subduction fluids and their interaction with the mantle...

Per. Mineral. (2007), 76, 2-3, 253-265 doi:10.2451/2007PM0028SPECIAL ISSUE: In honour of Ezio Callegari on his retirement http://go.to/permin

l PERIODICO di MINERALOGIAestablished in 1930

An International Journal ofMINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY,ORE DEPOSITS, PETROLOGY, VOLCANOLOGYandappliedtopicsonEnvironment,ArchaeometryandCultural Heritage

AbstrAct. — We review three case studiesemphasizing the role of ultramafic rocks in therecyclingofvolatilesandtraceelementsatconvergentplatemargins.Serpentinitesaremajorwatercarriersinsubductionzonesandtheirbreakdownliberateslarge quantities of water at sub-arc depths. Theincompatibleelementsincorporatedduringoceanicserpentinization are released into the fluid phaseproduced once antigorite dehydrates to olivine +orthopyroxene.Importantly,theantigoritebreakdowncan trigger either wet melting or production ofsupercriticalfluidsinalteredbasaltsandsediments.Theproduced fluidphases incorporate substantialamountsofincompatibleelement,initiallyresidinginthecrustalreservoirs.Thefluidphasewhichexitstheslabishighlyreactivewithrespecttotheoverlying,silica undersaturated,mantle rocks.This leads toformationofreactive(ortho)pyroxenitelayers,whichfiltertheuprisinghydrousmelt/supercriticalfluidtoproduce aqueous, solute-rich solutions.This fluidhasequilibratedwithperidotitesandismobileinthemantle.

Aconsequenceof thesesubductionfluid/mantlereactionsisthatthemantlewedgedomainsoverlyingtheslabscanbeheterogeneousincompositionandlayered,duetothepresenceofreactivepyroxenite

bodies.Anotheraspectregardsthedebatewhethersupercritical fluidsorhydrousmelts are effectivemediafortraceelementtransport.Sincebothagentsaresaturatedinsilica,theywillreactwiththesilica-undersaturatedmantlewedgeperidotitestoproduceaqueous, incompatible trace element-rich residualfluids.Hence,whilehydrousmeltand/orsupercriticalfluids are important for scavenging incompatibleelementsfromtheslab,theymaynotbetheagentsthattransferthemetasomaticsubductionsignaturetotheinnerpartsofthemantlewedges.

riAssunto.—Questocontributoriassumetrecasidistudiocheevidenzianoilruolodellerocceultrafemicheneiprocessidiriciclodellesostanzevolatiliedeglielementiintracciaaimarginidiplaccaconvergenti.Le serpentiniti sono i sistemi maggiormenteresponsabiliperiltrasportodell’acquanellezonedisubduzione,doveliberanograndiquantitàdiacquaaprofonditàdisub-arcoacausadelladisidratazionedelserpentino.Glielementiincompatibiliincorporatidaquesteroccedurantel’alterazioneoceanica,vengonorilasciatinelfluidoprodottodalladisidratazionedelserpentino. L’acqua rilasciata dall’antigorite puòinnescarelafusioneparzialeolaformazionedifluidisupercritici nei livelli di roccebasaltiche emeta-sedimentariecostituentilaplaccasubdotta.Ifusioifluidicosìprodottiincorporanoquantitàsignificativedielementimaggiori(oltreil50%inpeso)eintraccia

Subduction fluids and their interaction with the mantle wedge: a perspective from the study of high-pressure ultramafic rocks

MArco scAMbelluri 1 *, nAdiA MAlAspinA 2 and Joerg HerMAnn 3

1DipartimentoperloStudiodelTerritorioedellesueRisorse,UniversitàdiGenova,Italy2DipartimentodiScienzedellaTerra,UniversitàdiMilano,Italy

3ResearchSchoolofEarthSciences,TheAustralianNationalUniversity,Canberra,Australia

*Correspondingauthor,E-mail:[email protected]

