petrogenesis of oxide minerals in kimberlite, elliott county, kentucky

American Mineralogist, Volume 67, pages 2842, 1982

Petrogenesis of oxide minerals in kimberlite, Elliott County, Kentucky

JenrnBv J. Acne, Jeur,s R. GennrsoN, JR. eNo LewnnNcr, A. Tevron

Department of Geologicql SciencesThe University of TennesseeKnoxville. Tennessee 379 16

Abstract

Two kimberlite pipes in Elliott County, Kentucky contain inclusions of altered crustalrocks, ultramafic xenoliths, and a diverse megacryst suite in a groundmass of olivine, Mg-rich orthopyroxene, Mg-rich clinopyroxene, ilmenite, Cr-spinel, pleonaste, titanomagne-tite, perovskite, calcite, phologopite, vermiculite, pectolite, and septechlorite. The mega-cryst assemblage consists ofolivine, garnet, ilmenite, clinopyroxene, orthopyroxene, andphlogopite. Although rare, intergrowths of clinopyroxene + ilmenite and rutile + ilmenitealso occur.

Ilmenite megacrysts have reverse-zoned, Mg-rich rims at least 75-1fi) pm wide. Ilmenitemegacryst cores show a correlation of increasing Fe2O3 with decreasing MgO andMg/(Mg+Fe), which is interpreted as a fractionation trend. Ilmenites from clinopyroxene-ilmenite intergrowths are similar in composition to the more magnesian of the megacrystcore compositions. Clinopyroxenes from the intergrowths have compositions similar toclinopyroxene megacrysts; these clinopyroxenes from the intergrowths have estimatedequilibration temperatures of 1295"-1335 C. Groundmass ilmenites are more magnesianthan the ilmenite megacryst cores. Groundmass ilmenites can be grouped into twocompositionally distinct populations: (l) a high-MgO, low-Cr2O3 population (<0.73 wt.VoCr2O3) which is compositionally similar to the Mg-rich megacryst rims, and (2) a hieh-MgO,high-Cr2O3 population (1.43-3.76 wt.%o Cr2Ol which is compositionally similar to smallilmenite inclusions in groundmass olivines. The former group of groundmass ilmenites isinterpreted to be MgO-enriched fragments of disaggregated, recrystallized megacrysts; thelatter group is interpreted to be primary ilmenites that crystallized in equilibrium with thegroundmass assemblage from an MgO-enriched kimberlitic liquid. Estimated equilibrationtemperatures for the groundrnass assemblage are from 965'to 1085'C; at these tempera-tures, the position of the kimberlite solidus restricts the depth at which this crystallizationevent occurred to be less than 100 km.

Both megacryst and groundmass ilmenites have well-developed reaction rims of perov-skite + titanomagnetite + Mn-ilmenite. The Mn-ilmenite (17.9 mol% MnTiO:) is quitedistinct from either megacryst or groundmass ilmenites and is clearly of secondary origin.Discrete perovskites and titanomagnetites within the groundmass are similar in composi-tion to the phases within the reaction assemblage. Cr-spinel (63.6-68.1 molVo (Fe,Mg)Cr2O) also occurs in the groundmass; these Cr-spinels commonly contain overgrowths ofpleonaste.

Ilmenite megacrysts and clinopyroxene-ilmenite intergrowths are interpreted to haveformed, along with the other members of the silicate megacryst assemblage, from afractionating, mantle-derived liquid at about 150 km depth within the temperature intervalof 1165'-1390" C (Stage I of kimberlite development). Subsequently, the minerals experi-enced a sudden increase of the MgO/FeO ratio in the kimberlitic liquid (Stage II ofkimberlite evolution). This may have been due to upward movement of the megacrysts andxenocrysts into a diferent melt regime, possibly associated with assimilation of mantlewall-rock such as a carbonated peridotite. From this MgO-enriched liquid, a compositional-ly distinct suite of Mg-rich groundmass minerals precipitated and earlier formed olivine andilmenite megacrysts reacted with this liquid and formed high-MgO rims (Stage III). Thefinal recorded stage of kimberlite evolution (Stage IV) at Elliott County is the "autometaso-matic" reaction of ilmenite with a CaO-enriched fluid to form the reaction assemblageperovskite + titanomagnetite + Mn-ilmenite.

0003-.09x/82/0 102-002E$02. 00 2E

AGEE ET AL.: OXIDE MINERALS IN KIMBERLITE

Introduction

Important constituents of kimberlite are oxideminerals. Since compositions of oxide phases arevery susceptible to changes in temperature, bulkcomposition, zr:rd fsr, they provide a record of alarge portion of the crystallization history and theevolution of kimberlite.

Ilmenite is the most abundant oxide phase foundin kimberlite; it is present both as a megacryst (i.e.,a )l cm subrounded grain) and as a groundmassmineral. A diagnostic feature of kimberlitic ilmen-ites is their distinctive, high-MgO composition(Mitchell, 1973; 1977).In addition to rheir distinc-tive chemical systematics, they exhibit diverse tex-tural relationships. Megacrysts occur as large singlecrystals or may be totally recrystallized to mosaicpolycrystalline aggregates. Ilmenite also commonlyoccurs in lamellar or graphic intergrowths withclinopyroxene and/or orthopyroxene (e.g., Gurneyet al., 1973;'Dawson and Reid, 1970; Frick, 1973:Garrison and Taylor, 1981). Ilmenite can also occuras small spherical inclusions in silicate phases suchas olivine or garnet (e.9., Boyd and Nixon, 1973).

Spinels are restricted to the kimberlite ground-mass, although they may occur as exsolution lamel-lae in ilmenite (Danchin and D'Orey, 1972) or inlate-stage reaction zones around ilmenite. Ground-mass spinels show a wide variety of chemicalcompositions, ranging from Cr-spinel to pleonasteto titanomagnetite (Haggerty, 1976).

Perovskite is most commonly found in reactionzones associated with titanomagnetite, around bothmegacryst and groundmass ilmenites. Perovskitemay also occur as discrete grains in the kimberlitegroundmass (Boctor and Meyer, 1979; Boctor andBoyd,1980) .

The two kimberlite pipes examinedin this studyare located in Elliott County in eastern Kentuckyalong the eastern edge of the Appalachian Plateau.These kimberlites contain a variety of mantle-de-rived megacrysts and xenoliths, as well as frag-ments of highly metasomatized lower crustal andsupracrustal materials. Bolivar (1972) documentedthe kimberlitic nature of these bodies and describedtheir mineralogy. Zartman et al. (1972) reported aRb-Sr age of 257 -+22 m.y. for these bodies, indicat-ing an early Permian emplacement age.

The Elliott County kimberlites are unique inseveral aspects. The kimberlite is extremely fresh,providing an excellent opportunity to examine boththe megacrysts and the fine-grained groundmass

material. One pipe (Pipe 1), located 0.5 km south ofIson Creek, has abundant megacrysts and xeno-liths, both mantle and crustal, while the other (Pipe2), located along Ison Creek, is almost devoid ofxenolithic material. Mantle xenoliths are quitesmall, rarely exceeding 1 cm in diameter and formvolumetrically the smallest fraction of includedmaterial (Garrison and Taylor, 1980). In Pipe l, theabundance of highly metasomatized fragments ofsupracrustal limestones, siltstones, and shales (upto 20 cm in diameter) approaches 20% of the rock.This metasomatism of the sedimentary and crystal-line crustal materials suggests that the kimberlitemust have been hot at the time of xenolith incorpo-ration during the final emplacement (Garrison andTaylor, 1980; Garrison et al., 1980).

