexchange reaction at temperatures lower than 8508C(eg Smith 1999 and references therein) The lower tem-perature limit at which silicate melt metasomatism ispossible (ie defined by the meltrsquos solidus and viscosity)depends strongly on volatile and SiO2 contentsExperiments by Draper amp Green (1997) for exampleshowed that highly silicic melts (52^64wt SiO2) canbe in equilibrium with mantle harzburgite at temperaturesas low as 8508C (at 10^20GPa under anhydrous andC^O^H-fluid-bearing conditions) These are howeverextreme compositions for mantle melts The actual SiO2

content of the metasomatizing agent in this case is difficultto constrain from the metasomatic mineral assemblage ascompositions may have varied considerably during reac-tive percolation (eg including Si-releasing reactions suchas cpx replacement by amphibole versus Si-consumingreactions such as Opxm crystallization) Moreover it isunlikely that a melt persists at temperatures muchbelow 9008C which is the solidus of fertile lherzolite(ie primitive upper mantle composition) underH2O-saturated conditions at 15GPa (Grove et al 2006see Fig 2) Below the solidus residual volatile-enrichedfluids may percolate further for example forming the lessmetasomatized Type III-c xenoliths (eg Ke 785) as wellas Type III-b and -a peridotites (see also Bailey 1987)Relatively cold conditions would also explain the veryhigh compositional gradients developed on small scales inType III-a samples (Fig 15) Given the absence of chloritein all investigated samples the lower temperature limitof such fluid metasomatism is 8008C (eg Fumagalli ampPoli 2005 see Fig 2)

Table 7 Continued

Group III-b III-b III-c III-c III-c

Mineral Dol Mg-Cc Mg-Cc Mg-Cc Mg-Cc

Sample 195915 196515 785 19653 19653

Texture in glass in glass in glass in glass in glass

Hf 5016 50038 50036 50074 50013

Ta 0048 50012 50016 50031 0008

Pb 5024 858 5021 074 018

Th 0021 50020 50017 50028 50005

U 060 094 086 600 148

Mg-Cc Mg-calcite Dol dolomite nd not detectedna not analysed Ca-numberfrac14 100Ca(CathornMg)1Calculated from stoichiometry

Fig 14 Trace element patterns of glass-hosted carbonates in TypeIII-b and -c spinel harzburgite and dunite xenoliths from Marsabit[concentrations normalized to the primitive mantle values ofMcDonough amp Sun (1995)] The grey field outlines the compositionalrange of carbonates found in other peridotite xenoliths (Ionov ampHarmer 2002 and references therein) Carbonate material inter-preted as quenched carbonatite melt found in peridotite xenolithsfrom Kerguelen (Moine et al 2004) is shown for comparison(bold line)

25

Finally the presence of apatite even though oftentaken as indicator for carbonatite-type metasomatism(eg Yaxley et al 1998) is not in conflict with metasoma-tism by silicate melts Apatite has also been shown to crys-tallize from volatile and alkali-rich derivatives formedduring reactive porous flow of originally mafic silicatemelts (Woodland et al 1996 Bedini et al 1997 Bodinieret al 2004 Ionov et al 2006)

FORMATION OF THE LATE GLASS -BEAR ING ASSEMBLAGESTextures strongly suggest that the glass-bearing assem-blages formed later and at the expense of the early metaso-matic assemblages (Fig 4a and d) Features such as thereappearance of clinopyroxene (Na-rich Cr diopsidemicro-phenocrysts) a phase replaced by the early

Fig 15 Detailed REE systematics in primary clinopyroxene (cpx-I) in a porphyroclastic cryptically metasomatized spinel harzburgite(sample Ke 19651) showing the transition fromType III-a (Zone A and B) toType III-b (Zone C) on a scale of a few centimetres The numberson the photomicrographs (crossed polars) correspond to single laser ablation microprobe pits (for further explanation see text)

26

metasomatic assemblage indicate that melt pocket forma-tion reflects a significant change in P^T^X space in partsof the mantle beneath Marsabit after earlier metasoma-tism Based on geochemical arguments and mass-balanceconsiderations we will show that (1) the late glass-bearingassemblages are closely related to early metasomaticassemblages mainly representing breakdown productsand (2) breakdown was induced by infiltration of aCO2-rich fluid in the mantle probably followed andenhanced by heating and decompression melting duringxenolith transport in the host magma

Evidence for in situ partial meltingBSE imaging reveals that the phases becoming unstableare amphibole orthopyroxene and to minor extentsspl-I phlogopite cpx-I and apatite Compositional(inter- and intra-xenolith) heterogeneity of silicate glass istexture controlled and varies as a function of theassociated precursor phases (Fig 12a^f)TiO2 for exampleshows higher concentrations in glass from xenolith Ke196515 containing Ti-enriched amphibole (Fig 12c)whereas K2O contents are highest in phlogopite-rich sam-ples (eg Ke 785 Fig 12d) Elevated P2O5 contents arefound in glass adjacent to apatite (Fig 12e) whichmirrors at the same time the highTh and LREE contents

of apatite (Fig 12f) Significant Si enrichment is found inglass coating orthopyroxene (Fig 12d) supporting incon-gruent melting of orthopyroxene (see below) Even thoughthe melt pockets are connected along grain boundaries thestrong compositional variation (on inter- and intra-xeno-lith scale) suggests that the mobility of the melt was lim-ited or that the melting event was very short-lived (fortiming constraints see below)The co-variation plots in Fig 12 further reveal consider-

able compositional differences between all types of xeno-lith glass and the host basanite These data indicate thatthe glass cannot be the product of xenolith^host lava inter-action during ascent to the surface Also back-mixing ofmicro-phenocrysts into the melt fails to derive the glassfrom a host-related melt (Fig 12b) Glass from xenolithmargins however is compositionally closer to the bulkhost basanite (eg Fig 12b) in line with a direct derivationfrom xenolith^host lava interaction Its more evolvedmajor element composition (eg higher SiO2 lowerMg-number Fig 12b) is probably the result of fractiona-tion of basanite phenocrysts (eg olivine augite andtitanomagnetite Volker 1990) Host lava^xenolith interac-tion is further supported by the trace element signature ofglass at the margin of the xenoliths which is closely similarto that of the Marsabit basanites (Fig 13 Volker 1990)The same characteristics are observed for intra-xenolithglass in the unusual kaersutite-bearing sample Ke 195927

Fig 16 Oxygen fugacities relative to the quartz^fayalite^magnetitebuffer (QFM equation of OrsquoNeill 1987) as a function of temperatureat assumed pressure of 1GPa calculated using the composition ofcoexisting spinel and olivine (ol-I^spl-I pairs and Chrm^Olm pairsrespectively) based on the oxygen barometer of Ballhaus et al (1991)The graphite^CO2 equilibrium was calculated after Frost amp Wood(1998) and the graphite^carbonate^silicate equilibrium was calcu-lated using the equation of Eggler amp Baker (1982) (See text for furtherexplanation)

Fig 17 Calculated metasomatic agents in equilibrium with the euhe-dral Mg-katophorites from the Type III-c amphibole dunite Ke 196525 Amphibolehydrous fluid and amphibolecarbonatite melt parti-tion coefficients are from Adam et al (1997) and Adam amp Green(2001) respectively Silicate melts have been calculated using amphi-boleSi-rich melt partition coefficients from Chazot et al (1996b) andamphibolebasanite partition coefficients from LaTourrette et al(1995) The compositional range of the Marsabit basanites and alkalibasalts (Volker 1990) is shown for comparison

27

Xenolith glass unrelated to the host lava and melt pock-ets similar to those reported here in association with resi-dual phases (eg amphibole or clinopyroxene) has beenreported from many xenolith localities (eg Frey ampGreen 1974 Ionov et al 1994 1996 Chazot et al 1996a1996b Yaxley et al 1997 Laurora et al 2001 Bali et al2002) Residual phases within the melt pockets stronglyindicate in situ partial melting within the xenolithsHowever a variety of processes may trigger such in situ

melting including (1) closed-system decompression melt-ing of either anhydrous (Maale amp Printzlau 1979) orhydrous phases (Frey amp Green 1974 Francis 1976) duringxenolith ascent in the host lava (2) melting during interac-tion of the xenoliths with the host basaltic melt duringxenolith ascent (eg Zinngrebe amp Foley 1995 Shaw ampKlulaquo gel 2002 Shaw et al 2006) (3) melting in the mantleprior to xenolith entrainment induced by an infiltratingfluid (ie flux melting eg Jones et al 1983 Ionov et al1994 Demecurren ny et al 2004) However most studies agreethat that partial melting occurred very shortly beforeandor during xenolith exhumation by the host magmaThus a combination of metasomatically triggered meltingfollowed andor assisted by decompression melting duringxenolith transport may be a realistic alternative in somecases (eg Dawson 2002)Recently Ban et al (2005) have shown in a very detailed

mass-balance study on glass-bearing xenoliths from theWest Eifel that the weight proportions of glass and theassociated micro-phenocrysts match the composition ofthe preserved parent amphibole We used the sameapproach and performed simple mass-balance calculations(least-squares fitting) to constrain the conclusions drawnfrom petrography and to test which phase contributedand to what extent to the formation of the melt pocketsThe results reported in Table 8 show that in most casesamphibole-melting can account successfully for the meltpocket composition and only a minor amount (520) ofother phases (orthopyroxene andor phlogopite) needs tobe added to the reactant Such a contribution of otherphases agrees with textural observation (eg Fig 4cshowing Dim growing on resorbed Opxm) as well as withthe conclusions of Ban et al (2005) who stated that moreSi-rich glass (ie 454wt) necessitates orthopyroxenecontributing to the meltTrace element patterns of the bulk glassthornmicro-

phenocryst pockets calculated using the measured traceelement abundances of glass and micro-phenocrysts andthe respective mass-balance parameters (Table 8) arebroadly identical to the measured amphibole composition(Fig 18a and b) Higher LILE (Cs Rb Ba) concentrationsin the calculated bulk melt patches in sample Ke 785 canbe balanced by adding 7wt of phlogopite (Fig 18b)which is in line with mass balance (Table 8) The dominantcontrol of amphibole to account for the trace element

budget of the melt pockets is similar to results found inother suites of glass-bearing xenoliths (Chazot et al1996a Yaxley amp Kamenetsky 1999 Laurora et al 2001)Further the obtained phase proportions for the melt pock-ets are in good agreement with the results from image ana-lysis (Table 1) In addition they are similar to the glass^cpx^ol proportions recorded in recent experiments on the

Table 8 Results of mass-balance calculations to test melt

pocket formation by incongruent melting of the early metaso-

matic assemblages

Sample 195915 785 785 196525 196525 19653

Reactant

amph 1000 1000 0820 1000 0944 mdash

opx mdash mdash 0078 mdash 0056 0406

phl mdash mdash 0102 mdash mdash mdash

glass mdash mdash mdash mdash mdash 05941

SiO2 4484 4989 4964 5129 5167 5898

TiO2 014 039 036 025 024 018

Cr2O3 232 291 258 127 121 009

Al2O3 1368 762 769 800 757 961

FeO(tot) 336 314 333 343 357 394

MnO 006 008 008 012 012 010

NiO 009 013 014 011 011 004

MgO 1844 2029 2197 2040 2122 1625

CaO 854 783 644 966 913 078

Na2O 460 500 436 504 476 843

K2O 049 032 096 042 040 060

Product

glass 0389 0322 0335 0292 0284 07032

ol 0243 0259 0315 0266 0286 0273

cpx 0288 0373 0308 0442 0431 0024

spl 0052 0032 0028 mdash mdash mdash

carb 0028 0014 0014 mdash mdash mdash

SiO2 4477 5027 4986 5207 5181 5903

TiO2 013 041 039 015 014 017

Cr2O3 213 314 272 109 106 015

Al2O3 1364 764 770 798 776 957

FeO(tot) 374 331 351 322 332 350

MnO 007 007 007 009 009 006

NiO 011 013 015 011 012 007

MgO 1834 2024 2196 2063 2141 1635

CaO 854 759 632 952 929 080

Na2O 449 385 388 499 485 860

K2O 060 096 100 015 014 091

r2 023 200 035 084 028 033

Prefix Ke omitted1Low-Si glass 2High-Si glass (see text for explanation)

28

melting behaviour of amphibole in wehrlites at tempera-tures between 1175 and 12508C and pressures of05^10GPa (Mecurren dard et al 2006) These temperatures arein good agreement with the estimates for the formation ofolivine and chromite micro-phenocrysts in the melt

pockets (Fig 16) The heat supply required for amphibolemelting in the formerly low-T metasomatized Group IIIxenoliths (T 200^3008C) may come from local heatingshortly before or during xenolith entrainment in the hostlava or from fluid infiltration that triggered melting (seediscussion below)Finally formation of the most Si-enriched melt (adjacent

to Opxm in Group III-c samples) can be modelled assum-ing progressing interaction between infiltrating melt andorthopyroxene where the latter melts incongruently

opxthornmelt1 olthornmelt2 cpx ethseeTable 8THORN eth1THORN

To calculate reaction (1) we chose the least evolved glassobserved in melt patches in sample Ke 19653 (SiO2

5921wt Mg-number 6465) to represent the initialmelt1 whereas melt2 adjacent to opx is SiO2-rich(6530wt) Trace element abundances in glass next toorthopyroxene are shifted to lower concentrations(Table 6) which is in agreement with dilution caused byaddition of a trace element-poor orthopyroxene compo-nent The resultant phase proportions are similar to thoseobserved at orthopyroxene^melt interfaces (ie higholivine and low cpx abundances) and are in agreementwith experimental studies (Shaw et al 1998)

Partial melting triggered by fluid fluxMass-balance calculations provide an explanation for meltpocket formation by in situ partial melting Thus onemight conclude that melting is simply the result of decom-pression and heating during xenolith transport in the hostmagma However there are two points that argue thatmelting was triggered by the infiltration of a fluid phaseshortly before the xenoliths were sampled by the hostmagma (1) the presence of occasionally high amounts ofcarbonate included in the melt pockets (2) the extremelyNa-rich clinopyroxene (Dim) compositions either includedas micro-phenocrysts in the melt pocket or present asequilibrated grains around chromite in the single TypeIII-d xenolith (Fig 3f) Both arguments will be discussedin the next section

Formation of carbonates

Textures and compositions of carbonate globules can beexplained by carbonate precipitation from a CO2-bearingfluid As demonstrated by several studies highCa(CathornMg) values (Table 7) and low REE concentra-tions are strong arguments that the carbonates are cumu-late crystals precipitated from a CO2-rich melt or afluid and not quenched carbonated melts (Lee et al 2000Ionov amp Harmer 2002) Oscillatory zoning (Fig 5a and b)of individual carbonate crystals within singleglobules corroborates this interpretation The roundedcontacts toward glass (globules) are also in line with thesefindings Similar globular forms were reported fromexperimental studies and do not a priori imply the

Fig 18 Calculated trace element characteristics of reactants (in mostcases amphibolethorn orthopyroxene filled symbols) and products(melt pockets open symbols) of the lsquoamphibole meltingrsquo model[see equation (1) in text] The relative phase proportions wereobtained by mass-balance calculations and are given in Table 8 Thedashed lines in (b) show that the high LILE concentrations obtainedfor the product (bulk melt pocket) can be explained by adding about7wt of phlogopite to the reactant (amphibolethorn orthopyroxeneTable 8) Further explanation is given in the text

29

presence of immiscible carbonate and silicate melts(eg Lee amp Wyllie 1996)The high amount of carbonate in some melt pockets

however (eg Fig 4e) does need further explanation asthe incongruent melting reaction (1) does not account forthe CO2 required for carbonate precipitation duringamphibole melting Therefore amphibole breakdowneither occurred in an open system with addition of at leastCO2 (eg Laurora et al 2001 Ban et al 2005) or anotherpre-existing phase acted as a source of carbon Oxidationof graphite texturally identified as a part of the earlyassemblage (eg Fig 3d) could account for some of theCO2 necessary for carbonate formation A change in fO2

conditions is also indicated when the composition of glass-related spinel (Chrm) and Olm are used for oxygen baro-metry which yields oxygen fugacities well on the oxidizedside of the carbon^carbonate^silicate buffer (Fig 16)However increasing fO2 triggered by closed-systemdecompression-induced melting alone does not appear fea-sible Also textures such as straight contacts between gra-phite and Mg-calcite (Fig 4c) suggest that carbonategrows on graphite (ie indicating local fO2 conditions cor-responding to carbon^carbonate equilibrium) rather thanreplacing it We therefore interpret melt pocket formationto be triggered by the infiltration of a CO2-rich fluid(ie flux melting see also Ionov et al 1994) This interpre-tation also accounts for the presence of abundant vugs inthe glass that could have contained a fluid (Ionov et al1994 2006) which was presumably lost during sample pre-paration However we emphasize here that late fluid infil-tration had only subtle effects on the trace elementcomposition of Type III-b and -c xenoliths

Formation of Na-rich Cr diopside

Infiltration of CO2-rich fluidwould also explain the Na-richcomposition of Dim micro-phenocryst cores (Fig 6a and b)Ionov et al (2005) noted recently that clinopyroxenes inmeltpockets are commonly characterized by Na2O contents51wt mainly as a result of low cpxmelt partition coeffi-cients at low pressures (eg Blundy et al 1995) Thus veryhighNa2Ocontents in Dimmicro-phenocrysts (Fig6a) sug-gest replacement of amphibole by Dim and eventually Olmand Chrm during reaction with a fluid at subsolidus condi-tions An origin of micro-phenocrysts by replacementrather than crystallization from a melt is indicated by theidentical crystallographic orientation of individual Dimcrystals in singlemelt pockets (Fig4a)Once the solidus of the precursor phase(s) was reached

(ie 10508C for amphibole Niida amp Green 1999) silicatemelt was generated in situ and led to the formation of clin-opyroxene rims with lower Na contents (Fig 6b) and thedevelopment of euhedral shapes in contact with melt (egFig 4a and e) Eventual immiscibility between remainingfluid and newly formed melt (eg during xenolith ascent)would then explain the presence of carbonate globules and

vugs enclosed in glass Likewise different origins of meltand carbonate were recently proposed for similar meltpockets from Hungarian xenoliths by Demecurren ny et al(2004) who found oxygen isotope disequilibrium betweencarbonate globules and the host glass Such a two-stepmodel (fluid-induced replacement followed by melt gen-eration) can be explained by CO2-fluid infiltration occur-ring within the mantle at sub-solidus conditions ultimatelyfollowed by xenolith entrainment in the host magma andtransport to the surfaceThe host magma possibly suppliedthe additional heat necessary for melt generationFast decompression would further enhance melting andcould also account for the lower Na contents in Dimmicro-phenocryst rims as a result of decreasing cpxmeltpartition coefficients at decreasing pressures (Blundyet al 1995)Rapid replacement of amphibole ( phlogopite) by an

anhydrous phase assemblage indicates that the fluid waswater-poor (eg low H2OCO2 ratio see Ionov et al2006)This is corroborated by the fact that Mg-katophorite(presumably OH-rich see above) is occasionally rimmedby a residual kaersutitic selvage (Fig 4d) Such Ti-richmantle amphiboles are commonly OH-poor as a result ofpartial dehydrogenation (eg Tiepolo et al 2001 and refer-ences therein)Finally the presence of theType III-d xenolith contain-

ing texturally equilibrated clinopyroxene of almost identi-cal composition to Dim micro-phenocrysts in melt pockets(Table 2 Figs 6a and 7b) further strengthens the argu-ment that CO2-fluid percolation started in the mantlebefore entrainment (ie enough time elapsed to allow thisrock to anneal completely after the late metasomaticassemblage had formed) A similar conclusion can bedrawn for the clearly host-related glass found in xenolithKe 195927 (eg Fig 13b) ThisType III-c sample is charac-terized by the presence of Dim micro-phenocrysts partlyovergrown by mineral rims clearly related to the host basa-nite magma (kaersutite Ti-augite and olivine withFo885 Fig 4f Tables 3 and 4 and supplementary dataat httppetrologyoxfordjournalsorg) Kaersutite hasmajor and trace element characteristics similar to typicalbasanitic and alkali basaltic amphibole phenocrysts(Fig 9d) The special phase assemblages in this xenolithindicates that host-related basanitic melts intruded someType III-c xenoliths and partly overprinted the assemblageformed slightly earlier by in situ derived alkali-rich peralu-minous melts fully preserved in other samples (eg Ke19653 or Ke 785)

ORIGIN OF METASOMATIZ INGAGENTS T IM ING AND TECTONICSETT INGAs noted above late metasomatism was probably short-lived started at mantle depths and probably continued

30

Fig 12 Total alkalis vs SiO2 (a) Harker-type co-variation diagrams (b^e) and incompatible trace element co-variation diagram (f) showingthe compositional variability of silicate glass found in the porphyroclastic spinel harzburgites and dunites from Marsabit In (b) binary mixinglines between the typical composition of the alkali-rich glass (large open diamond) and the associated microlites are shown (each increment is10wt)They illustrate that the glass does not represent the derivate of host lava infiltration and fractional crystallization Subalkaline glass inGroup I (grt)^spl lherzolites from Marsabit is shown for comparison and is interpreted to derive from incongruent melting of Ti-pargasite(Kaeser 2006)

23

found as a reaction product between anhydrous mafic toultramafic rocks and H2O-rich evolved liquids (Costaet al 2001 and references therein) or H2O-bearing fluids(eg Odling 1994) The latter study was an experiment at20GPa simulating the effects of decreasing temperatureson the phase assemblages and their composition duringmetasomatism using a model peridotitethornC^O^H fluidThe coolest part of Odlingrsquos (1994) experiment (510758C)contained a strikingly similar phase assemblage (kato-phorite phlogopite with 2wt Na2O and Al-poororthopyroxene) to the Marsabit samples interpreted tohave formed during sub-solidus fluid^solid interactionA compositionally similar phase assemblage (highamounts of opx Si-rich hornblende Na-rich phlogopite)resulted from experiments simulating peridotite interac-tion with Si-rich melt (dacite) at lower P^T conditions(9508C at 092GPa Prouteau amp Scaillet 2003)In this study the trace element signatures of Group III-c

Mg-katophorite and the calculated metasomatic agentsfurther suggest that the metasomatizing agent was a meltrather than a fluid Hypothetical fluids in equilibrium witheuhedral Mg-katophorite from sample Ke 196525 havehigh NbLILE (eg Rb Ba) and low LILEREE ratios(Fig 17) Because experimental data predict the oppositefor fluids (LILE enriched over HFSE and REE egManning 2004 and references therein) a fluid with asignature such as that indicated in Fig 17 is unrealistic

Fig 13 Trace element patterns of silicate glass in porphyroclasticspinel harzburgites and dunites from Marsabit compared withthe trace element composition of associated clinopyroxene (Dim)micro-phenocrysts xenolith-hosting basanites from Marsabit andglass interpreted to represent xenolith^host interaction [concentra-tions normalized to the primitive mantle values of McDonough ampSun (1995)]

Table 7 Representative analyses of carbonates (oxides in

wt trace elements in mgg)

Group III-b III-b III-c III-c III-c

Mineral Dol Mg-Cc Mg-Cc Mg-Cc Mg-Cc

Sample 195915 196515 785 19653 19653

Texture in glass in glass in glass in glass in glass

SiO2 na 003 na nd 003

FeO(tot) 016 011 010 010 006

MnO 001 nd nd nd nd

MgO 1922 236 190 125 517

CaO 3189 5313 5377 5391 4853

Na2O 003 nd 004 nd 003

K2O na 002 na 001 nd

SrO 003 010 008 004 021

BaO 003 nd nd na na

CO21 4618 4444 4442 4377 4391

Total 9756 10021 10032 9909 9794

Mg-no 9954 9745 9713 9554 9931

Ca-no 5439 9418 9531 9687 8709

P 220 334 137 580 589

K 970 212 642 105 155

Sc 5065 50150 027 5027 50076

Ti 975 189 242 528 196

V 5085 0481 5042 5037 021

Ni 900 673 517 5294 593

Zn 459 5089 113 296 045

Ga na 5014 5013 na na

Rb 0084 50072 50074 50060 0046

Sr 593 860 371 251 4274

Y 012 011 130 704 015

Zr 261 024 012 097 756

Nb 144 019 0056 692 185

Cs 50063 50011 50018 50039 50008

Ba 126 230 5024 029 271

La 034 057 084 136 025

Ce 032 013 134 997 023

Pr 006 006 018 095 0026

Nd 030 013 068 228 0078

Sm 5044 50091 0074 048 50018

Eu 5012 50019 0051 020 50008

Gd 5037 50090 014 051 0026

Tb na 50010 0036 na na

Dy 5019 50039 0089 058 50038

Ho 50029 0013 0044 011 50006

Er 5012 50041 0083 033 50019

Tm 50052 50013 0016 0084 50006

Yb 5043 50063 014 027 50040

Lu 50072 50010 0011 0066 50008

(continued)

24

At the relatively low maximum pressures (515GPa) asupercritical liquid is not expected either (eg Kesselet al 2005)Finally a relatively hydrous metasomatizing agent

would be consistent with formation of Mg-katophoriteswith lowTi and presumably low Fe3thorn contents (given therelatively reducing conditions) which in turn imply high(OH)^ contents In other words partial dehydrogenationtypically observed in common (Ti-rich) mantle amphibole

(eg Popp et al 1995 Tiepolo et al 2001 and referencestherein) cannot have played an important role in thiscase as H deficiency is mainly the result of coupled substi-tutions involvingTi and Fe3thorn (Popp et al 1995) In conclu-sion the metasomatizing agent was most probably aNa2O^SiO2^CO2^H2O-rich meltComparison with experimental studies further con-

strains the low-temperature regime in which metasoma-tism occurred In ultramafic (eg Niida amp Green 1999)basaltic (eg Rapp 1995) and Si-enriched systems (egtrachybasaltic Barclay amp Carmichael 2004) amphibole(in the latter study together with phlogopite) is confinedto temperatures lower than 1000^10508C at pressures520GPa (Fig 2)Thus crystallization of the early metaso-matic phase assemblage can be explained by infiltration ofa silicate melt that rapidly cooled upon contact with theambient peridotite (750^8508C prior to metasomatismFig 2) However this low-T stage is based on Ca-in-opxthermometry (see Brey amp Kolaquo hler 1990 Kaeser et al2006 Table 1) and therefore probably underestimates theeffective temperature conditions during metasomatismThis can be explained by low diffusion rates and proximityto the closure temperature for the diopside^enstatiteexchange reaction at temperatures lower than 8508C(eg Smith 1999 and references therein) The lower tem-perature limit at which silicate melt metasomatism ispossible (ie defined by the meltrsquos solidus and viscosity)depends strongly on volatile and SiO2 contentsExperiments by Draper amp Green (1997) for exampleshowed that highly silicic melts (52^64wt SiO2) canbe in equilibrium with mantle harzburgite at temperaturesas low as 8508C (at 10^20GPa under anhydrous andC^O^H-fluid-bearing conditions) These are howeverextreme compositions for mantle melts The actual SiO2

content of the metasomatizing agent in this case is difficultto constrain from the metasomatic mineral assemblage ascompositions may have varied considerably during reac-tive percolation (eg including Si-releasing reactions suchas cpx replacement by amphibole versus Si-consumingreactions such as Opxm crystallization) Moreover it isunlikely that a melt persists at temperatures muchbelow 9008C which is the solidus of fertile lherzolite(ie primitive upper mantle composition) underH2O-saturated conditions at 15GPa (Grove et al 2006see Fig 2) Below the solidus residual volatile-enrichedfluids may percolate further for example forming the lessmetasomatized Type III-c xenoliths (eg Ke 785) as wellas Type III-b and -a peridotites (see also Bailey 1987)Relatively cold conditions would also explain the veryhigh compositional gradients developed on small scales inType III-a samples (Fig 15) Given the absence of chloritein all investigated samples the lower temperature limitof such fluid metasomatism is 8008C (eg Fumagalli ampPoli 2005 see Fig 2)

Table 7 Continued

Group III-b III-b III-c III-c III-c

Mineral Dol Mg-Cc Mg-Cc Mg-Cc Mg-Cc

Sample 195915 196515 785 19653 19653

Texture in glass in glass in glass in glass in glass

Hf 5016 50038 50036 50074 50013

Ta 0048 50012 50016 50031 0008

Pb 5024 858 5021 074 018

Th 0021 50020 50017 50028 50005

U 060 094 086 600 148

Mg-Cc Mg-calcite Dol dolomite nd not detectedna not analysed Ca-numberfrac14 100Ca(CathornMg)1Calculated from stoichiometry

Fig 14 Trace element patterns of glass-hosted carbonates in TypeIII-b and -c spinel harzburgite and dunite xenoliths from Marsabit[concentrations normalized to the primitive mantle values ofMcDonough amp Sun (1995)] The grey field outlines the compositionalrange of carbonates found in other peridotite xenoliths (Ionov ampHarmer 2002 and references therein) Carbonate material inter-preted as quenched carbonatite melt found in peridotite xenolithsfrom Kerguelen (Moine et al 2004) is shown for comparison(bold line)

25

Finally the presence of apatite even though oftentaken as indicator for carbonatite-type metasomatism(eg Yaxley et al 1998) is not in conflict with metasoma-tism by silicate melts Apatite has also been shown to crys-tallize from volatile and alkali-rich derivatives formedduring reactive porous flow of originally mafic silicatemelts (Woodland et al 1996 Bedini et al 1997 Bodinieret al 2004 Ionov et al 2006)

FORMATION OF THE LATE GLASS -BEAR ING ASSEMBLAGESTextures strongly suggest that the glass-bearing assem-blages formed later and at the expense of the early metaso-matic assemblages (Fig 4a and d) Features such as thereappearance of clinopyroxene (Na-rich Cr diopsidemicro-phenocrysts) a phase replaced by the early

Fig 15 Detailed REE systematics in primary clinopyroxene (cpx-I) in a porphyroclastic cryptically metasomatized spinel harzburgite(sample Ke 19651) showing the transition fromType III-a (Zone A and B) toType III-b (Zone C) on a scale of a few centimetres The numberson the photomicrographs (crossed polars) correspond to single laser ablation microprobe pits (for further explanation see text)

26

metasomatic assemblage indicate that melt pocket forma-tion reflects a significant change in P^T^X space in partsof the mantle beneath Marsabit after earlier metasoma-tism Based on geochemical arguments and mass-balanceconsiderations we will show that (1) the late glass-bearingassemblages are closely related to early metasomaticassemblages mainly representing breakdown productsand (2) breakdown was induced by infiltration of aCO2-rich fluid in the mantle probably followed andenhanced by heating and decompression melting duringxenolith transport in the host magma