254 M. scAMbelluri, n. MAlAspinAandJ. HerMAnn

originariamentepresentinelleroccecrostali.Lafasefluida rilasciata dallo slab subdotto è altamentereattivarispettoallesoprastantiroccedimantelloecausalaformazionedilivellireattiviaortopirosseno.Questi livelli ‘filtrano’ i fluidi supercritici e/o ifusiidratiuscentidalloslabperprodurreunfluidoacquoso residuale ricco in soluto: quest’ultimo sièequilibratoconleperidotititidimantelloedèingradodimigrareall’internodelwedgedimantello.Unaconseguenzadiquestereazionifluido/mantelloècheidominiidelcuneodimantellosoprastantiloslabsonocomposizionalmenteeterogeneie‘stratificati’a causa della presenza dei livelli di pirossenitireattive.Unaltroaspettodiquestericercheriguardal’efficienzadei fluidi supercritici odei fusi idraticomeagentiditrasportodeglielementiintraccianelmantello.Entrambigliagentisonoricchiinsiliceelalororeazioneconilmantelloliberafluidiacquosimobili arricchiti in elementi incompatibili. Diconseguenza,mentreifusiidratieifluidisupercriticisonoimportantiperincorporareelementidaiserbatoicrostali nello slab, essi non sono gli agenti chetrasferisconoallepartiinternedelcuneodimantellol’improntametasomaticasubduttiva.

introduction

Subduction zone fluids play a fundamentalrole in large-scale mass transfer at convergentplate margins, as they transfer volatiles andincompatibleelementsfromcrustalreservoirsinthesubductingplatestotheoverlyingmantle.Thefluidtransportleadstometasomatismofthemantlewedgeperidotitesandtriggerspartialmeltinginregionswhereperidotitesareabovethewetsolidustemperatures. �ased on detailed geochemical�ased on detailed geochemicalstudies of arc lavas, it has been inferred thatsubductionfluidsareenrichedinlargeionlitophile(LILE)andlightrareearths(LREE)relativetothehighfieldstrengthelements(HFSE;McCullochand Gamble, 1991; �renan et al., 1994). TheThecrust-to-mantleexchangeatsubductionzonesthusimpactsonmantlere-fertilizationandisamajordrivingforcefor thechemicaldifferentiationoftheEarth.Theroleoffluidsinsuchacyclehasbeenincreasinglyemphasizedinthelastdecadeand an ongoing debate concerns their nature,compositionandeffectivemobility(ScambelluriandPhilippot, 2001;Manning, 2004;Hermannet al., 2006; Zack and John, 2007). The clear

distinctionbetweenaqueousfluidsandhydroussilicatemelts,whichcharacterizesallrocksystemsat relatively low pressures and temperatures,vanishesatultrahigh-pressureconditions,wherecompletemiscibilitybetweenwaterandsilicatemeltshasbeenexperimentallyattainedinarangeofP-Tconditionsandofbulkrockcompositions(�ureauandKeppler,1999;Stalderet al.,2001;Schmidtet al.,2004;Hermannet al.,2006;Kesselet al.,2005).Theexistenceofasecondcriticalendpoint,wherethewetsolidusterminatesandasupercriticalliquidforms,openedthedebateontheroleofsupercriticalfluidphasesasmetasomaticagentsindeepsubductionenvironments.

Studiesofnaturaleclogite-faciesrocksprovideimportantconstraintstotheunderstandingofdeepsubductionfluidsandtheirinteractionwithslabandmantlewedgerocks.Thehigh(HP)andultrahigh-pressure (UHP) rocks exposed in orogenicterrainsprovideindependentconstraintsondeepmetamorphisminslabs,andrepresentexceptionalnaturallaboratoriesonsubduction-zoneprocessesinadepthwindowbetween50and200kilometers.Someultradeepcoesite-,diamond-andmajorite-bearingrockspreserveprimarysolidmultiphaseinclusions(VanRoermundet al.,2002;Stoeckertet al.,2001;Ferrandoet al.,2005;Malaspinaet al.,2006;Scambelluriet al.,2007),whichhavebeen interpreted in somecaseas remnantsofasupercriticalfluidphase.