This report summarizes petrographic and elec-tron microprobe data for a wide variety of oxideminerals in the kimberlite pipes. Detailed chemicaland textural studies of these kimberlites have al-lowed us to reconstruct a large portion of theircrystallization history, from the early megacryststage through the groundmass crystallization stageand the autometasomatic event associated withfinal emplacement.

Petrography

Megacrysts of olivine, ilmenite, garnet, clinopy-roxene, phlogopite, and orthopyroxene occur with-in the fine-grained kimberlitic groundmass. Thesemegacrysts average 0.5-l .0 cm in diameter, al-though discrete crystals of garnet, clinopyroxene,ilmenite, and phlogopite up to 1.5 cm in diameterare not uncommon. Pipe I contains megacrysts inthe following relative abundance: olivine > ilmenite) garnet > phlogopite > clinopyroxene > ortho-pyroxene (Garrison and Taylor, 1980). Pipe 2 hasthe relative abundance: olivine > ilmenite > phlog-opite > garnet > orthopyroxene > clinopyroxene.Olivine and phlogopite have corroded, fritted rimsreflecting reaction with the kimberlitic magma. Gar-net is always surrounded by a "kelyphitic" rim ofdark microcrystalline material, probably spinel +phyllosilicates. Olivine megacrysts commonly ex-hibit fracturing and kink banding; many have beentotally recrystallized to mosaic polycrystalline ag-gregates (dunites; Garrison and Taylor, 1980).Three graphic clinopyroxene-ilmenite intergrowths(Garrison and Taylor, 1981) and one rutile-ilmeniteintergrowths were recovered from the Elliott Coun-ty kimberlites.

The groundmass of the kimberlite consists of

30 AGEE ET AL.: OXIDE MINERALS IN KIMBERLITE

Fig. l. Reflected light photomicrographs of ilmenite texturaltypes. (a) Type I megacryst ilmenite; scale bar : I mm. (b) Type11 ilmenite in clinopyroxene-ilmenite intergrowth; il : ilmenite;cpx : clinopyroxene; scale bar = 2 mm. (c) Type III ilmenite inrutile-ilmenite intergrowth; il : ilmenite; ru : rutile; scale bar:100 pm. (d) Type IV groundmass ilmenite; il = ilmenite; mt =titanomagnetite; pf = perovskite; scale bar = 75 pm.

euhedral phenocrysts of olivine and subhedralgrains of olivine, ilmenite, Cr-spinel, pleonaste,Mg-rich orthopyroxene, Mg-rich clinopyroxene,perovskite, titano-magnetite, phlogopite, vermicu-lite, pectolite, calcite, and septechlorite. Locally,calcite occurs in veins and fractures. Possible de-watering of sedimentary inclusions has producedextensive serpentinization of the surroundinggroundmass olivines (to Al-poor lizardite).

Ultramafic xenoliths included in the kimberliteare small and usually represent incomplete perido-tite assemblages (Garrison and Taylor, 1980). Pipe 1contains the greatest abundance of ultramafic xeno-liths; these include garnet peridotite, spinel perido-tite, spinel-ilmenite websterite, and dunite. Onlytwo xenoliths were recovered from Pipe 2: a duniteand a serpentinized garnet peridotite.

Ilmenite

Ilmenite is the most abundant oxide phase in theElliott County kimberlites. Six textural types have

been identified: (I) subrounded megacrysts (0.5-1.5cm in diameter) which exhibit various degrees ofrecrystallization (Fig. la); (II) graphic intergrowthsof ilmenite in clinopyroxene (Fig. 1b); (III) thinlamellae of ilmenite in rutile "phenocrysts" (Fig.1c); (N) anhedral, single, groundmass crystals(0.05-1.0 mm) (Fig. 1d); (V) round inclusions (up to25 p,m) within euhedral to subhedral groundmassolivines (Fie. 2a); (VI) small ilmenite grains associ-ated with titanomagnetite and perovskite in reactionrims around megacryst and groundmass ilmenites(Fie.2b).

Ilmenite megacrysts have rounded anhedral tosubhedral shapes which are attributed to abrasionduring emplacement of the kimberlite (Mitchell,1973). Textures indicate a continuous variationfrom single crystals with slightly undulose extinc-tion to totally recrystallized polycrystalline mosaicswith sharp extinction. The following textural fea-tures provide evidence that the mosaic aggregatesof ilmenite within the Elliott County kimberlites arein fact recrystallized megacrysts: (1) twin lamellaeare occasionally present near the edges of mega-crysts (Fig. 3a), (2) bands of fine mosaic ilmenite cutlarge grains (Fig. 3b), (3) long bands or taperingwedges within grains have slightly different extinc-tion angle from the host (Fig. 3c), and (4) foliation insome megacrysts was formed by alignment of elon-gate sutured grains (Fig. 3d).

Three nodular clinopyroxene-ilmenite inter-growths were collected from the Elliott Countykimberlite Pipe I (Garrison and Taylor, 1981).These nodular intergrowths from Elliott County aretexturally similar to other such intergrowths from

Fig. 2. Reflected light photomicrographs of ilmenite texturaltypes. (a) Type V ilmenite inclusion in olivine; il : ilmenite; ol :

olivine; pf : perovskite; scale bar : 250 pm. (b) Type VIsecondary Mn-ilmenite; il : ilmenite; pf = perovskite; m-il :

Mn-ilmenite; scale bar : 100 u.m.

AGEE ET AL,: OXIDE MINERALS IN KIMBERLITE 3 I

kimberlites in South Africa (e.g., Gurney et al.,1973) and the United States (e.g., Eggler et al.,1979). The intergrowths from Pipe I range in sizefrom 0.5 cm to 2.0 cm in diameter and consist oflarge single crystals of pale-green clinopyroxenewith graphic intergrowths of ilmenite (Fig. lb).These graphic ilmenites (100-400 pm thick) formone optically continuous crystal (up to 30 modal Voof nodule), although occasionally they exhibit mi-nor recrystallization to polygonal mosaics or showundulose extinction. The host clinopyroxene com-monly shows undulose extinction but rarely recrys-tallization. The grain boundaries between thegraphic ilmenite and host clinopyroxene are gener-ally free of alteration, although in some cases,traces of phlogopite can be observed. The ilmenitesfrequently extend out into the host kimberlite be-yond the clinopyroxene; this is probably due tofragmentation and abrasion during transportthrough the kimberlite conduit.

The rutile-ilmenite intergrowth recovered fromElliott County Pipe 2 consists of small 5 pm x 10pm sigmoidal lenses of ilmenite in a rutile host. Therutile is mantled by a rim of polycrystalline ilmenite(Fig. 1c). This texture is very similar to that de-scribed from South African kimberlites by Haggerty(1975, 1976) and Mitchell (1973). Haggerty (1975)described two types of rutile-ilmenite intergrowths:(1) those in which rutile and ilmenite are equallyabundant, and (2) those in which ilmenite formsonly a few percent of the intergrowth. The ElliottCounty intergrowth belongs to the latter type.

Anhedral single crystals of ilmenite, ranging from0.05 to 1.0 mm in diameter, occur throughout thegroundmass of the kimberlite and in this study aretexturally referred to as groundmass ilmenites (Fig.1d). These groundmass ilmenites are never poly-crystalline and show uniform extinction. Theycould have two possible origins (Pasteris, 1980): (l)disaggregated recrystallized megacrysts, and/or (2)a compositionally distinct second generation ofilmenite that crystallized with the other groundmassphases.

