地球化学科学前沿-俯冲带地球化学：Subduction-Zone俯冲带

SubductionZone俯冲带Subduction

modellingThefateofsubductingslabsElements

mobilityNb

paradoxPlateBoundaries板块理论的两大基石:俯冲带和洋中脊

板块俯冲是地球上最重要的化学分异过程，对于大陆地壳的形成、地幔的演化、地幔柱的形成等都起着极其重要的作用。PlateBoundariesOcean–ContinentConvergentBoundariesContinent–ContinentConvergentBoundary板块俯冲引发的化学过程的三种主要研究方法：1）从岛弧火山岩入手，根据岛弧火山岩的成份来示踪板块俯冲过程中的地球化学分异作用。俯冲析出流体与地幔楔反应以后形成，经历了强烈的岩浆分异2）高温高压实验模拟：通过确定各种温压条件下元素在矿物和流体间的分配系数来研究各种元素在板块俯冲过程中的活动性。与实际俯冲过程有一定的差异3）通过研究蓝片岩、榴辉岩等俯冲产物，揭示元素和同位素地球化学分异现象。并不是板块俯冲的最终产物1,Subductionmodeling•Subductioninitiation.•Fluidandmelttransportinsubductionzones.•Subductionchannelprocessesandhistoriesofhigh-pressurerocksduringsubduction.•Small-scaleconvectionandthermal–chemicalplumesinthemantlewedge.•Crustalgrowthandmagmaticarcdevelopment.•Overridingplatedynamics.•Deepsubductionandslabbendingprocesses.•Terminationofsubductionandslabbreak-off.•SubductionintheearlyEarth.•Lateralvariabilityofsubductionprocessesin3D.Therewouldbenoplatetectonicsiftherewerenosubductionzones.Yethowasubductionzonebeginsremainspoorlyunderstood.（Niuetal.,2003）Gravitationalinstabilityofoldoceanicplatesprovidestheprimarydrivingforceforsubduction(VlaarandWortel,1976;Davies,1999).Twoforcesmustinitiallybeovercometomakeasubductionzone:faultfriction(orgrowthofalithosphere-cuttingshearzone)andplatebending.SubductioninitiationExamplesof2Dnumericalmodelsofthermal-chemicalplumesrisingfromsubductingslabs.(a)ModelbyGeryaetal.(2006),twotypesofplumesdevelopfromthe

slab:unmixedplumescomposedofhydratedpartiallymoltenmantleandmixedplumescomposedoftectonicmelangefrompartiallymoltenhydratedcrustaland

mantlerocks.(b)ModelbyRichardandBercovici(2009),sectionacrossthefloatingslabinthetransitionzoneismodeled,notenegativetemperatureanomalyassociated

withchemicallybuoyantwater-richplumesrisingfromtheslab.Terminationofsubductionandslabbreak-offColdvs.HotsubductionCold,oldslabYoung,hotslabMaruyamaandOkamoto,20072,ThefateofsubductingslabsthemodalcompositionofpyrolitewithdepthbasalticoceancrustDensities(g/cc)ofthermallyequilibratedbasalticoceancrust,subductedharzburgitelithospherecomparedwithundepletedpyrolitemantletodepthsof800kmDensitydifferencesbetweenbasalt-pyroliteandharzburgite-pyroliteasthesubductedoceancrustsinks.Thebasalticslabbecomeslessdensethanmantlepyroliteinthedepthrange650-750km.astrong'slab-pull'forcephasechangesinpyroliteatthe670kmdiscontinuitybasalticoceancrusttrappedatthe670kmdiscontinuityMantlestructureattheendoftheArchean(2500Maago)asaresultofsubductedmaficoceancrustpilingupatthe650kmdiscontinuity(Ringwoodetal.,1992)sourcefordiamond-bearingkimberlitemagmasEarthhistory,oceancrustaccumulatedatthe650kmdiscontinuity:atleast100kmthickharzburgiteisinherentlylessdenseandpotentiallymorebuoyantthanthesurroundingmantlelessdepletedwithincreasingdepthmuchmoredepletedharzburgiticmantlehighmantletemperatures----veryhighdegreesofmelting----high-Mgkomatiiticlavas----extremelydepletedpure-olivineduniticresidueSimple"boxmodel"ofmantleevolution,showinghowmeltingatspreadingridgesproducesoceancrust,whichisthenalteredbyhydrothermalactivityandthensubducted.Partofthissubductedcrustisthenmeltedtoformcontinentalcrust,andtheresiduesthensubductedtobecomepartofthereservoirofthedepleted(DMM)mantle.Smalldegreemeltsmigrateupwardstoenrichthesub-continentalmantleandprovidethesourceforalkalibasalts.Sedimentsubductionmaymodifythesub-continentallithosphere.(afterTarneyetal.1980)RingwooditeincludedindiamondcontainslargeamountofwaterPearsonetal.(2014)NatureInclusionExperimentsRingwooditeisahigh-pressurepolymorphofolivine(amagnesiumironsilicate)thatisformedathightemperaturesandpressuresoftheEarth'smantlebetween525and660kmdepth.3,Elements

mobilityNiu,2009汇聚板块边缘的岛弧钙碱性岩石富集大离子亲石元素(LILE)而亏损高场强元素(HFSE)这些岩石通常被认为是俯冲板片脱水，导致地幔楔熔融而来(e.g.,Tatsumi,1989;McCullochandGamble,1991).岛弧火山岩的亏损高场强元素特征与俯冲板片在脱水过程中金红石稳定不变有关(e.g.,RyersonandWatson,1987;ScambelluriandPhilippot,2001;TropperandManning,2005).因此，金红石控制了俯冲洋壳所释放流体中的HFSE含量

