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)而虧損高場強元素(HFSE)這些巖石通常被認為是俯沖板片脫水，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入虧損地幔中，對島弧巖漿作用做出貢獻；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