Spinel

Four different types of spinel can be distinguishedin the Elliott County kimberlites: (I) pale-blue chro-mite, (II) gray-blue pleonaste, (III) brown titano-magnetite, and (IV) brown spinel rodlets in ilmen-ite. Types I, II, and III occur in the groundmass ofthe kimberlites. Type 1 chromites (up to 50 p,m indiameter) are commonly rimmed by Type 11 pleo-

Fig. 3. Deformation and recrystallization textures in ilmenitemegacrysts. (a) Twin lamellae near the edge of a megacryst;scale bar = I mm. (b) Bands of mosaic polycrystalline ilmenitecrossing ilmenite megacryst; scale bar = I mm. (c) Unduloseextinction in large single crystal ilmenite megacryst; scale bar =

I mm. (d) Foliation in ilmenite megacryst outlined by elongatesutured grain boundaries; scale bar : I mm. All are reflectedlight photomicrographs.

naste (Fig. 4a). Type 11 pleonastes also occur asdiscrete single grains (up to 30 pm in diameter) inthe groundmass. Type lll titanomagnetites occur asdiscrete grains (up to 75 pm in diameter) in thegroundrnass and associated with perovskite in reac-tion rims around ilmenite (Figs. ld and 4a). Type IVspinel occurs as small (<1 prm thick) rodlets inilmenite (Fig. 4c). This texture is similar to thatdescribed from the Premier Mine, South Africa byDanchin and D'Orey (1972). These spinel rodletsare found in three of the six textural types ofilmenite in the Elliott County kimberlites; theyoccur in megacrysts, groundmass ilmenites, and inthe ilmenite inclusions in olivines.

Perovskite

Perovskite occurs as discrete, equant, anhedralblebs (up to 30 pm across) scattered throughout thegroundmass and in reaction rims around ground-mass and megacryst ilmenites (Fig. 4a, b). It is most


Fig. 4. Reflected light photomicrographs of spinel types andperovskite. (a) Composite spinel grain consisting of Type Ichromite rimmed by Type II pleonaste; mt = titanomagnetite; sp= pleonaste; cr : chromite; scale bar : 100 pm. (b) Reactionselvage on ilmenite megacryst. Note islands of ilmenite withinthe zone of perovskite and titanomagnetite; il : ilmenite; pf :perovskite; scale bar : 100 p,m. (c) Type 1V spinel rodlets inilmenite; sp = spinel; il : ilmenite; scale bar : 40 U,m.

common in the reaction rims around ilmenite. Thisreaction rim of perovskite is developed as a 50-75pm wide zone with a fritted, digitate boundary thatprojects into the ilmenite. The outer boundary ofthe perovskite zone (i.e., nearest the host kimber-lite) is in contact with a discontinuous zone ofsubhedral (up to 50 pm wide) Type III titanomagne-tite (Fig. 1d). Within these zones, optically continu-

ous fragments of the host ilmenite occur; theseislands of ilmenite indicate that extensive reactionmust have occurred. Between the spinel and per-ovskite zones are rare occurrences of secondaryType VI ilmenite (Fig. 2b).

Mineral chemistry

Minerals were analyzed with a MAC 400 S elec-tron microprobe utilizing the correction proceduresof Bence and Albee (1968) and the data of Albee andRay (1970). Except for the rims of ilmenite mega-crysts, all minerals were found to be extremelyhomogeneous. When grain size permitted, four toten analyses were made on gach grain and anaverage analysis and 1o standard deviation comput-ed. Representative average analyses of all mineralsare included in Tables I through 3.

Ilmenite

Representative average analyses of ilmenite meg-acrysts from the Elliott County kimberlites arepresented in Table 1. The compositional range ofilmenite megacrysts is shown in an expanded por-tion of the MgTiO3-FeTiO3-Fe2O3 ternary in Figure5. The compositions of the megacryst cores (31.6-46.3 molVo MgTiO3 and 6.3-14.9 molVo Fe2O) aresimilar to ilmenites reported from kimberlite pipesin West Africa and Lesotho (Fig. 6; Haggerty,1975). The ilmenite megacrysts show a correlationof increasing Fe2O3 Q.14-16.4 wt.%) with decreas-ing Mg/(Mg+Fe) (50.6-36.7) (Fig. 7) and decreasingMgO (13.2-8.63 wt.%). This correlation of decreas-ing MgO and Mg/(Mg+Fe) with increasing Fe2O3suggests that the ilmenite megacrysts are related bycrystal fractionation. As crystallizing silicates andilmenites deplete the melt in MgO and decreaseMg/(Mg+Fe), the preferential incorporation ofFe*2 into the silicate phases increases theFe+3/Fe+2 ratio in the remaining melt. Since ilmen-ite is the only major crystallizing phase that canincorporate substantial Fe+3, the ilmenites may beexpected to show an increase in Fe2O3 with differ-entiation (i.e., decreasing Mg/(Mg+Fe)).

The rims of ilmenite megacrysts have higherMgTiO3 content (44.3-53.1 molVo MgTiOr) than themegacryst cores (Table 1; Fig. 5); however, therims have a similar hematite content (4.4-13.3molVo Fe2Or). The megacryst rims exhibit a strongcorrelation of increasing Fe2O3 (5.15-15.0 wt.Vo)with decreasing MgO (15.6-12.6 wt.%); there ap-pears to be no correlation between Mg/(Mg+Fe)


Table 1. Representative average analyses ofilmenite megacrysts

J J

r l P t o e z

megacrys t megacrys tC p x - i l m

I n te rgrowthmegacrys t regacrys tr t m r t m r i m c o r e r i m

f io | 52.3 $r" 53. \ Q)4 1 2 0 3 0 . 6 3 ( 4 ) 0 . 6 2 ( 2 )

c r z o 3 0 . 0 7 ( 1 ) 0 . 0 7 ( 4 )

Fe203* * * 9 .44 8 .95

FeO 24.6 (6) 23.5 (81

M n o 0 . 2 8 ( 3 ) 0 . 4 3 ( 5 )

M s o 1 2 . 5 ( 1 ) 1 3 . 5 Q lN i 0 n . d . 0 . 0 4 ( 3 )

4 8 . 2 ( 2 ) 4 8 . 9 ( 5 )

0 . 3 9 ( 2 ) 0 . 5 2 ( 4 )

0 . 2 7 Q ) 0 . 3 0 ( 1 )

r 5 . 4 1 5 . 0

25 .7 r c , 21 .5 ( 9 )

g . 3 l ( 3 ) 0 . 4 1 ( 1 )

9 . 3 3 Q \ 1 2 . 5 ( 6 )

n . d . 0 . 1 0 ( 1 )

53 .0 ( 4 ) 53 .7 ( 2 )

0 . 6 5 ( 3 ) 0 . 5 5 ( 3 )

o . 0 9 ( 2 ) o . 0 9 ( 1 )

8 . 8 6 9 . 2 9

25.2 ( \ ) 21.5 (7)

o .29o ) 0 .39 (3 )

1 2 . 5 ( 1 ) 1 5 . 0 ( 3 )

n . d . 0 . 0 4 ( 2 )

52 .0 ( 2 ) f i . s ( 5 ) 54 .2 ( 3 )

0 . 6 1 ( l ) 0 . 5 3 ( r ) 0 . 5 7 ( 4 )

n . d . n o 0 . i l 5 1 j \ o . r ( z t

9 . 2 2 7 . 5 8 6 . 7 1

2 5 . 9 ( 8 ) 2 1 . 8 ( 5 ) 2 5 . 7 Q )o . 2 6 ( 2 ) 0 . 4 4 ( 2 ) o . 2 \ ( 3 )

1 1 . 7 ( 4 ) 1 4 . 6 ( 1 ) 1 3 . 0 ( r )

n . d . 0 . 0 7 ( 6 ) 0 . 0 9 ( 4 )

xC a t i o nB a s i s

A I

C r

t l

F e -

99.82

?