Ultramafic rocks play a fundamental role involatileandelementrecyclingatconvergentplatemargins.Fieldstudieshaveshownthatserpentiniteisstableateclogite-faciesconditionsandhencecantransportwaterintothemantle(Scambelluriet al.,1995).Experimentsdemonstratetheprolongedstabilityofantigoriteserpentineto200kmdepthandidentifyhydrousultramaficsystemsasexceptionalwatercarriersintotheEarth’smantle(UlmerandTrommsdorff,1995;WunderandSchreyer,1997;�romileyandPawley,2003).Thesefindingshaveimportantconsequencesonsubductiondynamicsbecause serpentinites provide a particularlyfertilewaterreservoirforarcmagmatism(Ulmerand Trommsdorff, 1995), and because theirdehydrationcangenerateintermediate-depth(50-200Km)earthquakes(Peacock,2001;Dobsonet al.,2002).Serpentinitesalsoactaslowdensityandlowviscositymediaenablingtheexhumationofhighandultrahighpressurerocks(Hermannet al.,

Subduction fluids and their interaction with the mantle wedge: a perspective from the study ... 255

2000;Guillotet al.,2001;Rupkeet al.,2004).Themantledomainsoverlyingthesubductingplatesareotherenvironmentswhereultramaficrocksplayakeyrole,asthefluid/peridotiteinteractionsattheslab/mantleinterfacecancontrolthecompositionoffluids,whicharetransferredtotheinnerpartsofthemantlewedges.However, the understandingHowever,theunderstandingofmechanismsrulingtheslab-to-mantleelementtransferisessentiallyhamperedbythepaucityofsuitable rock samples recording suchexchangereactions.Moststudiesofsupra-subductionzoneperidotitesinvestigatedeitherfore-arcxenoliths,orxenolithssampledatrelativelyshallowmantlelevels,wellabovearc-magmasources(Vidalet al..,1989;Mauryet al..,1992;Lauroraet al..,2001).InformationondeepmetasomatismofthemantlewedgecanbegainedbystudiesofHPandUHPterrains,where felsic rockshostmetasomatizedperidotites (�rueckner, 1998; Rampone andMorten,2001;Paquinet al..,2004;Scambelluriet al..,2006;Liouet al..,2004).Suchassociationsenable to study the element exchange betweencrustalandmantlerocksatpressurescorrespondingtothesub-arcdepthofthesubductedslab.

Toaddresstheroleofultramaficrocksin the fluid and element cycling in subductionzones,herewereviewthree field-basedstudiesconcerningfluidreleaseintheslabandpossiblefluid-rockinteractionsataslab-mantleinterface.Inthefirstpartofthispaperwediscusstheserpentinedehydrationreactionintheslab,anditspossibleconsequences in terms of interaction of de-serpentinizationfluidswithsedimentaryorgraniticlayersintheslab.Inthesecondpartwediscussthecaseofultrahigh-pressuregarnetorthopyroxenitesas proxies for the reaction between mantleperidotiteswithpercolating silicate-rich agentsreleasedfromfelsicand/ormetasedimentaryslabcomponents.

Fluid production in subducted ultrAMAFic rocks

Serpentinitesarekeylithologiesintheoverallwatercycleatoceanicandsubductionsettings.Severalmodelsassumeastratifiedstructureoftheslabs(e.g.PoliandSchmidt,2002),withalowerultramaficlayer,anoverlyingmaficcrustandanuppermostsedimentarylayer.Thisstructurecanbe

inheritedfromapreviousoceaniclithosphere,asdocumentedinpresent-dayfastspreadingridges.Insuchsettings,serpentinizationoftheoceanicmantleoccursatouterrises,wherefracturesinthebendingplatesenhanceseawaterinfiltrationatanddeepmantleserpentinization(Raneroet al.,2003;Peacock,2001).Alternatively,partofthelayeredslabarchitecturemaybeerasedbytectonicerosionand/orsubductiondeformation,e.g.boudinageofcompetentslablayersliketheoceaniccrust.Also,thelithosphereatslowandultraslowspreadingridgesisnotlayered,butischaracterizedbythevast exposure of serpentinized oceanicmantleattheseafloor(Cannatet al.,1995;Dicket al.,2003).Duringsubductionofthistypeofoceaniclithosphere,serpentinitesmaybeatthetopoftheslab.

