地球化學(xué)科學(xué)前沿-俯沖帶地球化學(xué)：Subduction-Zone俯沖帶

SubductionZone俯沖帶Subduction

modellingThefateofsubductingslabsElements

mobilityNb

paradoxPlateBoundaries板塊理論的兩大基石:俯沖帶和洋中脊

板塊俯沖是地球上最重要的化學(xué)分異過程，對于大陸地殼的形成、地幔的演化、地幔柱的形成等都起著極其重要的作用。PlateBoundariesOcean–ContinentConvergentBoundariesContinent–ContinentConvergentBoundary板塊俯沖引發(fā)的化學(xué)過程的三種主要研究方法：1）從島弧火山巖入手，根據(jù)島弧火山巖的成份來示蹤板塊俯沖過程中的地球化學(xué)分異作用。俯沖析出流體與地幔楔反應(yīng)以后形成，經(jīng)歷了強(qiáng)烈的巖漿分異2）高溫高壓實驗?zāi)M：通過確定各種溫壓條件下元素在礦物和流體間的分配系數(shù)來研究各種元素在板塊俯沖過程中的活動性。與實際俯沖過程有一定的差異3）通過研究藍(lán)片巖、榴輝巖等俯沖產(chǎn)物，揭示元素和同位素地球化學(xué)分異現(xiàn)象。并不是板塊俯沖的最終產(chǎn)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匯聚板塊邊緣的島弧鈣堿性巖石富集大離子親石元素(LILE)而虧損高場強(qiáng)元素(HFSE)這些巖石通常被認(rèn)為是俯沖板片脫水，導(dǎo)致地幔楔熔融而來(e.g.,Tatsumi,1989;McCullochandGamble,1991).島弧火山巖的虧損高場強(qiáng)元素特征與俯沖板片在脫水過程中金紅石穩(wěn)定不變有關(guān)(e.g.,RyersonandWatson,1987;ScambelluriandPhilippot,2001;TropperandManning,2005).因此，金紅石控制了俯沖洋殼所釋放流體中的HFSE含量

(Brenanetal.,1994;Ayers,1998).金紅石在水中的溶解度極低(AudetatandKeppler,2005;TropperandManning,2005)Ti,Nb和Ta在金紅石和流體之間的分配系數(shù)也較大，即這些元素在金紅石中極相容(Brenanetal.,1994;Stalderetal.,1998).俯沖洋殼在榴輝巖相條件下產(chǎn)生的高壓脈體通常被認(rèn)為是先前板片中的流體通道，從而可以反映流體遷移過程中與巖石間的交互反應(yīng)(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經(jīng)歷過蝕變和水化的洋殼到達(dá)匯聚板塊邊緣時，會攜帶上附沉積物。隨板片俯沖，部分上附沉積物會滯留在增生楔；板片中的基性巖石的含水量可達(dá)5%，lawsoniteblueschist-facies，一些低級含水礦物脫水會釋放出B和LILE；隨板片俯沖，持續(xù)脫水。主要的脫水過程發(fā)生在藍(lán)片巖相到榴輝巖相的轉(zhuǎn)變過程中,超過2%的水被釋放出來（Peacock,1993）；基性巖中的含水礦物分解或者超基性巖中的葉蛇紋石分解釋放流體，B,LILE,Th,U和LREE會隨流體遷移；富含以上元素的流體和熔體會進(jìn)入虧損地幔中，對島弧巖漿作用做出貢獻(xiàn)；一旦完全轉(zhuǎn)變到榴輝巖相，板片的密度大于周圍地幔，從而向更深俯沖至少到660KM（TatsumiandEggins,1995）。但其中仍有含水礦物，如多硅白云母，硬柱石等。這些含水礦物的脫水，會有助于弧后盆地區(qū)域的巖漿作用。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俯沖的沉積物中的金紅石，褐簾石，獨居石，鋯石和磷灰石等副礦物容納了主要微量元素；流體相中的K,Rb(LILE),LREE和Zr,Ti,P(HFSE)含量受控于這些副礦物和白云母。(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