0 . 0 1 7

0 . 0 0 1

0 . 908

0 . 1 5 4

o . 430

o -475

0 . 005

t 0 0 . 5 r

0 . 0 1 7

0 . 0 0 t

0 . 9 t 3

0 . 1 5 3

0 .459

0 , 0 0 1

0 . 448

0 .008

1 00 .50

)0 . 0 1 1

0 . 0 0 5

0 . 853

o . z 9 t

0 . 3 2 7

0 .506

0 .006

99.33

?

0 . 0 r 40 .005

o .856

o .251

0 .437

0 . 0 0 r

0 . 4 1 7

0 . 008

t00 .69

0 . 0 1 8

0 .002

0 . 9 1 0

0 .152

o . 4 3 r

0 .480

0 .006

1 00 .67

0 . 0 1 8

0 . 0 0 2

0 . 9 r 3

0 . 1 5 5

0 . 5 0 1

0 . 0 0 1

0 .404

0 . 007

q q ( q

3

0 . 0 t 7

0 .909

0 . 1 5 r

0 .404

0 . 504

0 .005

M9

N i_ + zf e

H N

98 .87 l oo .84

5 5

0 . 0 1 4 0 . 0 1 5

0 . 0 0 6 0 . 0 0 5

0 . 9 2 3 0 . 9 2 7

0 . r 3 r 0 . 1 1 50 . 4 r 8 0 . \ 4 20 . 0 0 1 0 . 0 0 1

0 . 4 1 8 0 . 4 9 0

0 . 009 0 .005

2 .000 2 .000 2 ,000 2 .000 2 .000 2 .000 2 .000 2 . 000 2 .000

* u n i t s i n 0 r e p r e s e n t 1 q s t a n d a r d d e v i a t i o n o f r e p l i c a t e a n a l y s e s* * n . d . = n o t d e t e c t e d ; l e s s t h a n 0 . 0 3 ? .* * * Fe de termined as Feo; ox ides based on c rys ta l chemis t ry .

i n te rms o f I eas t un i t s c i ted .

Table 2. Representative analyses of Type III, IV, V, and VI ilmenites

groundmass groundmass g roundmass groundmass

i n c l u s i o n l a m e l l a e r i mi n i n o f T y P e V l

o l i v i n e r u t i l e r u t i l e

P i p e 1 P ipe 2

T i 0 ^

A r ̂0 ^a 5L r - u -

FeZO3*

Fe0

Mn0

Hgo

N i 0

t

Cat I onB a s i s

A I

t lr 2

F e ' '

l"lg

N i_ + 2tse

l,tn

x

5 5 . U

o . 2 9

1 . 6 9

5 . 7 21 9 . 8

n q h

1 6 . 8

0 . 0 7

> 5 . 5

0 .55

0 . 1 3

8 .72

23.6

0 . 34

t J , o

n . d .

0 . 3 2

2 . 7 1

5 . 1 520.6

0 . 5 1

1 6 . 3

0 . l q

5 1 . 2

0 .95

0 . 3 4I ' E

2 1 . 5

0 . t t 5

t J . o

o . 1 2

E a

0 . 8 9

\ . \ 2

7 . 8 61 8 . 4

0 . 2 6

l 6 . 0

0 . r 6

58.7n . d .

0 . 0 6

28 .1n ? q

I 3 . 0

n . d .

57 . 9 48 . 30 . 3 4 o . 4 r

o . 2 1 0 . 9 5

r . 8 5 r 1 . 3

2 \ . 4 2 7 . o

0 .50 8 .69

1 5 . 4 4 . \ 7

n . d . n , d .

r 0 0 . 7 1

0 .008

0 . 030

0.932

o .096

u , ) ) o

0 . 0 0 1

0 .367

0 . 0 1 0

1 0 0 . 3 5

0 . 0 1 8

0 . 002

0 . 9 1 3

o . t 5 o

0 . 4 6 1

0 . 4 4 9

0 . 0 0 7

1 0 1 . 1 3

0 .008

0 .048

o.927

0 .086

0 . 5 3 8

0 .002

0.382

0 . 0 r 0

100 .67

o.026

0 .006

0 .874

o .21 \

o .462

0 .002

0 .408

0 .009

r00 .29

0 . 024

o . 078

0 .880

0 . 1 3 3

o .533

0 .002n ? ! q

0 .005

1O0.25

0 . 001

1 . 0 0 5

U . ) ' T

0 . 440

0 . 007

r 0 0 . 7 r l 0 l . 1 3

n n n q n n l t

0 . 0 0 4 0 . 0 1 8

0 .975 0 .879

0 .031 0 .207

0 . 5 1 3 0 . 1 5 1

0 . \ 56 0 .545

0 . 0 1 1 0 . 1 7 7

2 .000 2 . 0 0 0 2 .000 2 .000 2 .000 r . 9 9 0 2 ,000 2 .000

* Fe de termined as Feo; ox ides based on c rys ta l chemis t ry .t * n . d . = n o t d e t e c t e d ; l e s s t h a n 0 . 0 3 Z .


Table 3. Representative analyses of spinels, rutile, and perovskite

T y p e I s p i n e l T y p e l l s p i n e l T y p e l l l s p i n e l p e r o v s k i t e r u t l l e

s l 0 ^

T i 0 2

A l ̂ 0 -z 5Cr203

Fe^0-**z 5Fe0

lln0

M9o

N i 0

Ca0

t

Cat i onB a s i s

s l

A I

L T

T i+ ?

l'lg

N i

_ + 2t e

I'ln

C a

t

| . 5 2

8 .02

> t . 5

9 . 5 51 7 . 0o . 5 2

1 1 . 0

n . d .

3 . 8 58. 70

49 . 8

8 .24

t8 .2

o . 5 3

12.2

n . o .

n . a .

n . a ,

2 .05

50 .5

5 . 7 8

9 . 7 81 1 . 0

0 . 2 1

1 9 . 8

n . d .

n , a .

3 . 2 7q 6 . r

6 . o 7

12 .1

1 0 . 1

0 . 1 1

20.4

n . d .

n . a .

n , a .? 1 E

8 .48

2 . 0 7

27.8

1 7 . \

0 . 87

22.6

n . d .

n . a .

2 l . o

1 0 . 0

0 .46

25.5

22.7

0 .89

| 8 . 4

n . d .

0 .09

> I . o

o . 3 30 . 1 6

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n . d .

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97 .50 . i l

n . q ,

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n , d ,

n . d .

n . d .

9 8 . 9 |

c

0 .317

1 .363

0 .038

0 . 2 \1

0 .549

0 .478

0 . 0 r 5

10 r . 52

c

o.333

1 . 2 7 3

0 .095

0 .200

0 .590

0 .493

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4

1 .596

o .123

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0 . n 3

o ,245

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4

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0 .065

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100.72

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0 .305

0 .050

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0 .443

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{

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0.498

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'-liu

qn arr

0 .00 r

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0 ,994

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3 .000 3.000 3.000 3.000 3.000 3 .000 t . 9 9 1 1 .000

* n .a . = no t €na lyzed. * * Fe de termlned as Fe0; ox ldes based on c rys ta l chemls t ry . * * * n .d . = no t de tec ted '

I Megacryst coreO Cpx-llm int€rgrowthD Groundmassi l lmeni te in Ol iv ineO M€gacryst Rirns

7 0 3 0Fig. 5. Compositions of the ilmenite textural types plotted in

an expanded portion of the FeTiO3-MgTiO3-Fe2O3 temary. Alldata plotted as rnolVo.

and Fe2O3 such as that observed for the megacrystcores (Fig. 7).