Despite the considerable progress inunderstanding hydrous phase relations insubductedserpentinites,littleisyetknownabouttheirgeochemicalfeatures,particularlyaboutthetraceelementfingerprintsofthefluidsreleased.The only natural cases of analyzed fluidsproducedduringpartialtocompletebreakdownof antigorite at HP to UHP concern the Erro-Tobbio serpentinites (Western Alps) and thechloriteharzburgites from the�eticCordillera(SouthernSpain)(Scambelluriet al.,1997;2001;2004a; 2004b;Trommsdorffet al., 1998).Thehigh-pressureserpentinitecyclepasses throughtwodehydrationsteps(Fig.1):aminor“brucite-out” reaction leading to the firstappearanceofmetamorphicolivine+antigorite+fluid,andamajorfluidrelease(antigorite-out)relatedtofullantigoritedehydrationtoolivine+orthopyroxene+ fluid.The first reaction is recordedbymanyAlpineand�eticHPserpentinites(e.g.Liguria:Cimminoet al.,1979;Scambelluriet al.,1995;Zermatt:Liet al.,2004;Monviso:Lombardoet al.,1978;NevadoFilabride:Trommsdorffet al.,1998; Puga et al., 1999). In all these settingsthe serpentinites are associated with eclogites(Messigaet al.,1995;Messigaet al.1999;Pugaet al.,1999)anddisplayastableassemblageofolivine+antigorite+Ti-clinohumite+diopside+chlorite,crystallizedat500-650°Cand2-2.5GPa.The“brucite-out”reaction(Fig.1)causesalossofabout2wt%bulkwaterfromtheinitialserpentinites.�estrecordsofthisaqueousfluidareprimary fluid inclusionshostedbyolivine,


diopsideandTi-clinohumitecrystallizedinveinsintheErro-Tobbioserpentinites(Fig.2a,b).Theinclusionsgenerallydisplayasaltdaughtercrystal(Fig. 2a), locally associatedwith ilmenite andmagnetite:theirsalinitycanbeashighas50wt%NaClequivandthesaltcompositioncorrespondstoamixtureof(Na,K)ClandMgCl2(Scambelluriet al.,1997).Suchacompositionwastakenasevidencefordeeprecyclingofoceanicchlorineandalkaliesinthefluidphase(Scambelluriet al.,1997).Lower(present-day)averagesalinityof10wt%NaClequivalents pertain to primary fluid inclusionspresentinolivineanddiopsideinthe�etichigh-pressureserpentinites(Scambelluriet al.,2001a).Thefirstdehydrationfluidsreleasedduringtheserpentinitesubductioncyclethuscorrespondtoaqueoussolutionswhichconcentratehighamountsofhighlyincompatiblehalogenspecies.Thehighsalinityofsuchinclusionsmayreflectchangesinthecompositionofpristinefluidsdrivenbywater-consumingprocesses, suchashydrousmineralcrystallizationinveins,orhydrationofrelict(dry)mantleminerals(Scambelluriet al.,1997).

Theantigoritebreakdownisthesecondandthemostimportantdehydrationreactioninsubductedserpentinites(Fig.1),leadingtoabulklossof6.5-

