第07章-放射性同位素地球化学剖析课件

第七章 放射性同位素地球化学o同位素计时同位素计时o同位素示踪同位素示踪o能量特征能量特征第六章 放射性同位素地球化学I.放射性同位素衰变原理II.Rb-Sr同位素III.Sm-Nd同位素IV.U-Th-Pb同位素V.Rb-Sr-Pb同位素示踪VI.K-Ar同位素VII.C同位素I.衰变反应和衰变定律o不稳定的原子会自发地放射粒子和能量而转变成另外一种原子，这就是放射性衰变；I.Development of Atomic Structure In 1898,Marie Curie isolated polonium and In 1898,Marie Curie isolated polonium and radium from uranium ore which emitted radium from uranium ore which emitted rays that could darken photographic plates.rays that could darken photographic plates.B.RadioactivityMarie Curie(1867-1934)Marie Curie(1867-1934)Marie Curie postulated that these rays were omitted as the Marie Curie postulated that these rays were omitted as the atoms disintegrated.She named the phenomenon atoms disintegrated.She named the phenomenon radioactivityradioactivity.Curies discovery of Curies discovery of radioactivityradioactivity contradicted Daltons Atomic Theory contradicted Daltons Atomic Theory that stated that atoms were indivisible.that stated that atoms were indivisible.John Dalton(1766-1844)John Dalton(1766-1844)I.Development of Atomic Structure Curies early experiments identified three kinds of radiation.Curies early experiments identified three kinds of radiation.B.Radioactivity1.1.(alpha)particles(+2 charge and high mass)(alpha)particles(+2 charge and high mass)2.2.(beta)particles(-1 charge and low mass)(beta)particles(-1 charge and low mass)3.3.g g(gamma)particles(no detectable charge or mass)(gamma)particles(no detectable charge or mass)同位素地球化学基础1、自然核衰变的种类o衰变重核通过放出质点（He核），通式为：AZM =A-4Z-2M+,例如：22688Ra =22286Rn+Alpha DecayAvailable to elements with atomic numbers greater than 58I.衰变反应和衰变定律o衰变原子核中的中子分裂为质子和电子：AZM=AZ+1M+-,例如：8737Rb=8738Sr+-4019K=4020Ca+-同位素地球化学基础o电子捕获原子自发地从K或L层电子轨道吸取一个电子，使质子形成中子，通式为：AZM+e-=AZ-1M,例如：4019K+e-=4018Ar 13857La+e-=13856Bao核裂变Types of DecayAlphaBetahelium nucleuselectronpositronGamma Rayradiation同位素地球化学基础2、衰变定律母体和子体概念：o母体：放射性核素o子体：衰变产物The radioactive clockParent atomsDaughter atoms time zero after 1 half-life after 2 half-lives after 3 half-livesRadioactive decay can be used as a geologic clock because as time passes,the ratio of daughter/parent atoms increases.Furthermore the process occurs steadily through time,not influenced by any geological process or condition.同位素地球化学基础o卢瑟福（1902）实验结果：单位时间内衰变的原子数正比于放射性母体原子数。衰变的母体、子体原子数只与时间有关，与体系T,P等物理化学条件无关。