There occurs a regular, well-developed composi-tional gradient between the megacryst core and thehigh-MgO rim (Fig. 8). This profile is developedover a distance of at least 75-100 pm, although thetrue extent cannot be determined due to the exten-sive reaction rim of perovskite + titanomagnetite *Mn ilmenite. In many cases, the profiles can be

FeIiO3 MgIi03

Fig. 6. Ternary FeTiO3-MgTiO3-Fe2O3 plot showing thecompositions of the Eiliott County megacryst and groundmassilmenites relative to ilmenites from Africa (Ha€gerty, 1975;Mitchell, 1977). ,

50

50

AGEE ET AL.: OXIDE MINERALS IN KIMBERLITE 35

.6

.5ile

Itg. fe .l

.3

.2

t o o."ffi.e. .rl:_." 1... . '

t t

at '

I glrndilN. n4rcrrst cqlso ctr-iln!ilbo ilmdh h olirir!

o 2 4 6 t , l lort '

11 16 18 20

Fig. 7. A plot of Mg/(Mg*Fe) vs. wt.VoFe2O3 for the ElliottCounty ilmenites.

extended into the reaction zone by analyzing theremnant islands of ilmenite within the reactionzones (Fig. 4b). Haggerty et al. (1979) noted similarcompositional gradients in megacrysts from theMonastery Mine, South Africa; they referred to itas the magmatic reaction trend. Elthon and Ridley(1979) also found an increase in MgO near the rimsof ilmenites from the Premier Mine, South Africa.Boctor and Boyd (1980) reported similar composi-tional gradients developed within the outer 300-400pm of discrete ilmenite megacrysts from the Liqho-bong kimberlite, Lesotho. Since the perovskite +titanomagnetite + Mn-ilmenite reaction zone israrely over 100 pm wide, it appears that the compo-sitional gradients near the rims of the Elliott Countyilmenite megacrysts probably cannot be extendedover 200 pm inward, a distance somewhat less thanobserved for the Liqhobong ilmenite megacrysts.These compositional gradients to the high-MgOrims probably developed as the result of reaction ofthe ilmenite megacrysts with a MgO-enriched melt;therefore, they represent a diffusion profile betweenthe megacryst core and an overgrowth with a higherMgO content.

Ilmenites from the clinopyroxene-ilmenite inter-growths (44.7-44,6 mol% MgTiO3 and 5.4-5.8molVoFe2O) have compositions similar to the moreMgO-rich megacryst cores (Table 1; Figs. 5 andT).Although lower in Cr2O3 (0.06-0.38 wt.%o), theclinopyroxenes from the intergrowths have compo-sitions similar to the clinopyroxene megacrystsfrom Elliott County (0.56-0.88 wt.Vo); the clinopy-roxenes from the intergrowths have estimatedequilibration temperatures of 1295'-1335' C whichoverlap the range of temperatures estimated for thePipe I clinopyroxene megacrysts (1305"-1390' C),using the diopside-enstatite solvus of Lindsley and

Dixon (1976) with molVo Di : 2Cal(Ca+Mg+Fe)(Garrison and Taylor, 1980; 1981). Using the clino-pyroxene-ilmenite geothermometer of Bishop( 1980), with X|" r,o, : Fe * 2/(Fe* 2 + Mg *2 + 0' 5Fe *3)

and Xffi: [2Cal(Ca+ Mg+Fe)][Mg/(Mg+ Fe)], Gar-rison and Taylor (1981) estimated the equilibrationtemperatures (atP : 50 kbar) ofthe clinopyroxene-ilmenite pairs at 1210'-1245'C; these equilibrationtemperatures are generally consistent with the diop-side-cnstatite solvus temperature estimates consid-ering the uncertainties associated with applyingthese geothermometers. The compositional similar-ities between the clinopyroxenes and ilmenites fromthe intergrowths and the discrete ilmenite andclinopyroxene megacrysts from Elliott County sug-gest that the clinopyroxene-ilmenite intergrowthsand the clinopyroxene and ilmenite megacrysts aregenetically related (Garrison and Taylor, 1981).

The Elliott County Type IV groundmass ilmen-ites are distinctly richer in MgTiO3 @6.8-57 .3 mol%MgTiOl) than the ilmenite megacryst cores, al-though quite similar in MgTiO3 content to the MgO-enriched rims of the ilmenite megacrysts (Fig. 5;Table 2). The groundmass ilmenites show a widerange of Fe2O3 Q.9-13.9 molVo Fe2O3). Analogousto the MgO-enriched megacryst rims, the ground-mass ilmenites exhibit a correlation of increasingFe2O3 Q.29-15.7 wt.Vo) with decreasing MgO (15.7-13.5 wt.Vo), but no correlation between Fe2O3 andMg/(Mg+Fe). The groundmass ilmenites exhibit awider range of CrzOr (0.12-3.76 wt.%) than eitherthe megacryst cores (<0.03-1.30 wt.Vo CrzO) orthe megacryst rims (0.04-2.22 wt.Vo Cr2O). Thesimilarity in chemical systematics between theMgO-enriched rims of the megacrysts and the TypeIV groundmass ilmenites suggests that both mayhave equilibrated with the same MgO-enriched

1 0 0microns from rim

Fig. 8. Electron microprobe traverse from rim to core of aType I ilmenite megacryst showing the smooth MgOcompositional gradient.

oo

;s


melt. The core-rim-groundmass pattern of the El-liott County ilmenites is analogous to the chemicalsystematics displayed by the Elliott County olivinemegacrysts and groundmass olivines; the rims ofolivine megacrysts have Mg/(Mg+Fe) ratios of0.89-0.90, regardless of the core compositionswhich range in Mg/(Mg+Fe) from 0.85 to 0.93. Therims of these olivine megacrysts are similar to thelow-NiO groundmass olivines in Mg/(Mg+Fe),CaO, and NiO, reflecting a later-stage reaction ofthe olivine megacrysts with a liquid more enrichedin MgO than the liquid in equilibrium with the mostfractionated megacrysts (i.e., with the lowestMe/(Me+Fe)).

The ilmenites occurring as inclusions in ground-mass olivines (50.9-56.4 molVoMgTiO3 and 3.9-9.1molVo Fe2O3) have compositions that overlap themore MgO-rich groundmass ilmenites, althoughthey are slightly higher in Cr2O3 @.00-4.42 wt.%)(Figs. 5 andT; Table 2). This similarity suggests thatthese inclusions are genetically related to at leastsome of the Type IV groundmass ilmenites. Fur-thermore, this also suggests that at least some of thegroundmass ilmenites belong to a compositionallydistinct population of ilmenites formed during alater stage of ilmenite crystallization and are, infact, not MgO-enriched fragments of disaggregatedrecrystallized megacrysts. These high-Cr2O3 ilmen-ites actually approach the composition of the smallidiomorphic groundmass ilmenites reported fromthe Liqhobong kimberlite by Boctor and Boyd(1980), although somewhat more variable in Fe2O3.The Fe-Mg equilibria between groundmass olivine(Foss.o) and ilmenite inclusions, using the geother-mometer of Andersen and Lindsley (1979), suggestequilibration of these groundmass phases at 965" C(at p : 50 kbar). A 5 kbar change in the assumedpressure will change the estimated temperature byon ly 9 .5 'C.