12wt%water.Thisreaction is recordedby themetamorphicharzburgitesofthe�eticCordillera,a unique rock type (Fig. 2c) showing spinifextexturedolivine+orthopyroxene(Trommsdorffet al.,1998),aswellascoarsegranoblasticolivine,orthopyroxene,chlorite+Ti-clinohumite.Suchmetamorphicharzburgitescrystallizedat650-700°Cand2GPa(Trommsdorffet al.,1998;Pugaet al.,1999;LopezSanchez-Vizcainoet al.2005)(Fig.1,Field3);however,thisassemblagecanalsoformatUHPconditions(UlmerandTrommsdorff,1995).Olivineandorthopyroxeneintheserockscontainprimaryfluidinclusionsfilledwithsoliddaughter phases (olivine, magnetite-ilmenite,chlorite,apatite)andaninterstitialaqueousliquid.Theseinclusionsareremnantsofthefluidphasereleasedat theantigoritebreakdown.Aninitialsalinity range of 0.4 – 2 wt% NaClequiv wasestimated for this fluid from thebulk-rocknetdifferenceofwaterandchlorinebetweenantigoriteserpentinitesandharzburgites(Scambelluriet al.,2004a).ThetraceelementcompositionsoftheseinclusionsweremeasuredbyLaserAblation(LA)ICPMS,usingthe0.4–2wt%NaClequivestimatesof the initial fluid salinity as internal standard(Scambelluriet al.,2004a,b).ThespiderdiagramofFig.3showsthetraceelementcompositionsofinclusionsnormalizedtotheprimitivemantle.Thefluidinclusionsdisplayappreciableincompatibleelementcontents,thehighestamountspertainingto the light elements and the alkalies. In theinclusions,severalelements(e.g.�oron)displaylargevariations in theabsoluteconcentrations,whichmay spanover oneorder ofmagnitude.Allfluidinclusionsdisplaycomparablepatterns,whicharesystematicallyenrichedinLILE(Rb,�a,Cs,Sr),�andLiwithrespecttotheHFSE(Ti,Nb).Thesefeaturesaresimilartowhatisobservedin arc volcanics (Fig. 3) and are in excellentagreementwithexperimentalresults.TenthoreyandHermann(2004)analyzedfluidcompositionsthat were experimentally produced from thedehydrationofserpentinitesathighpressure.Theirstudyshowedthatincompatibleelements,whichcanbetakenupduringserpentinization,arefullyreleasedduringthebreakdownofantigorite.

Fig. 1 – Pressure-temperature diagram showing theevolutionarypathoftheoceanicmantleatthetransitionfromoceanicserpentinites(field1),tohigh-pressureantigoriteserpentinites(field2),toolivine-orthopyroxenerocks(field3).RedrawnafterHermannet al.(2000).


interAction oF Fluids witH subducted sediMents

Dependingon thestructureof thesubductinglithosphere,theserpentinebreakdownfluidsmayeither(i)direcly infiltrate themantlewedge,or(ii)reactwithcrustalrocksofthesubductedslab.Thefirstcaseoccursiftheslabisnotlayered(i.e.slabserpentinitesareclosetotheinterfacewiththemantlewedge)or if the fluid ischannelled.Thisleadstoadirectinteractionoftheantigorite-breakdownfluidswiththemantle,whichwouldacquirethesignatureshowninFig.3.Thesecond

caseoccursifslabsarelayeredandcrustalrocksareabovetheserpentinizedoceanicmantle,oraremixedwithserpentinitematerialinmélangezonesatthetopoftheslab(Spandleret al.,2007).Figure4portraysthewetsoliduscurvesforthevariousslabcomponents(Hermannet al.,2006;Kesselet al.,2006)withtheaimtoexplaintheinteractionbetweenserpentinitefluidsandcrustalslabrocks.ThewetperidotitesolidusreportedinFig.4isbyStalderet al.(2001),whodeterminedthesecondcriticalendpointforthissystem.AlsoreportedinFig.4aretheboxesreferringtothecrystallizationconditionsoftheHPErro-Tobbioserpentinites,of

Fig.2 – A: primary salt-bearing fluid inclusions in diopside from an olivine vein, Erro-Tobbio Unit (Scambelluri–A: primary salt-bearing fluid inclusions in diopside from an olivine vein, Erro-Tobbio Unit (ScambelluriA:primarysalt-bearingfluidinclusionsindiopsidefromanolivinevein,Erro-TobbioUnit(Scambelluriet al.,1997).�:olivine,magnetite,diopsideandTi-clinohumiteveininhigh-pressureserpentinite(ErroTobbioUnit,WesternAlps,Italy).C:Chloriteharzburgitewithspinifex-liketexture.Inlightgreyisorthopyroxeneinbrowngreyisolivine.(Cerro del Almirez,(CerrodelAlmirez,�èticCordillera,Spain;Trommsdorffet al.,1998).D: primary fluid inclusions rich of solid phases (magnetite, olivineD:primaryfluidinclusionsrichofsolidphases(magnetite,olivinechlorite)andwithaquoeusliquid,inolivinefromthechloriteharzburgites(Scambelluriet al.,2001;2004a).