Evidence for in situ partial meltingBSE imaging reveals that the phases becoming unstableare amphibole orthopyroxene and to minor extentsspl-I phlogopite cpx-I and apatite Compositional(inter- and intra-xenolith) heterogeneity of silicate glass istexture controlled and varies as a function of theassociated precursor phases (Fig 12a^f)TiO2 for exampleshows higher concentrations in glass from xenolith Ke196515 containing Ti-enriched amphibole (Fig 12c)whereas K2O contents are highest in phlogopite-rich sam-ples (eg Ke 785 Fig 12d) Elevated P2O5 contents arefound in glass adjacent to apatite (Fig 12e) whichmirrors at the same time the highTh and LREE contents

of apatite (Fig 12f) Significant Si enrichment is found inglass coating orthopyroxene (Fig 12d) supporting incon-gruent melting of orthopyroxene (see below) Even thoughthe melt pockets are connected along grain boundaries thestrong compositional variation (on inter- and intra-xeno-lith scale) suggests that the mobility of the melt was lim-ited or that the melting event was very short-lived (fortiming constraints see below)The co-variation plots in Fig 12 further reveal consider-

able compositional differences between all types of xeno-lith glass and the host basanite These data indicate thatthe glass cannot be the product of xenolith^host lava inter-action during ascent to the surface Also back-mixing ofmicro-phenocrysts into the melt fails to derive the glassfrom a host-related melt (Fig 12b) Glass from xenolithmargins however is compositionally closer to the bulkhost basanite (eg Fig 12b) in line with a direct derivationfrom xenolith^host lava interaction Its more evolvedmajor element composition (eg higher SiO2 lowerMg-number Fig 12b) is probably the result of fractiona-tion of basanite phenocrysts (eg olivine augite andtitanomagnetite Volker 1990) Host lava^xenolith interac-tion is further supported by the trace element signature ofglass at the margin of the xenoliths which is closely similarto that of the Marsabit basanites (Fig 13 Volker 1990)The same characteristics are observed for intra-xenolithglass in the unusual kaersutite-bearing sample Ke 195927

Fig 16 Oxygen fugacities relative to the quartz^fayalite^magnetitebuffer (QFM equation of OrsquoNeill 1987) as a function of temperatureat assumed pressure of 1GPa calculated using the composition ofcoexisting spinel and olivine (ol-I^spl-I pairs and Chrm^Olm pairsrespectively) based on the oxygen barometer of Ballhaus et al (1991)The graphite^CO2 equilibrium was calculated after Frost amp Wood(1998) and the graphite^carbonate^silicate equilibrium was calcu-lated using the equation of Eggler amp Baker (1982) (See text for furtherexplanation)

Fig 17 Calculated metasomatic agents in equilibrium with the euhe-dral Mg-katophorites from the Type III-c amphibole dunite Ke 196525 Amphibolehydrous fluid and amphibolecarbonatite melt parti-tion coefficients are from Adam et al (1997) and Adam amp Green(2001) respectively Silicate melts have been calculated using amphi-boleSi-rich melt partition coefficients from Chazot et al (1996b) andamphibolebasanite partition coefficients from LaTourrette et al(1995) The compositional range of the Marsabit basanites and alkalibasalts (Volker 1990) is shown for comparison

27

Xenolith glass unrelated to the host lava and melt pock-ets similar to those reported here in association with resi-dual phases (eg amphibole or clinopyroxene) has beenreported from many xenolith localities (eg Frey ampGreen 1974 Ionov et al 1994 1996 Chazot et al 1996a1996b Yaxley et al 1997 Laurora et al 2001 Bali et al2002) Residual phases within the melt pockets stronglyindicate in situ partial melting within the xenolithsHowever a variety of processes may trigger such in situ

melting including (1) closed-system decompression melt-ing of either anhydrous (Maale amp Printzlau 1979) orhydrous phases (Frey amp Green 1974 Francis 1976) duringxenolith ascent in the host lava (2) melting during interac-tion of the xenoliths with the host basaltic melt duringxenolith ascent (eg Zinngrebe amp Foley 1995 Shaw ampKlulaquo gel 2002 Shaw et al 2006) (3) melting in the mantleprior to xenolith entrainment induced by an infiltratingfluid (ie flux melting eg Jones et al 1983 Ionov et al1994 Demecurren ny et al 2004) However most studies agreethat that partial melting occurred very shortly beforeandor during xenolith exhumation by the host magmaThus a combination of metasomatically triggered meltingfollowed andor assisted by decompression melting duringxenolith transport may be a realistic alternative in somecases (eg Dawson 2002)Recently Ban et al (2005) have shown in a very detailed

mass-balance study on glass-bearing xenoliths from theWest Eifel that the weight proportions of glass and theassociated micro-phenocrysts match the composition ofthe preserved parent amphibole We used the sameapproach and performed simple mass-balance calculations(least-squares fitting) to constrain the conclusions drawnfrom petrography and to test which phase contributedand to what extent to the formation of the melt pocketsThe results reported in Table 8 show that in most casesamphibole-melting can account successfully for the meltpocket composition and only a minor amount (520) ofother phases (orthopyroxene andor phlogopite) needs tobe added to the reactant Such a contribution of otherphases agrees with textural observation (eg Fig 4cshowing Dim growing on resorbed Opxm) as well as withthe conclusions of Ban et al (2005) who stated that moreSi-rich glass (ie 454wt) necessitates orthopyroxenecontributing to the meltTrace element patterns of the bulk glassthornmicro-

phenocryst pockets calculated using the measured traceelement abundances of glass and micro-phenocrysts andthe respective mass-balance parameters (Table 8) arebroadly identical to the measured amphibole composition(Fig 18a and b) Higher LILE (Cs Rb Ba) concentrationsin the calculated bulk melt patches in sample Ke 785 canbe balanced by adding 7wt of phlogopite (Fig 18b)which is in line with mass balance (Table 8) The dominantcontrol of amphibole to account for the trace element

budget of the melt pockets is similar to results found inother suites of glass-bearing xenoliths (Chazot et al1996a Yaxley amp Kamenetsky 1999 Laurora et al 2001)Further the obtained phase proportions for the melt pock-ets are in good agreement with the results from image ana-lysis (Table 1) In addition they are similar to the glass^cpx^ol proportions recorded in recent experiments on the

Table 8 Results of mass-balance calculations to test melt

pocket formation by incongruent melting of the early metaso-

matic assemblages

Sample 195915 785 785 196525 196525 19653

Reactant

amph 1000 1000 0820 1000 0944 mdash

opx mdash mdash 0078 mdash 0056 0406

phl mdash mdash 0102 mdash mdash mdash

glass mdash mdash mdash mdash mdash 05941

SiO2 4484 4989 4964 5129 5167 5898

TiO2 014 039 036 025 024 018

Cr2O3 232 291 258 127 121 009

Al2O3 1368 762 769 800 757 961

FeO(tot) 336 314 333 343 357 394

MnO 006 008 008 012 012 010

NiO 009 013 014 011 011 004

MgO 1844 2029 2197 2040 2122 1625

CaO 854 783 644 966 913 078

Na2O 460 500 436 504 476 843

K2O 049 032 096 042 040 060

Product

glass 0389 0322 0335 0292 0284 07032

ol 0243 0259 0315 0266 0286 0273

cpx 0288 0373 0308 0442 0431 0024

spl 0052 0032 0028 mdash mdash mdash

carb 0028 0014 0014 mdash mdash mdash

SiO2 4477 5027 4986 5207 5181 5903

TiO2 013 041 039 015 014 017

Cr2O3 213 314 272 109 106 015

Al2O3 1364 764 770 798 776 957

FeO(tot) 374 331 351 322 332 350

MnO 007 007 007 009 009 006

NiO 011 013 015 011 012 007

MgO 1834 2024 2196 2063 2141 1635

CaO 854 759 632 952 929 080

Na2O 449 385 388 499 485 860

K2O 060 096 100 015 014 091

r2 023 200 035 084 028 033

Prefix Ke omitted1Low-Si glass 2High-Si glass (see text for explanation)

28

melting behaviour of amphibole in wehrlites at tempera-tures between 1175 and 12508C and pressures of05^10GPa (Mecurren dard et al 2006) These temperatures arein good agreement with the estimates for the formation ofolivine and chromite micro-phenocrysts in the melt

pockets (Fig 16) The heat supply required for amphibolemelting in the formerly low-T metasomatized Group IIIxenoliths (T 200^3008C) may come from local heatingshortly before or during xenolith entrainment in the hostlava or from fluid infiltration that triggered melting (seediscussion below)Finally formation of the most Si-enriched melt (adjacent

to Opxm in Group III-c samples) can be modelled assum-ing progressing interaction between infiltrating melt andorthopyroxene where the latter melts incongruently

opxthornmelt1 olthornmelt2 cpx ethseeTable 8THORN eth1THORN

To calculate reaction (1) we chose the least evolved glassobserved in melt patches in sample Ke 19653 (SiO2

5921wt Mg-number 6465) to represent the initialmelt1 whereas melt2 adjacent to opx is SiO2-rich(6530wt) Trace element abundances in glass next toorthopyroxene are shifted to lower concentrations(Table 6) which is in agreement with dilution caused byaddition of a trace element-poor orthopyroxene compo-nent The resultant phase proportions are similar to thoseobserved at orthopyroxene^melt interfaces (ie higholivine and low cpx abundances) and are in agreementwith experimental studies (Shaw et al 1998)

Partial melting triggered by fluid fluxMass-balance calculations provide an explanation for meltpocket formation by in situ partial melting Thus onemight conclude that melting is simply the result of decom-pression and heating during xenolith transport in the hostmagma However there are two points that argue thatmelting was triggered by the infiltration of a fluid phaseshortly before the xenoliths were sampled by the hostmagma (1) the presence of occasionally high amounts ofcarbonate included in the melt pockets (2) the extremelyNa-rich clinopyroxene (Dim) compositions either includedas micro-phenocrysts in the melt pocket or present asequilibrated grains around chromite in the single TypeIII-d xenolith (Fig 3f) Both arguments will be discussedin the next section

Formation of carbonates

Textures and compositions of carbonate globules can beexplained by carbonate precipitation from a CO2-bearingfluid As demonstrated by several studies highCa(CathornMg) values (Table 7) and low REE concentra-tions are strong arguments that the carbonates are cumu-late crystals precipitated from a CO2-rich melt or afluid and not quenched carbonated melts (Lee et al 2000Ionov amp Harmer 2002) Oscillatory zoning (Fig 5a and b)of individual carbonate crystals within singleglobules corroborates this interpretation The roundedcontacts toward glass (globules) are also in line with thesefindings Similar globular forms were reported fromexperimental studies and do not a priori imply the

Fig 18 Calculated trace element characteristics of reactants (in mostcases amphibolethorn orthopyroxene filled symbols) and products(melt pockets open symbols) of the lsquoamphibole meltingrsquo model[see equation (1) in text] The relative phase proportions wereobtained by mass-balance calculations and are given in Table 8 Thedashed lines in (b) show that the high LILE concentrations obtainedfor the product (bulk melt pocket) can be explained by adding about7wt of phlogopite to the reactant (amphibolethorn orthopyroxeneTable 8) Further explanation is given in the text

29

presence of immiscible carbonate and silicate melts(eg Lee amp Wyllie 1996)The high amount of carbonate in some melt pockets

however (eg Fig 4e) does need further explanation asthe incongruent melting reaction (1) does not account forthe CO2 required for carbonate precipitation duringamphibole melting Therefore amphibole breakdowneither occurred in an open system with addition of at leastCO2 (eg Laurora et al 2001 Ban et al 2005) or anotherpre-existing phase acted as a source of carbon Oxidationof graphite texturally identified as a part of the earlyassemblage (eg Fig 3d) could account for some of theCO2 necessary for carbonate formation A change in fO2

conditions is also indicated when the composition of glass-related spinel (Chrm) and Olm are used for oxygen baro-metry which yields oxygen fugacities well on the oxidizedside of the carbon^carbonate^silicate buffer (Fig 16)However increasing fO2 triggered by closed-systemdecompression-induced melting alone does not appear fea-sible Also textures such as straight contacts between gra-phite and Mg-calcite (Fig 4c) suggest that carbonategrows on graphite (ie indicating local fO2 conditions cor-responding to carbon^carbonate equilibrium) rather thanreplacing it We therefore interpret melt pocket formationto be triggered by the infiltration of a CO2-rich fluid(ie flux melting see also Ionov et al 1994) This interpre-tation also accounts for the presence of abundant vugs inthe glass that could have contained a fluid (Ionov et al1994 2006) which was presumably lost during sample pre-paration However we emphasize here that late fluid infil-tration had only subtle effects on the trace elementcomposition of Type III-b and -c xenoliths

Formation of Na-rich Cr diopside

Infiltration of CO2-rich fluidwould also explain the Na-richcomposition of Dim micro-phenocryst cores (Fig 6a and b)Ionov et al (2005) noted recently that clinopyroxenes inmeltpockets are commonly characterized by Na2O contents51wt mainly as a result of low cpxmelt partition coeffi-cients at low pressures (eg Blundy et al 1995) Thus veryhighNa2Ocontents in Dimmicro-phenocrysts (Fig6a) sug-gest replacement of amphibole by Dim and eventually Olmand Chrm during reaction with a fluid at subsolidus condi-tions An origin of micro-phenocrysts by replacementrather than crystallization from a melt is indicated by theidentical crystallographic orientation of individual Dimcrystals in singlemelt pockets (Fig4a)Once the solidus of the precursor phase(s) was reached

(ie 10508C for amphibole Niida amp Green 1999) silicatemelt was generated in situ and led to the formation of clin-opyroxene rims with lower Na contents (Fig 6b) and thedevelopment of euhedral shapes in contact with melt (egFig 4a and e) Eventual immiscibility between remainingfluid and newly formed melt (eg during xenolith ascent)would then explain the presence of carbonate globules and

vugs enclosed in glass Likewise different origins of meltand carbonate were recently proposed for similar meltpockets from Hungarian xenoliths by Demecurren ny et al(2004) who found oxygen isotope disequilibrium betweencarbonate globules and the host glass Such a two-stepmodel (fluid-induced replacement followed by melt gen-eration) can be explained by CO2-fluid infiltration occur-ring within the mantle at sub-solidus conditions ultimatelyfollowed by xenolith entrainment in the host magma andtransport to the surfaceThe host magma possibly suppliedthe additional heat necessary for melt generationFast decompression would further enhance melting andcould also account for the lower Na contents in Dimmicro-phenocryst rims as a result of decreasing cpxmeltpartition coefficients at decreasing pressures (Blundyet al 1995)Rapid replacement of amphibole ( phlogopite) by an

anhydrous phase assemblage indicates that the fluid waswater-poor (eg low H2OCO2 ratio see Ionov et al2006)This is corroborated by the fact that Mg-katophorite(presumably OH-rich see above) is occasionally rimmedby a residual kaersutitic selvage (Fig 4d) Such Ti-richmantle amphiboles are commonly OH-poor as a result ofpartial dehydrogenation (eg Tiepolo et al 2001 and refer-ences therein)Finally the presence of theType III-d xenolith contain-

ing texturally equilibrated clinopyroxene of almost identi-cal composition to Dim micro-phenocrysts in melt pockets(Table 2 Figs 6a and 7b) further strengthens the argu-ment that CO2-fluid percolation started in the mantlebefore entrainment (ie enough time elapsed to allow thisrock to anneal completely after the late metasomaticassemblage had formed) A similar conclusion can bedrawn for the clearly host-related glass found in xenolithKe 195927 (eg Fig 13b) ThisType III-c sample is charac-terized by the presence of Dim micro-phenocrysts partlyovergrown by mineral rims clearly related to the host basa-nite magma (kaersutite Ti-augite and olivine withFo885 Fig 4f Tables 3 and 4 and supplementary dataat httppetrologyoxfordjournalsorg) Kaersutite hasmajor and trace element characteristics similar to typicalbasanitic and alkali basaltic amphibole phenocrysts(Fig 9d) The special phase assemblages in this xenolithindicates that host-related basanitic melts intruded someType III-c xenoliths and partly overprinted the assemblageformed slightly earlier by in situ derived alkali-rich peralu-minous melts fully preserved in other samples (eg Ke19653 or Ke 785)

ORIGIN OF METASOMATIZ INGAGENTS T IM ING AND TECTONICSETT INGAs noted above late metasomatism was probably short-lived started at mantle depths and probably continued

30

found as a reaction product between anhydrous mafic toultramafic rocks and H2O-rich evolved liquids (Costaet al 2001 and references therein) or H2O-bearing fluids(eg Odling 1994) The latter study was an experiment at20GPa simulating the effects of decreasing temperatureson the phase assemblages and their composition duringmetasomatism using a model peridotitethornC^O^H fluidThe coolest part of Odlingrsquos (1994) experiment (510758C)contained a strikingly similar phase assemblage (kato-phorite phlogopite with 2wt Na2O and Al-poororthopyroxene) to the Marsabit samples interpreted tohave formed during sub-solidus fluid^solid interactionA compositionally similar phase assemblage (highamounts of opx Si-rich hornblende Na-rich phlogopite)resulted from experiments simulating peridotite interac-tion with Si-rich melt (dacite) at lower P^T conditions(9508C at 092GPa Prouteau amp Scaillet 2003)In this study the trace element signatures of Group III-c

Mg-katophorite and the calculated metasomatic agentsfurther suggest that the metasomatizing agent was a meltrather than a fluid Hypothetical fluids in equilibrium witheuhedral Mg-katophorite from sample Ke 196525 havehigh NbLILE (eg Rb Ba) and low LILEREE ratios(Fig 17) Because experimental data predict the oppositefor fluids (LILE enriched over HFSE and REE egManning 2004 and references therein) a fluid with asignature such as that indicated in Fig 17 is unrealistic

Fig 13 Trace element patterns of silicate glass in porphyroclasticspinel harzburgites and dunites from Marsabit compared withthe trace element composition of associated clinopyroxene (Dim)micro-phenocrysts xenolith-hosting basanites from Marsabit andglass interpreted to represent xenolith^host interaction [concentra-tions normalized to the primitive mantle values of McDonough ampSun (1995)]

Table 7 Representative analyses of carbonates (oxides in

wt trace elements in mgg)

Group III-b III-b III-c III-c III-c

Mineral Dol Mg-Cc Mg-Cc Mg-Cc Mg-Cc

Sample 195915 196515 785 19653 19653

Texture in glass in glass in glass in glass in glass

SiO2 na 003 na nd 003

FeO(tot) 016 011 010 010 006

MnO 001 nd nd nd nd

MgO 1922 236 190 125 517

CaO 3189 5313 5377 5391 4853

Na2O 003 nd 004 nd 003

K2O na 002 na 001 nd

SrO 003 010 008 004 021

BaO 003 nd nd na na

CO21 4618 4444 4442 4377 4391

Total 9756 10021 10032 9909 9794

Mg-no 9954 9745 9713 9554 9931

Ca-no 5439 9418 9531 9687 8709

P 220 334 137 580 589

K 970 212 642 105 155

Sc 5065 50150 027 5027 50076

Ti 975 189 242 528 196

V 5085 0481 5042 5037 021

Ni 900 673 517 5294 593

Zn 459 5089 113 296 045

Ga na 5014 5013 na na

Rb 0084 50072 50074 50060 0046

Sr 593 860 371 251 4274

Y 012 011 130 704 015

Zr 261 024 012 097 756

Nb 144 019 0056 692 185

Cs 50063 50011 50018 50039 50008

Ba 126 230 5024 029 271

La 034 057 084 136 025

Ce 032 013 134 997 023

Pr 006 006 018 095 0026

Nd 030 013 068 228 0078

Sm 5044 50091 0074 048 50018

Eu 5012 50019 0051 020 50008

Gd 5037 50090 014 051 0026

Tb na 50010 0036 na na

Dy 5019 50039 0089 058 50038

Ho 50029 0013 0044 011 50006

Er 5012 50041 0083 033 50019

Tm 50052 50013 0016 0084 50006

Yb 5043 50063 014 027 50040

Lu 50072 50010 0011 0066 50008

(continued)

24

At the relatively low maximum pressures (515GPa) asupercritical liquid is not expected either (eg Kesselet al 2005)Finally a relatively hydrous metasomatizing agent

would be consistent with formation of Mg-katophoriteswith lowTi and presumably low Fe3thorn contents (given therelatively reducing conditions) which in turn imply high(OH)^ contents In other words partial dehydrogenationtypically observed in common (Ti-rich) mantle amphibole

(eg Popp et al 1995 Tiepolo et al 2001 and referencestherein) cannot have played an important role in thiscase as H deficiency is mainly the result of coupled substi-tutions involvingTi and Fe3thorn (Popp et al 1995) In conclu-sion the metasomatizing agent was most probably aNa2O^SiO2^CO2^H2O-rich meltComparison with experimental studies further con-

strains the low-temperature regime in which metasoma-tism occurred In ultramafic (eg Niida amp Green 1999)basaltic (eg Rapp 1995) and Si-enriched systems (egtrachybasaltic Barclay amp Carmichael 2004) amphibole(in the latter study together with phlogopite) is confinedto temperatures lower than 1000^10508C at pressures520GPa (Fig 2)Thus crystallization of the early metaso-matic phase assemblage can be explained by infiltration ofa silicate melt that rapidly cooled upon contact with theambient peridotite (750^8508C prior to metasomatismFig 2) However this low-T stage is based on Ca-in-opxthermometry (see Brey amp Kolaquo hler 1990 Kaeser et al2006 Table 1) and therefore probably underestimates theeffective temperature conditions during metasomatismThis can be explained by low diffusion rates and proximityto the closure temperature for the diopside^enstatiteexchange reaction at temperatures lower than 8508C(eg Smith 1999 and references therein) The lower tem-perature limit at which silicate melt metasomatism ispossible (ie defined by the meltrsquos solidus and viscosity)depends strongly on volatile and SiO2 contentsExperiments by Draper amp Green (1997) for exampleshowed that highly silicic melts (52^64wt SiO2) canbe in equilibrium with mantle harzburgite at temperaturesas low as 8508C (at 10^20GPa under anhydrous andC^O^H-fluid-bearing conditions) These are howeverextreme compositions for mantle melts The actual SiO2

content of the metasomatizing agent in this case is difficultto constrain from the metasomatic mineral assemblage ascompositions may have varied considerably during reac-tive percolation (eg including Si-releasing reactions suchas cpx replacement by amphibole versus Si-consumingreactions such as Opxm crystallization) Moreover it isunlikely that a melt persists at temperatures muchbelow 9008C which is the solidus of fertile lherzolite(ie primitive upper mantle composition) underH2O-saturated conditions at 15GPa (Grove et al 2006see Fig 2) Below the solidus residual volatile-enrichedfluids may percolate further for example forming the lessmetasomatized Type III-c xenoliths (eg Ke 785) as wellas Type III-b and -a peridotites (see also Bailey 1987)Relatively cold conditions would also explain the veryhigh compositional gradients developed on small scales inType III-a samples (Fig 15) Given the absence of chloritein all investigated samples the lower temperature limitof such fluid metasomatism is 8008C (eg Fumagalli ampPoli 2005 see Fig 2)

Table 7 Continued

Group III-b III-b III-c III-c III-c

Mineral Dol Mg-Cc Mg-Cc Mg-Cc Mg-Cc

Sample 195915 196515 785 19653 19653

Texture in glass in glass in glass in glass in glass

Hf 5016 50038 50036 50074 50013

Ta 0048 50012 50016 50031 0008

Pb 5024 858 5021 074 018

Th 0021 50020 50017 50028 50005

U 060 094 086 600 148

Mg-Cc Mg-calcite Dol dolomite nd not detectedna not analysed Ca-numberfrac14 100Ca(CathornMg)1Calculated from stoichiometry

Fig 14 Trace element patterns of glass-hosted carbonates in TypeIII-b and -c spinel harzburgite and dunite xenoliths from Marsabit[concentrations normalized to the primitive mantle values ofMcDonough amp Sun (1995)] The grey field outlines the compositionalrange of carbonates found in other peridotite xenoliths (Ionov ampHarmer 2002 and references therein) Carbonate material inter-preted as quenched carbonatite melt found in peridotite xenolithsfrom Kerguelen (Moine et al 2004) is shown for comparison(bold line)

25

Finally the presence of apatite even though oftentaken as indicator for carbonatite-type metasomatism(eg Yaxley et al 1998) is not in conflict with metasoma-tism by silicate melts Apatite has also been shown to crys-tallize from volatile and alkali-rich derivatives formedduring reactive porous flow of originally mafic silicatemelts (Woodland et al 1996 Bedini et al 1997 Bodinieret al 2004 Ionov et al 2006)

FORMATION OF THE LATE GLASS -BEAR ING ASSEMBLAGESTextures strongly suggest that the glass-bearing assem-blages formed later and at the expense of the early metaso-matic assemblages (Fig 4a and d) Features such as thereappearance of clinopyroxene (Na-rich Cr diopsidemicro-phenocrysts) a phase replaced by the early

Fig 15 Detailed REE systematics in primary clinopyroxene (cpx-I) in a porphyroclastic cryptically metasomatized spinel harzburgite(sample Ke 19651) showing the transition fromType III-a (Zone A and B) toType III-b (Zone C) on a scale of a few centimetres The numberson the photomicrographs (crossed polars) correspond to single laser ablation microprobe pits (for further explanation see text)

26

metasomatic assemblage indicate that melt pocket forma-tion reflects a significant change in P^T^X space in partsof the mantle beneath Marsabit after earlier metasoma-tism Based on geochemical arguments and mass-balanceconsiderations we will show that (1) the late glass-bearingassemblages are closely related to early metasomaticassemblages mainly representing breakdown productsand (2) breakdown was induced by infiltration of aCO2-rich fluid in the mantle probably followed andenhanced by heating and decompression melting duringxenolith transport in the host magma

Evidence for in situ partial meltingBSE imaging reveals that the phases becoming unstableare amphibole orthopyroxene and to minor extentsspl-I phlogopite cpx-I and apatite Compositional(inter- and intra-xenolith) heterogeneity of silicate glass istexture controlled and varies as a function of theassociated precursor phases (Fig 12a^f)TiO2 for exampleshows higher concentrations in glass from xenolith Ke196515 containing Ti-enriched amphibole (Fig 12c)whereas K2O contents are highest in phlogopite-rich sam-ples (eg Ke 785 Fig 12d) Elevated P2O5 contents arefound in glass adjacent to apatite (Fig 12e) whichmirrors at the same time the highTh and LREE contents

of apatite (Fig 12f) Significant Si enrichment is found inglass coating orthopyroxene (Fig 12d) supporting incon-gruent melting of orthopyroxene (see below) Even thoughthe melt pockets are connected along grain boundaries thestrong compositional variation (on inter- and intra-xeno-lith scale) suggests that the mobility of the melt was lim-ited or that the melting event was very short-lived (fortiming constraints see below)The co-variation plots in Fig 12 further reveal consider-

able compositional differences between all types of xeno-lith glass and the host basanite These data indicate thatthe glass cannot be the product of xenolith^host lava inter-action during ascent to the surface Also back-mixing ofmicro-phenocrysts into the melt fails to derive the glassfrom a host-related melt (Fig 12b) Glass from xenolithmargins however is compositionally closer to the bulkhost basanite (eg Fig 12b) in line with a direct derivationfrom xenolith^host lava interaction Its more evolvedmajor element composition (eg higher SiO2 lowerMg-number Fig 12b) is probably the result of fractiona-tion of basanite phenocrysts (eg olivine augite andtitanomagnetite Volker 1990) Host lava^xenolith interac-tion is further supported by the trace element signature ofglass at the margin of the xenoliths which is closely similarto that of the Marsabit basanites (Fig 13 Volker 1990)The same characteristics are observed for intra-xenolithglass in the unusual kaersutite-bearing sample Ke 195927

Fig 16 Oxygen fugacities relative to the quartz^fayalite^magnetitebuffer (QFM equation of OrsquoNeill 1987) as a function of temperatureat assumed pressure of 1GPa calculated using the composition ofcoexisting spinel and olivine (ol-I^spl-I pairs and Chrm^Olm pairsrespectively) based on the oxygen barometer of Ballhaus et al (1991)The graphite^CO2 equilibrium was calculated after Frost amp Wood(1998) and the graphite^carbonate^silicate equilibrium was calcu-lated using the equation of Eggler amp Baker (1982) (See text for furtherexplanation)

Fig 17 Calculated metasomatic agents in equilibrium with the euhe-dral Mg-katophorites from the Type III-c amphibole dunite Ke 196525 Amphibolehydrous fluid and amphibolecarbonatite melt parti-tion coefficients are from Adam et al (1997) and Adam amp Green(2001) respectively Silicate melts have been calculated using amphi-boleSi-rich melt partition coefficients from Chazot et al (1996b) andamphibolebasanite partition coefficients from LaTourrette et al(1995) The compositional range of the Marsabit basanites and alkalibasalts (Volker 1990) is shown for comparison

27

Xenolith glass unrelated to the host lava and melt pock-ets similar to those reported here in association with resi-dual phases (eg amphibole or clinopyroxene) has beenreported from many xenolith localities (eg Frey ampGreen 1974 Ionov et al 1994 1996 Chazot et al 1996a1996b Yaxley et al 1997 Laurora et al 2001 Bali et al2002) Residual phases within the melt pockets stronglyindicate in situ partial melting within the xenolithsHowever a variety of processes may trigger such in situ

melting including (1) closed-system decompression melt-ing of either anhydrous (Maale amp Printzlau 1979) orhydrous phases (Frey amp Green 1974 Francis 1976) duringxenolith ascent in the host lava (2) melting during interac-tion of the xenoliths with the host basaltic melt duringxenolith ascent (eg Zinngrebe amp Foley 1995 Shaw ampKlulaquo gel 2002 Shaw et al 2006) (3) melting in the mantleprior to xenolith entrainment induced by an infiltratingfluid (ie flux melting eg Jones et al 1983 Ionov et al1994 Demecurren ny et al 2004) However most studies agreethat that partial melting occurred very shortly beforeandor during xenolith exhumation by the host magmaThus a combination of metasomatically triggered meltingfollowed andor assisted by decompression melting duringxenolith transport may be a realistic alternative in somecases (eg Dawson 2002)Recently Ban et al (2005) have shown in a very detailed

mass-balance study on glass-bearing xenoliths from theWest Eifel that the weight proportions of glass and theassociated micro-phenocrysts match the composition ofthe preserved parent amphibole We used the sameapproach and performed simple mass-balance calculations(least-squares fitting) to constrain the conclusions drawnfrom petrography and to test which phase contributedand to what extent to the formation of the melt pocketsThe results reported in Table 8 show that in most casesamphibole-melting can account successfully for the meltpocket composition and only a minor amount (520) ofother phases (orthopyroxene andor phlogopite) needs tobe added to the reactant Such a contribution of otherphases agrees with textural observation (eg Fig 4cshowing Dim growing on resorbed Opxm) as well as withthe conclusions of Ban et al (2005) who stated that moreSi-rich glass (ie 454wt) necessitates orthopyroxenecontributing to the meltTrace element patterns of the bulk glassthornmicro-

phenocryst pockets calculated using the measured traceelement abundances of glass and micro-phenocrysts andthe respective mass-balance parameters (Table 8) arebroadly identical to the measured amphibole composition(Fig 18a and b) Higher LILE (Cs Rb Ba) concentrationsin the calculated bulk melt patches in sample Ke 785 canbe balanced by adding 7wt of phlogopite (Fig 18b)which is in line with mass balance (Table 8) The dominantcontrol of amphibole to account for the trace element

budget of the melt pockets is similar to results found inother suites of glass-bearing xenoliths (Chazot et al1996a Yaxley amp Kamenetsky 1999 Laurora et al 2001)Further the obtained phase proportions for the melt pock-ets are in good agreement with the results from image ana-lysis (Table 1) In addition they are similar to the glass^cpx^ol proportions recorded in recent experiments on the