The ilmenite from the ilmenite-rutile intergrowth(55.0 mol% MgTiOs) contains no Fe2O3 Gable 2).The ilmenite that rims the rutile-ilmenite inter-growth is similar in composition (52.0 molVoMgTiO3 and 1.5 molVo Fe2Or) to the low-Fe2O3groundmass ilmenites. The absence of Fe2O3 in theilmenite lamellae in the rutile suggests that they arenot related to the rim ilmenite. The rim ilmenite isprobably related to the Type IV groundmass ilmen-ites and the MgO-enriched rims of the ilmenitemegacrysts, reflecting reaction of the rutile pheno-cryst with the MgO-enriched liquid. The ilmenitelamellae within the rutile were probably formed by

exsolution from the rutile. Energy Dispersive Anal-ysis (EDA) of the rutile indicates the presence ofsubstantial Nb: this is also reflected in the low sumof the rutile analysis in Table 3.

The high-MnO Type Vlilmenites found within theperovskite + titanomagnetite * Mn-ilmenite reac-tion rims around the ilmenite megacrysts and TypeN groundmass ilmenites contain 17.9 molVoMnTiO3, 16.3 molVo MgTiO3, and 10.5 molVoFe2O3.They are quite distinct from either megacryst orgroundmass ilmenite (Table 2). Their distinctivecomposition and textural relationship to perovs-kites and titanomagnetites indicate that these MnO-rich Type VI ilmenites are clearly of secondaryorigin. High-MnO ilmenites'have been described inkimberlite by Haggerty et al. (1979), Hsu andTaylor (1978), Boctor and Meyer (1979), and Wyatt(1979). In each case, they are associated with late-stage reaction rims or occur in cracks within ilmen-ite megacrysts.

Spinel

Representative analyses of spinels from the El-liott County kimberlites are presented in Table 3.The Type 1 chromites contain 63.6-68.1 mol%(Fe, Mg)CrzO 4, 10.0-12. 1 molVo (F e,Mg)FezO+, 3 . 8-ll.0 mol% (Fe,Mg)2TiOa, &rd 15.9-16.7 mol%(Fe,Mg)Al2Oa. The Type II pleonastes that occur asovergrowths on Type I chromites and as discretegroundmass grains contain 74.3-:79.8 molVo(Fe,Mg)Al2O 4, 6.2-6.6 molVo (Fe,Mg)Cr2Oa, 8.9-12.5 molVo (Fe,Mg)Fe2Oa, and 4.1-6.6 molVo(Fe,Mg)TiOc. Type III titanomagnetites that occurwithin the perovskite * titanomagnetite + Mn-ilmenite reaction zones and as discrete grains in thegroundmass are distinctly higher in TiO2 than theother spinels (38.0-53.0 molVo (Fe,Mg)2TiOa, 17.5-26.3 molVo (Fe,Mg)Al2Oa, and 3.0-6. I molVo(Fe,Mg)CrzO+). The compositional ranges for spin-els from the Elliott County kimberlites are plottedin the spinel prism of Figure 9 (Irvine, 1965). Thetypical kimberlite trend (Haggerty, 1976) of earlyCr- and Al-rich spinels followed by late-stage crys-tallization of spinels enriched in Fe*3 and Ti isreadily seen at Elliott County. This compositionalchange from ground mass Ty p e 1 Cr-spinel and Ty p e// pleonaste crystallization to the precipitation ofType III titanomagnetite reflects the breakdown ofilmenite in the presence of a CaO-enriched fluid.From the Elliott County data, it is not clear whetherthe change from Cr-spinel to pleonaste crystalliza-tion is continuous or discontinuous.

AGEE ET AL.: OXIDE MINERALS IN KIMBERLITE 37

Perovskite

The composition of perovskite shows little varia-tion whether in the reaction rim around ilmenite oras discrete groundmass grains (Table 3). Perovskitehas only minor FeTiO3 (2.6 nol%) and MgTiO3 (0.7mol%) in solid solution. Energy dispersive analysis(EDA) indicates the presence of substantial Nb andREEs. Boctor and Meyer (1979) considered theabundance of Nb and REEs in perovskite to be theresult of carbonate complexing of the REEs; thiscomplexing stabilizes the REEs in the late-stagekimberlitic fluids.

Discussion

Previous investigations of oxide minerals in kim-berlite (e.9., Haggerty, 1973; 1975; Danchin et al.,1975) have dealt with the mineralogy of the mega-crysts and/or groundmass, as well as compilationsof data from many localities. The Elliott Countykimberlite is well-preserved and permits a detailedinvestigation of both megacrysts and groundmassphases from the same kimberlite. The relationshipbetween these different generations of minerals iscomplex; however, when the oxide data are com-bined with the data on megacryst and groundmasssilicates by Garrison and Taylor (1980), the evolu-tion of the Elliott County kimberlite can be dividedinto four distinct stages: (I) the crystallization of themegacryst phases, (II) an increase in the MgO/FeOratio in the kimberlitic liquid, (III) the crystalliza-tion of the groundmass phases with the accompany-ing back-reaction of the megacrysts with the MgO-enriched liquid, and (IV) a late-stage "autometaso-matic" reaction of ilmenite with a CaO-enrichedfluid to form perovskite + titanomagnetite * Mn-ilmenite.

The ilmenite, as well as the silicate, megacrystscould have formed as the result of(1) disaggregationof peridotite to form discrete mineral grains, or (2)direct precipitation from a "proto-kimberlitic" liq-uid (Eggler et al.,19791' Garrison and Taylor, 1980).The latter origin was suggested for the Elliott Coun-ty silicate megacrysts (Garrison and Taylor, 1980)because: (1) the large grain size of the megacrysts isgenerally larger than the grain size expected fromdisaggregated peridotite; (2) the common inter-growths of the minerals is typical for phases co-precipitated from a liquid; (3) the megacrysts do notshow complete or nearly complete compositionaloverlap with the phases of included peridotite xeno-liths; and (4) the garnets, clinopyroxenes, olivines,

Fig. 9. Compositional fields of the different spinel typesplotted in the spinel prism (Irvine, 1965). The range of molVoFe2TiOa is given for each spinel type. l: Type 1 chromite; II :

Type II pleonaste; III : Type III titanomagnetite.

and to a lesser extent, the orthopyroxenes of themegacryst assemblage exhibit compositional trendsgenerally attributed to precipitation from a fraction-ating liquid.

The silicate megacrysts from Elliott County ex-hibit well-defined chemical trends which Garrisonand Taylor (1980) attribute to fractional crystalliza-tion from a "proto-kimberlitic" liquid. Garnet meg-acrysts show correlation of Mg/(Mg+Fe) (0.79-0.86) and CaO (4.54J.10 wt.Vo) with CrzOg (0.21-9.07 wt.%); the more magnesian garnets have moreuvarovite in solution. Olivine megacrysts show adecrease in NiO (0.5-0.05 wt.Vo) with decreasingMgi(Mg + Fe) (0. 93-0. 85). The clinopyroxene mega-crysts show increasing TiO2 (0.38-0.56 wt.Vo) anddecreasing Cr2O3 (0.88-0.56 wt.%) with decreasingMg/(Mg+Fe) (0.91-0.87). The high-AlzO:, high Z(1 165'-1255" C) orthopyroxene megacrysts exhibita general trend of decreasing TiO2 and Cr2O3 withdecreasing Me/(Mg+Fe).

