同位素讲座-1-2010.ppt

稳定同位素地球化学,储雪蕾InstituteofGeologyandGeophysicsChineseAcademyofSciences(联系电话：82998417；E-mail：xlchu),（第一讲）,I.稳定同位素基本原理,稳定同位素地球化学的诞生、发展离不开上个世纪3040年代两位著名的科学家：HaroldUrey(Univ.ofChicago)和AlfredNier(Univ.ofMinnesota)的贡献。1934年诺贝尔化学奖获得者Urey奠定了同位素取代的物理化学性质变化的理论基础，并把它用于地球科学。1946年他在英国皇家学会上发表了“TheThermodynamicPropertiesofIsotopicSubstances”，并理论上预示CaCO3和H2O的氧同位素比值（18O/16O）只依赖于温度的变化，提出了在海洋古温度上的应用。他与Epstein、McCrea建立了第一个碳酸盐的氧同位素实验室。,实现同位素分析始于质谱仪的发明与设计，Nier的贡献是最显著的。他设计和改进的Nier-型质谱仪一直是测定原子量的主要工具，也是测定重元素同位素的仪器，用于放射性同位素地质年代学和地球化学的研究。在他和他的同事测定轻元素的同位素组成时，发现了较大的变化。他们所测的灰岩比海水富集18O约3%，与Urey通过统计力学计算的分馏系数一致。因此，一门基于理论、实验和质谱分析技术的新学科稳定同位素地球化学诞生了。,稳定同位素地球化学在地球科学中的应用：1）同位素地质温度计；2）示踪剂（包括确定物质来源，物理化学条件与地质过程机制，等）。测定稳定同位素比值主要用气体离子源的同位素质谱仪。采用双进样同位素比值质谱仪，由于属大型仪器、贵重，只有国家级科研院、所的实验室从事这方面测试与研究。,本课程的内容主要是介绍稳定同位素地球化学原理与应用，重点介绍C、H、O、S同位素。,1同位素的基本概念,同位素的分类：(1)放射性同位素：原子核不稳定，能自发进行放射性衰变或核裂变，而转变为其它一类核素的同位素称为放射性同位素。(2)稳定同位素：原子核稳定，其本身不会自发进行放射性衰变或核裂变的同位素。,同位素的定义同位素定义：核内质子数相同而中子数不同的同一类原子。,传统的稳定同位素,非传统的稳定同位素,本课程,同位素效应(Isotopeeffect),同位素比值（Isotoperatio):R=重同位素丰度/轻同位素丰度,同位素分馏系数（Isotopefractionationfactor):A-B=RA/RB即值，表示某元素的同位素在两种物质（A和B）之间的分馏的程度。,同位素分馏（Isotopefractionation)：同位素在不同物质或不同物相间分配比例不同的现象称之为同位素分馏。,值：样品的同位素比值相对于标准样品同位素比值的千分偏差（）=（R样R标）/R标）X1000=（R样/R标）-1）X1000R样：样品的同位素比值R标：标准的同位素比值0表明样品相对标准富集重同位素0表明样品相对标准亏损重同位素=0表明样品与标准同位素比值相同,稳定同位素标准,2D/1HSMOW：Standardmeanofoceanwater(标准平均大洋水)18O/16OSMOW：Standardmeanofoceanwater(标准平均大洋水)PDB：BelemnitellaAmericana（美国北卡罗来纳州白垩系PeeDee建造美洲似箭石）13C/12CPDB：BelemnitellaAmericana（美国北卡罗来纳州白垩系PeeDee建造美洲似箭石）34S/32SCDT：美国亚利桑那州CanyonDiablo铁陨石中的陨硫铁(FeS),样品的值的计算需要引入一个标准。在对于样品的同位素组成进行比较时，必须采用同一的标准。国际选定的标准如下：,稳定同位素标准,D/H13C/12C15N/14N18O/16O34S/32S,dDd13Cd15Nd18Od34S,VSMOWVPDBAIRVSMOW,VPDBVCDT,1.5575x10-41.1237x10-23.677x10-32.0052x10-3,2.0672x10-34.5005x10-2,NIST:NationalInstituteofStandardsandTechnologyIAEA:InternationalAtomicEnergyAgency,同位素,比值,参考标准,丰度比值,鉴于原有的国际标准已用尽，国际原子能机构制做了下述标准供使用。目前，发表论文可用原标准和现标准两种方式发表，但推荐用现标准（即V标准）发表。,2同位素分馏机理,从严格意义上讲，在周期表中所有元素的不同种同位素由于其质量上存在差别，在自然界的各种物理，化学和生物的反应和过程中都会发生同位素分馏。这些反应和过程包括：蒸发作用，扩散作用，吸附作用，化学反应，生物化学反应等等。,自然界存在三种类型的同位素分馏:平衡分馏（equilibriumfractionation）动力（学）分馏（kineticfractionation）非质量依赖分馏（mass-independentfractionation),同位素分馏的类型,-主要由同位素取代所造成的气体、液体的分子和固体晶格中原子的振动能的差异造成动能的差异与质量有关体系趋向能态最低共价键具有大的平衡分馏，而离子键平衡分馏小，通常可忽略例如：,引自WilliamWhitestextbook(CornellUniv.),mostimp.,在25C达到平衡时，CO2的18O/16O比值比H2O高。,平衡分馏(Equilibriumfractionation),这什么会出现平衡分馏？,哪个化学键容易被打破？重同位素的分子具有比轻同位素的分子低的零点能。势能越高越容易脱离势阱，结合的键也越容易破裂。重同位素具有比轻同位素更强的结合能，即化学键能大，或键强度高。为什么与温度有关?轻、重同位素分子零点能差异随温度增加而减少。-键能在非常高的温度下趋近一致，所以同位素分馏系数将会趋近于1，即不产生分馏。