Table 8 Results of mass-balance calculations to test melt

pocket formation by incongruent melting of the early metaso-

matic assemblages

Sample 195915 785 785 196525 196525 19653

Reactant

amph 1000 1000 0820 1000 0944 mdash

opx mdash mdash 0078 mdash 0056 0406

phl mdash mdash 0102 mdash mdash mdash

glass mdash mdash mdash mdash mdash 05941

SiO2 4484 4989 4964 5129 5167 5898

TiO2 014 039 036 025 024 018

Cr2O3 232 291 258 127 121 009

Al2O3 1368 762 769 800 757 961

FeO(tot) 336 314 333 343 357 394

MnO 006 008 008 012 012 010

NiO 009 013 014 011 011 004

MgO 1844 2029 2197 2040 2122 1625

CaO 854 783 644 966 913 078

Na2O 460 500 436 504 476 843

K2O 049 032 096 042 040 060

Product

glass 0389 0322 0335 0292 0284 07032

ol 0243 0259 0315 0266 0286 0273

cpx 0288 0373 0308 0442 0431 0024

spl 0052 0032 0028 mdash mdash mdash

carb 0028 0014 0014 mdash mdash mdash

SiO2 4477 5027 4986 5207 5181 5903

TiO2 013 041 039 015 014 017

Cr2O3 213 314 272 109 106 015

Al2O3 1364 764 770 798 776 957

FeO(tot) 374 331 351 322 332 350

MnO 007 007 007 009 009 006

NiO 011 013 015 011 012 007

MgO 1834 2024 2196 2063 2141 1635

CaO 854 759 632 952 929 080

Na2O 449 385 388 499 485 860

K2O 060 096 100 015 014 091

r2 023 200 035 084 028 033

Prefix Ke omitted1Low-Si glass 2High-Si glass (see text for explanation)

28

melting behaviour of amphibole in wehrlites at tempera-tures between 1175 and 12508C and pressures of05^10GPa (Mecurren dard et al 2006) These temperatures arein good agreement with the estimates for the formation ofolivine and chromite micro-phenocrysts in the melt

pockets (Fig 16) The heat supply required for amphibolemelting in the formerly low-T metasomatized Group IIIxenoliths (T 200^3008C) may come from local heatingshortly before or during xenolith entrainment in the hostlava or from fluid infiltration that triggered melting (seediscussion below)Finally formation of the most Si-enriched melt (adjacent

to Opxm in Group III-c samples) can be modelled assum-ing progressing interaction between infiltrating melt andorthopyroxene where the latter melts incongruently

opxthornmelt1 olthornmelt2 cpx ethseeTable 8THORN eth1THORN

To calculate reaction (1) we chose the least evolved glassobserved in melt patches in sample Ke 19653 (SiO2

5921wt Mg-number 6465) to represent the initialmelt1 whereas melt2 adjacent to opx is SiO2-rich(6530wt) Trace element abundances in glass next toorthopyroxene are shifted to lower concentrations(Table 6) which is in agreement with dilution caused byaddition of a trace element-poor orthopyroxene compo-nent The resultant phase proportions are similar to thoseobserved at orthopyroxene^melt interfaces (ie higholivine and low cpx abundances) and are in agreementwith experimental studies (Shaw et al 1998)

Partial melting triggered by fluid fluxMass-balance calculations provide an explanation for meltpocket formation by in situ partial melting Thus onemight conclude that melting is simply the result of decom-pression and heating during xenolith transport in the hostmagma However there are two points that argue thatmelting was triggered by the infiltration of a fluid phaseshortly before the xenoliths were sampled by the hostmagma (1) the presence of occasionally high amounts ofcarbonate included in the melt pockets (2) the extremelyNa-rich clinopyroxene (Dim) compositions either includedas micro-phenocrysts in the melt pocket or present asequilibrated grains around chromite in the single TypeIII-d xenolith (Fig 3f) Both arguments will be discussedin the next section

Formation of carbonates

Textures and compositions of carbonate globules can beexplained by carbonate precipitation from a CO2-bearingfluid As demonstrated by several studies highCa(CathornMg) values (Table 7) and low REE concentra-tions are strong arguments that the carbonates are cumu-late crystals precipitated from a CO2-rich melt or afluid and not quenched carbonated melts (Lee et al 2000Ionov amp Harmer 2002) Oscillatory zoning (Fig 5a and b)of individual carbonate crystals within singleglobules corroborates this interpretation The roundedcontacts toward glass (globules) are also in line with thesefindings Similar globular forms were reported fromexperimental studies and do not a priori imply the

Fig 18 Calculated trace element characteristics of reactants (in mostcases amphibolethorn orthopyroxene filled symbols) and products(melt pockets open symbols) of the lsquoamphibole meltingrsquo model[see equation (1) in text] The relative phase proportions wereobtained by mass-balance calculations and are given in Table 8 Thedashed lines in (b) show that the high LILE concentrations obtainedfor the product (bulk melt pocket) can be explained by adding about7wt of phlogopite to the reactant (amphibolethorn orthopyroxeneTable 8) Further explanation is given in the text

29

presence of immiscible carbonate and silicate melts(eg Lee amp Wyllie 1996)The high amount of carbonate in some melt pockets

however (eg Fig 4e) does need further explanation asthe incongruent melting reaction (1) does not account forthe CO2 required for carbonate precipitation duringamphibole melting Therefore amphibole breakdowneither occurred in an open system with addition of at leastCO2 (eg Laurora et al 2001 Ban et al 2005) or anotherpre-existing phase acted as a source of carbon Oxidationof graphite texturally identified as a part of the earlyassemblage (eg Fig 3d) could account for some of theCO2 necessary for carbonate formation A change in fO2

conditions is also indicated when the composition of glass-related spinel (Chrm) and Olm are used for oxygen baro-metry which yields oxygen fugacities well on the oxidizedside of the carbon^carbonate^silicate buffer (Fig 16)However increasing fO2 triggered by closed-systemdecompression-induced melting alone does not appear fea-sible Also textures such as straight contacts between gra-phite and Mg-calcite (Fig 4c) suggest that carbonategrows on graphite (ie indicating local fO2 conditions cor-responding to carbon^carbonate equilibrium) rather thanreplacing it We therefore interpret melt pocket formationto be triggered by the infiltration of a CO2-rich fluid(ie flux melting see also Ionov et al 1994) This interpre-tation also accounts for the presence of abundant vugs inthe glass that could have contained a fluid (Ionov et al1994 2006) which was presumably lost during sample pre-paration However we emphasize here that late fluid infil-tration had only subtle effects on the trace elementcomposition of Type III-b and -c xenoliths

Formation of Na-rich Cr diopside

Infiltration of CO2-rich fluidwould also explain the Na-richcomposition of Dim micro-phenocryst cores (Fig 6a and b)Ionov et al (2005) noted recently that clinopyroxenes inmeltpockets are commonly characterized by Na2O contents51wt mainly as a result of low cpxmelt partition coeffi-cients at low pressures (eg Blundy et al 1995) Thus veryhighNa2Ocontents in Dimmicro-phenocrysts (Fig6a) sug-gest replacement of amphibole by Dim and eventually Olmand Chrm during reaction with a fluid at subsolidus condi-tions An origin of micro-phenocrysts by replacementrather than crystallization from a melt is indicated by theidentical crystallographic orientation of individual Dimcrystals in singlemelt pockets (Fig4a)Once the solidus of the precursor phase(s) was reached

(ie 10508C for amphibole Niida amp Green 1999) silicatemelt was generated in situ and led to the formation of clin-opyroxene rims with lower Na contents (Fig 6b) and thedevelopment of euhedral shapes in contact with melt (egFig 4a and e) Eventual immiscibility between remainingfluid and newly formed melt (eg during xenolith ascent)would then explain the presence of carbonate globules and

vugs enclosed in glass Likewise different origins of meltand carbonate were recently proposed for similar meltpockets from Hungarian xenoliths by Demecurren ny et al(2004) who found oxygen isotope disequilibrium betweencarbonate globules and the host glass Such a two-stepmodel (fluid-induced replacement followed by melt gen-eration) can be explained by CO2-fluid infiltration occur-ring within the mantle at sub-solidus conditions ultimatelyfollowed by xenolith entrainment in the host magma andtransport to the surfaceThe host magma possibly suppliedthe additional heat necessary for melt generationFast decompression would further enhance melting andcould also account for the lower Na contents in Dimmicro-phenocryst rims as a result of decreasing cpxmeltpartition coefficients at decreasing pressures (Blundyet al 1995)Rapid replacement of amphibole ( phlogopite) by an

anhydrous phase assemblage indicates that the fluid waswater-poor (eg low H2OCO2 ratio see Ionov et al2006)This is corroborated by the fact that Mg-katophorite(presumably OH-rich see above) is occasionally rimmedby a residual kaersutitic selvage (Fig 4d) Such Ti-richmantle amphiboles are commonly OH-poor as a result ofpartial dehydrogenation (eg Tiepolo et al 2001 and refer-ences therein)Finally the presence of theType III-d xenolith contain-

ing texturally equilibrated clinopyroxene of almost identi-cal composition to Dim micro-phenocrysts in melt pockets(Table 2 Figs 6a and 7b) further strengthens the argu-ment that CO2-fluid percolation started in the mantlebefore entrainment (ie enough time elapsed to allow thisrock to anneal completely after the late metasomaticassemblage had formed) A similar conclusion can bedrawn for the clearly host-related glass found in xenolithKe 195927 (eg Fig 13b) ThisType III-c sample is charac-terized by the presence of Dim micro-phenocrysts partlyovergrown by mineral rims clearly related to the host basa-nite magma (kaersutite Ti-augite and olivine withFo885 Fig 4f Tables 3 and 4 and supplementary dataat httppetrologyoxfordjournalsorg) Kaersutite hasmajor and trace element characteristics similar to typicalbasanitic and alkali basaltic amphibole phenocrysts(Fig 9d) The special phase assemblages in this xenolithindicates that host-related basanitic melts intruded someType III-c xenoliths and partly overprinted the assemblageformed slightly earlier by in situ derived alkali-rich peralu-minous melts fully preserved in other samples (eg Ke19653 or Ke 785)

ORIGIN OF METASOMATIZ INGAGENTS T IM ING AND TECTONICSETT INGAs noted above late metasomatism was probably short-lived started at mantle depths and probably continued

30

At the relatively low maximum pressures (515GPa) asupercritical liquid is not expected either (eg Kesselet al 2005)Finally a relatively hydrous metasomatizing agent

would be consistent with formation of Mg-katophoriteswith lowTi and presumably low Fe3thorn contents (given therelatively reducing conditions) which in turn imply high(OH)^ contents In other words partial dehydrogenationtypically observed in common (Ti-rich) mantle amphibole

(eg Popp et al 1995 Tiepolo et al 2001 and referencestherein) cannot have played an important role in thiscase as H deficiency is mainly the result of coupled substi-tutions involvingTi and Fe3thorn (Popp et al 1995) In conclu-sion the metasomatizing agent was most probably aNa2O^SiO2^CO2^H2O-rich meltComparison with experimental studies further con-

strains the low-temperature regime in which metasoma-tism occurred In ultramafic (eg Niida amp Green 1999)basaltic (eg Rapp 1995) and Si-enriched systems (egtrachybasaltic Barclay amp Carmichael 2004) amphibole(in the latter study together with phlogopite) is confinedto temperatures lower than 1000^10508C at pressures520GPa (Fig 2)Thus crystallization of the early metaso-matic phase assemblage can be explained by infiltration ofa silicate melt that rapidly cooled upon contact with theambient peridotite (750^8508C prior to metasomatismFig 2) However this low-T stage is based on Ca-in-opxthermometry (see Brey amp Kolaquo hler 1990 Kaeser et al2006 Table 1) and therefore probably underestimates theeffective temperature conditions during metasomatismThis can be explained by low diffusion rates and proximityto the closure temperature for the diopside^enstatiteexchange reaction at temperatures lower than 8508C(eg Smith 1999 and references therein) The lower tem-perature limit at which silicate melt metasomatism ispossible (ie defined by the meltrsquos solidus and viscosity)depends strongly on volatile and SiO2 contentsExperiments by Draper amp Green (1997) for exampleshowed that highly silicic melts (52^64wt SiO2) canbe in equilibrium with mantle harzburgite at temperaturesas low as 8508C (at 10^20GPa under anhydrous andC^O^H-fluid-bearing conditions) These are howeverextreme compositions for mantle melts The actual SiO2

content of the metasomatizing agent in this case is difficultto constrain from the metasomatic mineral assemblage ascompositions may have varied considerably during reac-tive percolation (eg including Si-releasing reactions suchas cpx replacement by amphibole versus Si-consumingreactions such as Opxm crystallization) Moreover it isunlikely that a melt persists at temperatures muchbelow 9008C which is the solidus of fertile lherzolite(ie primitive upper mantle composition) underH2O-saturated conditions at 15GPa (Grove et al 2006see Fig 2) Below the solidus residual volatile-enrichedfluids may percolate further for example forming the lessmetasomatized Type III-c xenoliths (eg Ke 785) as wellas Type III-b and -a peridotites (see also Bailey 1987)Relatively cold conditions would also explain the veryhigh compositional gradients developed on small scales inType III-a samples (Fig 15) Given the absence of chloritein all investigated samples the lower temperature limitof such fluid metasomatism is 8008C (eg Fumagalli ampPoli 2005 see Fig 2)

Table 7 Continued

Group III-b III-b III-c III-c III-c

Mineral Dol Mg-Cc Mg-Cc Mg-Cc Mg-Cc

Sample 195915 196515 785 19653 19653

Texture in glass in glass in glass in glass in glass

Hf 5016 50038 50036 50074 50013

Ta 0048 50012 50016 50031 0008

Pb 5024 858 5021 074 018

Th 0021 50020 50017 50028 50005

U 060 094 086 600 148

Mg-Cc Mg-calcite Dol dolomite nd not detectedna not analysed Ca-numberfrac14 100Ca(CathornMg)1Calculated from stoichiometry

Fig 14 Trace element patterns of glass-hosted carbonates in TypeIII-b and -c spinel harzburgite and dunite xenoliths from Marsabit[concentrations normalized to the primitive mantle values ofMcDonough amp Sun (1995)] The grey field outlines the compositionalrange of carbonates found in other peridotite xenoliths (Ionov ampHarmer 2002 and references therein) Carbonate material inter-preted as quenched carbonatite melt found in peridotite xenolithsfrom Kerguelen (Moine et al 2004) is shown for comparison(bold line)

25

Finally the presence of apatite even though oftentaken as indicator for carbonatite-type metasomatism(eg Yaxley et al 1998) is not in conflict with metasoma-tism by silicate melts Apatite has also been shown to crys-tallize from volatile and alkali-rich derivatives formedduring reactive porous flow of originally mafic silicatemelts (Woodland et al 1996 Bedini et al 1997 Bodinieret al 2004 Ionov et al 2006)

FORMATION OF THE LATE GLASS -BEAR ING ASSEMBLAGESTextures strongly suggest that the glass-bearing assem-blages formed later and at the expense of the early metaso-matic assemblages (Fig 4a and d) Features such as thereappearance of clinopyroxene (Na-rich Cr diopsidemicro-phenocrysts) a phase replaced by the early

Fig 15 Detailed REE systematics in primary clinopyroxene (cpx-I) in a porphyroclastic cryptically metasomatized spinel harzburgite(sample Ke 19651) showing the transition fromType III-a (Zone A and B) toType III-b (Zone C) on a scale of a few centimetres The numberson the photomicrographs (crossed polars) correspond to single laser ablation microprobe pits (for further explanation see text)

26

metasomatic assemblage indicate that melt pocket forma-tion reflects a significant change in P^T^X space in partsof the mantle beneath Marsabit after earlier metasoma-tism Based on geochemical arguments and mass-balanceconsiderations we will show that (1) the late glass-bearingassemblages are closely related to early metasomaticassemblages mainly representing breakdown productsand (2) breakdown was induced by infiltration of aCO2-rich fluid in the mantle probably followed andenhanced by heating and decompression melting duringxenolith transport in the host magma

Evidence for in situ partial meltingBSE imaging reveals that the phases becoming unstableare amphibole orthopyroxene and to minor extentsspl-I phlogopite cpx-I and apatite Compositional(inter- and intra-xenolith) heterogeneity of silicate glass istexture controlled and varies as a function of theassociated precursor phases (Fig 12a^f)TiO2 for exampleshows higher concentrations in glass from xenolith Ke196515 containing Ti-enriched amphibole (Fig 12c)whereas K2O contents are highest in phlogopite-rich sam-ples (eg Ke 785 Fig 12d) Elevated P2O5 contents arefound in glass adjacent to apatite (Fig 12e) whichmirrors at the same time the highTh and LREE contents

of apatite (Fig 12f) Significant Si enrichment is found inglass coating orthopyroxene (Fig 12d) supporting incon-gruent melting of orthopyroxene (see below) Even thoughthe melt pockets are connected along grain boundaries thestrong compositional variation (on inter- and intra-xeno-lith scale) suggests that the mobility of the melt was lim-ited or that the melting event was very short-lived (fortiming constraints see below)The co-variation plots in Fig 12 further reveal consider-

able compositional differences between all types of xeno-lith glass and the host basanite These data indicate thatthe glass cannot be the product of xenolith^host lava inter-action during ascent to the surface Also back-mixing ofmicro-phenocrysts into the melt fails to derive the glassfrom a host-related melt (Fig 12b) Glass from xenolithmargins however is compositionally closer to the bulkhost basanite (eg Fig 12b) in line with a direct derivationfrom xenolith^host lava interaction Its more evolvedmajor element composition (eg higher SiO2 lowerMg-number Fig 12b) is probably the result of fractiona-tion of basanite phenocrysts (eg olivine augite andtitanomagnetite Volker 1990) Host lava^xenolith interac-tion is further supported by the trace element signature ofglass at the margin of the xenoliths which is closely similarto that of the Marsabit basanites (Fig 13 Volker 1990)The same characteristics are observed for intra-xenolithglass in the unusual kaersutite-bearing sample Ke 195927

Fig 16 Oxygen fugacities relative to the quartz^fayalite^magnetitebuffer (QFM equation of OrsquoNeill 1987) as a function of temperatureat assumed pressure of 1GPa calculated using the composition ofcoexisting spinel and olivine (ol-I^spl-I pairs and Chrm^Olm pairsrespectively) based on the oxygen barometer of Ballhaus et al (1991)The graphite^CO2 equilibrium was calculated after Frost amp Wood(1998) and the graphite^carbonate^silicate equilibrium was calcu-lated using the equation of Eggler amp Baker (1982) (See text for furtherexplanation)

Fig 17 Calculated metasomatic agents in equilibrium with the euhe-dral Mg-katophorites from the Type III-c amphibole dunite Ke 196525 Amphibolehydrous fluid and amphibolecarbonatite melt parti-tion coefficients are from Adam et al (1997) and Adam amp Green(2001) respectively Silicate melts have been calculated using amphi-boleSi-rich melt partition coefficients from Chazot et al (1996b) andamphibolebasanite partition coefficients from LaTourrette et al(1995) The compositional range of the Marsabit basanites and alkalibasalts (Volker 1990) is shown for comparison

27

Xenolith glass unrelated to the host lava and melt pock-ets similar to those reported here in association with resi-dual phases (eg amphibole or clinopyroxene) has beenreported from many xenolith localities (eg Frey ampGreen 1974 Ionov et al 1994 1996 Chazot et al 1996a1996b Yaxley et al 1997 Laurora et al 2001 Bali et al2002) Residual phases within the melt pockets stronglyindicate in situ partial melting within the xenolithsHowever a variety of processes may trigger such in situ

melting including (1) closed-system decompression melt-ing of either anhydrous (Maale amp Printzlau 1979) orhydrous phases (Frey amp Green 1974 Francis 1976) duringxenolith ascent in the host lava (2) melting during interac-tion of the xenoliths with the host basaltic melt duringxenolith ascent (eg Zinngrebe amp Foley 1995 Shaw ampKlulaquo gel 2002 Shaw et al 2006) (3) melting in the mantleprior to xenolith entrainment induced by an infiltratingfluid (ie flux melting eg Jones et al 1983 Ionov et al1994 Demecurren ny et al 2004) However most studies agreethat that partial melting occurred very shortly beforeandor during xenolith exhumation by the host magmaThus a combination of metasomatically triggered meltingfollowed andor assisted by decompression melting duringxenolith transport may be a realistic alternative in somecases (eg Dawson 2002)Recently Ban et al (2005) have shown in a very detailed

mass-balance study on glass-bearing xenoliths from theWest Eifel that the weight proportions of glass and theassociated micro-phenocrysts match the composition ofthe preserved parent amphibole We used the sameapproach and performed simple mass-balance calculations(least-squares fitting) to constrain the conclusions drawnfrom petrography and to test which phase contributedand to what extent to the formation of the melt pocketsThe results reported in Table 8 show that in most casesamphibole-melting can account successfully for the meltpocket composition and only a minor amount (520) ofother phases (orthopyroxene andor phlogopite) needs tobe added to the reactant Such a contribution of otherphases agrees with textural observation (eg Fig 4cshowing Dim growing on resorbed Opxm) as well as withthe conclusions of Ban et al (2005) who stated that moreSi-rich glass (ie 454wt) necessitates orthopyroxenecontributing to the meltTrace element patterns of the bulk glassthornmicro-

phenocryst pockets calculated using the measured traceelement abundances of glass and micro-phenocrysts andthe respective mass-balance parameters (Table 8) arebroadly identical to the measured amphibole composition(Fig 18a and b) Higher LILE (Cs Rb Ba) concentrationsin the calculated bulk melt patches in sample Ke 785 canbe balanced by adding 7wt of phlogopite (Fig 18b)which is in line with mass balance (Table 8) The dominantcontrol of amphibole to account for the trace element

budget of the melt pockets is similar to results found inother suites of glass-bearing xenoliths (Chazot et al1996a Yaxley amp Kamenetsky 1999 Laurora et al 2001)Further the obtained phase proportions for the melt pock-ets are in good agreement with the results from image ana-lysis (Table 1) In addition they are similar to the glass^cpx^ol proportions recorded in recent experiments on the

Table 8 Results of mass-balance calculations to test melt

pocket formation by incongruent melting of the early metaso-

matic assemblages

Sample 195915 785 785 196525 196525 19653

Reactant

amph 1000 1000 0820 1000 0944 mdash

opx mdash mdash 0078 mdash 0056 0406

phl mdash mdash 0102 mdash mdash mdash

glass mdash mdash mdash mdash mdash 05941

SiO2 4484 4989 4964 5129 5167 5898

TiO2 014 039 036 025 024 018

Cr2O3 232 291 258 127 121 009

Al2O3 1368 762 769 800 757 961

FeO(tot) 336 314 333 343 357 394

MnO 006 008 008 012 012 010

NiO 009 013 014 011 011 004

MgO 1844 2029 2197 2040 2122 1625

CaO 854 783 644 966 913 078

Na2O 460 500 436 504 476 843

K2O 049 032 096 042 040 060

Product

glass 0389 0322 0335 0292 0284 07032

ol 0243 0259 0315 0266 0286 0273

cpx 0288 0373 0308 0442 0431 0024

spl 0052 0032 0028 mdash mdash mdash

carb 0028 0014 0014 mdash mdash mdash

SiO2 4477 5027 4986 5207 5181 5903

TiO2 013 041 039 015 014 017

Cr2O3 213 314 272 109 106 015

Al2O3 1364 764 770 798 776 957

FeO(tot) 374 331 351 322 332 350

MnO 007 007 007 009 009 006

NiO 011 013 015 011 012 007

MgO 1834 2024 2196 2063 2141 1635

CaO 854 759 632 952 929 080

Na2O 449 385 388 499 485 860

K2O 060 096 100 015 014 091

r2 023 200 035 084 028 033

Prefix Ke omitted1Low-Si glass 2High-Si glass (see text for explanation)

28

melting behaviour of amphibole in wehrlites at tempera-tures between 1175 and 12508C and pressures of05^10GPa (Mecurren dard et al 2006) These temperatures arein good agreement with the estimates for the formation ofolivine and chromite micro-phenocrysts in the melt

pockets (Fig 16) The heat supply required for amphibolemelting in the formerly low-T metasomatized Group IIIxenoliths (T 200^3008C) may come from local heatingshortly before or during xenolith entrainment in the hostlava or from fluid infiltration that triggered melting (seediscussion below)Finally formation of the most Si-enriched melt (adjacent

to Opxm in Group III-c samples) can be modelled assum-ing progressing interaction between infiltrating melt andorthopyroxene where the latter melts incongruently

opxthornmelt1 olthornmelt2 cpx ethseeTable 8THORN eth1THORN

To calculate reaction (1) we chose the least evolved glassobserved in melt patches in sample Ke 19653 (SiO2

5921wt Mg-number 6465) to represent the initialmelt1 whereas melt2 adjacent to opx is SiO2-rich(6530wt) Trace element abundances in glass next toorthopyroxene are shifted to lower concentrations(Table 6) which is in agreement with dilution caused byaddition of a trace element-poor orthopyroxene compo-nent The resultant phase proportions are similar to thoseobserved at orthopyroxene^melt interfaces (ie higholivine and low cpx abundances) and are in agreementwith experimental studies (Shaw et al 1998)

Partial melting triggered by fluid fluxMass-balance calculations provide an explanation for meltpocket formation by in situ partial melting Thus onemight conclude that melting is simply the result of decom-pression and heating during xenolith transport in the hostmagma However there are two points that argue thatmelting was triggered by the infiltration of a fluid phaseshortly before the xenoliths were sampled by the hostmagma (1) the presence of occasionally high amounts ofcarbonate included in the melt pockets (2) the extremelyNa-rich clinopyroxene (Dim) compositions either includedas micro-phenocrysts in the melt pocket or present asequilibrated grains around chromite in the single TypeIII-d xenolith (Fig 3f) Both arguments will be discussedin the next section

Formation of carbonates

Textures and compositions of carbonate globules can beexplained by carbonate precipitation from a CO2-bearingfluid As demonstrated by several studies highCa(CathornMg) values (Table 7) and low REE concentra-tions are strong arguments that the carbonates are cumu-late crystals precipitated from a CO2-rich melt or afluid and not quenched carbonated melts (Lee et al 2000Ionov amp Harmer 2002) Oscillatory zoning (Fig 5a and b)of individual carbonate crystals within singleglobules corroborates this interpretation The roundedcontacts toward glass (globules) are also in line with thesefindings Similar globular forms were reported fromexperimental studies and do not a priori imply the

Fig 18 Calculated trace element characteristics of reactants (in mostcases amphibolethorn orthopyroxene filled symbols) and products(melt pockets open symbols) of the lsquoamphibole meltingrsquo model[see equation (1) in text] The relative phase proportions wereobtained by mass-balance calculations and are given in Table 8 Thedashed lines in (b) show that the high LILE concentrations obtainedfor the product (bulk melt pocket) can be explained by adding about7wt of phlogopite to the reactant (amphibolethorn orthopyroxeneTable 8) Further explanation is given in the text

29

presence of immiscible carbonate and silicate melts(eg Lee amp Wyllie 1996)The high amount of carbonate in some melt pockets

however (eg Fig 4e) does need further explanation asthe incongruent melting reaction (1) does not account forthe CO2 required for carbonate precipitation duringamphibole melting Therefore amphibole breakdowneither occurred in an open system with addition of at leastCO2 (eg Laurora et al 2001 Ban et al 2005) or anotherpre-existing phase acted as a source of carbon Oxidationof graphite texturally identified as a part of the earlyassemblage (eg Fig 3d) could account for some of theCO2 necessary for carbonate formation A change in fO2

conditions is also indicated when the composition of glass-related spinel (Chrm) and Olm are used for oxygen baro-metry which yields oxygen fugacities well on the oxidizedside of the carbon^carbonate^silicate buffer (Fig 16)However increasing fO2 triggered by closed-systemdecompression-induced melting alone does not appear fea-sible Also textures such as straight contacts between gra-phite and Mg-calcite (Fig 4c) suggest that carbonategrows on graphite (ie indicating local fO2 conditions cor-responding to carbon^carbonate equilibrium) rather thanreplacing it We therefore interpret melt pocket formationto be triggered by the infiltration of a CO2-rich fluid(ie flux melting see also Ionov et al 1994) This interpre-tation also accounts for the presence of abundant vugs inthe glass that could have contained a fluid (Ionov et al1994 2006) which was presumably lost during sample pre-paration However we emphasize here that late fluid infil-tration had only subtle effects on the trace elementcomposition of Type III-b and -c xenoliths

Formation of Na-rich Cr diopside

Infiltration of CO2-rich fluidwould also explain the Na-richcomposition of Dim micro-phenocryst cores (Fig 6a and b)Ionov et al (2005) noted recently that clinopyroxenes inmeltpockets are commonly characterized by Na2O contents51wt mainly as a result of low cpxmelt partition coeffi-cients at low pressures (eg Blundy et al 1995) Thus veryhighNa2Ocontents in Dimmicro-phenocrysts (Fig6a) sug-gest replacement of amphibole by Dim and eventually Olmand Chrm during reaction with a fluid at subsolidus condi-tions An origin of micro-phenocrysts by replacementrather than crystallization from a melt is indicated by theidentical crystallographic orientation of individual Dimcrystals in singlemelt pockets (Fig4a)Once the solidus of the precursor phase(s) was reached

(ie 10508C for amphibole Niida amp Green 1999) silicatemelt was generated in situ and led to the formation of clin-opyroxene rims with lower Na contents (Fig 6b) and thedevelopment of euhedral shapes in contact with melt (egFig 4a and e) Eventual immiscibility between remainingfluid and newly formed melt (eg during xenolith ascent)would then explain the presence of carbonate globules and

vugs enclosed in glass Likewise different origins of meltand carbonate were recently proposed for similar meltpockets from Hungarian xenoliths by Demecurren ny et al(2004) who found oxygen isotope disequilibrium betweencarbonate globules and the host glass Such a two-stepmodel (fluid-induced replacement followed by melt gen-eration) can be explained by CO2-fluid infiltration occur-ring within the mantle at sub-solidus conditions ultimatelyfollowed by xenolith entrainment in the host magma andtransport to the surfaceThe host magma possibly suppliedthe additional heat necessary for melt generationFast decompression would further enhance melting andcould also account for the lower Na contents in Dimmicro-phenocryst rims as a result of decreasing cpxmeltpartition coefficients at decreasing pressures (Blundyet al 1995)Rapid replacement of amphibole ( phlogopite) by an

anhydrous phase assemblage indicates that the fluid waswater-poor (eg low H2OCO2 ratio see Ionov et al2006)This is corroborated by the fact that Mg-katophorite(presumably OH-rich see above) is occasionally rimmedby a residual kaersutitic selvage (Fig 4d) Such Ti-richmantle amphiboles are commonly OH-poor as a result ofpartial dehydrogenation (eg Tiepolo et al 2001 and refer-ences therein)Finally the presence of theType III-d xenolith contain-

ing texturally equilibrated clinopyroxene of almost identi-cal composition to Dim micro-phenocrysts in melt pockets(Table 2 Figs 6a and 7b) further strengthens the argu-ment that CO2-fluid percolation started in the mantlebefore entrainment (ie enough time elapsed to allow thisrock to anneal completely after the late metasomaticassemblage had formed) A similar conclusion can bedrawn for the clearly host-related glass found in xenolithKe 195927 (eg Fig 13b) ThisType III-c sample is charac-terized by the presence of Dim micro-phenocrysts partlyovergrown by mineral rims clearly related to the host basa-nite magma (kaersutite Ti-augite and olivine withFo885 Fig 4f Tables 3 and 4 and supplementary dataat httppetrologyoxfordjournalsorg) Kaersutite hasmajor and trace element characteristics similar to typicalbasanitic and alkali basaltic amphibole phenocrysts(Fig 9d) The special phase assemblages in this xenolithindicates that host-related basanitic melts intruded someType III-c xenoliths and partly overprinted the assemblageformed slightly earlier by in situ derived alkali-rich peralu-minous melts fully preserved in other samples (eg Ke19653 or Ke 785)