An origin by precipitation from a "proto-kimber-litic" liquid is also preferred for the Elliott Countyilmenite megacrysts because: (1) ilmenite is anextremely rare minor constituent of mantle perido-tite, (2) the size of the ilmenite megacrysts (0.5-1.5cm) is much larger than the grain size expected fromdisaggregated peridotite, and (3) the ilmenite mega-crysts display a correlation of increasing Fe2O3 withdecreasing MgO and Mg/(Mg+Fe), which can beattributed to fractional crvstallization.


Gurney et al. (1979) studied a large number ofcoexisting ilmenite-silicate pairs from the Monas-tery Mine and demonstrated a co-variance of MgOin ilmenite and Mg/(Mg+Fe) in the coexisting sili-cate phases. They suggested that the Fe2O3 varia-tion of the ilmenites could be due to increases in/p"as the kimberlitic liquid evolves. As discussed'earlier, we believe the increase in Fe2O3 withdecreasing MgO and Mg/(Mg+Fe) in the ElliottCounty ilrnenite megacrysts can be produced, as aresult of crystallizing silicates and ilmenites, deplet-ing the liquid in MgO and decreasing Mg/(Mg+Fe)slowly with fractionation. The preferential incorpo-ration of Fe+2 into the abundant silicate phases willproduce an increase in Fe+3/Fe*2 ratio in the re-maining melt. Since both mechanisms would pro-gressively enrich ilmenite in Fe2O3, the MgO-Fe2O3and Mg/(Mg+Fe)-Fe2O3 systematics may provevaluable indicators of the T-X trajectory of theevolving kimberlite liquid.

The silicate megacrysts from Elliott County areconsidered to be derived from a fractionating proto-kimberlitic liquid, representing the earliest stages ofkimberlite development (Garrison and Taylor, l9g0;1981); the data presented above suggest that theElliott County ilmenite megacrysts probably crys-tallized. from the same liquid. The compositionalsimilarities of ilmenite and clinopyroxene in thenodular graphic intergrowths to megacryst clino-pyroxene and ilmenite in the host kimberlite sup-port co-precipitation of clinopyroxene and ilmenite,at least during a portion of kimberlite crystalliza-tion. The clinopyroxene megacrysts generally havehigher estimated equilibration temperatures (l 305.-1390' C), by as much as 50o C, than the clinopyrox-enes from the nodular graphic intergrowths (1295'-1335" C) suggesting that clinopyroxene crystallizedover a short temperature interval before beingjoined by ilmenite. This sequence of events isconsistent with phase relations outlined by Wyatt(1977). The high Mg/(Mg+Fe) ratios (0.47-0.50) andMgO content (13.0-13.5 wt.%) and low Fe2O3 con-tent (6.14-6.71 wt.%) of the ilmenites from thegraphic intergrowths is consistent with the ilmenitesfrom the intergrowths representing some of theearliest formed ilmenites (j.e., least fractionated).The clinopyroxenes from the intergrowths havechemical systematics that suggest they representsome of the more fractionated members of theclinopyroxene population (Garrison and Taylor,1980). Ilmenite and clinopyroxene probably crystal-lized together over only a short temperature inter-

val. Data from the Elliott County high-Z orthopyr-oxene megacrysts (i.e., with estimated equilibrationtemperatures of 1165'-1255' C) suggest that Stage Imegacryst crystallization continued to about1165" C at pressures of 51-53 kbar. It is not clearwhether ilmenite megacrysts crystallized through-out this interval, along with orthopyroxene, garnet,phlogopite, and olivine.

C rystallization of groundmas s phas e s

The high-MgO Type 1V groundmass ilmenitesfrom the Elliott County kimberlite pipes could havetwo possible origins: (1) they may represent MgO-enriched fragments of disaggregated recryst allizedilmenite megacrysts, and/or (2) they may representa compositionally distinct second generation ofilmenite that crystallized with the other groundmassphases. The similarity in chemical systematics be-tween the MgO-enriched rims of the ilmenite mega-crysts and the Type IV groundmass ilmenites sug-gests that, regardless of their initial mode of origin,both probably equilibrated wirh the same MgO-enriched liquid.

By examining the Cr2O3 content of the Type IVgroundmass ilmenites, it is possible to delineate thesecond generation groundmass ilmenites from theMgO-enriched fragments of disaggregated mega-crysts. Since the Type V ilmenite inclusions ingroundmass olivines are high in Cr2O3 @.00-4.42wt.Vo), it is most probable that the ilmenites thatcrystallized in equilibrium with the groundmassassemblage are high in Cr2O3. Over 90Vo of theanalyzed, megacryst cores have between 0.03 and0.40 wt.Vo Cr2O3; over 86Vo of the analyzed mega-cryst rims have between 0.04 and 0.73 wt.Vo Cr2O3.This suggests that the megacryst ilmenites, as wellas their MgO-enriched rims, are dominantly of thelow-Cr2O3 variety. The low-Cr2O3 nature of theMgO-enriched megacryst rims probably reflects in-complete equilibration with the MgO- and Cr2O3-enriched liquid. Only 54% of the analyzed Type IVgroundmass ilmenites have Cr2O3 contents lessthan 0.73 wt.Vo; the remaining 46Vo have between1.43 and 3.76 wt.% Cr2O3. These high-Cr2O3 Type/Vgroundmass ilmenites are similar in Fe2O3 (4.58-8.58 wt.%) to the Type V ilmenite inclusions ingroundmass olivines (4.43-10.0 wt.Vo Fe2O), fur-ther substantiating a genetic relationship with thegroundmass mineral assemblage. As discussedabove, these high-Cr2O3 Type V and Type 1V ilmen-ites have compositions that approach the composi-


tions of the small idiomorphic groundmass ilmenitesreported by Boctor and Boyd (1980) from the Liq-hobong kimberlite. In contrast to the Liqhobonggroundmass ilmenites, the high-Cr2O3 Type IVgroundmass ilmenites from Elliott County do have awell-developed reaction assemblage of titanomag-netite + perovskite + Mn-ilmenite. This may sim-ply reflect the evolution of the late-stage kimberliticfluid in time and P-T-X space. We suggest thatkimberlite groundmass ilmenites sensu stricto canbe distinguished from small MgO-enriched frag-ments of disaggregated megacrysts by their distinct-ly higher Cr2O3 content (i.e., generally greater than1.00 wt.% CrzOr).

In the Elliott County kimberlites, the groundmassphases orthopyroxene (i.e., the low-T, high-mgGroup II orthopyroxenes of Garrison and Taylor,1980), olivine (Mg/(Mg+Fe) : 0.88-0.90), and high-Cr2O3 Type IV ilmenite are generally more magne-sian than the more fractionated members of thecorresponding megacryst phases (i.e., those withthe lowest Me/(Mg+Fe) ratios). Both ilmenite andolivine megacrysts exhibit MgO-enriched rims simi-lar in composition to the groundmass phases. Bothgroundmass ilmenite and orthopyroxene in the El-liott County groundmass assemblage are higher inCr2O3 than megacryst ilmenite and orthopyroxenesuggesting that the MgO-enriched liquid from whichthe groundmass assemblage crystallized was alsoenriched in Cr2O3. These chemical systematics indi-cate that the crystallization of groundmass phasesand the back-reaction of megacryst phases (i.e.,Stage III) was preceded by an effective increase inthe MgOiFeO ratio (and CrzOr) in the kimberliticliquid (i.e., Stage II).