同位素地球化学基础o衰变定律表达式：-dN/dt=N （原子数目与时间的关系）dN/N=-dt,左边由No积分到N,右边由0积分到 t,得：ln N-ln No=-tN/No =e-t 或 N=No e-t同位素地球化学基础当t=0时，D*=0,经过t时间后有：D*=No-N o代入上式得：oD*=N(e t-1)Variation of parent and daughter amounts with time3、放射性同位素测年的原理、条件和地质意义o测年原理：衰变定律Parent/Daughter Isotopes commonly used to date rocksParent IsotopeStable Daughter IsotopeHalf-Life in yearsUranium-238Lead-2064500MyUranium-235Lead-207704MyThorium-232Lead-20814000MyRubidium-87Strontium-8748800MyPotassium-40Argon-401250MyCarbon-14Nitrogen-145730y同位素地球化学基础同位素年代学的技术和条件o准确测定衰变常数o高精度、高灵敏度的测定技术o样品的封闭性：例如，对于岩浆作用可选择耐高温的岩石或矿物，如锆石等，含水矿物中，角闪石的封闭性好于云母o用作年龄测定的同位素半衰期应与所测的地质体的年龄相当；o必须准确知道放射性母体同位素的相对丰度，并有精确测定母、子体同位素含量的实验方法；o必须准确知道或能有效校正样品形成是就已经存在的子体同位素的初始含量同位素地球化学基础同位素测定方法o制样制成易离子化的化合物或气体o要求：试剂和设备不能带入污染处理过程不存在分馏作用o质谱仪测定见下图放射性年龄的地质意义o结晶年龄o变质结晶年龄o冷却年龄o变质年龄o地壳形成年龄o地壳存留年龄封闭温度及其意义o冷却年龄：是指岩石形成后冷却到基本上能完全保留放射性成因子体同位素的温度并开始放射性计时的年龄，o封闭温度：我们把上述开始计时的温度称为封闭温度。第六章 放射性同位素地球化学I.放射性同位素衰变原理II.Rb-Sr同位素III.Sm-Nd同位素IV.U-Th-Pb同位素V.Rb-Sr-Pb同位素示踪VI.K-Ar同位素VII.C同位素II、铷-锶同位素地球化学1.Rb-Sr衰变体系（1）Rb和Sr的同位素Rb:85Rb(72.15%)87Rb(27.85%)同位素地球化学基础锶的同位素:o84Sr 0.56%o86Sr 9.86%o87Sr 7.02%o88Sr 82.56%同位素地球化学基础(2)铷的衰变常数和半衰期o87Rb=87Sr+-oRb=1.3910-11 a-1o Rb=5.010-11 a-1同位素地球化学基础根据D*=N(et-1),有87Sr*=87Rb(et-1)t=1/ln(1+87Sr*/87Rb)o适用的样品：钾长石、白云母、天河石、钾盐等钙是常见组分同位素地球化学基础设87Sr=87Sro+87Sr*87Sr=87Sro+87Rb(et-1)用非放射性成因86Sr除，得到(87Sr/86Sr)=(87Sr/86Sr)o+(87Rb/86Sr)(et-1)同位素地球化学基础给定(87Sr/86Sr)o和根据地壳中平均(87Sr/86Sr)o=0.712地幔中平均(87Sr/86Sr)o=0.699由下式可得模式年龄:t=1/ln(87Sr/86Sr)-(87Sr/86Sr)o/(87Rb/86Sr)+1同位素地球化学基础2.Rb-Sr等时线法原理：(87Sr/86Sr)=(87Sr/86Sr)o+(87Rb/86Sr)(et-1)同位素地球化学基础方法的特点：o与不含初始锶方法和模式年龄相比，精度较高。o可得到初始86Sr/86Sr值（用于判断来源）。o根据数据点拟合好坏程度可以检验体系封闭性。同位素地球化学基础3.Rb-Sr等时线年龄研究的必要条件o体系封闭o体系封闭以前均一化o各样品之间的87Rb/86Sr值差别远远大于样品的87Sr/86Sr值误差范围。同位素地球化学基础4.Rb-Sr等时线年龄研究的注意问题（1）岩浆岩o可达到均一化o深成岩的结晶时间相对于其年龄可忽略。o喷出岩快速冷却，等时线年龄代表形成年龄。同位素地球化学基础（2）变质岩o变质作用对同位素体系变化很大，即使岩石组构没有影响时，同位素已发生巨大变化o变质作用会使Sr同位素重新均一化o小范围的均一化：全岩等时线为代表岩石形成年龄，矿物等时线为变质年龄。o大范围的均一化：均代表变质年龄。均一化后同位素地球化学基础（3）沉积岩o87Sr来源：o陆源碎屑物o自生矿物（海绿石等）同位素地球化学基础5.锶同位素地球化学演化陨石和整体地球的Sr同位素初始值BABIBABI-Basaltic Achondrite Best Initial=Bulk Earth,undifferentiated地幔地壳的Sr同位素组成和演化海水的Sr同位素组成和演化Sr同位素年龄标尺BABIIgneous Processes and 87Sr/86Sr ratios87Sr/86Sr ratios of igneous rocks:MORB0.7025Continents0.7119Ocean Islands0.704vs.Meteorites0.699*Remember that 87Rb likes meltMORBMore than just an age tool-tracking(87Sr/86Sr)i through timeBABI-Basaltic Achondrite Best Initial=Bulk Earth,undifferentiatedRb-Sr isochronFrom meteorites87Sr/86Sr ratios of igneous rocks:MORB0.7025Continents0.7119Ocean Islands0.704vs.Meteorites0.699T=4.5GaQuestions:1.Why are all present-day(87Sr/86Sr)values greater than BABI?2.Why are continental values the highest?