放射成因同位素在地学领域的应用

放射成因同位素在地学领域的两方面应用同位素地质年代学同位素地球化学示踪Radiometricdating-Atimemachinetothepast同位素年代学同位素地球化学示踪Howdoweknowwheretherockscamefrom?

同位素和同质异素相同质子数，不同中子数，87Rb和85Rb不同质子数，相同质量数，

87Rb和87Sr放射性核素衰变规律衰变速度与放射性核素含量成比例dNdtlN-dNNl-dt或同位素地球化学在地学领域的影响同位素地球化学在地学领域研究中的地位反映在对地学的基础问题研究的显著性的成功。同位素地球化学在将地质学由定性观察向现代的定量化科学的转变发挥重要的作用。同位素地球化学在下述领域产生影响：矿床成因研究、岩石成因研究、地壳演化、地幔动力学、全球构造演化和动力学、火山岩石学研究、水文学、古生物研究、海洋对流循环、人类学和考古学研究、古气候古环境研究、环境检测和保护、宇宙射线检测，等等。固体放射成因同位素地球化学分析流程化学分离与纯化质谱计同位素比值测定分析结果计算和评价离子色层分离溶解样品为什么超净化环境？－样品中待分析组分含量低－降低环境和人为因素对样品的影响，即降低实验室本底如何尽可能降低本底？－净化实验室空气－净化化学试剂和器皿超净化实验室环境下同位素分离为什么分离和纯化样品？－除去样品中其它元素的干扰如：Ca、Fe、Mg离子－除去同质异素的叠加如:87Rb对87Sr的叠加

144Sm对144Nd的叠加

150Sm对150Nd的叠加R-H++A+=R-A++H+离子交换树脂工作原理R-A++H+=R-H++A+铷－锶同位素分离纯化图例Ionchromatography质谱计的基本原理和主要组成DeviceusedtomeasurethemassofagiventypeofatomMassspectrum:

Positionofpeaksgivetheatommass

RelativeheightsofthepeaksindicaterelativenumberofatomsofdifferentmassesChargedParticleinMagnetFieldMass/Chargeratio(m/q)mB2

•r2q2V=MajorComponentsofMassSpectrometer同位素稀释法基本原理和应用Fordatingorcalculationofinitialisotopicratios,onehastoknow:Isotopiccompositionsofradioactiveparentanditsradiogenicproduct

TheirconcentrationsSample样品Spike稀释剂Red/blue=4Blue=6Red=24Howwecanknowtheconcentration?Howmany?Pb204Pb206Pb207Pb208PbPb205PbIsotopedilutionanalysis铷同位素为例Rbmixture

＝Rbsample+Rbspike87Rbsam+87Rbspi85Rbsam+85Rbspi87Rb85Rb()mix=87Rb85Rb()measured=测得某样品的铅同位素组成(原子数比）如：

a=206Pb/204Pb=18.8b=207Pb/204Pb=15.6c=208Pb/204Pb=39.2

204Pb/204Pb=1该样品的铅平均原子量（APb）？铅平均原子量的计算铅同位素的原子质量：

204Pb=203.97302(A204)

206Pb=205.97444(A206)

207Pb=206.97587(A207)

208Pb=207.97663(A208)axA206a+b+c+1bxA207a+b+c+1cxA208a+b+c+11xA204a+b+c+1+++APb=部分熔融过程微量元素的分馏作用及壳幔的组成特征PartialmeltingofrockMoltenrockPreferenceofcompatibleelementsMeltPreferenceofincompatibleelementsPartialmeltingCrystallizationofmeltPreferenceofincompatibleelementsMeltPreferenceofcompatibleelementsRockFractionalcrystallizationRbismoreincompatiblethanSr;NdismoreincompatiblethanSm;Duringpartialmeltingofmantleandformmagmaintrudingintocrust,Rb-SrandSm-Ndwillbefractionated.RbandNdareeasiertogointomeltrelativeSmandSr,therefore,mantlewillbedepletedinRbandNd(depletedmantle),whilethecrustwillbeenrichedinRbandNd.Differentiation