Garrison and Taylor (1980) have suggested thatthe Stage II increase in MgO/FeO ratio of thekimberlitic liquid was related to an increase in /6,and/or assimilation of carbonated peridotite. Theysuggested that the assimilation of a carbonatedperidotite such as dolomite or magnesite peridotitewould have the effect of adding the gaseous speciesCOz to the liquid. In the absence of graphite ordiamond (Eggler et al.,1980), this CO2 would be anoxidizing agent and would subsequently increasethe /6, and the Fe3+/Fe2+ ratio of the liquid. Thisincrease in Fe3* would have the effect of increasingthe MgO/FeO ratio. This mechanism seems incon-sistent with the observation that the high-Cr2O3Type IV and V ilmenites have Fe2O3 contents whichare generally lower than the Fe2O3 content of themore fractionated (i.e., lowest Mg/(Mg+Fe) ratio)

members of the ilmenite megacrysts. If the carbon-ated peridotite contains substantial MgO, in theform of easily assimilated dolomite or magnesite,assimilation would also add substantial MgO andsubsequently increase MgO/FeO. This increase inthe availability of divalent species may explain theobserved low-Fe2O3 content of the high-Cr2O3 TypeIV and V ilmenites. It should be realized that anysuch model invoked to explain the increase in MgOin the kimberlitic liquid at Elliott County shouldalso explain the apparently ubiquitious MgO-en-richment observed in kimberlite in other localities.

Temperature estimates for various pairs of StageIII groundmass minerals suggest equilibration of thegroundmass assemblage at temperatures as low as965" C. The Fe-Mg equilibria between groundmassolivine (Foas.o) and an ilmenite inclusion, using thegeothermometer of Andersen and Lindsley (1979),suggests equilibration at 965'C (at P : 50 kbar).This is consistent with the temperature estimates of970'-1020'C reported by Garrison and Taylor(1980) for the low-?, high-mg (Group II) orthopy-roxenes in the Elliott County kimberlite. Using theolivine-spinel geothermometer of Fabries (1979),the Fe-Mg equilibria between the Type l chromitesand groundmass olivines (Fosz.s to Foeq.6), repre-senting the entire range of observed groundmassolivine compositions, suggest equilibration at tem-peratures of 990"-1085'C; this range is consistentwith both of the previous temperature estimates forgroundmass phases.

If the Stage III groundmass mineral assemblageindeed crystallized from the MgO-enriched kimber-litic liquid at 965"-1085o C, it is possible to place aconstraint on the depth at which this event oc-curred. From the position of the univariant solidusfor kimberlite, as outlined by Eggler and Wendlandt(1979), it is clear that Stage III groundmass crystal-lization probably occurred at pressures not greaterthan 25-35 kbar. This suggests that Stage IIIgroundmass crystallization occurred at depths shal-lower than about 100 km, 50 km shallower than thedepth at which Stage I megacryst crystallizationoccurred. Assuming that the Group II low-? ortho-pyroxenes at Elliott County were in equilibriumwith garnet, Garrison and Taylor estimated that theGroup II orthopyroxenes crystallized at pressuresof 47-50 kbar using the geobarometer of Wood(1974). Since garnet does not belong to the Stage IIIgroundmass mineral assemblage, we feel that thegeobarometer of Wood (1974) is not applicable andthat the above maximum pressure estimate of 25-35


kbar for the Stage III groundmass crystallization ismore reasonable.

The presence of rutile in kimberlite has beenattributed to the breakdown of armalcolite((Fe,Mg)Ti2O5) when it is found in intergrowthscontaining approximately equal amounts of rutileand ilmenite (Haggerty, 1975). The rutile-ilmeniteintergrowth from Elliott County contains less than2Vo ilmenite lamellae and cannot be explained bythis mechanism. This intergrowth is best explainedas the result of exsolution of ilmenite from a rutilethat contained a small amount of FeO and MgO insolution. Haggerty (1976) and Mitchell (1973 1979)have noted similar intergrowths in kimberlites inSouth Africa and Canada: as in the case of theElliott County kimberlite, these intergrowths werealso rimmed by high-MgO ilmenite. This ilmeniterim probably formed at the time of Stage III ground-mass crystallization as the rutile reacted with theMgO-enriched liquid. The rutile host could be (1) axenocryst, or (2) a primary phase precipitated fromthe kimberlitic liquid.

Formation of the late-stage reaction assemblage

At Elliott County, the final recorded stage ofkimberlite evolution (Stage IV) is the reaction ofilmenite to form the assemblage perovskite + titan-omagnetite * Mn-ilmenite. This reaction suggeststhe presence of a late-stage CaO-enriched fluid; thepresence of Mn-ilmenite also suggests an increasein the availability of MnO in the fluid. The late-stagereaction of ilmenite with a CaO-enriched fluid toform perovskite and titanomagnetite may occuraccording to the following reactions:

2CaCO3= 2Caf,"]6 + Oz + CO2

2Ca*2 + 02 + [3(Fe,Mg)TiO3 * Fe2O3]." -

2CaTiO3 + [(Fe,Mg)2TiO4 + (Fe,Mg)Fe2Oa]..

These reactions explain the major element system-atics of the phases involved; minor elements suchas Al2O3, Ct2O3, and MnO are easily accommodat-ed into the reaction scheme. The availability ofCa*2 is controlled by the breakdown of calcite.Garrison (1981) has suggested that the breakdownof earlier-formed groundmass calcite may have in-creased the availability of Ca*2 in the kimberliticfluid; textural evidence suggests that the break-down of groundmass calcite may have been exten-sive in the Elliott County kimberlite. The early-formed groundmass calcite probably became unsta-ble as the late-stage kimberlitic fluid evolved in p-

T-X space. Thus, it appears that the metasomaticreaction of ilmenite to form the assemblage perov-skite + titanomagnetite * Mn-ilmenite may be anatural consequence of the evolution of the late-stage kimberlitic fluid in time and P-T-X space.

The discrete grains of perovskite and titanomag-netite fouhd throughout the groundmass of theElliott County kimberlite could be: (1) primaryphases precipitated from the late-stage kimberliticfluid, (2) disaggregated fragments of reaction zoneassemblages, and/or (3) the completely reacted rem-nants of very small groundmass ilmenite grains.These discrete groundmass grains are composition-ally similar to the phases within the reaction zoneassemblage suggesting that, regardless of mode oforigin, both textural types probably equilibratedwith the same fluid.

Since Mn-ilmenite is present in the reaction as-semblage around ilmenite, the activity of Mn*2must have also increased in the late-stage kimberlit-ic fluid. Wyatt (1979) suggested that Mn-rich ilmen-ites can form by interaction of kimberlite with localmeteoric waters. In a study of the Premier kimber-lite, Wyatt (1979) found the MnTiO3 content ofilmenites to increase toward the grain boundaries;these zoned grains were found only in the marginsof the kimberlite pipe and small kimberlite dikes.Wyatt concluded that these Mn-ilmenites formedduring post-magmatic interaction of the Premierkimberlite with local meteoric waters, the Mnsource being the local country rock. The intimatetextural relationship of the Elliott County Type VIMn-ilmenites to the minerals of the reaction zoneassemblage indicates a genetic relationship of Mn-ilmenite to the reaction zone perovskite and titano-magnetite. The ubiquitious occurrence of the per-ovskite * titanomagnetite reaction assemblage inkimberlites from other localities argues against acountry rock controlled metasomatic event as amechanism for the formation of the Mn-ilmenite. aswell as perovskite and titanomagnetite.

AcknowledgmentsThis study was supported in part by NSF Grant EAR-7906318,

a University of Tennessee Faculty Research Grant, and theDepartment of Geological Sciences discretionary fund. Criticalreviews by D. H. Eggler and N. Z. Boctor are gratefully ac-knowledged.

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Manuscript received, August 12, 19E0;accepted for publication, August 27, 1981 .

petrogenesis of oxide minerals in kimberlite, elliott county, kentucky

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