第六章 放射性同位素地球化学I.放射性同位素衰变原理II.Rb-Sr同位素III.Sm-Nd同位素IV.U-Th-Pb同位素V.Rb-Sr-Pb同位素示踪VI.K-Ar同位素VII.C同位素III、钐-钕同位素地球化学o1.钐-钕同位素o142Nd 27.16%o143Nd 12.18%o144Nd 23.80%(=2.01015)o145Nd 8.29%(1.01017)o146Nd 17.19%o148Nd 5.75%o150Nd 5.63%同位素地球化学基础o144Sm 3.10%o147Sm 15.1%(=1.071011)o148Sm 11.3%(3.01014)o149Sm 13.90%(1.01015)o150Sm 7.40%o152Sm 26.60%o154Sm 22.60%同位素地球化学基础o钐-钕的特点和基本性质：o丰度低，各岩石类型的Sm/Nd变化小o母子体性质相似o高价态，不易活动同位素地球化学基础o2.Sm-Nd法年龄测定根据D*=N(et-1),有143Nd*=147Sm(et-1)t=1/ln(1+143Nd*/147Sm)同位素地球化学基础o等时线年龄：类似于Rb-Sr等时线年龄计算公式，由143Nd*=147Sm(et-1)可以导出下式：(143Nd/144Nd)=(143Nd/144Nd)o+(147Sm/144Nd)(et-1)o模式年龄：同位素地球化学基础3.Sm-Nd法年龄测定的应用（1）Sm-Nd法的特点：o147Sm的半衰期长，是测定古老岩石年龄（10亿年）的有效手段。o镁铁质岩石中Sm/Nd比值的差异可满足等时线年龄研究的要求。o变质作用，蚀变作用和风化作用对Sm-Nd体系的影响很小。同位素地球化学基础（2）适用于Sm-Nd年龄测定的岩石和矿物样品o基性、超基性岩：岩石形成年龄o酸性岩：适于模式年龄o变质岩：原岩的年龄o沉积岩：物源的年龄Sm-Nd and Rb-Sr cross-plots第六章 放射性同位素地球化学I.放射性同位素衰变原理II.Rb-Sr同位素III.Sm-Nd同位素IV.U-Th-Pb同位素V.Rb-Sr-Pb同位素示踪VI.K-Ar同位素VII.C同位素同位素地球化学基础三、U-Th-Pb法定年和铅同位素地球化学1.U-Th-Pb的同位素oU：238U 99.27%(=1.5510-10,=4.47109)235U 0.70%(=9.8510-10,=7.04108)234U 0.006%同位素地球化学基础oTh:232Th(=4.9510-11,=1.41010)oPb:o204Pbo206Pbo207Pbo208Pb同位素地球化学基础o除204Pb外，余为放射性成因铅，反应总过程如下：o23892U=20682Pb+8 42He+6-+Eo23592U=20782Pb+7 42He+4-+Eo23290Th=20882Pb+6 42He+4-+E同位素地球化学基础2.U-Th-Pb法年龄测定根据基本公式：D*=N(et-1)(206Pb/204Pb)=(206Pb/204Pb)o+(238U/204Pb)(et-1)(207Pb/204Pb)=(207Pb/204Pb)o+(235U/204Pb)(et-1)(208Pb/204Pb)=(208Pb/204Pb)o+(232Th/204Pb)(et-1)同位素地球化学基础上式分别整理得：t=1/1 ln(206Pb/204Pb)-(206Pb/204Pb)o/(238U/204Pb)+1t=1/2 ln(207Pb/204Pb)-(207Pb/204Pb)o/(235U/204Pb)+1t=1/3 ln(208Pb/204Pb)-(208Pb/204Pb)o/(232Th/204Pb)+1可得到三个值适用的矿物：沥青铀矿，晶质铀矿，钍石，锆石，独居石，磷灰石等。同位素地球化学基础o三个年龄值不一致时，用两式联立得:(207Pb/204Pb)-(207Pb/204Pb)o 235U (e2t -1)_ =_ _ (206Pb/204Pb)-(206Pb/204Pb)o 238U (e1t -1)同位素地球化学基础3.普通铅法年龄测定（Pb-Pb法）（1）基本思路：地球上的铅同位素按正常母体量演化，直到形成含铅矿物时才脱离母体。（2）基本假设o地球形成初期U、Th、Pb分布均匀o地球的原始铅同位素组成为：同位素地球化学基础o地球的原始铅同位素组成为：ao=(206Pb/204Pb)o=9.307bo=(207Pb/204Pb)o=10.294co=(208Pb/204Pb)o=29.476o地球铅同位素的相对丰度变化是体系中放射性母体衰变的结果。o铅矿物形成后，铅同位素与放射性母体分离。同位素地球化学基础o T=45.5亿年 t 0亿年o时间 111o事件 111o 地球形成 矿物形成 测定同位素地球化学基础o计算公式：(206Pb/204Pb)=(206Pb/204Pb)o+(238U/204Pb)(e1T-1)-(238U/204Pb)(e1 t-1)(206Pb/204Pb)=(206Pb/204Pb)o+(238U/204Pb)(e1T-e1t)同位素地球化学基础o普通铅法的样品要求：o无U和Th的矿物，如方铅矿、黄铁矿等同位素地球化学基础o4.U-Pb谐和曲线法(一致曲线法)o206Pb*/238U=e1t-1o207Pb*/235U=e2t-1同位素地球化学基础5.