,zeropointenergy,平衡分馏的温度依赖性,harmonicoscilllatormodel,harmonicoscilllatormodel,data,data,简谐振荡模型给出lna高温下与T2成反比，低温下与T成反比。,（T200C）,因此，在较低的温度上会有更严重的同位素分馏。,Generalruleofthumb:theheavyisotopewillbeconcentratedinthephaseinwhichitismoststronglybound(orlowestenergystate).Solidliquidgas,covalentionic,etc.Ex:18Oincarbonates-heavilyenrichedincarbonatebecauseOtightlybondedtosmall,highlychargedC4+,vs.weakerH+-soD18Ocal-water=d18Ocarb-d18Owater=30Ex:quartz(SiO2)mostenrichedmineralLatticeconfiguration(aragonitevs.calcite)playsasecondaryrole(D18Oarag-cal=0.5)Chemicalsubstitutionsinthelattice(ie.BainsteadofCa)alsohaveasmalleffect:D18OBa-cal-water=25(vs.30forCa-cal),富集规律（平衡分馏）,规律：重同位素相对富集在化学键强或能态最低的物相中。,同位素平衡分馏小结不同物质或物相间的同位素比值达到恒定不变时，即达到了同位素平衡状态，这种状态的分馏称为同位素平衡分馏。一旦同位素平衡状态建立后，只要体系的物理化学性质不变化，则在不同矿物或物相中同位素组成就维持不变，这是同位素平衡分馏的特点。同位素平衡分馏与路径、同位素交换速率、压力等都无关，而仅与温度有关。同位素平衡分馏的研究只考虑过程的始态与终态，对其演化过程及时间不予考虑。因此，同位素平衡分馏又称热力学分馏，是同位素地质温度计的理论依据。,动力分馏(Kineticfractionation),起因：由速度、单向、不完全的反应或过程引起（包括生物为媒介的反应或过程）。例如：伴随着蒸发过程、扩散过程、分解反应过程，及光合过程等等发生的同位素分馏都属于动力分馏。,由于轻同位素取代具有相对高的势能，因此它相对“活泼”，优先反应。例如，C-H键比C-D键容易破裂，它容易反应。反应没有达到平衡时，轻同位素相对富集在产物中，而重同位素则在反应物中相对富集。通常生物为媒介的氧化还原反应中会产生大的动力分馏，例如：光合作用生成的有机体贫13C，细菌还原产生的硫化物贫34S。,考虑两个CO2分子:12C16O2(质量数=12+2*16=44)13C16O2(质量数=13+2*16=45)假定为理想气体，动能相同时则:它们的速度比:,如此，12C16O2比13C16O2扩散的速度要快1.1%。,不是理想气体，由于气体的碰撞使这两种分子运动速度的差异减小，分馏减小。,气体分子的速度差异-理想气体的动能是相同的。-因此，重同位素与轻同位素的质量之不同是通过速度来补尝的，即,同位素动力分馏小结一些物理-化学（如蒸发、扩散、单向或未完成的化学反应等）过程和生物（如光合作用、呼吸作用和细菌硫酸盐还原等）过程中伴随发生的同位素分馏称之为同位素动力分馏。这些过程往往受化学反应动力学控制，其造成的同位素分馏受扩散速度或反应速度控制，依赖于路径、时间与速度。生物参与的化学过程，一般同位素动力分馏明显，这在C和S同位素分馏的研究中占有重要位置。,Closed-andopen-systemfractionation,瑞利同位素分馏(Rayleighisotopefractionation),推导：,ThiemensandHeidenreich,1983;Theimens,1999(review),在陨石、大气光化学反应的产物中观察到了非质量依赖同位素分馏。非质量依赖分馏要通过三个或三个以上同位素的体系研究来确定，如16O、17O和18O体系；32S、33S、34S和36S体系。机制是光子的量子效应造成光化学反应,或自由基参与的化学反应。这些反应与同位素的质量无关。用途：天体化学、地球早期大气氧的增加、大气化学（如气溶胶）等。,非质量依赖分馏(Mass-independentfractionation),质量相关定则,对于小的同位素分馏（20）的三同位体系的同位素比值是各种同位素质量倒数之差的函数。如分子氧（氧气）来讲有三种稳定同位素：16O16O、16O17O和16O18O，遵守质量相关定则的地球上物质普遍有d17O/d18O(1/32-1/33)/(1/32-1/34)=0.516即d17O=0.516d18O,地球样品普遍满足质量相关分馏线或质量分馏线。质量分馏线的斜率在0.500到0.526范围内。,质量分馏线,D33S和D36S定义,D33S=(33S/32S)sample/(33S/32S)ref(34S/32S)sample/(34S/32S)ref0.515103D36S=(36S/32S)sample/(36S/32S)ref(34S/32S)sample/(34S/32S)ref1.9103,硫的质量相关和非质量相关同位素分馏,3同位素地质温度计原理,值：（）=(R样/R标）-1）X1000同位素分馏系数与值的关系：103lnA-BA-B=DA-B即lnA-B与A,B两种物质的值之差相关。,同位素平衡分馏系数与温度的关系：103ln=a/T2+b/T+c（T:K)其中a，b，c分别为常数。