ORIGIN OF METASOMATIZ INGAGENTS T IM ING AND TECTONICSETT INGAs noted above late metasomatism was probably short-lived started at mantle depths and probably continued

30

Finally the presence of apatite even though oftentaken as indicator for carbonatite-type metasomatism(eg Yaxley et al 1998) is not in conflict with metasoma-tism by silicate melts Apatite has also been shown to crys-tallize from volatile and alkali-rich derivatives formedduring reactive porous flow of originally mafic silicatemelts (Woodland et al 1996 Bedini et al 1997 Bodinieret al 2004 Ionov et al 2006)

FORMATION OF THE LATE GLASS -BEAR ING ASSEMBLAGESTextures strongly suggest that the glass-bearing assem-blages formed later and at the expense of the early metaso-matic assemblages (Fig 4a and d) Features such as thereappearance of clinopyroxene (Na-rich Cr diopsidemicro-phenocrysts) a phase replaced by the early

Fig 15 Detailed REE systematics in primary clinopyroxene (cpx-I) in a porphyroclastic cryptically metasomatized spinel harzburgite(sample Ke 19651) showing the transition fromType III-a (Zone A and B) toType III-b (Zone C) on a scale of a few centimetres The numberson the photomicrographs (crossed polars) correspond to single laser ablation microprobe pits (for further explanation see text)

26

metasomatic assemblage indicate that melt pocket forma-tion reflects a significant change in P^T^X space in partsof the mantle beneath Marsabit after earlier metasoma-tism Based on geochemical arguments and mass-balanceconsiderations we will show that (1) the late glass-bearingassemblages are closely related to early metasomaticassemblages mainly representing breakdown productsand (2) breakdown was induced by infiltration of aCO2-rich fluid in the mantle probably followed andenhanced by heating and decompression melting duringxenolith transport in the host magma

Evidence for in situ partial meltingBSE imaging reveals that the phases becoming unstableare amphibole orthopyroxene and to minor extentsspl-I phlogopite cpx-I and apatite Compositional(inter- and intra-xenolith) heterogeneity of silicate glass istexture controlled and varies as a function of theassociated precursor phases (Fig 12a^f)TiO2 for exampleshows higher concentrations in glass from xenolith Ke196515 containing Ti-enriched amphibole (Fig 12c)whereas K2O contents are highest in phlogopite-rich sam-ples (eg Ke 785 Fig 12d) Elevated P2O5 contents arefound in glass adjacent to apatite (Fig 12e) whichmirrors at the same time the highTh and LREE contents

of apatite (Fig 12f) Significant Si enrichment is found inglass coating orthopyroxene (Fig 12d) supporting incon-gruent melting of orthopyroxene (see below) Even thoughthe melt pockets are connected along grain boundaries thestrong compositional variation (on inter- and intra-xeno-lith scale) suggests that the mobility of the melt was lim-ited or that the melting event was very short-lived (fortiming constraints see below)The co-variation plots in Fig 12 further reveal consider-

able compositional differences between all types of xeno-lith glass and the host basanite These data indicate thatthe glass cannot be the product of xenolith^host lava inter-action during ascent to the surface Also back-mixing ofmicro-phenocrysts into the melt fails to derive the glassfrom a host-related melt (Fig 12b) Glass from xenolithmargins however is compositionally closer to the bulkhost basanite (eg Fig 12b) in line with a direct derivationfrom xenolith^host lava interaction Its more evolvedmajor element composition (eg higher SiO2 lowerMg-number Fig 12b) is probably the result of fractiona-tion of basanite phenocrysts (eg olivine augite andtitanomagnetite Volker 1990) Host lava^xenolith interac-tion is further supported by the trace element signature ofglass at the margin of the xenoliths which is closely similarto that of the Marsabit basanites (Fig 13 Volker 1990)The same characteristics are observed for intra-xenolithglass in the unusual kaersutite-bearing sample Ke 195927

Fig 16 Oxygen fugacities relative to the quartz^fayalite^magnetitebuffer (QFM equation of OrsquoNeill 1987) as a function of temperatureat assumed pressure of 1GPa calculated using the composition ofcoexisting spinel and olivine (ol-I^spl-I pairs and Chrm^Olm pairsrespectively) based on the oxygen barometer of Ballhaus et al (1991)The graphite^CO2 equilibrium was calculated after Frost amp Wood(1998) and the graphite^carbonate^silicate equilibrium was calcu-lated using the equation of Eggler amp Baker (1982) (See text for furtherexplanation)

Fig 17 Calculated metasomatic agents in equilibrium with the euhe-dral Mg-katophorites from the Type III-c amphibole dunite Ke 196525 Amphibolehydrous fluid and amphibolecarbonatite melt parti-tion coefficients are from Adam et al (1997) and Adam amp Green(2001) respectively Silicate melts have been calculated using amphi-boleSi-rich melt partition coefficients from Chazot et al (1996b) andamphibolebasanite partition coefficients from LaTourrette et al(1995) The compositional range of the Marsabit basanites and alkalibasalts (Volker 1990) is shown for comparison

27

Xenolith glass unrelated to the host lava and melt pock-ets similar to those reported here in association with resi-dual phases (eg amphibole or clinopyroxene) has beenreported from many xenolith localities (eg Frey ampGreen 1974 Ionov et al 1994 1996 Chazot et al 1996a1996b Yaxley et al 1997 Laurora et al 2001 Bali et al2002) Residual phases within the melt pockets stronglyindicate in situ partial melting within the xenolithsHowever a variety of processes may trigger such in situ

melting including (1) closed-system decompression melt-ing of either anhydrous (Maale amp Printzlau 1979) orhydrous phases (Frey amp Green 1974 Francis 1976) duringxenolith ascent in the host lava (2) melting during interac-tion of the xenoliths with the host basaltic melt duringxenolith ascent (eg Zinngrebe amp Foley 1995 Shaw ampKlulaquo gel 2002 Shaw et al 2006) (3) melting in the mantleprior to xenolith entrainment induced by an infiltratingfluid (ie flux melting eg Jones et al 1983 Ionov et al1994 Demecurren ny et al 2004) However most studies agreethat that partial melting occurred very shortly beforeandor during xenolith exhumation by the host magmaThus a combination of metasomatically triggered meltingfollowed andor assisted by decompression melting duringxenolith transport may be a realistic alternative in somecases (eg Dawson 2002)Recently Ban et al (2005) have shown in a very detailed

mass-balance study on glass-bearing xenoliths from theWest Eifel that the weight proportions of glass and theassociated micro-phenocrysts match the composition ofthe preserved parent amphibole We used the sameapproach and performed simple mass-balance calculations(least-squares fitting) to constrain the conclusions drawnfrom petrography and to test which phase contributedand to what extent to the formation of the melt pocketsThe results reported in Table 8 show that in most casesamphibole-melting can account successfully for the meltpocket composition and only a minor amount (520) ofother phases (orthopyroxene andor phlogopite) needs tobe added to the reactant Such a contribution of otherphases agrees with textural observation (eg Fig 4cshowing Dim growing on resorbed Opxm) as well as withthe conclusions of Ban et al (2005) who stated that moreSi-rich glass (ie 454wt) necessitates orthopyroxenecontributing to the meltTrace element patterns of the bulk glassthornmicro-

phenocryst pockets calculated using the measured traceelement abundances of glass and micro-phenocrysts andthe respective mass-balance parameters (Table 8) arebroadly identical to the measured amphibole composition(Fig 18a and b) Higher LILE (Cs Rb Ba) concentrationsin the calculated bulk melt patches in sample Ke 785 canbe balanced by adding 7wt of phlogopite (Fig 18b)which is in line with mass balance (Table 8) The dominantcontrol of amphibole to account for the trace element

budget of the melt pockets is similar to results found inother suites of glass-bearing xenoliths (Chazot et al1996a Yaxley amp Kamenetsky 1999 Laurora et al 2001)Further the obtained phase proportions for the melt pock-ets are in good agreement with the results from image ana-lysis (Table 1) In addition they are similar to the glass^cpx^ol proportions recorded in recent experiments on the

Table 8 Results of mass-balance calculations to test melt

pocket formation by incongruent melting of the early metaso-

matic assemblages

Sample 195915 785 785 196525 196525 19653

Reactant

amph 1000 1000 0820 1000 0944 mdash

opx mdash mdash 0078 mdash 0056 0406

phl mdash mdash 0102 mdash mdash mdash

glass mdash mdash mdash mdash mdash 05941

SiO2 4484 4989 4964 5129 5167 5898

TiO2 014 039 036 025 024 018

Cr2O3 232 291 258 127 121 009

Al2O3 1368 762 769 800 757 961

FeO(tot) 336 314 333 343 357 394

MnO 006 008 008 012 012 010

NiO 009 013 014 011 011 004

MgO 1844 2029 2197 2040 2122 1625

CaO 854 783 644 966 913 078

Na2O 460 500 436 504 476 843

K2O 049 032 096 042 040 060

Product

glass 0389 0322 0335 0292 0284 07032

ol 0243 0259 0315 0266 0286 0273

cpx 0288 0373 0308 0442 0431 0024

spl 0052 0032 0028 mdash mdash mdash

carb 0028 0014 0014 mdash mdash mdash

SiO2 4477 5027 4986 5207 5181 5903

TiO2 013 041 039 015 014 017

Cr2O3 213 314 272 109 106 015

Al2O3 1364 764 770 798 776 957

FeO(tot) 374 331 351 322 332 350

MnO 007 007 007 009 009 006

NiO 011 013 015 011 012 007

MgO 1834 2024 2196 2063 2141 1635

CaO 854 759 632 952 929 080

Na2O 449 385 388 499 485 860

K2O 060 096 100 015 014 091

r2 023 200 035 084 028 033

Prefix Ke omitted1Low-Si glass 2High-Si glass (see text for explanation)

28

melting behaviour of amphibole in wehrlites at tempera-tures between 1175 and 12508C and pressures of05^10GPa (Mecurren dard et al 2006) These temperatures arein good agreement with the estimates for the formation ofolivine and chromite micro-phenocrysts in the melt

pockets (Fig 16) The heat supply required for amphibolemelting in the formerly low-T metasomatized Group IIIxenoliths (T 200^3008C) may come from local heatingshortly before or during xenolith entrainment in the hostlava or from fluid infiltration that triggered melting (seediscussion below)Finally formation of the most Si-enriched melt (adjacent

to Opxm in Group III-c samples) can be modelled assum-ing progressing interaction between infiltrating melt andorthopyroxene where the latter melts incongruently

opxthornmelt1 olthornmelt2 cpx ethseeTable 8THORN eth1THORN

To calculate reaction (1) we chose the least evolved glassobserved in melt patches in sample Ke 19653 (SiO2

5921wt Mg-number 6465) to represent the initialmelt1 whereas melt2 adjacent to opx is SiO2-rich(6530wt) Trace element abundances in glass next toorthopyroxene are shifted to lower concentrations(Table 6) which is in agreement with dilution caused byaddition of a trace element-poor orthopyroxene compo-nent The resultant phase proportions are similar to thoseobserved at orthopyroxene^melt interfaces (ie higholivine and low cpx abundances) and are in agreementwith experimental studies (Shaw et al 1998)

Partial melting triggered by fluid fluxMass-balance calculations provide an explanation for meltpocket formation by in situ partial melting Thus onemight conclude that melting is simply the result of decom-pression and heating during xenolith transport in the hostmagma However there are two points that argue thatmelting was triggered by the infiltration of a fluid phaseshortly before the xenoliths were sampled by the hostmagma (1) the presence of occasionally high amounts ofcarbonate included in the melt pockets (2) the extremelyNa-rich clinopyroxene (Dim) compositions either includedas micro-phenocrysts in the melt pocket or present asequilibrated grains around chromite in the single TypeIII-d xenolith (Fig 3f) Both arguments will be discussedin the next section

Formation of carbonates

Textures and compositions of carbonate globules can beexplained by carbonate precipitation from a CO2-bearingfluid As demonstrated by several studies highCa(CathornMg) values (Table 7) and low REE concentra-tions are strong arguments that the carbonates are cumu-late crystals precipitated from a CO2-rich melt or afluid and not quenched carbonated melts (Lee et al 2000Ionov amp Harmer 2002) Oscillatory zoning (Fig 5a and b)of individual carbonate crystals within singleglobules corroborates this interpretation The roundedcontacts toward glass (globules) are also in line with thesefindings Similar globular forms were reported fromexperimental studies and do not a priori imply the

Fig 18 Calculated trace element characteristics of reactants (in mostcases amphibolethorn orthopyroxene filled symbols) and products(melt pockets open symbols) of the lsquoamphibole meltingrsquo model[see equation (1) in text] The relative phase proportions wereobtained by mass-balance calculations and are given in Table 8 Thedashed lines in (b) show that the high LILE concentrations obtainedfor the product (bulk melt pocket) can be explained by adding about7wt of phlogopite to the reactant (amphibolethorn orthopyroxeneTable 8) Further explanation is given in the text

29

presence of immiscible carbonate and silicate melts(eg Lee amp Wyllie 1996)The high amount of carbonate in some melt pockets

however (eg Fig 4e) does need further explanation asthe incongruent melting reaction (1) does not account forthe CO2 required for carbonate precipitation duringamphibole melting Therefore amphibole breakdowneither occurred in an open system with addition of at leastCO2 (eg Laurora et al 2001 Ban et al 2005) or anotherpre-existing phase acted as a source of carbon Oxidationof graphite texturally identified as a part of the earlyassemblage (eg Fig 3d) could account for some of theCO2 necessary for carbonate formation A change in fO2

conditions is also indicated when the composition of glass-related spinel (Chrm) and Olm are used for oxygen baro-metry which yields oxygen fugacities well on the oxidizedside of the carbon^carbonate^silicate buffer (Fig 16)However increasing fO2 triggered by closed-systemdecompression-induced melting alone does not appear fea-sible Also textures such as straight contacts between gra-phite and Mg-calcite (Fig 4c) suggest that carbonategrows on graphite (ie indicating local fO2 conditions cor-responding to carbon^carbonate equilibrium) rather thanreplacing it We therefore interpret melt pocket formationto be triggered by the infiltration of a CO2-rich fluid(ie flux melting see also Ionov et al 1994) This interpre-tation also accounts for the presence of abundant vugs inthe glass that could have contained a fluid (Ionov et al1994 2006) which was presumably lost during sample pre-paration However we emphasize here that late fluid infil-tration had only subtle effects on the trace elementcomposition of Type III-b and -c xenoliths

Formation of Na-rich Cr diopside

Infiltration of CO2-rich fluidwould also explain the Na-richcomposition of Dim micro-phenocryst cores (Fig 6a and b)Ionov et al (2005) noted recently that clinopyroxenes inmeltpockets are commonly characterized by Na2O contents51wt mainly as a result of low cpxmelt partition coeffi-cients at low pressures (eg Blundy et al 1995) Thus veryhighNa2Ocontents in Dimmicro-phenocrysts (Fig6a) sug-gest replacement of amphibole by Dim and eventually Olmand Chrm during reaction with a fluid at subsolidus condi-tions An origin of micro-phenocrysts by replacementrather than crystallization from a melt is indicated by theidentical crystallographic orientation of individual Dimcrystals in singlemelt pockets (Fig4a)Once the solidus of the precursor phase(s) was reached

(ie 10508C for amphibole Niida amp Green 1999) silicatemelt was generated in situ and led to the formation of clin-opyroxene rims with lower Na contents (Fig 6b) and thedevelopment of euhedral shapes in contact with melt (egFig 4a and e) Eventual immiscibility between remainingfluid and newly formed melt (eg during xenolith ascent)would then explain the presence of carbonate globules and

vugs enclosed in glass Likewise different origins of meltand carbonate were recently proposed for similar meltpockets from Hungarian xenoliths by Demecurren ny et al(2004) who found oxygen isotope disequilibrium betweencarbonate globules and the host glass Such a two-stepmodel (fluid-induced replacement followed by melt gen-eration) can be explained by CO2-fluid infiltration occur-ring within the mantle at sub-solidus conditions ultimatelyfollowed by xenolith entrainment in the host magma andtransport to the surfaceThe host magma possibly suppliedthe additional heat necessary for melt generationFast decompression would further enhance melting andcould also account for the lower Na contents in Dimmicro-phenocryst rims as a result of decreasing cpxmeltpartition coefficients at decreasing pressures (Blundyet al 1995)Rapid replacement of amphibole ( phlogopite) by an

anhydrous phase assemblage indicates that the fluid waswater-poor (eg low H2OCO2 ratio see Ionov et al2006)This is corroborated by the fact that Mg-katophorite(presumably OH-rich see above) is occasionally rimmedby a residual kaersutitic selvage (Fig 4d) Such Ti-richmantle amphiboles are commonly OH-poor as a result ofpartial dehydrogenation (eg Tiepolo et al 2001 and refer-ences therein)Finally the presence of theType III-d xenolith contain-

ing texturally equilibrated clinopyroxene of almost identi-cal composition to Dim micro-phenocrysts in melt pockets(Table 2 Figs 6a and 7b) further strengthens the argu-ment that CO2-fluid percolation started in the mantlebefore entrainment (ie enough time elapsed to allow thisrock to anneal completely after the late metasomaticassemblage had formed) A similar conclusion can bedrawn for the clearly host-related glass found in xenolithKe 195927 (eg Fig 13b) ThisType III-c sample is charac-terized by the presence of Dim micro-phenocrysts partlyovergrown by mineral rims clearly related to the host basa-nite magma (kaersutite Ti-augite and olivine withFo885 Fig 4f Tables 3 and 4 and supplementary dataat httppetrologyoxfordjournalsorg) Kaersutite hasmajor and trace element characteristics similar to typicalbasanitic and alkali basaltic amphibole phenocrysts(Fig 9d) The special phase assemblages in this xenolithindicates that host-related basanitic melts intruded someType III-c xenoliths and partly overprinted the assemblageformed slightly earlier by in situ derived alkali-rich peralu-minous melts fully preserved in other samples (eg Ke19653 or Ke 785)

ORIGIN OF METASOMATIZ INGAGENTS T IM ING AND TECTONICSETT INGAs noted above late metasomatism was probably short-lived started at mantle depths and probably continued

30

metasomatic assemblage indicate that melt pocket forma-tion reflects a significant change in P^T^X space in partsof the mantle beneath Marsabit after earlier metasoma-tism Based on geochemical arguments and mass-balanceconsiderations we will show that (1) the late glass-bearingassemblages are closely related to early metasomaticassemblages mainly representing breakdown productsand (2) breakdown was induced by infiltration of aCO2-rich fluid in the mantle probably followed andenhanced by heating and decompression melting duringxenolith transport in the host magma

Evidence for in situ partial meltingBSE imaging reveals that the phases becoming unstableare amphibole orthopyroxene and to minor extentsspl-I phlogopite cpx-I and apatite Compositional(inter- and intra-xenolith) heterogeneity of silicate glass istexture controlled and varies as a function of theassociated precursor phases (Fig 12a^f)TiO2 for exampleshows higher concentrations in glass from xenolith Ke196515 containing Ti-enriched amphibole (Fig 12c)whereas K2O contents are highest in phlogopite-rich sam-ples (eg Ke 785 Fig 12d) Elevated P2O5 contents arefound in glass adjacent to apatite (Fig 12e) whichmirrors at the same time the highTh and LREE contents

of apatite (Fig 12f) Significant Si enrichment is found inglass coating orthopyroxene (Fig 12d) supporting incon-gruent melting of orthopyroxene (see below) Even thoughthe melt pockets are connected along grain boundaries thestrong compositional variation (on inter- and intra-xeno-lith scale) suggests that the mobility of the melt was lim-ited or that the melting event was very short-lived (fortiming constraints see below)The co-variation plots in Fig 12 further reveal consider-

able compositional differences between all types of xeno-lith glass and the host basanite These data indicate thatthe glass cannot be the product of xenolith^host lava inter-action during ascent to the surface Also back-mixing ofmicro-phenocrysts into the melt fails to derive the glassfrom a host-related melt (Fig 12b) Glass from xenolithmargins however is compositionally closer to the bulkhost basanite (eg Fig 12b) in line with a direct derivationfrom xenolith^host lava interaction Its more evolvedmajor element composition (eg higher SiO2 lowerMg-number Fig 12b) is probably the result of fractiona-tion of basanite phenocrysts (eg olivine augite andtitanomagnetite Volker 1990) Host lava^xenolith interac-tion is further supported by the trace element signature ofglass at the margin of the xenoliths which is closely similarto that of the Marsabit basanites (Fig 13 Volker 1990)The same characteristics are observed for intra-xenolithglass in the unusual kaersutite-bearing sample Ke 195927

Fig 16 Oxygen fugacities relative to the quartz^fayalite^magnetitebuffer (QFM equation of OrsquoNeill 1987) as a function of temperatureat assumed pressure of 1GPa calculated using the composition ofcoexisting spinel and olivine (ol-I^spl-I pairs and Chrm^Olm pairsrespectively) based on the oxygen barometer of Ballhaus et al (1991)The graphite^CO2 equilibrium was calculated after Frost amp Wood(1998) and the graphite^carbonate^silicate equilibrium was calcu-lated using the equation of Eggler amp Baker (1982) (See text for furtherexplanation)

Fig 17 Calculated metasomatic agents in equilibrium with the euhe-dral Mg-katophorites from the Type III-c amphibole dunite Ke 196525 Amphibolehydrous fluid and amphibolecarbonatite melt parti-tion coefficients are from Adam et al (1997) and Adam amp Green(2001) respectively Silicate melts have been calculated using amphi-boleSi-rich melt partition coefficients from Chazot et al (1996b) andamphibolebasanite partition coefficients from LaTourrette et al(1995) The compositional range of the Marsabit basanites and alkalibasalts (Volker 1990) is shown for comparison

27

Xenolith glass unrelated to the host lava and melt pock-ets similar to those reported here in association with resi-dual phases (eg amphibole or clinopyroxene) has beenreported from many xenolith localities (eg Frey ampGreen 1974 Ionov et al 1994 1996 Chazot et al 1996a1996b Yaxley et al 1997 Laurora et al 2001 Bali et al2002) Residual phases within the melt pockets stronglyindicate in situ partial melting within the xenolithsHowever a variety of processes may trigger such in situ

melting including (1) closed-system decompression melt-ing of either anhydrous (Maale amp Printzlau 1979) orhydrous phases (Frey amp Green 1974 Francis 1976) duringxenolith ascent in the host lava (2) melting during interac-tion of the xenoliths with the host basaltic melt duringxenolith ascent (eg Zinngrebe amp Foley 1995 Shaw ampKlulaquo gel 2002 Shaw et al 2006) (3) melting in the mantleprior to xenolith entrainment induced by an infiltratingfluid (ie flux melting eg Jones et al 1983 Ionov et al1994 Demecurren ny et al 2004) However most studies agreethat that partial melting occurred very shortly beforeandor during xenolith exhumation by the host magmaThus a combination of metasomatically triggered meltingfollowed andor assisted by decompression melting duringxenolith transport may be a realistic alternative in somecases (eg Dawson 2002)Recently Ban et al (2005) have shown in a very detailed

mass-balance study on glass-bearing xenoliths from theWest Eifel that the weight proportions of glass and theassociated micro-phenocrysts match the composition ofthe preserved parent amphibole We used the sameapproach and performed simple mass-balance calculations(least-squares fitting) to constrain the conclusions drawnfrom petrography and to test which phase contributedand to what extent to the formation of the melt pocketsThe results reported in Table 8 show that in most casesamphibole-melting can account successfully for the meltpocket composition and only a minor amount (520) ofother phases (orthopyroxene andor phlogopite) needs tobe added to the reactant Such a contribution of otherphases agrees with textural observation (eg Fig 4cshowing Dim growing on resorbed Opxm) as well as withthe conclusions of Ban et al (2005) who stated that moreSi-rich glass (ie 454wt) necessitates orthopyroxenecontributing to the meltTrace element patterns of the bulk glassthornmicro-

phenocryst pockets calculated using the measured traceelement abundances of glass and micro-phenocrysts andthe respective mass-balance parameters (Table 8) arebroadly identical to the measured amphibole composition(Fig 18a and b) Higher LILE (Cs Rb Ba) concentrationsin the calculated bulk melt patches in sample Ke 785 canbe balanced by adding 7wt of phlogopite (Fig 18b)which is in line with mass balance (Table 8) The dominantcontrol of amphibole to account for the trace element

budget of the melt pockets is similar to results found inother suites of glass-bearing xenoliths (Chazot et al1996a Yaxley amp Kamenetsky 1999 Laurora et al 2001)Further the obtained phase proportions for the melt pock-ets are in good agreement with the results from image ana-lysis (Table 1) In addition they are similar to the glass^cpx^ol proportions recorded in recent experiments on the

Table 8 Results of mass-balance calculations to test melt

pocket formation by incongruent melting of the early metaso-

matic assemblages

Sample 195915 785 785 196525 196525 19653

Reactant

amph 1000 1000 0820 1000 0944 mdash

opx mdash mdash 0078 mdash 0056 0406

phl mdash mdash 0102 mdash mdash mdash

glass mdash mdash mdash mdash mdash 05941

SiO2 4484 4989 4964 5129 5167 5898

TiO2 014 039 036 025 024 018

Cr2O3 232 291 258 127 121 009

Al2O3 1368 762 769 800 757 961

FeO(tot) 336 314 333 343 357 394

MnO 006 008 008 012 012 010

NiO 009 013 014 011 011 004

MgO 1844 2029 2197 2040 2122 1625

CaO 854 783 644 966 913 078

Na2O 460 500 436 504 476 843

K2O 049 032 096 042 040 060

Product

glass 0389 0322 0335 0292 0284 07032

ol 0243 0259 0315 0266 0286 0273

cpx 0288 0373 0308 0442 0431 0024

spl 0052 0032 0028 mdash mdash mdash

carb 0028 0014 0014 mdash mdash mdash

SiO2 4477 5027 4986 5207 5181 5903

TiO2 013 041 039 015 014 017

Cr2O3 213 314 272 109 106 015

Al2O3 1364 764 770 798 776 957

FeO(tot) 374 331 351 322 332 350

MnO 007 007 007 009 009 006

NiO 011 013 015 011 012 007

MgO 1834 2024 2196 2063 2141 1635

CaO 854 759 632 952 929 080

Na2O 449 385 388 499 485 860

K2O 060 096 100 015 014 091

r2 023 200 035 084 028 033

Prefix Ke omitted1Low-Si glass 2High-Si glass (see text for explanation)

28

melting behaviour of amphibole in wehrlites at tempera-tures between 1175 and 12508C and pressures of05^10GPa (Mecurren dard et al 2006) These temperatures arein good agreement with the estimates for the formation ofolivine and chromite micro-phenocrysts in the melt

pockets (Fig 16) The heat supply required for amphibolemelting in the formerly low-T metasomatized Group IIIxenoliths (T 200^3008C) may come from local heatingshortly before or during xenolith entrainment in the hostlava or from fluid infiltration that triggered melting (seediscussion below)Finally formation of the most Si-enriched melt (adjacent

to Opxm in Group III-c samples) can be modelled assum-ing progressing interaction between infiltrating melt andorthopyroxene where the latter melts incongruently

opxthornmelt1 olthornmelt2 cpx ethseeTable 8THORN eth1THORN

To calculate reaction (1) we chose the least evolved glassobserved in melt patches in sample Ke 19653 (SiO2

5921wt Mg-number 6465) to represent the initialmelt1 whereas melt2 adjacent to opx is SiO2-rich(6530wt) Trace element abundances in glass next toorthopyroxene are shifted to lower concentrations(Table 6) which is in agreement with dilution caused byaddition of a trace element-poor orthopyroxene compo-nent The resultant phase proportions are similar to thoseobserved at orthopyroxene^melt interfaces (ie higholivine and low cpx abundances) and are in agreementwith experimental studies (Shaw et al 1998)

Partial melting triggered by fluid fluxMass-balance calculations provide an explanation for meltpocket formation by in situ partial melting Thus onemight conclude that melting is simply the result of decom-pression and heating during xenolith transport in the hostmagma However there are two points that argue thatmelting was triggered by the infiltration of a fluid phaseshortly before the xenoliths were sampled by the hostmagma (1) the presence of occasionally high amounts ofcarbonate included in the melt pockets (2) the extremelyNa-rich clinopyroxene (Dim) compositions either includedas micro-phenocrysts in the melt pocket or present asequilibrated grains around chromite in the single TypeIII-d xenolith (Fig 3f) Both arguments will be discussedin the next section

Formation of carbonates

Textures and compositions of carbonate globules can beexplained by carbonate precipitation from a CO2-bearingfluid As demonstrated by several studies highCa(CathornMg) values (Table 7) and low REE concentra-tions are strong arguments that the carbonates are cumu-late crystals precipitated from a CO2-rich melt or afluid and not quenched carbonated melts (Lee et al 2000Ionov amp Harmer 2002) Oscillatory zoning (Fig 5a and b)of individual carbonate crystals within singleglobules corroborates this interpretation The roundedcontacts toward glass (globules) are also in line with thesefindings Similar globular forms were reported fromexperimental studies and do not a priori imply the

Fig 18 Calculated trace element characteristics of reactants (in mostcases amphibolethorn orthopyroxene filled symbols) and products(melt pockets open symbols) of the lsquoamphibole meltingrsquo model[see equation (1) in text] The relative phase proportions wereobtained by mass-balance calculations and are given in Table 8 Thedashed lines in (b) show that the high LILE concentrations obtainedfor the product (bulk melt pocket) can be explained by adding about7wt of phlogopite to the reactant (amphibolethorn orthopyroxeneTable 8) Further explanation is given in the text