theUHPCignanaophiolites,andoftheHP�èticharzburgites.Allpeakconditionsareclosetothewet solidus and to the secondcritical endpointof pelitic and granitic systems.�ecause of theinverted isotherms in subduction zones, fluidsreleasedat650-700°Cduringantigoritebreakdownwillheatupastheyrise.Asaconsequence,thesetheseuprisingfluidswilllikelytriggertheproductionofmeltsorsupercriticalliquidsinthesedimentarylayers at 750–800°C (Hermann and Green,2001).Subductedsedimentsandalteredoceaniccrust are themajorhostsof incompatible traceelements in thesubductedcrust (TenthoreyandHermann,2004).ThegreatmajorityofLILEishostedinphengiteintheserocktypes(Hermann,2002;Spandleret al.., 2003).�ecausephengiteisstable to temperaturesof950-1000°Catsub-arcpressures(Schmidtet al..2004;HermannandGreen,2001), fluid-absentmeltingofphengite-bearingmetasedimentsandalteredoceaniccrustishardlyachievedatsub-arcconditionsanditisnotregardedtobeaviableprocesstoproduceLILEenrichedslabfluids.Incontrast,inthepresenceofanexternally-derivedfluid,sedimentsandalteredoceaniccrustwillundergofluid-presentmelting

and LILE will preferentially partition into thefluidphase(HermannandGreen,2001;Kesselet al..,2005).Thisdemonstratesthatfluidsliberatedfromsubductedultramaficrocksplayakeyroleinscavengingtraceelementsfromfertilesubductionlithologiessuchasalteredbasaltsandsediments.

interAction oF subduction zone Fluids witH tHe MAntle wedge

Understandingtheinteractionofhydrousmelts/supercriticalfluidswiththemantlewedgeiscrucialtodefinethevolatileandtraceelementrecyclinginsubductionzones.Thegarnetorthopyroxenitesfrom the Maowu Ultramafic Complex (DabieShan,China)areexcellentproxiestounraveltheslab-to-mantleelementtransferatUHPconditions.Thisbodyconsistsoflayeredmeta-harzburgites,garnetorthopyroxenitesandwebsteritesassociatedwith coesite-eclogites. Pyroxenites are locallyboundedbyphlogopite-richlayers,andarehostedbygarnet-coesite-bearinggneisses.Inthisterrane,gneisses andultramafic rocks share a commonUHPhistorywithpeakconditionsof4-6GPaand

Fig.3–Traceelementcompositionsofinclusionsinolivinefromthechloriteharzburgites(Scambelluriet al.,2004b).


700-750°C (Liou and Zhang, 1998).Althoughthese rocks are not direct samples of amantlewedge,texturalandgeochemicaldatademonstratethat they represent former garnet–peridotitesmetasomatizedbyacrust-derivedSiO2-richfluidphaseat~4.0GPaand~750°C(Malaspinaet al.,2006).Thepetrographicobservationsindicatethatthegarnetorthopyroxenitespreservearelictparagenesisconsistingofolivine+orthopyroxene1+ garnet1 ± clinopyroxene ± Ti-clinohumiteovergrown by coarse-grained orthopyroxene2(Fig. 5a, b) associated with porphyroblasticinclusion-richgarnet2(Fig.5c).Orthopyroxene2replacesearlierolivine(Fig.5b)andcanincludefine-grained garnet1 and orthopyroxene1. Thisindicatesthatformermantlephases(i.e.olivine,garnet1andorthopyroxene1)werereplacedbyaSi-enrichedphase.Themajorandtraceelementcompositionsofwhole-rocksandmineralphases

