中国东部下地壳麻粒岩矿物的水含量和H-O-Li同位素组成.doc

中国东部下地壳麻粒岩矿物的水含量和H-O-Li同位素组成 应用傅里叶变换红外光谱FTIR和离子探针SIMS技术分别对大陆下地壳麻粒岩以及少量地幔橄榄岩样品主要组成矿物单斜辉石,斜方辉石和斜长石中的结构水含量和REE,Li含量以及H,O,Li同位素组成进行了系统性的测定分析,旨在更好地理解下地壳中的水含量,深部岩石圈中的水分布,下地壳的H和Li同位素组成以及可能的流体活动.研究结果表明下地壳麻粒岩中的这些名义上无水矿物普遍含有一定量以OH和/H_2O形式存在的结构水,其含量通常以H_2O的重量百分含量形式表示分别为单斜辉石,200-2330ppm;斜方辉石,60-1875ppm;斜长石,65-900ppm;全岩含量可达1000ppm以上155-1100ppm.这表明下地壳中水的分布是显著不均一的.这些麻粒岩的平均水含量为450ppm,由此估计蕴含在大陆下地壳中镁铁质麻粒岩中的水含量至少为4.261018kg.形成于前寒武纪和显牛宙的大陆下地壳具有截然不同的水含量前者较后者明显偏高,可能指示早期的下地壳更加富水.下地壳中单矿物以及估测的全岩结构水含量比岩石圈地幔中显著偏高,表明深部岩石圈中水含量的垂向不均一性.这种不均一性可能与两个圈层中岩石的形成过程以及部分熔融有关.下地壳和岩石圈地幔之间水含量的差异能对其流变性质产生直接影响,并可能进一步影响深部岩石圈内的动力学过程,使得岩石圈处于减薄或增厚状态.麻粒岩中辉石的氧同位素组成为4.5至12.5‰考虑到这些共存矿物可能处于氧同位素的平衡状态,其全岩氧同位素组成也可能在这个范围内.相较正常地幔而言5.70.5‰,一些产地的样品具有显著偏高的氧同位素组成,而一些产地则与之相当,前者指示了再循环的地壳物质的影响.样品中的辉石颗粒无例外的具有均一的氧同位素组成颗粒内部,但颗粒之间同种矿物甚至在同一样品中＜1cm尺度内的差异可达2-3‰,暗示颗粒之间的不均一性.辉石氧同位素平衡温度和Fe-Mg离子交换平衡温度之间的一致性指示峰期变质作用时样品的氧同位素组成没有被后期过程改变.相较正常地幔而言-90～-60‰,下地壳中矿物的氢同位素组成显著偏高-80～-10‰.在仪器误差范围内,多数颗粒氢同位素组成自核至边部均一分布,但有少数颗粒显出较大的变化,可能是由样品出露前一些近期事件所致;但矿物之间平均的氢同位素组成却处于平衡状态,暗示其初始组成并没有被显著改变.下地壳矿物中普遍偏高的D/H比值可能同麻粒岩成岩过程中的脱水作用有关;该作用可能发生在熔体的分异结晶过程中,可能主要表现为H以还原性组分形式如H_2,H_2S等脱离体系.下地壳矿物中单斜辉石最为富Li,其次是斜方辉石,而斜长石相对最为亏损,其含量变化分别为单斜辉石,3.2～34.2ppm;斜方辉石,0.6～9.1ppm;斜长石,0.2～12.1ppm;由此估计的全岩含量为1.9～12.6ppm平均含量约5.2ppm.这些矿物的δ7Li为-13～4.7‰估计的全岩组成为-11～2‰,多数明显低于正常地幔2～6‰.个别颗粒表现出大的δ7Li变化同氢同位素结果相似,可能是由样品出露前一些近期事件所致;但矿物间Li同位素平均组成多数处于平衡状态,暗示其经历了封闭体系的Li同位素分馏.下地壳矿物中较低的δ7Li组成可能同麻粒岩样品出露前或出露过程中Li同位素扩散有关的分馏效应有关.基于H,O,Li同位素组成的研究初步指示无论是大尺度如不同产地间还是小尺度如同一样品内部不同颗粒间上,流体都不可能在下地壳中普遍存在;麻粒岩成岩过程中的脱水作用能强烈影响其H和Li同位素组成;脱水作用可能主要发生在麻粒岩矿物自熔体中结晶之前,这意味着麻粒岩相变质作用应该是在相对干态的条件下进行的.遗憾的是,本文用于H-O-Li同位素研究的样品只取自华北克拉通,而下地壳的成因,组成和演化等是非常复杂的,这些结论是否具有全球尺度上的通用性还需要更多深入的研究.;ReplaceFontEnDivSummary,SystematicFouriertransinfraredFTIRandionmicroprobeSIMSinvestigationsofcommonlowercrustalmineralspyroxenesandplagioclase,aswellasmantleperidotitemineralspyroxenesandolivine,wereundertakentobetterunderstandthecompositionofstructurallyboundH-specieswithinthesenominallyanhydrousminerals,thepossiblelateralandverticalvariationsofwaterinthedeepcontinentallithosphere,andtheH-,O-,andLi-isotopicandREEcompositionsofthecoexistingmineralswithinthecontinentallowercrust.Thenominallyanhydrousminerals,suchaspyroxenesandplagioclase,inthelowercrustalgranulitesgenerallycontaintraceamountsofwaterinthemannerofstructurallyboundhydroxylandlessmolecularwater,withtheircontentsH_2Obywt.varyingfrom200to2330ppmforclinopyroxene,60to1875ppmfororthopyroxene,65to900ppmforplagioclaseand155to1100ppmfortheestimatedbulkcompositions.Theaveragebulkcontentisabout450ppm,andthereforethelowercrustisestimatedtocontain～4.261018kgH_2Ointhenominallyanhydrousminerals.AsignificantcontrastinwatercontentisobservedbetweenPrecambrianandPhanerozoiccontinentaldeepcrust,implyingamorehydrousancientlowercrustrelativetothemodernone.H_2Ocontentofthemaincontinentallowercrustalminerals,andtheirbulkconcentrations