29

presence of immiscible carbonate and silicate melts(eg Lee amp Wyllie 1996)The high amount of carbonate in some melt pockets

however (eg Fig 4e) does need further explanation asthe incongruent melting reaction (1) does not account forthe CO2 required for carbonate precipitation duringamphibole melting Therefore amphibole breakdowneither occurred in an open system with addition of at leastCO2 (eg Laurora et al 2001 Ban et al 2005) or anotherpre-existing phase acted as a source of carbon Oxidationof graphite texturally identified as a part of the earlyassemblage (eg Fig 3d) could account for some of theCO2 necessary for carbonate formation A change in fO2

conditions is also indicated when the composition of glass-related spinel (Chrm) and Olm are used for oxygen baro-metry which yields oxygen fugacities well on the oxidizedside of the carbon^carbonate^silicate buffer (Fig 16)However increasing fO2 triggered by closed-systemdecompression-induced melting alone does not appear fea-sible Also textures such as straight contacts between gra-phite and Mg-calcite (Fig 4c) suggest that carbonategrows on graphite (ie indicating local fO2 conditions cor-responding to carbon^carbonate equilibrium) rather thanreplacing it We therefore interpret melt pocket formationto be triggered by the infiltration of a CO2-rich fluid(ie flux melting see also Ionov et al 1994) This interpre-tation also accounts for the presence of abundant vugs inthe glass that could have contained a fluid (Ionov et al1994 2006) which was presumably lost during sample pre-paration However we emphasize here that late fluid infil-tration had only subtle effects on the trace elementcomposition of Type III-b and -c xenoliths

Formation of Na-rich Cr diopside

Infiltration of CO2-rich fluidwould also explain the Na-richcomposition of Dim micro-phenocryst cores (Fig 6a and b)Ionov et al (2005) noted recently that clinopyroxenes inmeltpockets are commonly characterized by Na2O contents51wt mainly as a result of low cpxmelt partition coeffi-cients at low pressures (eg Blundy et al 1995) Thus veryhighNa2Ocontents in Dimmicro-phenocrysts (Fig6a) sug-gest replacement of amphibole by Dim and eventually Olmand Chrm during reaction with a fluid at subsolidus condi-tions An origin of micro-phenocrysts by replacementrather than crystallization from a melt is indicated by theidentical crystallographic orientation of individual Dimcrystals in singlemelt pockets (Fig4a)Once the solidus of the precursor phase(s) was reached

(ie 10508C for amphibole Niida amp Green 1999) silicatemelt was generated in situ and led to the formation of clin-opyroxene rims with lower Na contents (Fig 6b) and thedevelopment of euhedral shapes in contact with melt (egFig 4a and e) Eventual immiscibility between remainingfluid and newly formed melt (eg during xenolith ascent)would then explain the presence of carbonate globules and

vugs enclosed in glass Likewise different origins of meltand carbonate were recently proposed for similar meltpockets from Hungarian xenoliths by Demecurren ny et al(2004) who found oxygen isotope disequilibrium betweencarbonate globules and the host glass Such a two-stepmodel (fluid-induced replacement followed by melt gen-eration) can be explained by CO2-fluid infiltration occur-ring within the mantle at sub-solidus conditions ultimatelyfollowed by xenolith entrainment in the host magma andtransport to the surfaceThe host magma possibly suppliedthe additional heat necessary for melt generationFast decompression would further enhance melting andcould also account for the lower Na contents in Dimmicro-phenocryst rims as a result of decreasing cpxmeltpartition coefficients at decreasing pressures (Blundyet al 1995)Rapid replacement of amphibole ( phlogopite) by an

anhydrous phase assemblage indicates that the fluid waswater-poor (eg low H2OCO2 ratio see Ionov et al2006)This is corroborated by the fact that Mg-katophorite(presumably OH-rich see above) is occasionally rimmedby a residual kaersutitic selvage (Fig 4d) Such Ti-richmantle amphiboles are commonly OH-poor as a result ofpartial dehydrogenation (eg Tiepolo et al 2001 and refer-ences therein)Finally the presence of theType III-d xenolith contain-

ing texturally equilibrated clinopyroxene of almost identi-cal composition to Dim micro-phenocrysts in melt pockets(Table 2 Figs 6a and 7b) further strengthens the argu-ment that CO2-fluid percolation started in the mantlebefore entrainment (ie enough time elapsed to allow thisrock to anneal completely after the late metasomaticassemblage had formed) A similar conclusion can bedrawn for the clearly host-related glass found in xenolithKe 195927 (eg Fig 13b) ThisType III-c sample is charac-terized by the presence of Dim micro-phenocrysts partlyovergrown by mineral rims clearly related to the host basa-nite magma (kaersutite Ti-augite and olivine withFo885 Fig 4f Tables 3 and 4 and supplementary dataat httppetrologyoxfordjournalsorg) Kaersutite hasmajor and trace element characteristics similar to typicalbasanitic and alkali basaltic amphibole phenocrysts(Fig 9d) The special phase assemblages in this xenolithindicates that host-related basanitic melts intruded someType III-c xenoliths and partly overprinted the assemblageformed slightly earlier by in situ derived alkali-rich peralu-minous melts fully preserved in other samples (eg Ke19653 or Ke 785)

ORIGIN OF METASOMATIZ INGAGENTS T IM ING AND TECTONICSETT INGAs noted above late metasomatism was probably short-lived started at mantle depths and probably continued

30

Xenolith glass unrelated to the host lava and melt pock-ets similar to those reported here in association with resi-dual phases (eg amphibole or clinopyroxene) has beenreported from many xenolith localities (eg Frey ampGreen 1974 Ionov et al 1994 1996 Chazot et al 1996a1996b Yaxley et al 1997 Laurora et al 2001 Bali et al2002) Residual phases within the melt pockets stronglyindicate in situ partial melting within the xenolithsHowever a variety of processes may trigger such in situ

melting including (1) closed-system decompression melt-ing of either anhydrous (Maale amp Printzlau 1979) orhydrous phases (Frey amp Green 1974 Francis 1976) duringxenolith ascent in the host lava (2) melting during interac-tion of the xenoliths with the host basaltic melt duringxenolith ascent (eg Zinngrebe amp Foley 1995 Shaw ampKlulaquo gel 2002 Shaw et al 2006) (3) melting in the mantleprior to xenolith entrainment induced by an infiltratingfluid (ie flux melting eg Jones et al 1983 Ionov et al1994 Demecurren ny et al 2004) However most studies agreethat that partial melting occurred very shortly beforeandor during xenolith exhumation by the host magmaThus a combination of metasomatically triggered meltingfollowed andor assisted by decompression melting duringxenolith transport may be a realistic alternative in somecases (eg Dawson 2002)Recently Ban et al (2005) have shown in a very detailed

mass-balance study on glass-bearing xenoliths from theWest Eifel that the weight proportions of glass and theassociated micro-phenocrysts match the composition ofthe preserved parent amphibole We used the sameapproach and performed simple mass-balance calculations(least-squares fitting) to constrain the conclusions drawnfrom petrography and to test which phase contributedand to what extent to the formation of the melt pocketsThe results reported in Table 8 show that in most casesamphibole-melting can account successfully for the meltpocket composition and only a minor amount (520) ofother phases (orthopyroxene andor phlogopite) needs tobe added to the reactant Such a contribution of otherphases agrees with textural observation (eg Fig 4cshowing Dim growing on resorbed Opxm) as well as withthe conclusions of Ban et al (2005) who stated that moreSi-rich glass (ie 454wt) necessitates orthopyroxenecontributing to the meltTrace element patterns of the bulk glassthornmicro-

phenocryst pockets calculated using the measured traceelement abundances of glass and micro-phenocrysts andthe respective mass-balance parameters (Table 8) arebroadly identical to the measured amphibole composition(Fig 18a and b) Higher LILE (Cs Rb Ba) concentrationsin the calculated bulk melt patches in sample Ke 785 canbe balanced by adding 7wt of phlogopite (Fig 18b)which is in line with mass balance (Table 8) The dominantcontrol of amphibole to account for the trace element

budget of the melt pockets is similar to results found inother suites of glass-bearing xenoliths (Chazot et al1996a Yaxley amp Kamenetsky 1999 Laurora et al 2001)Further the obtained phase proportions for the melt pock-ets are in good agreement with the results from image ana-lysis (Table 1) In addition they are similar to the glass^cpx^ol proportions recorded in recent experiments on the

Table 8 Results of mass-balance calculations to test melt

pocket formation by incongruent melting of the early metaso-

matic assemblages

Sample 195915 785 785 196525 196525 19653

Reactant

amph 1000 1000 0820 1000 0944 mdash

opx mdash mdash 0078 mdash 0056 0406

phl mdash mdash 0102 mdash mdash mdash

glass mdash mdash mdash mdash mdash 05941

SiO2 4484 4989 4964 5129 5167 5898

TiO2 014 039 036 025 024 018

Cr2O3 232 291 258 127 121 009

Al2O3 1368 762 769 800 757 961

FeO(tot) 336 314 333 343 357 394

MnO 006 008 008 012 012 010

NiO 009 013 014 011 011 004

MgO 1844 2029 2197 2040 2122 1625

CaO 854 783 644 966 913 078

Na2O 460 500 436 504 476 843

K2O 049 032 096 042 040 060

Product

glass 0389 0322 0335 0292 0284 07032

ol 0243 0259 0315 0266 0286 0273

cpx 0288 0373 0308 0442 0431 0024

spl 0052 0032 0028 mdash mdash mdash

carb 0028 0014 0014 mdash mdash mdash

SiO2 4477 5027 4986 5207 5181 5903

TiO2 013 041 039 015 014 017

Cr2O3 213 314 272 109 106 015

Al2O3 1364 764 770 798 776 957

FeO(tot) 374 331 351 322 332 350

MnO 007 007 007 009 009 006

NiO 011 013 015 011 012 007

MgO 1834 2024 2196 2063 2141 1635

CaO 854 759 632 952 929 080

Na2O 449 385 388 499 485 860

K2O 060 096 100 015 014 091

r2 023 200 035 084 028 033

Prefix Ke omitted1Low-Si glass 2High-Si glass (see text for explanation)

28

melting behaviour of amphibole in wehrlites at tempera-tures between 1175 and 12508C and pressures of05^10GPa (Mecurren dard et al 2006) These temperatures arein good agreement with the estimates for the formation ofolivine and chromite micro-phenocrysts in the melt

pockets (Fig 16) The heat supply required for amphibolemelting in the formerly low-T metasomatized Group IIIxenoliths (T 200^3008C) may come from local heatingshortly before or during xenolith entrainment in the hostlava or from fluid infiltration that triggered melting (seediscussion below)Finally formation of the most Si-enriched melt (adjacent

to Opxm in Group III-c samples) can be modelled assum-ing progressing interaction between infiltrating melt andorthopyroxene where the latter melts incongruently

opxthornmelt1 olthornmelt2 cpx ethseeTable 8THORN eth1THORN

To calculate reaction (1) we chose the least evolved glassobserved in melt patches in sample Ke 19653 (SiO2

5921wt Mg-number 6465) to represent the initialmelt1 whereas melt2 adjacent to opx is SiO2-rich(6530wt) Trace element abundances in glass next toorthopyroxene are shifted to lower concentrations(Table 6) which is in agreement with dilution caused byaddition of a trace element-poor orthopyroxene compo-nent The resultant phase proportions are similar to thoseobserved at orthopyroxene^melt interfaces (ie higholivine and low cpx abundances) and are in agreementwith experimental studies (Shaw et al 1998)

Partial melting triggered by fluid fluxMass-balance calculations provide an explanation for meltpocket formation by in situ partial melting Thus onemight conclude that melting is simply the result of decom-pression and heating during xenolith transport in the hostmagma However there are two points that argue thatmelting was triggered by the infiltration of a fluid phaseshortly before the xenoliths were sampled by the hostmagma (1) the presence of occasionally high amounts ofcarbonate included in the melt pockets (2) the extremelyNa-rich clinopyroxene (Dim) compositions either includedas micro-phenocrysts in the melt pocket or present asequilibrated grains around chromite in the single TypeIII-d xenolith (Fig 3f) Both arguments will be discussedin the next section

Formation of carbonates

Textures and compositions of carbonate globules can beexplained by carbonate precipitation from a CO2-bearingfluid As demonstrated by several studies highCa(CathornMg) values (Table 7) and low REE concentra-tions are strong arguments that the carbonates are cumu-late crystals precipitated from a CO2-rich melt or afluid and not quenched carbonated melts (Lee et al 2000Ionov amp Harmer 2002) Oscillatory zoning (Fig 5a and b)of individual carbonate crystals within singleglobules corroborates this interpretation The roundedcontacts toward glass (globules) are also in line with thesefindings Similar globular forms were reported fromexperimental studies and do not a priori imply the

Fig 18 Calculated trace element characteristics of reactants (in mostcases amphibolethorn orthopyroxene filled symbols) and products(melt pockets open symbols) of the lsquoamphibole meltingrsquo model[see equation (1) in text] The relative phase proportions wereobtained by mass-balance calculations and are given in Table 8 Thedashed lines in (b) show that the high LILE concentrations obtainedfor the product (bulk melt pocket) can be explained by adding about7wt of phlogopite to the reactant (amphibolethorn orthopyroxeneTable 8) Further explanation is given in the text

29

presence of immiscible carbonate and silicate melts(eg Lee amp Wyllie 1996)The high amount of carbonate in some melt pockets

however (eg Fig 4e) does need further explanation asthe incongruent melting reaction (1) does not account forthe CO2 required for carbonate precipitation duringamphibole melting Therefore amphibole breakdowneither occurred in an open system with addition of at leastCO2 (eg Laurora et al 2001 Ban et al 2005) or anotherpre-existing phase acted as a source of carbon Oxidationof graphite texturally identified as a part of the earlyassemblage (eg Fig 3d) could account for some of theCO2 necessary for carbonate formation A change in fO2

conditions is also indicated when the composition of glass-related spinel (Chrm) and Olm are used for oxygen baro-metry which yields oxygen fugacities well on the oxidizedside of the carbon^carbonate^silicate buffer (Fig 16)However increasing fO2 triggered by closed-systemdecompression-induced melting alone does not appear fea-sible Also textures such as straight contacts between gra-phite and Mg-calcite (Fig 4c) suggest that carbonategrows on graphite (ie indicating local fO2 conditions cor-responding to carbon^carbonate equilibrium) rather thanreplacing it We therefore interpret melt pocket formationto be triggered by the infiltration of a CO2-rich fluid(ie flux melting see also Ionov et al 1994) This interpre-tation also accounts for the presence of abundant vugs inthe glass that could have contained a fluid (Ionov et al1994 2006) which was presumably lost during sample pre-paration However we emphasize here that late fluid infil-tration had only subtle effects on the trace elementcomposition of Type III-b and -c xenoliths

Formation of Na-rich Cr diopside

Infiltration of CO2-rich fluidwould also explain the Na-richcomposition of Dim micro-phenocryst cores (Fig 6a and b)Ionov et al (2005) noted recently that clinopyroxenes inmeltpockets are commonly characterized by Na2O contents51wt mainly as a result of low cpxmelt partition coeffi-cients at low pressures (eg Blundy et al 1995) Thus veryhighNa2Ocontents in Dimmicro-phenocrysts (Fig6a) sug-gest replacement of amphibole by Dim and eventually Olmand Chrm during reaction with a fluid at subsolidus condi-tions An origin of micro-phenocrysts by replacementrather than crystallization from a melt is indicated by theidentical crystallographic orientation of individual Dimcrystals in singlemelt pockets (Fig4a)Once the solidus of the precursor phase(s) was reached

(ie 10508C for amphibole Niida amp Green 1999) silicatemelt was generated in situ and led to the formation of clin-opyroxene rims with lower Na contents (Fig 6b) and thedevelopment of euhedral shapes in contact with melt (egFig 4a and e) Eventual immiscibility between remainingfluid and newly formed melt (eg during xenolith ascent)would then explain the presence of carbonate globules and

vugs enclosed in glass Likewise different origins of meltand carbonate were recently proposed for similar meltpockets from Hungarian xenoliths by Demecurren ny et al(2004) who found oxygen isotope disequilibrium betweencarbonate globules and the host glass Such a two-stepmodel (fluid-induced replacement followed by melt gen-eration) can be explained by CO2-fluid infiltration occur-ring within the mantle at sub-solidus conditions ultimatelyfollowed by xenolith entrainment in the host magma andtransport to the surfaceThe host magma possibly suppliedthe additional heat necessary for melt generationFast decompression would further enhance melting andcould also account for the lower Na contents in Dimmicro-phenocryst rims as a result of decreasing cpxmeltpartition coefficients at decreasing pressures (Blundyet al 1995)Rapid replacement of amphibole ( phlogopite) by an

anhydrous phase assemblage indicates that the fluid waswater-poor (eg low H2OCO2 ratio see Ionov et al2006)This is corroborated by the fact that Mg-katophorite(presumably OH-rich see above) is occasionally rimmedby a residual kaersutitic selvage (Fig 4d) Such Ti-richmantle amphiboles are commonly OH-poor as a result ofpartial dehydrogenation (eg Tiepolo et al 2001 and refer-ences therein)Finally the presence of theType III-d xenolith contain-

ing texturally equilibrated clinopyroxene of almost identi-cal composition to Dim micro-phenocrysts in melt pockets(Table 2 Figs 6a and 7b) further strengthens the argu-ment that CO2-fluid percolation started in the mantlebefore entrainment (ie enough time elapsed to allow thisrock to anneal completely after the late metasomaticassemblage had formed) A similar conclusion can bedrawn for the clearly host-related glass found in xenolithKe 195927 (eg Fig 13b) ThisType III-c sample is charac-terized by the presence of Dim micro-phenocrysts partlyovergrown by mineral rims clearly related to the host basa-nite magma (kaersutite Ti-augite and olivine withFo885 Fig 4f Tables 3 and 4 and supplementary dataat httppetrologyoxfordjournalsorg) Kaersutite hasmajor and trace element characteristics similar to typicalbasanitic and alkali basaltic amphibole phenocrysts(Fig 9d) The special phase assemblages in this xenolithindicates that host-related basanitic melts intruded someType III-c xenoliths and partly overprinted the assemblageformed slightly earlier by in situ derived alkali-rich peralu-minous melts fully preserved in other samples (eg Ke19653 or Ke 785)

ORIGIN OF METASOMATIZ INGAGENTS T IM ING AND TECTONICSETT INGAs noted above late metasomatism was probably short-lived started at mantle depths and probably continued

30

melting behaviour of amphibole in wehrlites at tempera-tures between 1175 and 12508C and pressures of05^10GPa (Mecurren dard et al 2006) These temperatures arein good agreement with the estimates for the formation ofolivine and chromite micro-phenocrysts in the melt

pockets (Fig 16) The heat supply required for amphibolemelting in the formerly low-T metasomatized Group IIIxenoliths (T 200^3008C) may come from local heatingshortly before or during xenolith entrainment in the hostlava or from fluid infiltration that triggered melting (seediscussion below)Finally formation of the most Si-enriched melt (adjacent

to Opxm in Group III-c samples) can be modelled assum-ing progressing interaction between infiltrating melt andorthopyroxene where the latter melts incongruently

opxthornmelt1 olthornmelt2 cpx ethseeTable 8THORN eth1THORN

To calculate reaction (1) we chose the least evolved glassobserved in melt patches in sample Ke 19653 (SiO2

5921wt Mg-number 6465) to represent the initialmelt1 whereas melt2 adjacent to opx is SiO2-rich(6530wt) Trace element abundances in glass next toorthopyroxene are shifted to lower concentrations(Table 6) which is in agreement with dilution caused byaddition of a trace element-poor orthopyroxene compo-nent The resultant phase proportions are similar to thoseobserved at orthopyroxene^melt interfaces (ie higholivine and low cpx abundances) and are in agreementwith experimental studies (Shaw et al 1998)

Partial melting triggered by fluid fluxMass-balance calculations provide an explanation for meltpocket formation by in situ partial melting Thus onemight conclude that melting is simply the result of decom-pression and heating during xenolith transport in the hostmagma However there are two points that argue thatmelting was triggered by the infiltration of a fluid phaseshortly before the xenoliths were sampled by the hostmagma (1) the presence of occasionally high amounts ofcarbonate included in the melt pockets (2) the extremelyNa-rich clinopyroxene (Dim) compositions either includedas micro-phenocrysts in the melt pocket or present asequilibrated grains around chromite in the single TypeIII-d xenolith (Fig 3f) Both arguments will be discussedin the next section

Formation of carbonates

Textures and compositions of carbonate globules can beexplained by carbonate precipitation from a CO2-bearingfluid As demonstrated by several studies highCa(CathornMg) values (Table 7) and low REE concentra-tions are strong arguments that the carbonates are cumu-late crystals precipitated from a CO2-rich melt or afluid and not quenched carbonated melts (Lee et al 2000Ionov amp Harmer 2002) Oscillatory zoning (Fig 5a and b)of individual carbonate crystals within singleglobules corroborates this interpretation The roundedcontacts toward glass (globules) are also in line with thesefindings Similar globular forms were reported fromexperimental studies and do not a priori imply the

Fig 18 Calculated trace element characteristics of reactants (in mostcases amphibolethorn orthopyroxene filled symbols) and products(melt pockets open symbols) of the lsquoamphibole meltingrsquo model[see equation (1) in text] The relative phase proportions wereobtained by mass-balance calculations and are given in Table 8 Thedashed lines in (b) show that the high LILE concentrations obtainedfor the product (bulk melt pocket) can be explained by adding about7wt of phlogopite to the reactant (amphibolethorn orthopyroxeneTable 8) Further explanation is given in the text

29

presence of immiscible carbonate and silicate melts(eg Lee amp Wyllie 1996)The high amount of carbonate in some melt pockets

however (eg Fig 4e) does need further explanation asthe incongruent melting reaction (1) does not account forthe CO2 required for carbonate precipitation duringamphibole melting Therefore amphibole breakdowneither occurred in an open system with addition of at leastCO2 (eg Laurora et al 2001 Ban et al 2005) or anotherpre-existing phase acted as a source of carbon Oxidationof graphite texturally identified as a part of the earlyassemblage (eg Fig 3d) could account for some of theCO2 necessary for carbonate formation A change in fO2

conditions is also indicated when the composition of glass-related spinel (Chrm) and Olm are used for oxygen baro-metry which yields oxygen fugacities well on the oxidizedside of the carbon^carbonate^silicate buffer (Fig 16)However increasing fO2 triggered by closed-systemdecompression-induced melting alone does not appear fea-sible Also textures such as straight contacts between gra-phite and Mg-calcite (Fig 4c) suggest that carbonategrows on graphite (ie indicating local fO2 conditions cor-responding to carbon^carbonate equilibrium) rather thanreplacing it We therefore interpret melt pocket formationto be triggered by the infiltration of a CO2-rich fluid(ie flux melting see also Ionov et al 1994) This interpre-tation also accounts for the presence of abundant vugs inthe glass that could have contained a fluid (Ionov et al1994 2006) which was presumably lost during sample pre-paration However we emphasize here that late fluid infil-tration had only subtle effects on the trace elementcomposition of Type III-b and -c xenoliths

Formation of Na-rich Cr diopside

Infiltration of CO2-rich fluidwould also explain the Na-richcomposition of Dim micro-phenocryst cores (Fig 6a and b)Ionov et al (2005) noted recently that clinopyroxenes inmeltpockets are commonly characterized by Na2O contents51wt mainly as a result of low cpxmelt partition coeffi-cients at low pressures (eg Blundy et al 1995) Thus veryhighNa2Ocontents in Dimmicro-phenocrysts (Fig6a) sug-gest replacement of amphibole by Dim and eventually Olmand Chrm during reaction with a fluid at subsolidus condi-tions An origin of micro-phenocrysts by replacementrather than crystallization from a melt is indicated by theidentical crystallographic orientation of individual Dimcrystals in singlemelt pockets (Fig4a)Once the solidus of the precursor phase(s) was reached

(ie 10508C for amphibole Niida amp Green 1999) silicatemelt was generated in situ and led to the formation of clin-opyroxene rims with lower Na contents (Fig 6b) and thedevelopment of euhedral shapes in contact with melt (egFig 4a and e) Eventual immiscibility between remainingfluid and newly formed melt (eg during xenolith ascent)would then explain the presence of carbonate globules and

vugs enclosed in glass Likewise different origins of meltand carbonate were recently proposed for similar meltpockets from Hungarian xenoliths by Demecurren ny et al(2004) who found oxygen isotope disequilibrium betweencarbonate globules and the host glass Such a two-stepmodel (fluid-induced replacement followed by melt gen-eration) can be explained by CO2-fluid infiltration occur-ring within the mantle at sub-solidus conditions ultimatelyfollowed by xenolith entrainment in the host magma andtransport to the surfaceThe host magma possibly suppliedthe additional heat necessary for melt generationFast decompression would further enhance melting andcould also account for the lower Na contents in Dimmicro-phenocryst rims as a result of decreasing cpxmeltpartition coefficients at decreasing pressures (Blundyet al 1995)Rapid replacement of amphibole ( phlogopite) by an

anhydrous phase assemblage indicates that the fluid waswater-poor (eg low H2OCO2 ratio see Ionov et al2006)This is corroborated by the fact that Mg-katophorite(presumably OH-rich see above) is occasionally rimmedby a residual kaersutitic selvage (Fig 4d) Such Ti-richmantle amphiboles are commonly OH-poor as a result ofpartial dehydrogenation (eg Tiepolo et al 2001 and refer-ences therein)Finally the presence of theType III-d xenolith contain-

ing texturally equilibrated clinopyroxene of almost identi-cal composition to Dim micro-phenocrysts in melt pockets(Table 2 Figs 6a and 7b) further strengthens the argu-ment that CO2-fluid percolation started in the mantlebefore entrainment (ie enough time elapsed to allow thisrock to anneal completely after the late metasomaticassemblage had formed) A similar conclusion can bedrawn for the clearly host-related glass found in xenolithKe 195927 (eg Fig 13b) ThisType III-c sample is charac-terized by the presence of Dim micro-phenocrysts partlyovergrown by mineral rims clearly related to the host basa-nite magma (kaersutite Ti-augite and olivine withFo885 Fig 4f Tables 3 and 4 and supplementary dataat httppetrologyoxfordjournalsorg) Kaersutite hasmajor and trace element characteristics similar to typicalbasanitic and alkali basaltic amphibole phenocrysts(Fig 9d) The special phase assemblages in this xenolithindicates that host-related basanitic melts intruded someType III-c xenoliths and partly overprinted the assemblageformed slightly earlier by in situ derived alkali-rich peralu-minous melts fully preserved in other samples (eg Ke19653 or Ke 785)

ORIGIN OF METASOMATIZ INGAGENTS T IM ING AND TECTONICSETT INGAs noted above late metasomatism was probably short-lived started at mantle depths and probably continued

30

presence of immiscible carbonate and silicate melts(eg Lee amp Wyllie 1996)The high amount of carbonate in some melt pockets

however (eg Fig 4e) does need further explanation asthe incongruent melting reaction (1) does not account forthe CO2 required for carbonate precipitation duringamphibole melting Therefore amphibole breakdowneither occurred in an open system with addition of at leastCO2 (eg Laurora et al 2001 Ban et al 2005) or anotherpre-existing phase acted as a source of carbon Oxidationof graphite texturally identified as a part of the earlyassemblage (eg Fig 3d) could account for some of theCO2 necessary for carbonate formation A change in fO2

conditions is also indicated when the composition of glass-related spinel (Chrm) and Olm are used for oxygen baro-metry which yields oxygen fugacities well on the oxidizedside of the carbon^carbonate^silicate buffer (Fig 16)However increasing fO2 triggered by closed-systemdecompression-induced melting alone does not appear fea-sible Also textures such as straight contacts between gra-phite and Mg-calcite (Fig 4c) suggest that carbonategrows on graphite (ie indicating local fO2 conditions cor-responding to carbon^carbonate equilibrium) rather thanreplacing it We therefore interpret melt pocket formationto be triggered by the infiltration of a CO2-rich fluid(ie flux melting see also Ionov et al 1994) This interpre-tation also accounts for the presence of abundant vugs inthe glass that could have contained a fluid (Ionov et al1994 2006) which was presumably lost during sample pre-paration However we emphasize here that late fluid infil-tration had only subtle effects on the trace elementcomposition of Type III-b and -c xenoliths

Formation of Na-rich Cr diopside

Infiltration of CO2-rich fluidwould also explain the Na-richcomposition of Dim micro-phenocryst cores (Fig 6a and b)Ionov et al (2005) noted recently that clinopyroxenes inmeltpockets are commonly characterized by Na2O contents51wt mainly as a result of low cpxmelt partition coeffi-cients at low pressures (eg Blundy et al 1995) Thus veryhighNa2Ocontents in Dimmicro-phenocrysts (Fig6a) sug-gest replacement of amphibole by Dim and eventually Olmand Chrm during reaction with a fluid at subsolidus condi-tions An origin of micro-phenocrysts by replacementrather than crystallization from a melt is indicated by theidentical crystallographic orientation of individual Dimcrystals in singlemelt pockets (Fig4a)Once the solidus of the precursor phase(s) was reached

(ie 10508C for amphibole Niida amp Green 1999) silicatemelt was generated in situ and led to the formation of clin-opyroxene rims with lower Na contents (Fig 6b) and thedevelopment of euhedral shapes in contact with melt (egFig 4a and e) Eventual immiscibility between remainingfluid and newly formed melt (eg during xenolith ascent)would then explain the presence of carbonate globules and

vugs enclosed in glass Likewise different origins of meltand carbonate were recently proposed for similar meltpockets from Hungarian xenoliths by Demecurren ny et al(2004) who found oxygen isotope disequilibrium betweencarbonate globules and the host glass Such a two-stepmodel (fluid-induced replacement followed by melt gen-eration) can be explained by CO2-fluid infiltration occur-ring within the mantle at sub-solidus conditions ultimatelyfollowed by xenolith entrainment in the host magma andtransport to the surfaceThe host magma possibly suppliedthe additional heat necessary for melt generationFast decompression would further enhance melting andcould also account for the lower Na contents in Dimmicro-phenocryst rims as a result of decreasing cpxmeltpartition coefficients at decreasing pressures (Blundyet al 1995)Rapid replacement of amphibole ( phlogopite) by an

anhydrous phase assemblage indicates that the fluid waswater-poor (eg low H2OCO2 ratio see Ionov et al2006)This is corroborated by the fact that Mg-katophorite(presumably OH-rich see above) is occasionally rimmedby a residual kaersutitic selvage (Fig 4d) Such Ti-richmantle amphiboles are commonly OH-poor as a result ofpartial dehydrogenation (eg Tiepolo et al 2001 and refer-ences therein)Finally the presence of theType III-d xenolith contain-

ing texturally equilibrated clinopyroxene of almost identi-cal composition to Dim micro-phenocrysts in melt pockets(Table 2 Figs 6a and 7b) further strengthens the argu-ment that CO2-fluid percolation started in the mantlebefore entrainment (ie enough time elapsed to allow thisrock to anneal completely after the late metasomaticassemblage had formed) A similar conclusion can bedrawn for the clearly host-related glass found in xenolithKe 195927 (eg Fig 13b) ThisType III-c sample is charac-terized by the presence of Dim micro-phenocrysts partlyovergrown by mineral rims clearly related to the host basa-nite magma (kaersutite Ti-augite and olivine withFo885 Fig 4f Tables 3 and 4 and supplementary dataat httppetrologyoxfordjournalsorg) Kaersutite hasmajor and trace element characteristics similar to typicalbasanitic and alkali basaltic amphibole phenocrysts(Fig 9d) The special phase assemblages in this xenolithindicates that host-related basanitic melts intruded someType III-c xenoliths and partly overprinted the assemblageformed slightly earlier by in situ derived alkali-rich peralu-minous melts fully preserved in other samples (eg Ke19653 or Ke 785)