oftheEarthMeltsextractedfromthemantlerisetothecrust,carryingwiththemtheir“enrichment”inincompatibleelementsContinentalcrustbecomes“incompatibleelementenriched”Mantlebecomes“incompatibleelementdepleted”地壳和地幔Nd和Sr同位素体系特征HigherRb/Srand87Sr/86SrratioLowerSm/Ndand143Nd/144NdratioLowerRb/Srand87Sr/86SrratioLowerSm/Ndand143Nd/144NdratioLowerRb/Srand87Sr/86SrratioHigherSm/Ndand143Nd/144NdratioLowRb/Srand87Sr/86SrratioHighSm/Ndand143Nd/144NdratioSm-NdandRb-SrIsotopeSystematicsUpperCrustLowerCrustUpperMantleLowerMantleLithosphereEnrichedDepletedPrimitiveAsthenosphere放射成因同位素地球化学在岩石成因研究的应用MagmadifferentiationFractionalCrystallizationAssimilation,fractionalcrystallizationSiO2(wt.%)87Sr/86Sr50607080.704.706.708FractionationtrendContaminationtrendMagmamixing87Sr/86Sr0.7040.7060.7080.710-404MixingtrendeNdEndmember1Endmember2e.g.depletedmantlee.g.crustalmaterial等时线原理、类型和基本要求Y＝c+mXSlopeofthislinem=elt-1c:initialisotopicratioelt=1+lt++(lt)22!(lt)33!+...elt1+lt

~~whenlt<<1,D=D0+N(elt-1)

=D0+NltBasisforisochron(等时线）87Sr/87Sr=(87Sr/86Sr)0+(87Rb/86Sr)ltRb-Sr

isochrondating5100.710.720.7387Rb/86Sr87Sr/86Srm=(elt-1)Initial87Sr/87SrratioTheisotopesystemofinterestwasatisotopicequilibriumattimet=0.Isotopicequilibriuminthiscasemeansthesystemhadahomogeneous,uniformvalueofD0.Theisotopesystemasawholeandeachanalyzedpartofitwasclosedbetweent=0andtimet(usuallythepresenttime).By“closedsystem”wemeantherehasbeennotransferoftheparentorthedaughterelementintooroutofthesystem.TypeofisochronInternalisochron(内部等时线)Allanalyzedfractionscomefromonesample,e.g.,wholerockpowderanditsmineralsExternalisochron(外部等时线)Allanalyzedfractionscomefromdifferentsamples,whicharecollectedfromdifferentpartsofagenetic-relatedgeologicalbody,e.g.,agranitepluton

地球Sr和Nd同位素演化87Sr/87Sr=(87Sr/86Sr)0+(87Rb/86Sr)ltbasalticachondrite

bestinitialTime(Ma)87Sr/86Sr143Nd/144NdTime(Ga)同位素初始比值计算测得样品的现在87Sr/86Sr和143Nd/144Nd比值测得样品的现在Rb、Sr、Sm、Nd含量已知样品的形成时代(87Sr/86Sr)0=87Sr/86Sr-87Rb/86Sr(elt–1)(143Nd/144Nd)0=143Nd/144Nd-147Sm/144Nd(elt–1)为什么计算同位素初始比值？eNd值和Nd模式年龄计算原理现代eNd值：(143Nd/144Nd)sample(143Nd/144Nd)CHUReNd-1=x10000CHUR:143Nd/144Nd=0.512638

147Sm/144Nd=0.1967初始eNd值：(143Nd/144Nd)0,

sample(143Nd/144Nd)0,

CHUReNd(t)-1=x10000CHUR:143Nd/144Nd=0.512638

147Sm/144Nd=0.1967(143Nd/144Nd)0,sample=143Nd/144Nd-147Sm/144Nd(elt–1)(143Nd/144Nd)0,CHUR=143Nd/144Nd-147Sm/144Nd(elt–1)Nd模式年龄计算原理TCHUR和TDM可观的

U和Th含量较常见的微量矿物同位素体系封闭温度高238U206Pb235U207Pb232Th208Pb锆石铀－铅定年基本原理、方法和应用

207Pb*=235U(el5t–1)

206Pb*=238U(el8t–1)

207Pb*

206Pb*=235U(el5t–1)238U(el8t–1)=1137.88(el5t–1)(el8t–1)U-PbconcordiadiagramZircondatingmethods

U-Pb同位素稀释法定年

Pb-Pb蒸发定年

微区原位U-Pb定年

(如：SHRIMP)岩浆岩成岩年龄变质作用年龄碎屑锆石定年和物源问题40Ar-39Ar定年基本原理和优点Thekeyofthismethodistheproductionof39Arbyanuclearreactionon39K,byirradiatingasamplewithneutronsinareactor.39K(n,p)39ArTheproductionof39Arfrom39K:e:theneutronenergy;fe:thefluxofneutronswithenergye;se:thecapturecrosssectionforthatenergy;t:irradiationtime.39Ar=39Ktfe

sede40Ar*36Arlle=39Ktfe

sede40K(elt-1)C=tfe

sede1lleC=40Ar*36Ar(elt-1)39K40K40Ar/39Aragespectrumproducedbystepheatingofabiotitefromagraniticgneiss.McD