(Brenanetal.,1994;Ayers,1998).金红石在水中的溶解度极低(AudetatandKeppler,2005;TropperandManning,2005)Ti,Nb和Ta在金红石和流体之间的分配系数也较大，即这些元素在金红石中极相容(Brenanetal.,1994;Stalderetal.,1998).俯冲洋壳在榴辉岩相条件下产生的高压脉体通常被认为是先前板片中的流体通道，从而可以反映流体迁移过程中与岩石间的交互反应(e.g.,Beckeretal.,1999;GaoandKlemd,2001;Scambellurietal.,2001;JohnandSchenk,2003,2006;Johnetal.,2007).(Gaoetal.,2007)Fibrousomphacitesandrutilesinthesegregationclosetothehosteclogite;(a–e)markthewholerocksamplestakenforchemicalcompositionanalysisalongaprofilewithvaryingdistancetothesegregation.Oriented,acicularcentimeter-sizedrutilecrystalsoccurasinclusionsinankeriteinanirregularlyshapedsegregationinthecentreofaneclogiteboudin,whichitselfisenclosedbyblueschist(Gaoetal.,2007)Chemicalcompositionofthesegregationhosteclogitealongaprofilewithvariabledistancetothesegregation.(a)LILEand(b)HFSE.(Gaoetal.,2007)transportofTi,Nb,andTafromthehostrockintothesegregation.Spandleretal.,2004Spandleretal.,2004经历过蚀变和水化的洋壳到达汇聚板块边缘时，会携带上附沉积物。随板片俯冲，部分上附沉积物会滞留在增生楔；板片中的基性岩石的含水量可达5%，lawsoniteblueschist-facies，一些低级含水矿物脱水会释放出B和LILE；随板片俯冲，持续脱水。主要的脱水过程发生在蓝片岩相到榴辉岩相的转变过程中,超过2%的水被释放出来（Peacock,1993）；基性岩中的含水矿物分解或者超基性岩中的叶蛇纹石分解释放流体，B,LILE,Th,U和LREE会随流体迁移；富含以上元素的流体和熔体会进入亏损地幔中，对岛弧岩浆作用做出贡献；一旦完全转变到榴辉岩相，板片的密度大于周围地幔，从而向更深俯冲至少到660KM（TatsumiandEggins,1995）。但其中仍有含水矿物，如多硅白云母，硬柱石等。这些含水矿物的脱水，会有助于弧后盆地区域的岩浆作用。Spandleretal.,2004Ba,Cs,

Rb,K,Pb,andSrarenotsignificantlycorrelated

withNbandZr：Mobilized

during

eitherprogradeorretrogrademetamorphism,or

priorto

subduction-zonemetamorphismThepositiveSm/Nd-Lu/Hfcorrelationisconsistentwiththeserocksbeingpotentialsourcematerialsforocean-islandbasalts(OIBs)whensubductedwithapositiveHf-Ndisotopicrelationship,butthelackofRb/Sr-Sm/Nd(andLu/Hf)correlationisinconsistentwiththerocksbeingthemajorsourcematerialsforoceanicbasaltsthatshowsignificantinverseSr-NdandSr-Hfisotopicsystematics.Zacketal.,2002俯冲的沉积物中的金红石，褐帘石，独居石，锆石和磷灰石等副矿物容纳了主要微量元素；流体相中的K,Rb(LILE),LREE和Zr,Ti,P(HFSE)含量受控于这些副矿物和白云母。(HermannandRubatto,2009)ThedepletedmantleandthecontinentalcrustarethoughttocounterbalancethebulksilicateEarthcompositionwithrespecttomostrefractorylithophileelements.Thisrelationshipisvalidforanumberoftraceelementsandisotoperatios,suchastherareearthelements(REE)andSr–Ndisotopesystematics(Hofmann,1988;Hofmann,1997).4,NbparadoxHighfieldstrengthelementsarecommonlyconsideredasmembersofthegroupoftraceelementsthatpossesschondriticBSEratios;e.g.Nb/Uratioshavebeenusedtodemonstratethecomplementaryrelationshipofthecontinentalcrustandthedepletedmantle(Hofmannetal.,1986).However,allofthemajorsilicatereservoirsonEarthappeartoshowsubchondriticNb/Taratios,aphenomenoncommonlyreferredtoasthe‘‘Nbparadox”(Barthetal.,2000;KamberandCollerson,2000;Rudnicketal.,2000;Mu¨nkeretal.,2003;Rappetal.,2003).Rudnicketal.(2000)suggestedthatrefractoryeclogiteproducedbyslabmeltingisthe“missingreservoir”toaccommodatethemassimbalanceinNb/Tabetweencontinentalcrustanddepletedmantle.ThisreservoirpossiblyexistsintheEarth‘sdeepmantle.Niobiumandtantalumhavethesameionicchargeandnearlyidenticalionicradii.Consequently,theydisplayaverysimilargeochemicalbehavior(e.g.duringthemagmaticproc