ORIGIN OF METASOMATIZ INGAGENTS T IM ING AND TECTONICSETT INGAs noted above late metasomatism was probably short-lived started at mantle depths and probably continued

30

during xenolith transport in the host magma The relationbetween fluid infiltration and xenolith entrainment sug-gests that the source of the fluid is intimately related tothe host magma Volatile components such as CO2 play amajor role during wall-rock disruption and xenolith sam-pling at depth (eg Spera 1984 Lensky et al 2006) Thusthe CO2-rich fluids may be derived directly from therising basanitic magma that ultimately sampled the xeno-liths (Fig 19) Alternatively the CO2-fluid may be derivedfrom decarbonation reactions (CO2 release) at greaterdepths where rising carbonate-rich melts reach the solidusof carbonated lherzolite (see Fig 2 and Lee ampWyllie 2000and references therein) Finally we would like to stress thatsilicate melt (now glass) was formed in situ within thexenoliths and thus does not represent an extraneouslyderived metasomatizing agentIn contrast to the melt pockets the age of formation of

the early metasomatic assemblages is more difficult toassess Relative age information can be obtained from tex-tural relationships Metasomatism partly overprints thestrongly deformed fabric of the pre-metasomatic GroupIII rocks Because pervasive deformation probablyoccurred during the development of the Mesozoic Anza

Graben (Kaeser et al 2006 Fig 1) metasomatism shouldbe younger In the geodynamic context of Marsabitthis would then most probably be in association with theyoung (Miocene^Pleistocene) magmatic activity of theKenya Rift (EARS)The occurrence of orthopyroxene-saturated volatile-rich

melts in a region characterized by voluminous alkalinemagmatism is enigmatic however It seems most plausiblethat such melts evolved from alkaline mafic melts throughmelt^rock reaction as in the models of Bedini et al (1997)or Bodinier et al (2004) In their study Bedini et al (1997)explained metasomatism in the enriched and deformedharzburgite xenoliths from southern Ethiopia (200 kmnorth of Marsabit) by complex reactive porous flow pro-cesses starting with an lsquoordinaryrsquo ocean island basalt typemafic silicate meltTheir hypothetical melts in equilibriumwith the metasomatized xenoliths have trace element com-positions strikingly similar to the calculated silicate meltsin equilibrium with euhedral Mg-katophorite (Fig 17)In Marsabit the heated (1000^12008C) and recrystallizedGroup II lherzolites provide evidence for recentEARS-related metasomatism through alkaline maficmelts possibly parental to the basanites extruded at the

Fig 19 Schematic illustration of the possible spatial and temporal relationships between the different metasomatic processes recorded by theinvestigated mantle xenoliths from Marsabit (a) State of the lithosphere beneath Marsabit after Mesozoic rifting (decompression and coolingKaeser et al 2006) LAB lithosphere^asthenosphere boundary (Henjes-Kunst amp Altherr 1992) Penetrative deformation is indicated by finehorizontal lines Dark grey boudins indicate compositional heterogeneities including deformed pyroxenite bands and cpx-rich lherzolite(Group I xenoliths) including associated Ti-pargasite and phlogopite (b) Schematic representation of the EARS-related heating as a resultof mafic silicate melt impregnation (Kaeser et al 2006 bold arrow) and propagation (and associated melt^rock reaction) of residual volatileSi-rich melts in colder and relatively reduced lithospheric domains (early metasomatism this study) L A and P represent possibleyet unconstrained components in the initial melt either deriving from the lower lithosphere the asthenosphere or a mantle plumerespectively Close-ups 1 possible location of statically recrystallized silicate melt metasomatized Group II lherzolite (Kaeser et al 2006)2 SiO2^Na2O^CO2^H2O-rich melt responsible for metasomatism in Group III peridotites 3 position of Type III-c amphibole-harzburgiteand dunite (close to the metasomatizing agent) 4 position of Type III-b harzburgite 5 position of Type III-a harzburgite (least metasomatized)Percolation through andor reaction with pre-existing heterogeneities (dashed lines) may account for several trace element characteristics(see text) Small arrows indicate melt or fluid percolation away from the initial source (eg a melt conduit) (c) Sampling of the xenolithsby Quaternary basanites and alkali basalts (Brotzu et al1984) i Group II (grt)-spinel lherzolites ii non-metasomatized Group I (grt)-spinellherzolites iii strongly metasomatized Type III-a to -d harzburgites and dunites Close-up melt pocket formation during this late-stage eventtriggered by CO2-rich fluid It should be noted that the vertical differences are hypothetical owing to the lack of accurate constraints on thesampling depth The relationship between Group II and III could also be lateral (eg away from a larger melt conduit)

31

surface (Kaeser et al 2006) Similar to the process invokedfor the Ethiopian xenoliths continuous melt^rock reactionand fractionation could have led to the formation of smallfractions of highly evolved melts infiltrating andmetasomatizing the cold and deformed uppermost mantlerepresented by Group III harzburgites (Fig 19 see alsoIonov et al 2006 for a similar model) A relationshipbetween metasomatism in Group II and III peridotites issupported by several xenoliths showing transitionaltextures and trace element signatures (ie strong U^Th^LREE enrichment in some Group II clinopyroxenesKaeser et al 2006)Alternatively Si- and volatile-enriched melts may be

residual liquids from the crystallization of alkali basalticor basanitic veins (ie hornblendite or clinopyroxenite)ultimately metasomatizing the adjacent peridotitic wall-rock (eg Harte et al 1993 Wulff-Pedersen et al 1999Bodinier et al 2004) This remains speculative howeveras such veins or dykes have not yet been reported fromMarsabit It can thus not be resolved whether early meta-somatism reflects a large kilometre-scale process (eg anupwelling melt percolation front as proposed for themantle beneath southern Ethiopia Bedini et al 1997) orwhether metasomatism is restricted to narrower zonesadjacent to small (centimetre- to metre-scale) melt con-duits (eg wall-rock metasomatism)Extensive melt^rock reaction in the mantle beneath the

EARS however is indicated by numerous xenolith suitescontaining abundant and variably metasomatized rocks(eg Tanzania Dawson amp Smith 1988 Rudnick et al1993 Lee amp Rudnick 1999 Dawson 2002 and referencestherein Ethiopia Bedini et al1997 Rooney et al 2005) Inthe case of northern Kenya seismic studies point to abroad zone of low seismic wave velocities in the relativelyshallow mantle 600 km across and reaching depths of540 km depth beneath LakeTurkana and 90 km beneathMarsabit (see Simiyu amp Keller 1997) This anomaly iscommonly interpreted as a zone of hot rising mantle con-taining small amounts (3^5) of melt (Green et al 1991)Upwelling of such melts into the lithospheric mantle fol-lowed by reaction could thus account for the metasoma-tism seen in the Marsabit mantle xenoliths Suchinteraction of sub-lithospheric melts or fluids (astheno-spheric or plume-related) with colder lithosphericmaterial is one of the major processes invoked toaccount for the heterogeneous isotopic composition ofprimitive lavas from numerous parts of the EARS(eg Macdonald et al 2001 Furman et al 2006 and refer-ences therein)

ACKNOWLEDGEMENTSWe would like to thank Edwin Gnos and Alfons Berger(Bern) and Hans-Peter Meyer (Heidelberg) for assistancewith microprobe work as well as Andrecurren Villard

who provided top-quality thin sections Discussion withOthmar Mulaquo ntener Arjan Dijkstra Rainer Altherr Jean-Louis Bodinier and Alan Woodland helped to clarifymany aspects of this work We thank Hilary DownesTanya Furman and Dmitri Ionov for constructive reviewsand MarjorieWilson for valuable comments and editorialhandling This publication is part of a project financed bythe Swiss National Science Foundation (grant No 200021-1006471 to AK) Further financial support from the SNF(grants 21-2657989 and 200021-1034791) for the electronmicroprobe facility at the Institute of Geological SciencesUniversity of Bern is acknowledged

SUPPLEMENTARY DATASupplementary data for this paper are available at Journalof Petrology online

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Barclay J amp Carmichael I S E (2004) A hornblende basaltfrom western Mexico water-saturated phase relations constrain apressure^temperature window of eruptibility Journal of Petrology

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Evolution of LILE-enriched small melt fractions in the lithosphericmantle a case study from the East African Rift Earth and PlanetaryScience Letters 153 67^83

Blundy J Falloon T J Wood B J amp Dalton J A (1995) Sodiumpartitioning between clinopyroxene and silicate melts Journal ofGeophysical Research 100 15501^15515

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Bodinier J-L Menzies M A Shimizu N Frey F A ampMcPherson E (2004) Silicate hydrous and carbonate

32

metasomatism at Lherz France contemporaneous derivates of sili-cate melt^harzburgite reaction Journal of Petrology 45 299^320

Brey G P amp Kolaquo hler T (1990) Geothermobarometry in four-phaselherzolites II New thermobarometers and practical assessment ofexisting thermobarometers Journal of Petrology 31 1353^1378

Brotzu P Morbideli L Nicoletti M Piccirillo E M ampTraversa G (1984) Miocene to Quaternary volcanism in easternAfrica sequence and chronologyTectonophysics 101 75^86

Chazot G Menzies M A amp Harte B (1996a) Determination ofpartition coefficients between apatite clinopyroxene amphiboleand melt in natural spinel lherzolites fromYemen implications forwet melting of the lithospheric mantle Geochimica et Cosmochimica

Acta 60 423^437Chazot G Menzies M A amp Harte B (1996b) Silicate glasses in

spinel lherzolites from Yemen origin and chemical compositionChemical Geology 134 159^179

Coltorti M Bonadiman C Hinton RW Siena F amp Upton B GJ (1999) Carbonatite metasomatism of the oceanic uppermantle evidence from clinopyroxenes and glasses in ultramaficxenoliths of Grande Comore Indian Ocean Journal of Petrology 40133^165

Coltorti M Beccaluva L Bonadiman C Salvini L amp Siena F(2000) Glasses in mantle xenoliths as geochemical indicatorsof metasomatic agents Earth and Planetary Science Letters 183303^320

Costa F Dungan M A amp Singer B S (2001) Magmatic Na-richphlogopite in a suite of gabbroic crustal xenoliths fromVolcacurren n SanPedro Chilean Andes evidence for a solvus relation between phlo-gopite and aspidolite American Mineralogist 86 29^35

Dautria J-M Dupuy C Takherist D amp Dostal J (1992)Carbonate metasomatism in the lithospheric mantle peridotiticxenoliths from a melilitic district of the Sahara basin Contributionsto Mineralogy and Petrology 111 37^52

Dawson J B (2002) Metasomatism and partial melting in upper-mantle peridotite xenoliths from the Lashaine volcano NorthernTanzania Journal of Petrology 43 1749^1777

Dawson J B amp Smith JV (1988) Metasomatized and veined uppermantle xenoliths from Pello Hill Tanzania evidence for anoma-lously light mantle beneath the Tanzanian sector of the EastAfrican rift valley Contributions to Mineralogy and Petrology 100510^527

Demecurren ny A Vennemann T W Hegner E Nagy G Milton J AEmbey-Isztin A Homonnay Z amp Dobosi G (2004) Traceelement and C^O^Sr^Nd isotope evidence for subduction relatedcarbonate^silicate melts in mantle xenoliths (Pannonian BasinHungary) Lithos 75 89^113

Draper D S amp Green T H (1997) P^T phase relations of silicicalkaline aluminous mantle-xenolith glasses under anhydrous andC^O^H fluid-saturated conditions Journal of Petrology 381187^1224

Eggler D H amp Baker D R (1982) Reduced volatiles in the systemC^O^H implications to mantle melting fluid formation and dia-mond genesis In Akimoto S amp Manghnani M H (eds) High

Pressure Research in Geophysics Tokyo Center for AcademicPublications Japan pp 237^250

Ferraris C Grobecurren ty B Frulaquo h-Green G L amp Wessicken R (2004)Intergrowth of graphite within phlogopite from Finero ultramaficcomplex (ItalianWestern Alps) implications for mantle crystalliza-tion of primary-texture mica European Journal of Mineralogy 16899^908

Francis D M (1976) Amphibole pyroxenite xenoliths cumulate orreplacement phenomena from the upper mantle Nunivak IslandAlaska Contributions to Mineralogy and Petrology 58 51^61

Frey F A amp Green D H (1974) Mineralogy geochemistry andorigin of lherzolite inclusions in Victorian basanites Geochimica et

Cosmochimica Acta 38 1023^1059Frost D J amp Wood B J (1998) The fugacity of carbon dioxide and

the graphitediamond CO equilibrium between 35 and 77 kbar at925 to 16508C (vol 61 pg 1565 1997) Geochimica et Cosmochimica Acta62 725^725

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198 495^510Ionov D A amp Hofmann A W (1995) Nb^Ta-rich mantle amphi-

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33

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Carbonate-bearing mantle peridotite xenoliths from Spitsbergenphase relationship mineral compositions and trace-element resi-dence Contributions to Mineralogy and Petrology 125 375^392

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Petrology 43 2219^2259Ionov D A Prikhodko V S Bodinier J L Sobolev A V amp

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Varela M E Clocchiatti R Kurat G amp Schiano P (1999)Silicic glasses in hydrous and anhydrous mantle xenoliths fromWestern Victoria at least two different sources Chemical Geology

153 151^169Vernie res J Godard M amp Bodinier J L (1997) A plate model for

the simulation of trace element fractionation during partial melt-ing and magma transport in the Earthrsquos upper mantle Journal ofGeophysical Research 102 24771^24784

Volker F (1990) Geochemie der quartalaquo ren Vulkanite aufder Ostschulter des Kenia-Rifts PhD thesis University ofKarlsruhe

Witt-Eickschen G Seck H A Mezger K Eggins S M ampAltherr R (2003) Lithospheric mantle evolution beneath theEifel (Germany) constraints from Sr^Nd^Pb isotopes and traceelement abundances in spinel peridotite and pyroxenite xenolithsJournal of Petrology 44 1077^1095

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63 263^349Wood B J amp Virgo D (1989) Upper mantle oxidation state ferric

iron contents of lherzolite spinels by 57Fe Molaquo ssbauer spectroscopyand resultant oxygen fugacities Geochimica et Cosmochimica Acta 531277^1291

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Yaxley G M amp Kemenetsky V (1999) In situ origin for glass inmantle xenoliths from southeastern Australia insights from traceelement compositions of glasses and metasomatic phases Earth andPlanetary Science Letters 172 97^109

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Letters 148 433^446Yaxley G M Green D H amp Kamenetsky V (1998) Carbonatite

metasomatism in the southeastern Australian lithosphere Journalof Petrology 39 1917^1930

Zanetti A Mazzucchelli M Rivalenti G amp Vannucci R (1999)The Finero phlogopite^peridotite massif an example of subduc-tion-related metasomatism Contributions to Mineralogy and Petrology

134 107^122Zanetti Z Vannucci R Bottazzi P Oberti R amp Ottolini L

(1996) Infiltration metasomatism at Lherz as monitored by sys-tematic ion-microprobe investigations close to a hornblendite veinChemical Geology 134 113^133

Zinngrebe E amp Foley S F (1995) Metasomatism in mantle xenolithsfrom Gees West Eifel Germany evidence for the genesis of calc-alkaline glasses and metasomatic Ca-enrichment Contributions to

Mineralogy and Petrology 122 79^96

36

surface (Kaeser et al 2006) Similar to the process invokedfor the Ethiopian xenoliths continuous melt^rock reactionand fractionation could have led to the formation of smallfractions of highly evolved melts infiltrating andmetasomatizing the cold and deformed uppermost mantlerepresented by Group III harzburgites (Fig 19 see alsoIonov et al 2006 for a similar model) A relationshipbetween metasomatism in Group II and III peridotites issupported by several xenoliths showing transitionaltextures and trace element signatures (ie strong U^Th^LREE enrichment in some Group II clinopyroxenesKaeser et al 2006)Alternatively Si- and volatile-enriched melts may be

residual liquids from the crystallization of alkali basalticor basanitic veins (ie hornblendite or clinopyroxenite)ultimately metasomatizing the adjacent peridotitic wall-rock (eg Harte et al 1993 Wulff-Pedersen et al 1999Bodinier et al 2004) This remains speculative howeveras such veins or dykes have not yet been reported fromMarsabit It can thus not be resolved whether early meta-somatism reflects a large kilometre-scale process (eg anupwelling melt percolation front as proposed for themantle beneath southern Ethiopia Bedini et al 1997) orwhether metasomatism is restricted to narrower zonesadjacent to small (centimetre- to metre-scale) melt con-duits (eg wall-rock metasomatism)Extensive melt^rock reaction in the mantle beneath the

EARS however is indicated by numerous xenolith suitescontaining abundant and variably metasomatized rocks(eg Tanzania Dawson amp Smith 1988 Rudnick et al1993 Lee amp Rudnick 1999 Dawson 2002 and referencestherein Ethiopia Bedini et al1997 Rooney et al 2005) Inthe case of northern Kenya seismic studies point to abroad zone of low seismic wave velocities in the relativelyshallow mantle 600 km across and reaching depths of540 km depth beneath LakeTurkana and 90 km beneathMarsabit (see Simiyu amp Keller 1997) This anomaly iscommonly interpreted as a zone of hot rising mantle con-taining small amounts (3^5) of melt (Green et al 1991)Upwelling of such melts into the lithospheric mantle fol-lowed by reaction could thus account for the metasoma-tism seen in the Marsabit mantle xenoliths Suchinteraction of sub-lithospheric melts or fluids (astheno-spheric or plume-related) with colder lithosphericmaterial is one of the major processes invoked toaccount for the heterogeneous isotopic composition ofprimitive lavas from numerous parts of the EARS(eg Macdonald et al 2001 Furman et al 2006 and refer-ences therein)

ACKNOWLEDGEMENTSWe would like to thank Edwin Gnos and Alfons Berger(Bern) and Hans-Peter Meyer (Heidelberg) for assistancewith microprobe work as well as Andrecurren Villard

who provided top-quality thin sections Discussion withOthmar Mulaquo ntener Arjan Dijkstra Rainer Altherr Jean-Louis Bodinier and Alan Woodland helped to clarifymany aspects of this work We thank Hilary DownesTanya Furman and Dmitri Ionov for constructive reviewsand MarjorieWilson for valuable comments and editorialhandling This publication is part of a project financed bythe Swiss National Science Foundation (grant No 200021-1006471 to AK) Further financial support from the SNF(grants 21-2657989 and 200021-1034791) for the electronmicroprobe facility at the Institute of Geological SciencesUniversity of Bern is acknowledged

SUPPLEMENTARY DATASupplementary data for this paper are available at Journalof Petrology online

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Frey F A amp Green D H (1974) Mineralogy geochemistry andorigin of lherzolite inclusions in Victorian basanites Geochimica et

Cosmochimica Acta 38 1023^1059Frost D J amp Wood B J (1998) The fugacity of carbon dioxide and

the graphitediamond CO equilibrium between 35 and 77 kbar at925 to 16508C (vol 61 pg 1565 1997) Geochimica et Cosmochimica Acta62 725^725

Fumagalli P amp Poli S (2005) Experimentally determined phaserelations in hydrous peridotites to 65GPa and their consequenceson the dynamics of subduction zones Journal of Petrology 46555^578

Furman T Kaleta K M Bryce J G amp Hanan B B (2006)Tertiary mafic lavas of Turkana Kenya constraints on EastAfrican plume structure and the occurrence of high-mu volcanismin Africa Journal of Petrology 47 1221^1244

Gasparik T (1987) Orthopyroxene thermobarometry in simple andcomplex systems Contributions to Mineralogy and Petrology 96 357^370

Godard M Bodinier J-L amp Vasseur G (1995) Effects of mineral-ogical reactions on trace element redistributions in mantle rocksduring percolation processes a chromatographic approach Earthand Planetary Science Letters 133 449^461

Green D H amp Wallace M E (1988) Mantle metasomatism byephemeral carbonatite melts Nature 336 459^462

GreenWV Achauer U amp Meyer R P (1991) A 3-dimensional seis-mic image of the crust and upper mantle beneath the Kenya riftNature 354 199^203

Grecurren goire M McInnes B I A amp OrsquoReilly S Y (2001) Hydrousmetasomatism of oceanic sub-arc mantle Lihir Papua NewGuinea Part 2 Trace element characteristics of slab-derivedfluids Lithos 59 91^108

Grecurren goire M Bell D R amp Le Roex A P (2002) Trace element geo-chemistry of phlogopite-rich mafic mantle xenoliths their classifi-cation and their relationship to phlogopite-bearing peridotites andkimberlites Contributions to Mineralogy and Petrology 142 603^625

GroveT L Chatterjee N Parman SW amp Mecurren dard E (2006) Theinfluence of H2O on mantle wedge melting Earth and Planetary

Science Letters 249 74^89Harte B Hunter R H amp Kinny P D (1993) Melt geometry move-

ment and crystallization in relation to mantle dykes veins andmetasomatism Philosophical Transactions of the Royal Society of LondonSeries A 342 1^21

Hauri E H Shimizu N Dieu J J amp Hart S R (1993) Evidencefor hotspot-related carbonatite metasomatism in the oceanic uppermantle Nature 365 221^227

Henjes-Kunst F amp Altherr R (1992) Metamorphic petrology ofxenoliths from Kenya and northernTanzania and implications forgeotherms and lithospheric structures Journal of Petrology 331125^1156

Ionov D A (1998) Trace element composition of mantle-derived car-bonates and coexisting phases in peridotite xenoliths from alkalibasalts Journal of Petrology 39 1931^1941

Ionov D A amp Harmer R E (2002) Trace element distributionin calcite^dolomite carbonatites from Spitskop inferences fordifferentiation of carbonatite magmas and the origin ofcarbonates in mantle xenoliths Earth and Planetary Science Letters

198 495^510Ionov D A amp Hofmann A W (1995) Nb^Ta-rich mantle amphi-

boles and micasccedilimplications for subduction-related metasomatictrace-element fractionations Earth and Planetary Science Letters 131341^356

Ionov D A Dupuy C OrsquoReilly S Y Kopylova M G ampGenshaft Y S (1993) Carbonated peridotite xenoliths fromSpitsbergen implications for trace element signature of mantle

33

carbonate metasomatism Earth and Planetary Science Letters 119283^297

Ionov D A Hofmann AW amp Shimizu N (1994) Metasomatism-induced melting in mantle xenoliths from Mongolia Journal of

Petrology 35 753^785Ionov D A OrsquoReilly SY GenshaftY S amp Kopylova M G (1996)

Carbonate-bearing mantle peridotite xenoliths from Spitsbergenphase relationship mineral compositions and trace-element resi-dence Contributions to Mineralogy and Petrology 125 375^392

Ionov D A Griffin W L amp OrsquoReilly S Y (1997) Volatile-bearingminerals and lithophile trace elements in the upper mantleChemical Geology 141 153^184

Ionov D A Bodinier J-L Mukasa S B amp Zanetti A (2002)Mechanisms and sources of mantle metasomatism major andtrace element compositions of peridotite xenoliths fromSpitsbergen in the context of numerical modelling Journal of

Petrology 43 2219^2259Ionov D A Prikhodko V S Bodinier J L Sobolev A V amp

Weis D (2005) Lithospheric mantle beneath the south-easternSiberian craton petrology of peridotite xenoliths in basalts fromthe Tokinsky Stanovik Contributions to Mineralogy and Petrology 149647^665

Ionov D A Chazot G Chauvel C Merlet C amp Bodinier J L(2006) Trace element distribution in peridotite xenoliths fromTokSE Siberian craton a record of pervasive multi-stage metasoma-tism in shallow refractory mantle Geochimica et Cosmochimica Acta

70 1231^1260Jones A P Smith J V amp Dawson J B (1983) Glasses in mantle

xenoliths from Olmani Tanzania Journal of Geology 91 167^178Kaeser B (2006) Mantle xenoliths from the Marsabit volcanic field a

case study on the evolution of the lithospheric mantle in a continen-tal rift environment PhD thesis University of Neuchaldquo tel 207 pp

Kaeser B Kalt A amp Pettke T (2006) Evolution of the lithosphericmantle beneath the Marsabit volcanic field (northern Kenya) con-straints from textural P^T and geochemical studies on xenolithsJournal of Petrology 47 2149^2184

Kelemen P B Whitehead J A Aharonov E amp Jordahl K A(1995) Experiments on flow focusing in soluble porous mediawith applications to melt extraction from the mantle Journal ofGeophysical Research 100 475^496

Kessel R Ulmer P Pettke T Schmidt MW amp Thompson A B(2005) The water^basalt system at 4 to 6GPa phase relations andsecond critical endpoint in a K-free eclogite at 700 to 14008C Earthand Planetary Science Letters 237 873^892

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King B C (1970) Vulcanicity and rift tectonics in east Africa InClifford T N amp Gass I G (eds) African Magmatism and TectonicsEdinburgh Oliver amp Boyd pp 263^285

Klemme S amp OrsquoNeill H S (2000) The near-solidus transition fromgarnet lherzolite to spinel lherzolite Contributions to Mineralogy and

Petrology 138 237^248LaTourretteT Hervig R L amp Holloway J R (1995) Trace-element

partitioning between amphibole phlogopite and basanite meltEarth and Planetary Science Letters 135 13^30

Laurora A Mazzucchelli M Rivalenti G Vannucci RZanetti A Barbieri M A amp Cingolani C A (2001)Metasomatism and melting in carbonated peridotite xenolithsfrom the mantle wedge the Gobernador Gregores case (southernPatagonia) Journal of Petrology 42 69^87

Leake B EWoolley A R Arps C E S et al (1997) Nomenclatureof amphiboles report of the subcommittee on amphiboles of the

International Mineralogical Association commission on newminerals and mineral names American Mineralogist 82 1019^1037

Lee C-T amp Rudnick R (1999) Compositionally stratified cratoniclithosphere petrology and geochemistry of peridotite xenolithsfrom the Labait Volcano Tanzanie In Gurney J Gurney J LPascoe M D amp Richardson S H (eds) Proceedings of the 7th inter-national Kimberlite Conference CapeTown Red Roof Design 503^521

Lee C-T Rudnick R L McDonough W F amp Horn I (2000)Petrologic and geochemical investigation of carbonates in perido-tite xenoliths from northeastern Tanzania Contributions to

Mineralogy and Petrology 139 470^484Lee W J amp Wyllie P J (1996) Liquid immiscibility in the join

NaAlSi3O8^CaCO3 to 25GPa and the origin of calciocarbonatitemagmas Journal of Petrology 37 1125^1152

Lee W J amp Wyllie P J (2000) The system CaO^MgO^SiO2^CO2

at 1GPa metasomatic wehrlites and primary carbonatite magmasContributions to Mineralogy and Petrology 138 214^228

LenskyNGNieboRWHolloway JRLyakhovskyVampNavonO(2006) Bubble nucleation as a trigger for xenolith entrapment inmantlemeltsEarth andPlanetary Science Letters 245 278^288

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Manning C E (2004) The chemistry of subduction-zone fluids Earthand Planetary Science Letters 223 1^16

Maury R C Defant M J amp Joron J L (1992) Metasomatism ofthe sub-arc mantle inferred from trace-elements in Philippinexenoliths Nature 360 661^663

McDonoughW F amp Sun S S (1995) The composition of the EarthChemical Geology 120 223^253

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Mecurren dard E Schmidt M W Schiano P amp Ottolini L (2006)Melting of amphibole-bearing wehrlites an experimental study onthe origin of ultra-calcic nepheline-normative melts Journal of

Petrology 47 481^504Meert J G (2003) A synopsis of events related to the assembly of

eastern GondwanaTectonophysics 362 1^40Menzies M A amp Hawkesworth C J (1987) Mantle Metsomatism

London Academic PressMoine B N Grecurren goire M OrsquoReilly S Y Sheppard S M F amp

Cottin JY (2001) High field strength element fractionation in theupper mantle evidence from amphibole-rich composite mantlexenoliths from the Kerguelen Islands (Indian Ocean) Journal ofPetrology 42 2145^2167

Moine B N Grecurren goire M OrsquoReilly SY Delpech G Sheppard SM F Lorand J P Renac C Giret A amp Cottin J Y (2004)Carbonatite melt in the upper mantle beneath the KerguelenArchipelago Lithos 75 239^252

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34

Morley C K (1999) Tectonic evolution of the East African RiftSystem and the modifying influence of magmatism a reviewActaVulcanologica 11 1^19

Navon O amp Stolper E (1987) Geochemical consequences of meltpercolation the upper mantle as a chromatographic columnJournal of Geology 95 285^307

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Neumann E-RWulff-Pedersen E Pearson N J amp Spencer E A(2002) Mantle xenoliths from Tenerife (Canary Islands)evidence for reactions between mantle peridotites and siliciccarbonatite melts inducing Ca metasomatism Journal of Petrology43 825^857

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metasomatism American Mineralogist 79 148^153Olker B (2001) Entwicklung und Anwendung eines Computerpro-

grammes zur numerischen Modellierung von Diffusionsprofilen inMineralkolaquo rnern PhD thesis University of Heidelberg 269 pp

OrsquoNeill H S (1987) Quartz^fayalite^iron and quartz^fayalite^magnetite equilibria and the free-energy of formation of fayalite(Fe2SiO4) and magnetite (Fe3O4) American Mineralogist 72 67^75

OrsquoReilly SY amp GriffinW L (2000) Apatite in the mantle implica-tions for metasomatic processes and high heat production inPhanerozoic mantle Lithos 53 217^232

Parkinson I J Arculus R J amp Eggins S M (2003) Peridotite xeno-liths from Grenada Lesser Antilles island arc Contributions to

Mineralogy and Petrology 146 241^262Pearson D G Boyd F R Haggerty S E Pasteris J D Field S

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Popp R KVirgo D amp Phillips MW (1995) H deficiency in kaer-sutitic amphiboles experimental verification American Mineralogist

80 1347^1350Powell W Zhang M OrsquoReilly S Y amp Tiepolo M (2004) Mantle

amphibole trace-element and isotope signatures trace multiplemetasomatic episodes in lithospheric mantle western VictoriaAustralia Lithos 75 141^171

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and source permeability Journal of Geophysical ResearchccedilSolid Earth

100 15601^15610Rivalenti G Zanetti A Mazzucchelli M Vannucci R amp

Cingolani C A (2004) Equivocal carbonatite markers in themantle xenoliths of the Patagonia backarc the GobernadorGregores case (Santa Cruz Province Argentina) Contributions toMineralogy and Petrology 147 647^670

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Science Letters 114 463^475Rudnick R L IrelandT R Gehrels G Irving A J Chesley JT

amp Hanchar J M (1999) Dating mantle metasomatism U^Pb geo-chronology of zircons in cratonic mantle xenoliths from Montanaand Tanzania In Gurney J Gurney J L Pascoe M D ampRichardson S H (eds) Proceedings of the 7th international Kimberlite

Conference CapeTown Red Roof Design pp 728^735Shaw C S J amp Eyzaguirre J (2000) Origin of megacrysts in the

mafic alkaline lavas of the West Eifel volcanic field GermanyLithos 50 75^95

Shaw C S amp Klulaquo gel A (2002) The pressure and temperature condi-tions and timing of glass formation in mantle-derived xenolithsfrom Baarley West Eifel Germany the case for amphibole break-down lava infiltration and mineral^melt reaction Mineralogy and

Petrology 74 163^187Shaw C S Thibault Y Edgar A D amp Lloyd F E (1998)

Mechanism of orthopyroxene dissolution in silica-undersaturatedmelts at 1 atmosphere and implications for the origin of silica-richglass in mantle xenoliths Contributions to Mineralogy and Petrology 132354^370

Shaw S J Heidelbach F amp Dingwell D B (2006) The origin ofreaction textures in mantle peridotite xenoliths from Sal IslandCape Verde the case for lsquometasomatismrsquo by the host lavaContributions to Mineralogy and Petrology 151 681^697

Simiyu S M amp Keller G R (1997) An integrated analysis of lithos-pheric structure across the East African plateau based on gravityanomalies and recent seismic studiesTectonophysics 278 291^313

Smith D (1999) Temperatures and pressures of mineral equilibrationin peridotite xenoliths review discussion and implications In FeiY Bertka C M amp Mysen B O (eds) Mantle Petrology Field

Observation and High-Pressure Experimentation A Tribute to Francis R

(Joe) Boyd Geochemical Society Special Publications 6 171^188Smith D Riter J C A amp Mertzman S A (1999)Water^rock inter-

actions orthopyroxene growth and Si-enrichment in the mantleevidence in xenoliths from the Colorado Plateau southwesternUnited States (vol 165 pg 45 1999) Earth and Planetary Science

Letters 167 347^356Spera F J (1984) Carbon dioxide in petrogenesis 3 Role of volatiles

in the ascent of alkaline magma with special reference to xenolith-bearing mafic lavas Contributions to Mineralogy and Petrology 88217^232

Tiepolo M Bottazzi P Foley S F Oberti R Vannucci R ampZanetti A (2001) Fractionation of Nb and Ta from Zr and Hf atmantle depths the role of titanian pargasite and kaersutiteJournal of Petrology 42 221^232

Vannucci R Piccardo G B Rivalenti G Zanetti ARampone E Ottolini L Oberti R Mazzucchelli M ampBottazzi P (1995) Origin of LREE-depleted amphiboles inthe subcontinental mantle Geochimica et Cosmochimica Acta 591763^1771

35

Varela M E Clocchiatti R Kurat G amp Schiano P (1999)Silicic glasses in hydrous and anhydrous mantle xenoliths fromWestern Victoria at least two different sources Chemical Geology

153 151^169Vernie res J Godard M amp Bodinier J L (1997) A plate model for

the simulation of trace element fractionation during partial melt-ing and magma transport in the Earthrsquos upper mantle Journal ofGeophysical Research 102 24771^24784

Volker F (1990) Geochemie der quartalaquo ren Vulkanite aufder Ostschulter des Kenia-Rifts PhD thesis University ofKarlsruhe

Witt-Eickschen G Seck H A Mezger K Eggins S M ampAltherr R (2003) Lithospheric mantle evolution beneath theEifel (Germany) constraints from Sr^Nd^Pb isotopes and traceelement abundances in spinel peridotite and pyroxenite xenolithsJournal of Petrology 44 1077^1095

Woermann E amp Rosenhauer M (1985) Fluid phases and the redoxstate of the Earthrsquos mantleccedilextrapolations based on experimentalphase-theoretical and petrological data Fortschritte der Mineralogie

63 263^349Wood B J amp Virgo D (1989) Upper mantle oxidation state ferric

iron contents of lherzolite spinels by 57Fe Molaquo ssbauer spectroscopyand resultant oxygen fugacities Geochimica et Cosmochimica Acta 531277^1291

Woodland A B Kornprobst J McPherson E Bodinier J L ampMenzies A (1996) Metasomatic interactions in the lithosphericmantle petrologic evidence from the Lherz massif FrenchPyrenees Chemical Geology 134 83^112

Wulff-Pedersen E Neumann E-R Vannucci R Bottazzi P ampOttolini L (1999) Silicicmelts producedby reactionbetweenperido-tite and infiltrating basaltic melts ion probe data on glasses andminerals in veined xenoliths from La Palma Canary IslandsContributions toMineralogyandPetrology137 59^82

Yaxley G M amp Kemenetsky V (1999) In situ origin for glass inmantle xenoliths from southeastern Australia insights from traceelement compositions of glasses and metasomatic phases Earth andPlanetary Science Letters 172 97^109

Yaxley G M Kamenetsky V Green D H amp Falloon T J (1997)Glasses in mantle xenoliths from western Victoria Australia andtheir relevance to mantle processes Earth and Planetary Science

Letters 148 433^446Yaxley G M Green D H amp Kamenetsky V (1998) Carbonatite

metasomatism in the southeastern Australian lithosphere Journalof Petrology 39 1917^1930

Zanetti A Mazzucchelli M Rivalenti G amp Vannucci R (1999)The Finero phlogopite^peridotite massif an example of subduc-tion-related metasomatism Contributions to Mineralogy and Petrology

134 107^122Zanetti Z Vannucci R Bottazzi P Oberti R amp Ottolini L

(1996) Infiltration metasomatism at Lherz as monitored by sys-tematic ion-microprobe investigations close to a hornblendite veinChemical Geology 134 113^133

Zinngrebe E amp Foley S F (1995) Metasomatism in mantle xenolithsfrom Gees West Eifel Germany evidence for the genesis of calc-alkaline glasses and metasomatic Ca-enrichment Contributions to

Mineralogy and Petrology 122 79^96

36

metasomatism at Lherz France contemporaneous derivates of sili-cate melt^harzburgite reaction Journal of Petrology 45 299^320

Brey G P amp Kolaquo hler T (1990) Geothermobarometry in four-phaselherzolites II New thermobarometers and practical assessment ofexisting thermobarometers Journal of Petrology 31 1353^1378

Brotzu P Morbideli L Nicoletti M Piccirillo E M ampTraversa G (1984) Miocene to Quaternary volcanism in easternAfrica sequence and chronologyTectonophysics 101 75^86

Chazot G Menzies M A amp Harte B (1996a) Determination ofpartition coefficients between apatite clinopyroxene amphiboleand melt in natural spinel lherzolites fromYemen implications forwet melting of the lithospheric mantle Geochimica et Cosmochimica

Acta 60 423^437Chazot G Menzies M A amp Harte B (1996b) Silicate glasses in

spinel lherzolites from Yemen origin and chemical compositionChemical Geology 134 159^179

Coltorti M Bonadiman C Hinton RW Siena F amp Upton B GJ (1999) Carbonatite metasomatism of the oceanic uppermantle evidence from clinopyroxenes and glasses in ultramaficxenoliths of Grande Comore Indian Ocean Journal of Petrology 40133^165

Coltorti M Beccaluva L Bonadiman C Salvini L amp Siena F(2000) Glasses in mantle xenoliths as geochemical indicatorsof metasomatic agents Earth and Planetary Science Letters 183303^320

Costa F Dungan M A amp Singer B S (2001) Magmatic Na-richphlogopite in a suite of gabbroic crustal xenoliths fromVolcacurren n SanPedro Chilean Andes evidence for a solvus relation between phlo-gopite and aspidolite American Mineralogist 86 29^35

Dautria J-M Dupuy C Takherist D amp Dostal J (1992)Carbonate metasomatism in the lithospheric mantle peridotiticxenoliths from a melilitic district of the Sahara basin Contributionsto Mineralogy and Petrology 111 37^52

Dawson J B (2002) Metasomatism and partial melting in upper-mantle peridotite xenoliths from the Lashaine volcano NorthernTanzania Journal of Petrology 43 1749^1777

Dawson J B amp Smith JV (1988) Metasomatized and veined uppermantle xenoliths from Pello Hill Tanzania evidence for anoma-lously light mantle beneath the Tanzanian sector of the EastAfrican rift valley Contributions to Mineralogy and Petrology 100510^527

Demecurren ny A Vennemann T W Hegner E Nagy G Milton J AEmbey-Isztin A Homonnay Z amp Dobosi G (2004) Traceelement and C^O^Sr^Nd isotope evidence for subduction relatedcarbonate^silicate melts in mantle xenoliths (Pannonian BasinHungary) Lithos 75 89^113

Draper D S amp Green T H (1997) P^T phase relations of silicicalkaline aluminous mantle-xenolith glasses under anhydrous andC^O^H fluid-saturated conditions Journal of Petrology 381187^1224

Eggler D H amp Baker D R (1982) Reduced volatiles in the systemC^O^H implications to mantle melting fluid formation and dia-mond genesis In Akimoto S amp Manghnani M H (eds) High

Pressure Research in Geophysics Tokyo Center for AcademicPublications Japan pp 237^250

Ferraris C Grobecurren ty B Frulaquo h-Green G L amp Wessicken R (2004)Intergrowth of graphite within phlogopite from Finero ultramaficcomplex (ItalianWestern Alps) implications for mantle crystalliza-tion of primary-texture mica European Journal of Mineralogy 16899^908

Francis D M (1976) Amphibole pyroxenite xenoliths cumulate orreplacement phenomena from the upper mantle Nunivak IslandAlaska Contributions to Mineralogy and Petrology 58 51^61

Frey F A amp Green D H (1974) Mineralogy geochemistry andorigin of lherzolite inclusions in Victorian basanites Geochimica et

Cosmochimica Acta 38 1023^1059Frost D J amp Wood B J (1998) The fugacity of carbon dioxide and

the graphitediamond CO equilibrium between 35 and 77 kbar at925 to 16508C (vol 61 pg 1565 1997) Geochimica et Cosmochimica Acta62 725^725

Fumagalli P amp Poli S (2005) Experimentally determined phaserelations in hydrous peridotites to 65GPa and their consequenceson the dynamics of subduction zones Journal of Petrology 46555^578

Furman T Kaleta K M Bryce J G amp Hanan B B (2006)Tertiary mafic lavas of Turkana Kenya constraints on EastAfrican plume structure and the occurrence of high-mu volcanismin Africa Journal of Petrology 47 1221^1244

Gasparik T (1987) Orthopyroxene thermobarometry in simple andcomplex systems Contributions to Mineralogy and Petrology 96 357^370

Godard M Bodinier J-L amp Vasseur G (1995) Effects of mineral-ogical reactions on trace element redistributions in mantle rocksduring percolation processes a chromatographic approach Earthand Planetary Science Letters 133 449^461

Green D H amp Wallace M E (1988) Mantle metasomatism byephemeral carbonatite melts Nature 336 459^462

GreenWV Achauer U amp Meyer R P (1991) A 3-dimensional seis-mic image of the crust and upper mantle beneath the Kenya riftNature 354 199^203

Grecurren goire M McInnes B I A amp OrsquoReilly S Y (2001) Hydrousmetasomatism of oceanic sub-arc mantle Lihir Papua NewGuinea Part 2 Trace element characteristics of slab-derivedfluids Lithos 59 91^108

Grecurren goire M Bell D R amp Le Roex A P (2002) Trace element geo-chemistry of phlogopite-rich mafic mantle xenoliths their classifi-cation and their relationship to phlogopite-bearing peridotites andkimberlites Contributions to Mineralogy and Petrology 142 603^625

GroveT L Chatterjee N Parman SW amp Mecurren dard E (2006) Theinfluence of H2O on mantle wedge melting Earth and Planetary

Science Letters 249 74^89Harte B Hunter R H amp Kinny P D (1993) Melt geometry move-

ment and crystallization in relation to mantle dykes veins andmetasomatism Philosophical Transactions of the Royal Society of LondonSeries A 342 1^21

Hauri E H Shimizu N Dieu J J amp Hart S R (1993) Evidencefor hotspot-related carbonatite metasomatism in the oceanic uppermantle Nature 365 221^227

Henjes-Kunst F amp Altherr R (1992) Metamorphic petrology ofxenoliths from Kenya and northernTanzania and implications forgeotherms and lithospheric structures Journal of Petrology 331125^1156

Ionov D A (1998) Trace element composition of mantle-derived car-bonates and coexisting phases in peridotite xenoliths from alkalibasalts Journal of Petrology 39 1931^1941

Ionov D A amp Harmer R E (2002) Trace element distributionin calcite^dolomite carbonatites from Spitskop inferences fordifferentiation of carbonatite magmas and the origin ofcarbonates in mantle xenoliths Earth and Planetary Science Letters

198 495^510Ionov D A amp Hofmann A W (1995) Nb^Ta-rich mantle amphi-

boles and micasccedilimplications for subduction-related metasomatictrace-element fractionations Earth and Planetary Science Letters 131341^356

Ionov D A Dupuy C OrsquoReilly S Y Kopylova M G ampGenshaft Y S (1993) Carbonated peridotite xenoliths fromSpitsbergen implications for trace element signature of mantle

33

carbonate metasomatism Earth and Planetary Science Letters 119283^297

Ionov D A Hofmann AW amp Shimizu N (1994) Metasomatism-induced melting in mantle xenoliths from Mongolia Journal of

Petrology 35 753^785Ionov D A OrsquoReilly SY GenshaftY S amp Kopylova M G (1996)

Carbonate-bearing mantle peridotite xenoliths from Spitsbergenphase relationship mineral compositions and trace-element resi-dence Contributions to Mineralogy and Petrology 125 375^392

Ionov D A Griffin W L amp OrsquoReilly S Y (1997) Volatile-bearingminerals and lithophile trace elements in the upper mantleChemical Geology 141 153^184

Ionov D A Bodinier J-L Mukasa S B amp Zanetti A (2002)Mechanisms and sources of mantle metasomatism major andtrace element compositions of peridotite xenoliths fromSpitsbergen in the context of numerical modelling Journal of

Petrology 43 2219^2259Ionov D A Prikhodko V S Bodinier J L Sobolev A V amp

Weis D (2005) Lithospheric mantle beneath the south-easternSiberian craton petrology of peridotite xenoliths in basalts fromthe Tokinsky Stanovik Contributions to Mineralogy and Petrology 149647^665

Ionov D A Chazot G Chauvel C Merlet C amp Bodinier J L(2006) Trace element distribution in peridotite xenoliths fromTokSE Siberian craton a record of pervasive multi-stage metasoma-tism in shallow refractory mantle Geochimica et Cosmochimica Acta

70 1231^1260Jones A P Smith J V amp Dawson J B (1983) Glasses in mantle

xenoliths from Olmani Tanzania Journal of Geology 91 167^178Kaeser B (2006) Mantle xenoliths from the Marsabit volcanic field a

case study on the evolution of the lithospheric mantle in a continen-tal rift environment PhD thesis University of Neuchaldquo tel 207 pp

Kaeser B Kalt A amp Pettke T (2006) Evolution of the lithosphericmantle beneath the Marsabit volcanic field (northern Kenya) con-straints from textural P^T and geochemical studies on xenolithsJournal of Petrology 47 2149^2184

Kelemen P B Whitehead J A Aharonov E amp Jordahl K A(1995) Experiments on flow focusing in soluble porous mediawith applications to melt extraction from the mantle Journal ofGeophysical Research 100 475^496

Kessel R Ulmer P Pettke T Schmidt MW amp Thompson A B(2005) The water^basalt system at 4 to 6GPa phase relations andsecond critical endpoint in a K-free eclogite at 700 to 14008C Earthand Planetary Science Letters 237 873^892

Key R M Rop B P amp Rundle C C (1987)The development of thelate Cenozoic alkali basaltic Marsabit shield volcano northernKenya Journal of African Earth Sciences 6 475^491

King B C (1970) Vulcanicity and rift tectonics in east Africa InClifford T N amp Gass I G (eds) African Magmatism and TectonicsEdinburgh Oliver amp Boyd pp 263^285

Klemme S amp OrsquoNeill H S (2000) The near-solidus transition fromgarnet lherzolite to spinel lherzolite Contributions to Mineralogy and

Petrology 138 237^248LaTourretteT Hervig R L amp Holloway J R (1995) Trace-element

partitioning between amphibole phlogopite and basanite meltEarth and Planetary Science Letters 135 13^30

Laurora A Mazzucchelli M Rivalenti G Vannucci RZanetti A Barbieri M A amp Cingolani C A (2001)Metasomatism and melting in carbonated peridotite xenolithsfrom the mantle wedge the Gobernador Gregores case (southernPatagonia) Journal of Petrology 42 69^87

Leake B EWoolley A R Arps C E S et al (1997) Nomenclatureof amphiboles report of the subcommittee on amphiboles of the

International Mineralogical Association commission on newminerals and mineral names American Mineralogist 82 1019^1037

Lee C-T amp Rudnick R (1999) Compositionally stratified cratoniclithosphere petrology and geochemistry of peridotite xenolithsfrom the Labait Volcano Tanzanie In Gurney J Gurney J LPascoe M D amp Richardson S H (eds) Proceedings of the 7th inter-national Kimberlite Conference CapeTown Red Roof Design 503^521

Lee C-T Rudnick R L McDonough W F amp Horn I (2000)Petrologic and geochemical investigation of carbonates in perido-tite xenoliths from northeastern Tanzania Contributions to

Mineralogy and Petrology 139 470^484Lee W J amp Wyllie P J (1996) Liquid immiscibility in the join

NaAlSi3O8^CaCO3 to 25GPa and the origin of calciocarbonatitemagmas Journal of Petrology 37 1125^1152

Lee W J amp Wyllie P J (2000) The system CaO^MgO^SiO2^CO2

at 1GPa metasomatic wehrlites and primary carbonatite magmasContributions to Mineralogy and Petrology 138 214^228

LenskyNGNieboRWHolloway JRLyakhovskyVampNavonO(2006) Bubble nucleation as a trigger for xenolith entrapment inmantlemeltsEarth andPlanetary Science Letters 245 278^288

Luth RW (2003) Mantle volatilesccedildistribution and consequencesIn Holland H D amp Turekian K K (eds) Treatise on GeochemistryAmsterdam Elsevier pp 319^361

Maale S amp Printzlau I (1979) Natural partial melting of spinellherzolite Journal of Petrology 20 727^741

Macdonald R Rogers N W Fitton J G Black S amp Smith M(2001) Plume^lithosphere interactions in the generation of thebasalts of the Kenya Rift East Africa Journal of Petrology 42877^900

Manning C E (2004) The chemistry of subduction-zone fluids Earthand Planetary Science Letters 223 1^16

Maury R C Defant M J amp Joron J L (1992) Metasomatism ofthe sub-arc mantle inferred from trace-elements in Philippinexenoliths Nature 360 661^663

McDonoughW F amp Sun S S (1995) The composition of the EarthChemical Geology 120 223^253

McInnes B I A Grecurren goire M Binns R A Herzig P M ampHannington M D (2001) Hydrous metasomatism of oceanic sub-arc mantle Lihir Papua New Guinea petrology and geochemistryof fluid-metasomatised mantle wedge xenoliths Earth and PlanetaryScience Letters 188 169^183

Mecurren dard E Schmidt M W Schiano P amp Ottolini L (2006)Melting of amphibole-bearing wehrlites an experimental study onthe origin of ultra-calcic nepheline-normative melts Journal of

Petrology 47 481^504Meert J G (2003) A synopsis of events related to the assembly of

eastern GondwanaTectonophysics 362 1^40Menzies M A amp Hawkesworth C J (1987) Mantle Metsomatism

London Academic PressMoine B N Grecurren goire M OrsquoReilly S Y Sheppard S M F amp

Cottin JY (2001) High field strength element fractionation in theupper mantle evidence from amphibole-rich composite mantlexenoliths from the Kerguelen Islands (Indian Ocean) Journal ofPetrology 42 2145^2167

Moine B N Grecurren goire M OrsquoReilly SY Delpech G Sheppard SM F Lorand J P Renac C Giret A amp Cottin J Y (2004)Carbonatite melt in the upper mantle beneath the KerguelenArchipelago Lithos 75 239^252

Morishita T Arai S amp Green D H (2003) Evolution oflow-Al orthopyroxene in the Horoman peridotite Japan an unu-sual indicator of metasomatizing fluids Journal of Petrology 441237^1246

34

Morley C K (1999) Tectonic evolution of the East African RiftSystem and the modifying influence of magmatism a reviewActaVulcanologica 11 1^19

Navon O amp Stolper E (1987) Geochemical consequences of meltpercolation the upper mantle as a chromatographic columnJournal of Geology 95 285^307

Neumann E-R amp Wulff-Pedersen E (1997) The origin of highlysilicic glass in mantle xenoliths from the Canary Islands Journal ofPetrology 38 1513^1539

Neumann E-RWulff-Pedersen E Pearson N J amp Spencer E A(2002) Mantle xenoliths from Tenerife (Canary Islands)evidence for reactions between mantle peridotites and siliciccarbonatite melts inducing Ca metasomatism Journal of Petrology43 825^857

Niida K amp Green D H (1999) Stability and chemical compositionof pargasitic amphibole in MORB pyrolite under upper mantleconditions Contributions to Mineralogy and Petrology 135 18^40

Norman M D (1998) Melting and metasomatism in the continentallithosphere laser ablation ICPMS analysis of minerals in spinellherzolites from eastern Australia Contributions to Mineralogy and

Petrology 130 240^255Odling NW A (1994) An experimental simulation of upper-mantle

metasomatism American Mineralogist 79 148^153Olker B (2001) Entwicklung und Anwendung eines Computerpro-

grammes zur numerischen Modellierung von Diffusionsprofilen inMineralkolaquo rnern PhD thesis University of Heidelberg 269 pp

OrsquoNeill H S (1987) Quartz^fayalite^iron and quartz^fayalite^magnetite equilibria and the free-energy of formation of fayalite(Fe2SiO4) and magnetite (Fe3O4) American Mineralogist 72 67^75

OrsquoReilly SY amp GriffinW L (2000) Apatite in the mantle implica-tions for metasomatic processes and high heat production inPhanerozoic mantle Lithos 53 217^232

Parkinson I J Arculus R J amp Eggins S M (2003) Peridotite xeno-liths from Grenada Lesser Antilles island arc Contributions to

Mineralogy and Petrology 146 241^262Pearson D G Boyd F R Haggerty S E Pasteris J D Field S

W Nixon P H amp Pokhilenko N P (1994) The characterizationand origin of graphite in cratonic lithospheric mantleccedila petrologi-cal carbon-isotope and Raman-spectroscopic study Contributions toMineralogy and Petrology 115 449^466

PearsonDG CanilDampShirey S B (2003)Mantle samples includedin volcanic rocks xenoliths and diamonds In Carlson RW (ed)Treatise on Geochemistry Amsterdam Elsevier pp171^275

Pettke T Halter W E Webster J D Aigner-Torres M ampHeinrich C A (2004) Accurate quantification of melt inclusionchemistry by LA-ICPMS a comparison with EMP and SIMS andthe adventages and possible limitations of these methods Lithos 78333^361

Popp R KVirgo D amp Phillips MW (1995) H deficiency in kaer-sutitic amphiboles experimental verification American Mineralogist

80 1347^1350Powell W Zhang M OrsquoReilly S Y amp Tiepolo M (2004) Mantle

amphibole trace-element and isotope signatures trace multiplemetasomatic episodes in lithospheric mantle western VictoriaAustralia Lithos 75 141^171

Prouteau G amp Scaillet B (2003) Experimental constraints onthe origin of the 1991 Pinatubo dacite Journal of Petrology 442203^2241

Rampone E Bottazzi P amp Ottolini L (1991) Complementary Tiand Zr anomalies in orthopyroxene and clinopyroxene frommantle peridotites Nature 354 518^520

Rapp R P (1995) Amphibole-out phase-boundary in partiallymelted metabasalt its control over liquid fraction and composition

and source permeability Journal of Geophysical ResearchccedilSolid Earth

100 15601^15610Rivalenti G Zanetti A Mazzucchelli M Vannucci R amp

Cingolani C A (2004) Equivocal carbonatite markers in themantle xenoliths of the Patagonia backarc the GobernadorGregores case (Santa Cruz Province Argentina) Contributions toMineralogy and Petrology 147 647^670

RooneyT O FurmanTYirgu G amp Ayalew D (2005) Structure ofthe Ethiopian lithosphere xenolith evidence in the Main EthiopianRift Geochimica et Cosmochimica Acta 69 3889^3910

Rudnick R L McDonough W F amp Chappell B W (1993)Carbonatite metasomatism in the northernTanzanian mantle pet-rographic and geochemical characteristics Earth and Planetary

Science Letters 114 463^475Rudnick R L IrelandT R Gehrels G Irving A J Chesley JT

amp Hanchar J M (1999) Dating mantle metasomatism U^Pb geo-chronology of zircons in cratonic mantle xenoliths from Montanaand Tanzania In Gurney J Gurney J L Pascoe M D ampRichardson S H (eds) Proceedings of the 7th international Kimberlite

Conference CapeTown Red Roof Design pp 728^735Shaw C S J amp Eyzaguirre J (2000) Origin of megacrysts in the

mafic alkaline lavas of the West Eifel volcanic field GermanyLithos 50 75^95

Shaw C S amp Klulaquo gel A (2002) The pressure and temperature condi-tions and timing of glass formation in mantle-derived xenolithsfrom Baarley West Eifel Germany the case for amphibole break-down lava infiltration and mineral^melt reaction Mineralogy and

Petrology 74 163^187Shaw C S Thibault Y Edgar A D amp Lloyd F E (1998)

Mechanism of orthopyroxene dissolution in silica-undersaturatedmelts at 1 atmosphere and implications for the origin of silica-richglass in mantle xenoliths Contributions to Mineralogy and Petrology 132354^370

Shaw S J Heidelbach F amp Dingwell D B (2006) The origin ofreaction textures in mantle peridotite xenoliths from Sal IslandCape Verde the case for lsquometasomatismrsquo by the host lavaContributions to Mineralogy and Petrology 151 681^697

Simiyu S M amp Keller G R (1997) An integrated analysis of lithos-pheric structure across the East African plateau based on gravityanomalies and recent seismic studiesTectonophysics 278 291^313

Smith D (1999) Temperatures and pressures of mineral equilibrationin peridotite xenoliths review discussion and implications In FeiY Bertka C M amp Mysen B O (eds) Mantle Petrology Field

Observation and High-Pressure Experimentation A Tribute to Francis R

(Joe) Boyd Geochemical Society Special Publications 6 171^188Smith D Riter J C A amp Mertzman S A (1999)Water^rock inter-

actions orthopyroxene growth and Si-enrichment in the mantleevidence in xenoliths from the Colorado Plateau southwesternUnited States (vol 165 pg 45 1999) Earth and Planetary Science

Letters 167 347^356Spera F J (1984) Carbon dioxide in petrogenesis 3 Role of volatiles

in the ascent of alkaline magma with special reference to xenolith-bearing mafic lavas Contributions to Mineralogy and Petrology 88217^232

Tiepolo M Bottazzi P Foley S F Oberti R Vannucci R ampZanetti A (2001) Fractionation of Nb and Ta from Zr and Hf atmantle depths the role of titanian pargasite and kaersutiteJournal of Petrology 42 221^232

Vannucci R Piccardo G B Rivalenti G Zanetti ARampone E Ottolini L Oberti R Mazzucchelli M ampBottazzi P (1995) Origin of LREE-depleted amphiboles inthe subcontinental mantle Geochimica et Cosmochimica Acta 591763^1771

35

Varela M E Clocchiatti R Kurat G amp Schiano P (1999)Silicic glasses in hydrous and anhydrous mantle xenoliths fromWestern Victoria at least two different sources Chemical Geology

153 151^169Vernie res J Godard M amp Bodinier J L (1997) A plate model for

the simulation of trace element fractionation during partial melt-ing and magma transport in the Earthrsquos upper mantle Journal ofGeophysical Research 102 24771^24784

Volker F (1990) Geochemie der quartalaquo ren Vulkanite aufder Ostschulter des Kenia-Rifts PhD thesis University ofKarlsruhe

Witt-Eickschen G Seck H A Mezger K Eggins S M ampAltherr R (2003) Lithospheric mantle evolution beneath theEifel (Germany) constraints from Sr^Nd^Pb isotopes and traceelement abundances in spinel peridotite and pyroxenite xenolithsJournal of Petrology 44 1077^1095

Woermann E amp Rosenhauer M (1985) Fluid phases and the redoxstate of the Earthrsquos mantleccedilextrapolations based on experimentalphase-theoretical and petrological data Fortschritte der Mineralogie

63 263^349Wood B J amp Virgo D (1989) Upper mantle oxidation state ferric

iron contents of lherzolite spinels by 57Fe Molaquo ssbauer spectroscopyand resultant oxygen fugacities Geochimica et Cosmochimica Acta 531277^1291

Woodland A B Kornprobst J McPherson E Bodinier J L ampMenzies A (1996) Metasomatic interactions in the lithosphericmantle petrologic evidence from the Lherz massif FrenchPyrenees Chemical Geology 134 83^112

Wulff-Pedersen E Neumann E-R Vannucci R Bottazzi P ampOttolini L (1999) Silicicmelts producedby reactionbetweenperido-tite and infiltrating basaltic melts ion probe data on glasses andminerals in veined xenoliths from La Palma Canary IslandsContributions toMineralogyandPetrology137 59^82

Yaxley G M amp Kemenetsky V (1999) In situ origin for glass inmantle xenoliths from southeastern Australia insights from traceelement compositions of glasses and metasomatic phases Earth andPlanetary Science Letters 172 97^109

Yaxley G M Kamenetsky V Green D H amp Falloon T J (1997)Glasses in mantle xenoliths from western Victoria Australia andtheir relevance to mantle processes Earth and Planetary Science

Letters 148 433^446Yaxley G M Green D H amp Kamenetsky V (1998) Carbonatite

metasomatism in the southeastern Australian lithosphere Journalof Petrology 39 1917^1930

Zanetti A Mazzucchelli M Rivalenti G amp Vannucci R (1999)The Finero phlogopite^peridotite massif an example of subduc-tion-related metasomatism Contributions to Mineralogy and Petrology

134 107^122Zanetti Z Vannucci R Bottazzi P Oberti R amp Ottolini L

(1996) Infiltration metasomatism at Lherz as monitored by sys-tematic ion-microprobe investigations close to a hornblendite veinChemical Geology 134 113^133

Zinngrebe E amp Foley S F (1995) Metasomatism in mantle xenolithsfrom Gees West Eifel Germany evidence for the genesis of calc-alkaline glasses and metasomatic Ca-enrichment Contributions to

Mineralogy and Petrology 122 79^96

36

carbonate metasomatism Earth and Planetary Science Letters 119283^297

Ionov D A Hofmann AW amp Shimizu N (1994) Metasomatism-induced melting in mantle xenoliths from Mongolia Journal of