铅同位素的演化原始铅：地球形成时的初始铅放射铅：地球形成后放射性母体产物第六章 放射性同位素地球化学I.放射性同位素衰变原理II.Rb-Sr同位素III.Sm-Nd同位素IV.U-Th-Pb同位素V.Rb-Sr-Pb同位素示踪VI.K-Ar同位素VII.C同位素White and Stan Hart mantle end-member reservoirsMantle tetrahedronSr-Nd isotopic spaceSr-Nd IsotopesFigure 13-12.Figure 13-12.Data from Ito et al.(1987)Data from Ito et al.(1987)Chemical Geology,62,157-176;Chemical Geology,62,157-176;and and LeRoex LeRoex et al.(1983)et al.(1983)J.Petrol.,24,267-318.J.Petrol.,24,267-318.Figure 14-6.After Zindler and Hart(1986),Staudigel et al.(1984),Hamelin et al.(1986)and Wilson(1989).m mMantle Reservoirs1.DM(Depleted Mantle)=N-MORB sourceFigure 14-6.After Zindler and Hart(1986),Staudigel et al.(1984),Hamelin et al.(1986)and Wilson(1989).2.BSE(Bulk Silicate Earth)or the Primary Uniform ReservoirFigure 14-6.After Zindler and Hart(1986),Staudigel et al.(1984),Hamelin et al.(1986)and Wilson(1989).3.EMI=enriched mantle type I has lower 87Sr/86Sr(near primordial)4.EMII=enriched mantle type II has higher 87Sr/86Sr(0.720,well above any reasonable mantle sourcesFigure 14-6.After Zindler and Hart(1986),Staudigel et al.(1984),Hamelin et al.(1986)and Wilson(1989).5.PREMA(PREvalent MAntle)Figure 14-6.After Zindler and Hart(1986),Staudigel et al.(1984),Hamelin et al.(1986)and Wilson(1989).Figure 14-6.After Zindler and Hart(1986),Staudigel et al.(1984),Hamelin et al.(1986)and Wilson(1989).Figure 14-8.After Wilson(1989)Igneous Petrogenesis.Kluwer.Data from Hamelin and Allgre(1985),Hart(1984),Vidal et al.(1984).A Model for Oceanic MagmatismDMDMOIBOIBContinentalContinentalReservoirsReservoirsEM and HIMU from EM and HIMU from crustalcrustal sources(subducted OC+CC sources(subducted OC+CC sedsseds)Figure 14-10.Nomenclature from Zindler and Hart(1986).After Wilson(1989)and Rollinson(1993).四、钾-氩法定年1.K,Ar同位素39K,93.08%40K,0.0019%41K,6.91%大气氩的相对丰度：40Ar,99.60%38Ar,0.063%36Ar,0.377%2.40K的衰变反应4019K+e-40Ar (12%)(e=5.8510-11/y)o 4020Ca+-(88%)(=4.7210-10/y)o计算公式：根据 D*=N(et-1)，有40Ar*=e/(e+)40K(et-1)t=1/ln 1+(40Ar*/40K)e/(e+)3.K-Ar 法的应用o一般情况下，Ar不进入样品中，可视为无初始氩。o适用的样品：角闪石，辉石，白云母，黑云母，长石类，全岩样品，海绿石，钾盐等。同位素地球化学基础o关于氩丢失和过剩氩问题：等时线法同位素地球化学基础o六、14C法o1.碳的同位素o12C 98.89%o13C 1.108%o14C 1.2 10-10%同位素地球化学基础2.14C的产生和衰变o(=537040a;=1.209 10-4/a)o由宇宙射线作用：o 147N+n=146C+11Ho14C的衰变反应：o146C=147N+-同位素地球化学基础3.14C法年龄测定及其原理大气中14C的丰度处于动态平衡中，因此可以设：o大气中的14C是恒定。o含碳样品“死亡”后，其14C即按衰变规律减少。14C Cycle