1）在一般低温下，a/T2可以忽略，简化：103ln=b/T+c2）在高温下，b/T可以忽略，简化：103ln=a/T2+c,4同位素样品制备与质谱分析,AvacuumsystemforSO2preparation,Sulfideminerals:suchaspyrite,galena,sphalerite,etc.oxidizingagent:CuO,Cu2O,orV2O5temperature:900to1100CSulfateminerals:suchasbarite,gypsum,anhydriteoxidizingagent:Cu2O,orV2O5+SiO2cover:Cutemperature:1100to1200C,Ceramicboat,Ironring,SulfurisoiopicanalysisforSO2gas,Inionsource,SO2gasisionizedtopositivelychargedparticles,whichareacceleratedthroughavoltagegradient.,TheSO2+ionbeampassesthroughamagneticfield,whichcausesseparationofvariousmassessuchas64(32S16O2)and66(34S16O2,34S18O16O).,ThebeamcurrentsaremeasuredinFaradaycupsandcanberelatedtotheisotopicratiowhenthesampleandstandardgasesarecompared.,GasBenchII,MS+EA,TC/EA,GeochemistryofStableIsotopes,On-linesulfurisotopedeterminationusingEA-IRMS-Anewmethodofrapidanalyses,Thismothodisveryusefulininvestigationsonenvironment,ecologyandmineralresources.,Advantages:1)impurity:wholerock,suchasblackshale;2)smallamountofsample:1mg(10mgSinsample);3)rapidly,continuously,andautomaticallyDisadvantage:loweranalyticalprecision:0.2-0.5ford34S,II.硫同位素地球化学,Threeprocessescausetheisotopefractionationbetweentwosubstancesinnature:Isotopeexchangereactions;Kineticprocessesduringachemicalreactionorphysicalprocess,suchasfreeze,evaporation,etc.;Biologicalprocesses,Thed34SdistributioninthenatureThed34Ssecularvariationsofmarineevaporites,1.Sulfurisotopevariationsingeologicalsystems,Sulfurispresentinnearlyallnaturalenvironments:asaminorcomponentinigneousandmetamorphicrocks,mostlyassulfides;inthebiosphereandrelatedorganicsubstances,likecrudeoilandcoal;inoceanwaterassulfateandinmarinesedimentsasbothsulfideandsulfate.Itmaybeamajorcomponentinoredeposits,whereitisthedominantnon-metalassulfatesinevaporites.Inaddition,varioussulfideoredepositsareeconomicallyveryimportantsourcesforCu,Pb,Zn,Ag,andothermetals.Theseoccurrencescoverthewholetemperaturerangeofgeologicinterest.Sulfurisboundinvariousoxidationstates,fromsulfidestoelementalsulfur,tosulfates.Fromthesefactsitisquiteclearthatsulfurisofspecialinterestinstableisotopegeochemistry.,Thed34Sdistributioninnature,Thecommonreferencereservoirs1)Meteoriticsulfur:0,suchasCanonDiablotroiliteMeteoritesapproximatelyhavethesamed34SvaluesoftheEarthsbulk.Theironmeteoriteshaveanaverageisotopecompositionof0.20.2.Theaveraged34Svalueofmid-oceanridgebasaltsis0.30.5.2)Sea-watersulfate:21,inmodernocean,Geochemicalprocesses,themostnotableofwhichareoxidationandreduction,profoundlyfractionatesulfurisotopesawayfrombulk-Earthvaluesingeologicalsystems.Oxidationprocessesproducespeciesthatareenrichedin34Srelativetothestartingmaterial,whereasreductionproducesspeciesthataredepletedin34S.But,greatisotopefractionationsarerelatedcloselytoabiologicalprocess,i.e.,bacterialsulfatereduction.,Thed34Sofsulfateinancientoceansasrecordedbymarineevaporitesequences(Claypooletal.1980)hasvariedfromalowofapproximately10duringPermianandTriassictimetoahighof35duringCambriantime.Becausetheisotopefractionationbetweenthesulfate-containingevaporiteandthesulfateinoceanwaterisalmostnegligible,theobservedtrendinevaporitesulfateshouldcloselyreflectfluctuationsinthesulfurisotopecompositionofmarinesulfatethroughgeologictime.,Thed34Ssecularvariationsofmarineevaporites,Thed34Ssecularvariationsofmarineevaporites,Changesinthed34Sofmarinesulfateduringthegeologicpastmaybecausedbymajorchangesinthebudgetbetweentheindividualreservoirs:duringperiodsofhighbiologicalsulfatereduction(),whichshouldtakeplaceunderfavorablepaleogeographicconditions,thed34Sofoceanwatershouldincrease.Incontrast,periodsofextendedweathering()introduceadditionallightcontinentalsulfurintotheoceanwhichdecreasesthed34Svalueofoceansulfate.Suchperiodsofextendedweatheringaregeologicallyplausibleinperiodsofhightectonic,mountain-buildingactivity.,Sulfurcycleinnature,Whilethepartialcyclebetweenoceanandevaporitesonlyinvolvessulfatetransferfromonereservoirtotheother,bacterialsulfatereduction,aswellastheweatheringofsulfidesfromargillaceoussediments,changethevalencestateofthesulfur.Therefore,duringaperiodwithincreasedrateofoneofthesetwoprocesses,appreciableamountseitheroforganiccompoundsoroffreeatmosphericoxygenareneeded.Especiallyinthelattercase,oxygenconsumptionduringweatheringisappreciable.,2.Factorscontrollingsulfurisotopefractionation,Isotopeequilibriumfractionation:equilibriumfractionationfactorandisotopegeothermometerIsotopekineticfractionationIsotopefractionationduringbacterialsulfatereductionRayleighisotopefractionation,Thefractionationfactor（a）isdefinedastheratioofthenumbersofanytwoisotopesinonechemicalcompoundAdividedbythecorrespondingratioforanotherchemicalcompoundB:aA-B=RA/RBwhereRis34S/32S.ThisequationcanberecastintermsofdvaluesasaA-B=(1+dA/1000)/(1+dB/1000)=(1000+dA)/(1000+dB),Valuesofaaretypicallynearunity,withvariationsnormallyinthethirddecimalplace(1.00 x).ThevalueDa-bisdefinedasDa-b=dA-dBBecause1000ln(1.00 