Geochemistry-of-subduction-zone-serpentinites--A-r

InvitedreviewarticleGeochemistryofsubductionzoneserpentinites:AreviewFabienDeschampsa,⁎,MargueriteGodarda,StéphaneGuillotb,KéikoHattoricaGéosciencesMontpellier,UniversitéMontpellier2,CNRS,cc060,PlaceE.Bataillon,34095Montpellier,FrancebISTerre,UniversitéGrenobleI,CNRS,1381ruedelaPiscine,38400GrenobleCedex09,FrancecDepartmentofEarthSciences,UniversityofOttawa,Ottawa,OntarioK1N6N5,CanadaabstractarticleinfoArticlehistory:Received5October2012Accepted31May2013Availableonline11June2013Keywords:SubductedserpentinitesMantlewedgeserpentinitesAbyssalperidotitesSerpentinephasesSubductionzonesGeochemistryOverthelastdecades,numerousstudieshaveemphasizedtheroleofserpentinitesinthesubductionzonegeodynamics.Theirpresenceandroleinsubductionenvironmentsarerecognizedthroughgeophysical,geo-chemicalandfieldobservationsofmodernandancientsubductionzonesandlargeamountsofgeochemicaldatabaseofserpentiniteshavebeencreated.Here,wepresentareviewofthegeochemistryofserpentinites,basedonthecompilationof~900geochemicaldataofabyssal,mantlewedgeandexhumedserpentinitesaftersubduction.Theaimwastobetterunderstandthegeochemicalevolutionoftheserocksduringtheirsubductionaswellastheirimpactintheglobalgeochemicalcycle.Whenstudyingserpentinites,itisessentialtodeterminetheirprotolithsandtheirgeologicalhistorybeforeserpentinization.Thegeochemicaldataofserpentinitesshowslittlemobilityofcompatibleandrareearthel-ements(REE)atthescaleofhand-specimenduringtheirserpentinization.Thus,REEabundancecanbeusedtoidentifytheprotolithforserpentinites,aswellasmagmaticprocessessuchasmelt/rockinteractionsbeforeserpentinization.Inthecaseofsubductedserpentinites,theinterpretationoftraceelementdataisdifficultduetotheenrichmentsoflightREE,independentofthenatureoftheprotolith.Weproposethatenrichmentsareprobablynotrelatedtoserpentinizationitself,butmostlydueto(sedimentary-derived)fluid/rockinterac-tionswithinthesubductionchannelaftertheserpentinization.Itisalsopossiblethattheenrichmentreflectsthegeochemicalsignatureofthemantleprotolithitselfwhichcouldderivefromthelessrefractorycontinen-tallithosphereexhumedattheocean–continenttransition.Additionally,duringthelasttenyears,numerousanalyseshavebeencarriedout,notablyusinginsituap-proaches,tobetterconstrainthebehavioroffluid-mobileelements(FME;e.g.B,Li,Cl,As,Sb,U,Th,Sr)incor-poratedinserpentinephases.Theabundanceoftheseelementsprovidesinformationrelatedtothefluid/rockinteractionsduringserpentinizationandthebehaviorofFME,fromtheirincorporationtotheirgradualre-leaseduringsubduction.SerpentinitesareconsideredasareservoiroftheFMEinsubductionzonesandtheirrole,notablyonarcmagmacomposition,isunderestimatedpresentlyintheglobalgeochemicalcycle.©2013ElsevierB.V.Allrightsreserved.Contents1.Introduction...............................................................971.1.Subductionzones.........................................................971.2.Water:animportantcomponentinsubductionzone........................................971.3.Subduction-relatedserpentinites:waterandfluid-mobileelementcarriers..............................992.Nature,formationandlocationofsubduction-relatedserpentinites.....................................992.1.Abyssalserpentinites.......................................................992.2.Mantlewedgeserpentinites...................................................1002.3.Subductionzone-relatedserpentinites..............................................1012.4.Compilationofserpentinitesgeochemistry............................................1053.Inheritedgeochemicalsignatureofserpentinites.............................................1053.1.Geochemicalconsequencesduetoserpentinizationandprotolithfingerprints............................1053.1.1.Majorelements.....................................................1053.1.2.Rareearthelementsandothertraceelements......................................109Lithos178(2013)96–127⁎Correspondingauthor.E-mailaddress:[e-mail](F.Deschamps).0024-4937/$–seefrontmatter©2013ElsevierB.V.Allrightsreserved.:(lizardite,chrysotileandantigorite)...........................1153.2.1.Majorelements....................................................1153.2.2.Rareearthelementsandmoderatelyincompatibleelements...............................1153.3.Determinationofserpentinizationenvironmentsusinggeochemicalcompositions.........................1153.3.1.Inheritedsignatureofserpentiniteprotoliths.......................................1153.3.2.Chromianspinels...................................................1163.3.3.Redoxconditions....................................................1173.3.4.Platinumgroupelements(PGE).............................................1174.Serpentinites:asink/sourcevectorforfluid-mobileelements.......................................1174.1.Fluid-mobileenrichmentsinbulkserpentinitesandrelatedserpentinephas