Petrology 35 753^785Ionov D A OrsquoReilly SY GenshaftY S amp Kopylova M G (1996)

Carbonate-bearing mantle peridotite xenoliths from Spitsbergenphase relationship mineral compositions and trace-element resi-dence Contributions to Mineralogy and Petrology 125 375^392

Ionov D A Griffin W L amp OrsquoReilly S Y (1997) Volatile-bearingminerals and lithophile trace elements in the upper mantleChemical Geology 141 153^184

Ionov D A Bodinier J-L Mukasa S B amp Zanetti A (2002)Mechanisms and sources of mantle metasomatism major andtrace element compositions of peridotite xenoliths fromSpitsbergen in the context of numerical modelling Journal of

Petrology 43 2219^2259Ionov D A Prikhodko V S Bodinier J L Sobolev A V amp

Weis D (2005) Lithospheric mantle beneath the south-easternSiberian craton petrology of peridotite xenoliths in basalts fromthe Tokinsky Stanovik Contributions to Mineralogy and Petrology 149647^665

Ionov D A Chazot G Chauvel C Merlet C amp Bodinier J L(2006) Trace element distribution in peridotite xenoliths fromTokSE Siberian craton a record of pervasive multi-stage metasoma-tism in shallow refractory mantle Geochimica et Cosmochimica Acta

70 1231^1260Jones A P Smith J V amp Dawson J B (1983) Glasses in mantle

xenoliths from Olmani Tanzania Journal of Geology 91 167^178Kaeser B (2006) Mantle xenoliths from the Marsabit volcanic field a

case study on the evolution of the lithospheric mantle in a continen-tal rift environment PhD thesis University of Neuchaldquo tel 207 pp

Kaeser B Kalt A amp Pettke T (2006) Evolution of the lithosphericmantle beneath the Marsabit volcanic field (northern Kenya) con-straints from textural P^T and geochemical studies on xenolithsJournal of Petrology 47 2149^2184

Kelemen P B Whitehead J A Aharonov E amp Jordahl K A(1995) Experiments on flow focusing in soluble porous mediawith applications to melt extraction from the mantle Journal ofGeophysical Research 100 475^496

Kessel R Ulmer P Pettke T Schmidt MW amp Thompson A B(2005) The water^basalt system at 4 to 6GPa phase relations andsecond critical endpoint in a K-free eclogite at 700 to 14008C Earthand Planetary Science Letters 237 873^892

Key R M Rop B P amp Rundle C C (1987)The development of thelate Cenozoic alkali basaltic Marsabit shield volcano northernKenya Journal of African Earth Sciences 6 475^491

King B C (1970) Vulcanicity and rift tectonics in east Africa InClifford T N amp Gass I G (eds) African Magmatism and TectonicsEdinburgh Oliver amp Boyd pp 263^285

Klemme S amp OrsquoNeill H S (2000) The near-solidus transition fromgarnet lherzolite to spinel lherzolite Contributions to Mineralogy and

Petrology 138 237^248LaTourretteT Hervig R L amp Holloway J R (1995) Trace-element

partitioning between amphibole phlogopite and basanite meltEarth and Planetary Science Letters 135 13^30

Laurora A Mazzucchelli M Rivalenti G Vannucci RZanetti A Barbieri M A amp Cingolani C A (2001)Metasomatism and melting in carbonated peridotite xenolithsfrom the mantle wedge the Gobernador Gregores case (southernPatagonia) Journal of Petrology 42 69^87

Leake B EWoolley A R Arps C E S et al (1997) Nomenclatureof amphiboles report of the subcommittee on amphiboles of the

International Mineralogical Association commission on newminerals and mineral names American Mineralogist 82 1019^1037

Lee C-T amp Rudnick R (1999) Compositionally stratified cratoniclithosphere petrology and geochemistry of peridotite xenolithsfrom the Labait Volcano Tanzanie In Gurney J Gurney J LPascoe M D amp Richardson S H (eds) Proceedings of the 7th inter-national Kimberlite Conference CapeTown Red Roof Design 503^521

Lee C-T Rudnick R L McDonough W F amp Horn I (2000)Petrologic and geochemical investigation of carbonates in perido-tite xenoliths from northeastern Tanzania Contributions to

Mineralogy and Petrology 139 470^484Lee W J amp Wyllie P J (1996) Liquid immiscibility in the join

NaAlSi3O8^CaCO3 to 25GPa and the origin of calciocarbonatitemagmas Journal of Petrology 37 1125^1152

Lee W J amp Wyllie P J (2000) The system CaO^MgO^SiO2^CO2

at 1GPa metasomatic wehrlites and primary carbonatite magmasContributions to Mineralogy and Petrology 138 214^228

LenskyNGNieboRWHolloway JRLyakhovskyVampNavonO(2006) Bubble nucleation as a trigger for xenolith entrapment inmantlemeltsEarth andPlanetary Science Letters 245 278^288

Luth RW (2003) Mantle volatilesccedildistribution and consequencesIn Holland H D amp Turekian K K (eds) Treatise on GeochemistryAmsterdam Elsevier pp 319^361

Maale S amp Printzlau I (1979) Natural partial melting of spinellherzolite Journal of Petrology 20 727^741

Macdonald R Rogers N W Fitton J G Black S amp Smith M(2001) Plume^lithosphere interactions in the generation of thebasalts of the Kenya Rift East Africa Journal of Petrology 42877^900

Manning C E (2004) The chemistry of subduction-zone fluids Earthand Planetary Science Letters 223 1^16

Maury R C Defant M J amp Joron J L (1992) Metasomatism ofthe sub-arc mantle inferred from trace-elements in Philippinexenoliths Nature 360 661^663

McDonoughW F amp Sun S S (1995) The composition of the EarthChemical Geology 120 223^253

McInnes B I A Grecurren goire M Binns R A Herzig P M ampHannington M D (2001) Hydrous metasomatism of oceanic sub-arc mantle Lihir Papua New Guinea petrology and geochemistryof fluid-metasomatised mantle wedge xenoliths Earth and PlanetaryScience Letters 188 169^183

Mecurren dard E Schmidt M W Schiano P amp Ottolini L (2006)Melting of amphibole-bearing wehrlites an experimental study onthe origin of ultra-calcic nepheline-normative melts Journal of

Petrology 47 481^504Meert J G (2003) A synopsis of events related to the assembly of

eastern GondwanaTectonophysics 362 1^40Menzies M A amp Hawkesworth C J (1987) Mantle Metsomatism

London Academic PressMoine B N Grecurren goire M OrsquoReilly S Y Sheppard S M F amp

Cottin JY (2001) High field strength element fractionation in theupper mantle evidence from amphibole-rich composite mantlexenoliths from the Kerguelen Islands (Indian Ocean) Journal ofPetrology 42 2145^2167

Moine B N Grecurren goire M OrsquoReilly SY Delpech G Sheppard SM F Lorand J P Renac C Giret A amp Cottin J Y (2004)Carbonatite melt in the upper mantle beneath the KerguelenArchipelago Lithos 75 239^252

Morishita T Arai S amp Green D H (2003) Evolution oflow-Al orthopyroxene in the Horoman peridotite Japan an unu-sual indicator of metasomatizing fluids Journal of Petrology 441237^1246

34

Morley C K (1999) Tectonic evolution of the East African RiftSystem and the modifying influence of magmatism a reviewActaVulcanologica 11 1^19

Navon O amp Stolper E (1987) Geochemical consequences of meltpercolation the upper mantle as a chromatographic columnJournal of Geology 95 285^307

Neumann E-R amp Wulff-Pedersen E (1997) The origin of highlysilicic glass in mantle xenoliths from the Canary Islands Journal ofPetrology 38 1513^1539

Neumann E-RWulff-Pedersen E Pearson N J amp Spencer E A(2002) Mantle xenoliths from Tenerife (Canary Islands)evidence for reactions between mantle peridotites and siliciccarbonatite melts inducing Ca metasomatism Journal of Petrology43 825^857

Niida K amp Green D H (1999) Stability and chemical compositionof pargasitic amphibole in MORB pyrolite under upper mantleconditions Contributions to Mineralogy and Petrology 135 18^40

Norman M D (1998) Melting and metasomatism in the continentallithosphere laser ablation ICPMS analysis of minerals in spinellherzolites from eastern Australia Contributions to Mineralogy and

Petrology 130 240^255Odling NW A (1994) An experimental simulation of upper-mantle

metasomatism American Mineralogist 79 148^153Olker B (2001) Entwicklung und Anwendung eines Computerpro-

grammes zur numerischen Modellierung von Diffusionsprofilen inMineralkolaquo rnern PhD thesis University of Heidelberg 269 pp

OrsquoNeill H S (1987) Quartz^fayalite^iron and quartz^fayalite^magnetite equilibria and the free-energy of formation of fayalite(Fe2SiO4) and magnetite (Fe3O4) American Mineralogist 72 67^75

OrsquoReilly SY amp GriffinW L (2000) Apatite in the mantle implica-tions for metasomatic processes and high heat production inPhanerozoic mantle Lithos 53 217^232

Parkinson I J Arculus R J amp Eggins S M (2003) Peridotite xeno-liths from Grenada Lesser Antilles island arc Contributions to

Mineralogy and Petrology 146 241^262Pearson D G Boyd F R Haggerty S E Pasteris J D Field S

W Nixon P H amp Pokhilenko N P (1994) The characterizationand origin of graphite in cratonic lithospheric mantleccedila petrologi-cal carbon-isotope and Raman-spectroscopic study Contributions toMineralogy and Petrology 115 449^466

PearsonDG CanilDampShirey S B (2003)Mantle samples includedin volcanic rocks xenoliths and diamonds In Carlson RW (ed)Treatise on Geochemistry Amsterdam Elsevier pp171^275

Pettke T Halter W E Webster J D Aigner-Torres M ampHeinrich C A (2004) Accurate quantification of melt inclusionchemistry by LA-ICPMS a comparison with EMP and SIMS andthe adventages and possible limitations of these methods Lithos 78333^361

Popp R KVirgo D amp Phillips MW (1995) H deficiency in kaer-sutitic amphiboles experimental verification American Mineralogist

80 1347^1350Powell W Zhang M OrsquoReilly S Y amp Tiepolo M (2004) Mantle

amphibole trace-element and isotope signatures trace multiplemetasomatic episodes in lithospheric mantle western VictoriaAustralia Lithos 75 141^171

Prouteau G amp Scaillet B (2003) Experimental constraints onthe origin of the 1991 Pinatubo dacite Journal of Petrology 442203^2241

Rampone E Bottazzi P amp Ottolini L (1991) Complementary Tiand Zr anomalies in orthopyroxene and clinopyroxene frommantle peridotites Nature 354 518^520

Rapp R P (1995) Amphibole-out phase-boundary in partiallymelted metabasalt its control over liquid fraction and composition

and source permeability Journal of Geophysical ResearchccedilSolid Earth

100 15601^15610Rivalenti G Zanetti A Mazzucchelli M Vannucci R amp

Cingolani C A (2004) Equivocal carbonatite markers in themantle xenoliths of the Patagonia backarc the GobernadorGregores case (Santa Cruz Province Argentina) Contributions toMineralogy and Petrology 147 647^670

RooneyT O FurmanTYirgu G amp Ayalew D (2005) Structure ofthe Ethiopian lithosphere xenolith evidence in the Main EthiopianRift Geochimica et Cosmochimica Acta 69 3889^3910

Rudnick R L McDonough W F amp Chappell B W (1993)Carbonatite metasomatism in the northernTanzanian mantle pet-rographic and geochemical characteristics Earth and Planetary

Science Letters 114 463^475Rudnick R L IrelandT R Gehrels G Irving A J Chesley JT

amp Hanchar J M (1999) Dating mantle metasomatism U^Pb geo-chronology of zircons in cratonic mantle xenoliths from Montanaand Tanzania In Gurney J Gurney J L Pascoe M D ampRichardson S H (eds) Proceedings of the 7th international Kimberlite

Conference CapeTown Red Roof Design pp 728^735Shaw C S J amp Eyzaguirre J (2000) Origin of megacrysts in the

mafic alkaline lavas of the West Eifel volcanic field GermanyLithos 50 75^95

Shaw C S amp Klulaquo gel A (2002) The pressure and temperature condi-tions and timing of glass formation in mantle-derived xenolithsfrom Baarley West Eifel Germany the case for amphibole break-down lava infiltration and mineral^melt reaction Mineralogy and

Petrology 74 163^187Shaw C S Thibault Y Edgar A D amp Lloyd F E (1998)

Mechanism of orthopyroxene dissolution in silica-undersaturatedmelts at 1 atmosphere and implications for the origin of silica-richglass in mantle xenoliths Contributions to Mineralogy and Petrology 132354^370

Shaw S J Heidelbach F amp Dingwell D B (2006) The origin ofreaction textures in mantle peridotite xenoliths from Sal IslandCape Verde the case for lsquometasomatismrsquo by the host lavaContributions to Mineralogy and Petrology 151 681^697

Simiyu S M amp Keller G R (1997) An integrated analysis of lithos-pheric structure across the East African plateau based on gravityanomalies and recent seismic studiesTectonophysics 278 291^313

Smith D (1999) Temperatures and pressures of mineral equilibrationin peridotite xenoliths review discussion and implications In FeiY Bertka C M amp Mysen B O (eds) Mantle Petrology Field

Observation and High-Pressure Experimentation A Tribute to Francis R

(Joe) Boyd Geochemical Society Special Publications 6 171^188Smith D Riter J C A amp Mertzman S A (1999)Water^rock inter-

actions orthopyroxene growth and Si-enrichment in the mantleevidence in xenoliths from the Colorado Plateau southwesternUnited States (vol 165 pg 45 1999) Earth and Planetary Science

Letters 167 347^356Spera F J (1984) Carbon dioxide in petrogenesis 3 Role of volatiles

in the ascent of alkaline magma with special reference to xenolith-bearing mafic lavas Contributions to Mineralogy and Petrology 88217^232

Tiepolo M Bottazzi P Foley S F Oberti R Vannucci R ampZanetti A (2001) Fractionation of Nb and Ta from Zr and Hf atmantle depths the role of titanian pargasite and kaersutiteJournal of Petrology 42 221^232

Vannucci R Piccardo G B Rivalenti G Zanetti ARampone E Ottolini L Oberti R Mazzucchelli M ampBottazzi P (1995) Origin of LREE-depleted amphiboles inthe subcontinental mantle Geochimica et Cosmochimica Acta 591763^1771

35

Varela M E Clocchiatti R Kurat G amp Schiano P (1999)Silicic glasses in hydrous and anhydrous mantle xenoliths fromWestern Victoria at least two different sources Chemical Geology

153 151^169Vernie res J Godard M amp Bodinier J L (1997) A plate model for

the simulation of trace element fractionation during partial melt-ing and magma transport in the Earthrsquos upper mantle Journal ofGeophysical Research 102 24771^24784

Volker F (1990) Geochemie der quartalaquo ren Vulkanite aufder Ostschulter des Kenia-Rifts PhD thesis University ofKarlsruhe

Witt-Eickschen G Seck H A Mezger K Eggins S M ampAltherr R (2003) Lithospheric mantle evolution beneath theEifel (Germany) constraints from Sr^Nd^Pb isotopes and traceelement abundances in spinel peridotite and pyroxenite xenolithsJournal of Petrology 44 1077^1095

Woermann E amp Rosenhauer M (1985) Fluid phases and the redoxstate of the Earthrsquos mantleccedilextrapolations based on experimentalphase-theoretical and petrological data Fortschritte der Mineralogie

63 263^349Wood B J amp Virgo D (1989) Upper mantle oxidation state ferric

iron contents of lherzolite spinels by 57Fe Molaquo ssbauer spectroscopyand resultant oxygen fugacities Geochimica et Cosmochimica Acta 531277^1291

Woodland A B Kornprobst J McPherson E Bodinier J L ampMenzies A (1996) Metasomatic interactions in the lithosphericmantle petrologic evidence from the Lherz massif FrenchPyrenees Chemical Geology 134 83^112

Wulff-Pedersen E Neumann E-R Vannucci R Bottazzi P ampOttolini L (1999) Silicicmelts producedby reactionbetweenperido-tite and infiltrating basaltic melts ion probe data on glasses andminerals in veined xenoliths from La Palma Canary IslandsContributions toMineralogyandPetrology137 59^82

Yaxley G M amp Kemenetsky V (1999) In situ origin for glass inmantle xenoliths from southeastern Australia insights from traceelement compositions of glasses and metasomatic phases Earth andPlanetary Science Letters 172 97^109

Yaxley G M Kamenetsky V Green D H amp Falloon T J (1997)Glasses in mantle xenoliths from western Victoria Australia andtheir relevance to mantle processes Earth and Planetary Science

Letters 148 433^446Yaxley G M Green D H amp Kamenetsky V (1998) Carbonatite

metasomatism in the southeastern Australian lithosphere Journalof Petrology 39 1917^1930

Zanetti A Mazzucchelli M Rivalenti G amp Vannucci R (1999)The Finero phlogopite^peridotite massif an example of subduc-tion-related metasomatism Contributions to Mineralogy and Petrology

134 107^122Zanetti Z Vannucci R Bottazzi P Oberti R amp Ottolini L

(1996) Infiltration metasomatism at Lherz as monitored by sys-tematic ion-microprobe investigations close to a hornblendite veinChemical Geology 134 113^133

Zinngrebe E amp Foley S F (1995) Metasomatism in mantle xenolithsfrom Gees West Eifel Germany evidence for the genesis of calc-alkaline glasses and metasomatic Ca-enrichment Contributions to

Mineralogy and Petrology 122 79^96

36

Morley C K (1999) Tectonic evolution of the East African RiftSystem and the modifying influence of magmatism a reviewActaVulcanologica 11 1^19

Navon O amp Stolper E (1987) Geochemical consequences of meltpercolation the upper mantle as a chromatographic columnJournal of Geology 95 285^307

Neumann E-R amp Wulff-Pedersen E (1997) The origin of highlysilicic glass in mantle xenoliths from the Canary Islands Journal ofPetrology 38 1513^1539

Neumann E-RWulff-Pedersen E Pearson N J amp Spencer E A(2002) Mantle xenoliths from Tenerife (Canary Islands)evidence for reactions between mantle peridotites and siliciccarbonatite melts inducing Ca metasomatism Journal of Petrology43 825^857

Niida K amp Green D H (1999) Stability and chemical compositionof pargasitic amphibole in MORB pyrolite under upper mantleconditions Contributions to Mineralogy and Petrology 135 18^40

Norman M D (1998) Melting and metasomatism in the continentallithosphere laser ablation ICPMS analysis of minerals in spinellherzolites from eastern Australia Contributions to Mineralogy and

Petrology 130 240^255Odling NW A (1994) An experimental simulation of upper-mantle

metasomatism American Mineralogist 79 148^153Olker B (2001) Entwicklung und Anwendung eines Computerpro-

grammes zur numerischen Modellierung von Diffusionsprofilen inMineralkolaquo rnern PhD thesis University of Heidelberg 269 pp

OrsquoNeill H S (1987) Quartz^fayalite^iron and quartz^fayalite^magnetite equilibria and the free-energy of formation of fayalite(Fe2SiO4) and magnetite (Fe3O4) American Mineralogist 72 67^75

OrsquoReilly SY amp GriffinW L (2000) Apatite in the mantle implica-tions for metasomatic processes and high heat production inPhanerozoic mantle Lithos 53 217^232

Parkinson I J Arculus R J amp Eggins S M (2003) Peridotite xeno-liths from Grenada Lesser Antilles island arc Contributions to

Mineralogy and Petrology 146 241^262Pearson D G Boyd F R Haggerty S E Pasteris J D Field S

W Nixon P H amp Pokhilenko N P (1994) The characterizationand origin of graphite in cratonic lithospheric mantleccedila petrologi-cal carbon-isotope and Raman-spectroscopic study Contributions toMineralogy and Petrology 115 449^466

PearsonDG CanilDampShirey S B (2003)Mantle samples includedin volcanic rocks xenoliths and diamonds In Carlson RW (ed)Treatise on Geochemistry Amsterdam Elsevier pp171^275

Pettke T Halter W E Webster J D Aigner-Torres M ampHeinrich C A (2004) Accurate quantification of melt inclusionchemistry by LA-ICPMS a comparison with EMP and SIMS andthe adventages and possible limitations of these methods Lithos 78333^361

Popp R KVirgo D amp Phillips MW (1995) H deficiency in kaer-sutitic amphiboles experimental verification American Mineralogist

80 1347^1350Powell W Zhang M OrsquoReilly S Y amp Tiepolo M (2004) Mantle

amphibole trace-element and isotope signatures trace multiplemetasomatic episodes in lithospheric mantle western VictoriaAustralia Lithos 75 141^171

Prouteau G amp Scaillet B (2003) Experimental constraints onthe origin of the 1991 Pinatubo dacite Journal of Petrology 442203^2241

Rampone E Bottazzi P amp Ottolini L (1991) Complementary Tiand Zr anomalies in orthopyroxene and clinopyroxene frommantle peridotites Nature 354 518^520

Rapp R P (1995) Amphibole-out phase-boundary in partiallymelted metabasalt its control over liquid fraction and composition

and source permeability Journal of Geophysical ResearchccedilSolid Earth

100 15601^15610Rivalenti G Zanetti A Mazzucchelli M Vannucci R amp

Cingolani C A (2004) Equivocal carbonatite markers in themantle xenoliths of the Patagonia backarc the GobernadorGregores case (Santa Cruz Province Argentina) Contributions toMineralogy and Petrology 147 647^670

RooneyT O FurmanTYirgu G amp Ayalew D (2005) Structure ofthe Ethiopian lithosphere xenolith evidence in the Main EthiopianRift Geochimica et Cosmochimica Acta 69 3889^3910

Rudnick R L McDonough W F amp Chappell B W (1993)Carbonatite metasomatism in the northernTanzanian mantle pet-rographic and geochemical characteristics Earth and Planetary

Science Letters 114 463^475Rudnick R L IrelandT R Gehrels G Irving A J Chesley JT

amp Hanchar J M (1999) Dating mantle metasomatism U^Pb geo-chronology of zircons in cratonic mantle xenoliths from Montanaand Tanzania In Gurney J Gurney J L Pascoe M D ampRichardson S H (eds) Proceedings of the 7th international Kimberlite

Conference CapeTown Red Roof Design pp 728^735Shaw C S J amp Eyzaguirre J (2000) Origin of megacrysts in the

mafic alkaline lavas of the West Eifel volcanic field GermanyLithos 50 75^95

Shaw C S amp Klulaquo gel A (2002) The pressure and temperature condi-tions and timing of glass formation in mantle-derived xenolithsfrom Baarley West Eifel Germany the case for amphibole break-down lava infiltration and mineral^melt reaction Mineralogy and

Petrology 74 163^187Shaw C S Thibault Y Edgar A D amp Lloyd F E (1998)

Mechanism of orthopyroxene dissolution in silica-undersaturatedmelts at 1 atmosphere and implications for the origin of silica-richglass in mantle xenoliths Contributions to Mineralogy and Petrology 132354^370

Shaw S J Heidelbach F amp Dingwell D B (2006) The origin ofreaction textures in mantle peridotite xenoliths from Sal IslandCape Verde the case for lsquometasomatismrsquo by the host lavaContributions to Mineralogy and Petrology 151 681^697

Simiyu S M amp Keller G R (1997) An integrated analysis of lithos-pheric structure across the East African plateau based on gravityanomalies and recent seismic studiesTectonophysics 278 291^313

Smith D (1999) Temperatures and pressures of mineral equilibrationin peridotite xenoliths review discussion and implications In FeiY Bertka C M amp Mysen B O (eds) Mantle Petrology Field

Observation and High-Pressure Experimentation A Tribute to Francis R

(Joe) Boyd Geochemical Society Special Publications 6 171^188Smith D Riter J C A amp Mertzman S A (1999)Water^rock inter-

actions orthopyroxene growth and Si-enrichment in the mantleevidence in xenoliths from the Colorado Plateau southwesternUnited States (vol 165 pg 45 1999) Earth and Planetary Science

Letters 167 347^356Spera F J (1984) Carbon dioxide in petrogenesis 3 Role of volatiles

in the ascent of alkaline magma with special reference to xenolith-bearing mafic lavas Contributions to Mineralogy and Petrology 88217^232

Tiepolo M Bottazzi P Foley S F Oberti R Vannucci R ampZanetti A (2001) Fractionation of Nb and Ta from Zr and Hf atmantle depths the role of titanian pargasite and kaersutiteJournal of Petrology 42 221^232

Vannucci R Piccardo G B Rivalenti G Zanetti ARampone E Ottolini L Oberti R Mazzucchelli M ampBottazzi P (1995) Origin of LREE-depleted amphiboles inthe subcontinental mantle Geochimica et Cosmochimica Acta 591763^1771

35

Varela M E Clocchiatti R Kurat G amp Schiano P (1999)Silicic glasses in hydrous and anhydrous mantle xenoliths fromWestern Victoria at least two different sources Chemical Geology

153 151^169Vernie res J Godard M amp Bodinier J L (1997) A plate model for

the simulation of trace element fractionation during partial melt-ing and magma transport in the Earthrsquos upper mantle Journal ofGeophysical Research 102 24771^24784

Volker F (1990) Geochemie der quartalaquo ren Vulkanite aufder Ostschulter des Kenia-Rifts PhD thesis University ofKarlsruhe

Witt-Eickschen G Seck H A Mezger K Eggins S M ampAltherr R (2003) Lithospheric mantle evolution beneath theEifel (Germany) constraints from Sr^Nd^Pb isotopes and traceelement abundances in spinel peridotite and pyroxenite xenolithsJournal of Petrology 44 1077^1095

Woermann E amp Rosenhauer M (1985) Fluid phases and the redoxstate of the Earthrsquos mantleccedilextrapolations based on experimentalphase-theoretical and petrological data Fortschritte der Mineralogie

63 263^349Wood B J amp Virgo D (1989) Upper mantle oxidation state ferric

iron contents of lherzolite spinels by 57Fe Molaquo ssbauer spectroscopyand resultant oxygen fugacities Geochimica et Cosmochimica Acta 531277^1291

Woodland A B Kornprobst J McPherson E Bodinier J L ampMenzies A (1996) Metasomatic interactions in the lithosphericmantle petrologic evidence from the Lherz massif FrenchPyrenees Chemical Geology 134 83^112

Wulff-Pedersen E Neumann E-R Vannucci R Bottazzi P ampOttolini L (1999) Silicicmelts producedby reactionbetweenperido-tite and infiltrating basaltic melts ion probe data on glasses andminerals in veined xenoliths from La Palma Canary IslandsContributions toMineralogyandPetrology137 59^82

Yaxley G M amp Kemenetsky V (1999) In situ origin for glass inmantle xenoliths from southeastern Australia insights from traceelement compositions of glasses and metasomatic phases Earth andPlanetary Science Letters 172 97^109

Yaxley G M Kamenetsky V Green D H amp Falloon T J (1997)Glasses in mantle xenoliths from western Victoria Australia andtheir relevance to mantle processes Earth and Planetary Science

Letters 148 433^446Yaxley G M Green D H amp Kamenetsky V (1998) Carbonatite

metasomatism in the southeastern Australian lithosphere Journalof Petrology 39 1917^1930

Zanetti A Mazzucchelli M Rivalenti G amp Vannucci R (1999)The Finero phlogopite^peridotite massif an example of subduc-tion-related metasomatism Contributions to Mineralogy and Petrology

134 107^122Zanetti Z Vannucci R Bottazzi P Oberti R amp Ottolini L

(1996) Infiltration metasomatism at Lherz as monitored by sys-tematic ion-microprobe investigations close to a hornblendite veinChemical Geology 134 113^133

Zinngrebe E amp Foley S F (1995) Metasomatism in mantle xenolithsfrom Gees West Eifel Germany evidence for the genesis of calc-alkaline glasses and metasomatic Ca-enrichment Contributions to

Mineralogy and Petrology 122 79^96

36

Varela M E Clocchiatti R Kurat G amp Schiano P (1999)Silicic glasses in hydrous and anhydrous mantle xenoliths fromWestern Victoria at least two different sources Chemical Geology

153 151^169Vernie res J Godard M amp Bodinier J L (1997) A plate model for

the simulation of trace element fractionation during partial melt-ing and magma transport in the Earthrsquos upper mantle Journal ofGeophysical Research 102 24771^24784

Volker F (1990) Geochemie der quartalaquo ren Vulkanite aufder Ostschulter des Kenia-Rifts PhD thesis University ofKarlsruhe

Witt-Eickschen G Seck H A Mezger K Eggins S M ampAltherr R (2003) Lithospheric mantle evolution beneath theEifel (Germany) constraints from Sr^Nd^Pb isotopes and traceelement abundances in spinel peridotite and pyroxenite xenolithsJournal of Petrology 44 1077^1095

Woermann E amp Rosenhauer M (1985) Fluid phases and the redoxstate of the Earthrsquos mantleccedilextrapolations based on experimentalphase-theoretical and petrological data Fortschritte der Mineralogie

63 263^349Wood B J amp Virgo D (1989) Upper mantle oxidation state ferric

iron contents of lherzolite spinels by 57Fe Molaquo ssbauer spectroscopyand resultant oxygen fugacities Geochimica et Cosmochimica Acta 531277^1291

Woodland A B Kornprobst J McPherson E Bodinier J L ampMenzies A (1996) Metasomatic interactions in the lithosphericmantle petrologic evidence from the Lherz massif FrenchPyrenees Chemical Geology 134 83^112

Wulff-Pedersen E Neumann E-R Vannucci R Bottazzi P ampOttolini L (1999) Silicicmelts producedby reactionbetweenperido-tite and infiltrating basaltic melts ion probe data on glasses andminerals in veined xenoliths from La Palma Canary IslandsContributions toMineralogyandPetrology137 59^82

Yaxley G M amp Kemenetsky V (1999) In situ origin for glass inmantle xenoliths from southeastern Australia insights from traceelement compositions of glasses and metasomatic phases Earth andPlanetary Science Letters 172 97^109

Yaxley G M Kamenetsky V Green D H amp Falloon T J (1997)Glasses in mantle xenoliths from western Victoria Australia andtheir relevance to mantle processes Earth and Planetary Science

Letters 148 433^446Yaxley G M Green D H amp Kamenetsky V (1998) Carbonatite

metasomatism in the southeastern Australian lithosphere Journalof Petrology 39 1917^1930

Zanetti A Mazzucchelli M Rivalenti G amp Vannucci R (1999)The Finero phlogopite^peridotite massif an example of subduc-tion-related metasomatism Contributions to Mineralogy and Petrology

134 107^122Zanetti Z Vannucci R Bottazzi P Oberti R amp Ottolini L

(1996) Infiltration metasomatism at Lherz as monitored by sys-tematic ion-microprobe investigations close to a hornblendite veinChemical Geology 134 113^133

Zinngrebe E amp Foley S F (1995) Metasomatism in mantle xenolithsfrom Gees West Eifel Germany evidence for the genesis of calc-alkaline glasses and metasomatic Ca-enrichment Contributions to

Mineralogy and Petrology 122 79^96

36