supportthetexturalevidencethattheprotolithofthesepyroxeniteswasaperidotite.TheydisplayhighMg#andNiconcentrationsandchondrite-normalizedREEpatternsresemblingtheonesofadepletedmantle(Malaspinaet al.,2006).Withrespect to the inferred harzburgite protholith,however, the Maowu orthopyroxenites areenriched in SiO2 andAl2O3. LREE enrichmentcharacterizesthebulkrocksaswellasthereplaciveorthopyroxene2. Such major and trace elementcompositionsofthegarnet–orthopyroxeniteshavebeenattributedbyMalaspinaet al.(2006)totheinfiltrationofametasomaticmelt-likefluidphaserich inSiO2,Al2O3, and incompatible elementssourcedfromthecountry-rockgneissesatpeakUHP conditions. Reaction of such a SiO2-richhydrousfluidphasewiththeperidotiteresultsinaSiO2-andAl2O3-lossfromthefluidphasetoformthegarnetorthopyroxenites.Ontheotherhand,partof theH2Ocomponentof themetasomaticagentcannotbeaccomodatedbythenewlyformedanhydrousphasesOpx2andGrt2,andevolvesintoa residualaqueous fluid.This free fluid is thenoccasionally trapped by the growing Grt2 intoprimarypolyphaseinclusions(Fig.5c,d,e,f).

The polyphase inclusions in the Maowuorthopyroxenites thereforeprovide informationonthenatureandcompositionoftheresidualfluidproducedafterinteractionoftheperidotitewiththemelt-likefluidphase.Theydisplayregularnegativecrystalshapes(Fig.5c,d,e,f)andcontainasolidassemblage(oxide+amphibole+chlorite±talc±mica±apatite)showingconstantvolumeratios,tosuggestthattheyrepresentdaughterphasesthatformed from a compositionally homogeneousfluidphaseprimarilytrappedbytheUHPgarnet.Toconstrainthenatureofsuchafluidphasetheinclusionshavebeenre-homogenizedinapistoncylinderexperimentatP=3.5GPaandT=900°C(Malaspinaet al.,2006).There-homogenizationexperimentproducedahydrousporousquench,indicatingthattheUHPfluidintheinclusionswasasolute-richaqueousfluidratherthanahydrousmelt. The composition of this fluid has beeninvestigatedwithLA-ICP-MSanalysesperformedonthebulkofbothpolyphaseandexperimentallyre-homogenizedinclusions,followingthemethoddevelopedbyHeinrichet al.(2003).Theresultsgive a reliable estimate of the trace elementcompositionofthetrappedresidualfluid.Asshown

Fig.4–Wetsoliduscurvesforultramafic,mafic,peliticandgraniticsystems,showingthesecondcriticalendpoints.Theendpoint forultramaficsystems liesatmuchhigherpressures(10GPa).�oxesrefertothepeakP-TconditionsfordifferentserpentiniteandcrustalHPandUHPunitsoftheAlpsandofthe�eticCordillera.RedrawnandmodifiedafterHermannet al.(2006).


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in Fig. 6, both polyphase and re-homogenizedinclusions display very high concentrations ofincompatibleandfluid-mobiletraceelements,withpositivespikesinCs,�a,Pb,Sr,andahighU–Thratios. These chemical characteristics provideevidencethatthemetasomaticagentleadingtotheformationof theorthopyroxeniteshadacrustalaffinity.Theresidualaqueousfluidretainedmostoftheincompatibleelementsthatwerepresentinthereactinghydrous-melt.Infact,thewhole-rockcomposition,representedbythewhitediamondsin Fig. 6, shows relative enrichments only inLREE,whereasmostofthefluid-mobileelementsarebelowthedetectionlimit.Thereasonforthisobservation is that the rock-forming minerals– orthopyroxene and garnet – are not able toincorporatetheseincompatibleelements.ThelowLILEcontentsinthewhole-rocksimplythatsuchLILE-enrichedfluid largelyescaped thesystemandwasonlyoccasionally trappedingarnet2 toformthepolyphaseinclusions.Thisfluidisabletomigrateupintothemantlewhereitmayenhancecrystallizationofmetasomaticamphibole(below3GPa;FumagalliandPoli,2005)and/orphlogopite,toultimatelyreachthelocusofpartialmeltinginthemantlewedge.

discussion And conclusions

The presented case studies highlight theimportanceofultramafic rocks in the recyclingof volatiles and trace elements at convergentplatemargins.Serpentinitesarethemaincarriers

ofwaterinsubductionzonesandthebreakdownofantigoriteliberateslargequantitiesofwateratsub-arcdepths.Moreover,incompatibleelementsincorporated during oceanic serpentinizationwill be released into the HP-UHP fluid phaseonceantigoritebreaksdown.Probablythemostimportanteffectonthetraceelementrecyclinginsubductionzonesisthatfluidsderivedfromtheantigoritebreakdownwilltriggerwetmeltinginalteredbasaltsandsediments.Theproducedmelts/supercriticalfluidsincorporatesubstantialamountsofincompatibleelements,intiallyresidinginthecrustalreservoirs.Fig.7describesapossibledeepsubductionenvironment,wheretheaqueousfluidsreleasedbytheserpentinitesinfiltrateanoverlyingmeta-sedimentarylayertoenhanceeitheritspartialmeltingortheproductionofsilicate-richfluids.Thefluidphasesthatexittheslabwillbehighlyreactive to the surrounding mantle peridotites.This will produce the orthopyroxenite layersdescribedintheMaowuexamplesandfiltertheuprisinghydrousmelt-supercritical fluidphase.Thefluiduprisingfromsuchfilteringzonewillbeanaqueous,solute-richsolutionwithcompositioncomparablewiththeoneportrayedinFig.6.Thisfluidequilibratedwithmantlerocksthroughthefilteringprocessandismobileinthemantle.

A first implication of our case studies thusconcerns the mantle wedges above subductingslabs,whichcanbeheterogeneousincompositionand layered, due to the presence of reactivepyroxenitebodies.Si-enrichmentinthemantlewasclaimedbypreviousstudiesofsupra-subductionmantledomainsaffectedbyre-fertilizationand/orformationofreactivepyroxenites(Kelemenet al.,1998;YaxleyandGreen,1998;Garridoand�odinier,1999;Grooveet al.,2005).Also,Sobolevet al. (2005) proposed that interaction between(2005)proposed that interactionbetweenrecycled crust andmantle peridotites producespyroxenitesinthedeepmantle,andmeltingofsuchamantlemightcontributetothegenesisofoceanislandbasalts.Herewehavedetailedafeasiblemechanismbywhichthesupra-subductionmantlereactswithslab-derivedsilicateagentstoproducelargezoneswherepyroxenitesdominate.

Oneinterestingaspectofthisworkisthatthedebatewhethertraceelementsaretransportedinsupercriticalfluidsorhydrousmeltsisafterallnotthatessential.�ecauseboththeseagentsaresaturatedinsilica,theywillinevitablyreactwith

Fig.6 – Trace element compositions of multiphase inclusions–Trace element compositions of multiphase inclusionsTraceelementcompositionsofmultiphaseinclusionsfromMaowuorthopyroxenitesandofthehostrocks.AfterAfterMalaspinaet al.(2006).


thesilica-undersaturatedmantlewedgeasoutlinedintheMaowucasestudy.Duringthisreactionthefluidphasewillbetransformed,asSiandAlareextractedfromthemelt/supercriticalfluid,leavinganaqueous,incompatibletraceelementenrichedresidualfluid.Hence,whilehydrousmeltand/orsupercriticalfluidsareimportantforscavengingincompatible elements from the slab, they areunlikelytheagentsthattransportthemetasomatictraceelementsignature to thesourceof thearcmagmas.Onlyinthecasethatthereleasedfluidsarechannelledinpyroxenitedikesinthemantlewedge, the reactionwithmantle olivinewouldbe inhibited and the slab-derivedSi- and traceelement-richliquidsmightmigrateintooverlyinghotterregionsofthemantlewedge.

AcknowledgeMents

It isagreatpleasure forus toparticipate in theSpecialVolumeinhonourofProfessorEzioCallegari,andweacknowledgeR.CompagnoniandD.Castellifor the invitation to write this contribution. M.Scambelluri,inparticular,hasbeenstudentofProf.

Callegari duringhisUniversity studies inTorino,wherehemettheenthusiasmandpassionofEziointeachingpetrographyintheclassandinthefield.TheauthorsthankO.MüntenerandA.Zanettifortheirconstructivereviews.Thisworkhasbeenfinanciallysupportedby the ItalianMIUR, theUniversityofGenovaandbytheAustralianResearchCouncil.

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