x)isapproximatelyequaltox,Da-b1000lnaA-B.,Example:Foranisotopeexchangereaction32SO42-+H234S=34SO42-+H232Stheequilibriumfractionationfactorbetweensulfateandsulfide(i.e.,asulfate-sulfide)isabout1.075at25C(TudgeandThode1950).,Howtoobtainequilibriumisotopicfractionationfactor(threeways):,(1)experimentaldetermination;(2)theoreticalestimationusingcalculatedbondstrengthsorstatisticalmechanicalcalculationsbasedondataonvibrationalfrequenciesofcompounds;(3)analysisofnaturalsamplesforwhichindependentestimatesoftemperatureareavailable.,1)themagnitudeoffractionationfactordependsprimarilyontemperature,becomingsmallerwithincreasingtemperature;2)wheninequilibrium,sulfurspeciesofhighervalence(i.e.,moreoxidized)trendtobemoreenrichedintheheavierisotopes,suchthatd34SSO4(andsulfateminerals)d34SSO2d34SSSH2S(andsulfideminerals)3)thefractionationfactorsbetweensulfatemineralsandSO42-arequitesmall,butthoseamongsomesulfidemineralsandaqueoussulfidesareverysignificant.,Sulfurisotopicfeaturesofequilibriumfractionation:,Sulfurisotopegeothermometry,Sulfurisotopegeothermometryistypicallybasedontheisotopicpartitioningbetweentwosulfur-bearingminerals,foranexample,bariteandpyrite.Anequationtocalculatethetemperaturerecordedbyacoexistingpairofbarite(Ba)andpyrite(Py)canderivedasfollows:1000lnaBa-PyDBa-Py=d34SBa-d34SPy(1)Thus,DBa-Py1000lnaBa-H2S-1000lnaPy-H2S(2)SubstitutingfromtheaboveTableyieldsDBa-Py=(6.463x106)/T2+0.56(0.40 x106)/T2=(6.063x106)/T2+0.56(3)withTinK.SolvingforT,andconvertingtoCyields:T(C)=(6.063x106/(DBa-Py-0.56)1/2-273.15(4)Forexample,foramineralpairwithd34SBa=21.0andd34SPy=5.1,atemperatueof356CiscalculatedusingEquation(4).,Isotopekineticfractionation,Duringnonequilibrium,unidirectionalchemicalreactions,thefractionationofsulfurisotopesarisesfromthefactthatchemicalreactionratesaremassdependentandthatoneisotopicspeciesreactsmorerapidlythananother.Ingeneral,themoleculescontainingthelighterisotopewillhavethefasterreactionrate.Consequently,theproducttendstobeenrichedinthelighterisotope.Forexample,oxidationofsulfidetosulfatecanbeconsideredastwoseparatereactionswithdifferentrateconstants:k1H232S32SO42-k2H234S34SO42-Theratiooftworateconstantsk1/k2isequaltothekineticisotopiceffect,i.e.,kineticfractionationfactor,a=k1/k2.,Sulfurisotopekineticfractionation,1)Low-temperatureoxidativealterationofsulfidemineralstosulfateminerals:Isotopekineticeffectiscommonlynegligible,i.e.,d34Sproduct(sulfate)d34Sreactant(sulfide)2)Thermochemicalreductionofsulfateduetointeractionwithorganicmatter:Thekineticfractionationwasless10duringthisreduction.,Bacterialsulfatereduction-isotopekineticeffects,Thefractionationofsulfurisotopesbetweensulfateandsulfideduringbacterialsulfatereductionisakineticallycontrolledprocessinwhich34Sisenrichedinthesulfaterelativetothesulfide.Thesulfate-reducingbacteriamorereadilymetabolize32Srelativeto34S.Thus,thed34Softheresidualaqueoussulfateincreaseduringthereactionprogress.Thefractionationassociatedwithbacterialsulfatereduction(1000lnaSO4-H2S)typicallyrangesfrom15to60(GoldhaberandKaplan1975)inmarinesettings,comparedtoanequilibrium,abioticfractionationofapproximately73at25C.Themagnitudeofthefractionationhasbeenshowntobeafunctionoftherateofsulfatereduction,whichcanberelatedtosedimentationrates.Thesmallerfractionations(15)correspondtofasterratesofsulfatereductionandsedimentation,whereasthelargerfractionations(60)correspondtoslowerratesofsulfatereductionandsedimentation(GoldhaberandKaplan1975).,Recently,Canfieldetal.(1998,2001)havedevelopedanargumentbasedontheSisotopiccompositionofbiogenicsedimentarysulfides,whichreflectSO42-availabilityandredoxconditionsattheirtimeofformation.WhentheavailabilityofSO42-isstronglylimited(SO42-concentration45betweensedimentarysulfidesandsulfatesmayindicateincreasedoxygenationoftheenvironment.,CanfieldandThamdrup(1994),细菌还原、氧化和岐化作用,天然和人工培养的细菌硫酸盐还原实验证实，最大的硫同位素分馏为46。So的细菌岐化作用实验也证实，还能产生17的同位素分馏。因此，只有伴随着H2S氧化这个中间过程的BSR和岐化作用才能造成60的SO4H2S硫同位素分馏。,大气圈氧增加与硫同位素分馏,硫酸盐浓度1mM，BSR造成的同位素分馏在446之间，平均在18。硫酸盐浓度1mM，BSR造成的同位素分馏4。能够发生H2S氧化为S0时，BSR和岐化作用造成的同位素分馏46，达到60。,Isotopecompositionofsedimentarysulfidesofbiologicaloriginovergeologicaltime(Canfield,1998),7,200,Habichtetal（2002）的新观测和实验数据表明细菌硫酸盐还原的阀值是硫酸盐浓度为200mM，大于此值可产生30的分馏，小于此值分馏不超过7。,扬子地区九龙湾剖面陡山沱组地层层序记录的海洋几次脉冲式氧化,McFadden,Huangb)20,inwhichthesulfurshouldbederivedfromoceanwater;c)othervaluebesides0and20,inwhichthesulfurmaybereceivedfromlocalcountryrocksorfrommixingofseveralsources.3)ThepHvalueoftheore-formingfluidandtheproportionsofoxidizedandreducedsulfurspecies,whichhavethesameimportanceininterpretingd34Svaluesinhydrothermaloredeposits.,Controllingfactors,Forexample:SulfurisotopestudyontheDajingoredeposit,TheDajingoredepositisaCu-polymetallicoredepositwithvaluableCu,Sn,Ag,ZnandPbmetals,whichislocatedinLinxicounty,InnerMongoliaAutonomousRegion,China.,Forallsulfideminerals,thed34Svaluesexhibitanarrowdistributionfrom-6to+4withasharppeakat+1(Fig.A).Thisdistributionofd34Svaluesstronglysuggeststhatthesulfurinoresulfidescanbederivedfromahypomagmaticsource.Becausethed34Svalueoftheore-formingfluidisestimatedtobeabout+1,i.e.,theaverageofthed34Svaluesofsulfides,andisveryclosetotherangeof01ofthenormalmantle(Eldridgeetal.,1991).FortheDajingoredeposit,thed34Svaluesofthemagmaareabsolutelywithintherangeof03,thuswecanexcludetheinvolvementofthecountryrocks,especiallythehostedcountryrocksoftheLinxiFormationthroughassimilationandcontamination.,TheDajingoredepositiscompletelyimpossibletobeacontemporaneousdeposition-hydrothermalreformati