,areobviouslyhigherthanthoseintheunderlyinglithosphericmantle,despitetheirlargelateralvariations,suggestingverticalvariationsofH_2Ocontentinthedeepcontinentallithospherelowercrustvs.uppermantle.Suchwatercontrastmayberelatedwiththepetrogenesisoftheserocks,andcanstronglyaffecttherheologicalbehaviorofthedeepcontinentallithospherebelowtheNorthChinaCraton,whichprobablyresultsindifferentlithosphericstructuresandprocessesbetweendifferenttectoniczonesbeneaththiscraton,e.g.lithosphericthinningvs.thickening.TheO-isotopicratiosofstudiedpyroxenes,andprobablythebulksamples,rangefrom～4.5to12.5‰,higherthanornearlycomparabletothatofnormalmantle5.70.5‰,whichindicatesinvolvementofrecycledcrustalmaterialsduringtheirpetrogenesisforsampleswithhighδ18O.Intra-grainδ18Ovariationsupto2-3‰wereobservedforthesamemineralineven＜1cmscaleinsomesamples,incontrasttotheirnearlyunidistributiononeachgrain,implyinggrain-scaleheterogeneity.ConsistencybetweenO-isotopicandcationexchangetemperaturessuggeststhepreservationofpeak-metamorphiccompositions.ThelowercrustalmineralsarecharacterizedbyrelativelyhighδDvaluescomparedtothenormalmantle,e.g.-80～-10‰fortheervs.-90～-60‰forthelatter.LargevariationsofδDwithinsomegrainswereprobablycausedbydiffusion-inducedprocessesshortlybeforeorduringtheirentrainmentorexhumation,however,themineralswereusuallyinequilibriumwitheachotherwithrespecttotheiraverageH-isotopiccompositions,indicatingweakinfluencesfromsuchdiffusionprocesses.TherelativelyheavierD/Hratiosofthelowercrustalmineralscanbeaccountedforbymeltdehydration,e.g.inthecrystallizationofgranulitephases,butmainlythroughthelossofreductiveH-speciese.g.H_2,H_2S.Lithiumcontentvariesfrom3.2to34.2ppmforcpx,0.6to9.1ppmforopx,0.2to12.1ppmforplagand～1.9to12.6ppmforbulkcompositions,withanaveragebulkvalueof～5.2ppminthelowercrust.ThestudiedmineralsarecharacterizedbydepletedLi-isotopicvaluesrelativetonormalmantle,e.g.-13to4.7‰withestimatedbulkvaluesof-11to2‰vs.2to6‰.Largevariationsinδ7Liobservedonsomegrainswereprobablyrelatedwithdiffusion-drivenprocesses;however,thepreservationofequilibriumδ7Lifractionationbetweencoexistingmineralsindicatesthatclosed-systemisotopicfractionationshaveoccurredtothestudiedgranulites.Thelighterδ7Licompositionswereprobablycausedbykineticfractionationsassociatedwithhigh-temperatureLidiffusion.ThepreliminaryresultsfromtheH-O-Liisotopicsystematicsindicatethatthatpervasivefluidsareabsentinthecontinentallowercrusteitheronlargeorlocalscales.Thedehydrationduringtheationofgranulitesoccursprobablyintheiroriginalmeltpriortothecrystallizationofgranuliteminerals,mainlyinthemannerofwaterlossthroughreductiveH-speciese.g.H_2etc..However,ithastobeassertedthattheseconclusionsarebasedonlyonsamplesfromtheNorthChinaCraton,anditisnotclearthatiftheyareapplicabletothelowercrustgloballybecauseofthecomplexitiesinvolvedinthecomposition,ationandevolutionofthedeepcrust.;