同位素讲座-1-2010

1、稳定同位素地球化学稳定同位素地球化学储雪蕾（第一讲）（第一讲）I. I. 稳定同位素基本原理稳定同位素基本原理IntroductionStable isotope geochemistry is concerned with variations of the isotopiccompositions of elements arising from physicochemical processes ratherthan nuclear processes. Here, we will find that the very small differences inthe chemical b

2、ehavior of different isotopes of an element can provide avery large amount of useful information about chemical (both geochemicaland biochemical) processes.The origins of stable isotope geochemistry are closely tied to thedevelopment of modern physics in the first half of the 20th century. Thereal h

3、istory of stable isotope geochemistry begins in 1947 with the HaroldUreys publication of a paper entitled “The Thermodynamic Properties ofIsotopic Substances”. Urey not only showed why, on theoretical grounds,isotope fractionations could be expected, but also suggested that thesefractionations could

4、 provide useful geological information. Urey then setup a laboratory to determine the isotopic compositions of naturalsubstances and experimentally determine the temperature dependence ofthese fractionations, in the process establishing the field of stable isotopegeochemistry. Stable isotope geochem

5、istry has become an essential part of not onlygeochemistry, but the earth sciences as a whole. 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：Harold Urey(Univ. of Chicago)和 Alfred Nier(Univ. of Minnesota)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“The Thermodynamic Properties of

6、Isotopic Substances”，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

7、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，定同位素地球化学原理与应用，重点介绍重点介绍C C、H H 、 O O 、 S S同位素。同位素。1 同位素的基本概念同位素的基本概念同位素的分类同位素的分类： (1) 放射性同位素：原子核不稳定，能自发进放射性同

8、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。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。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的定义 同位素定义：核内质子数相同而中子数不同的同同位素定义：核内质子数相同而中子数不同的同一类原子。一类原子。传统的稳定同位素非传统的稳定同位素本课程 H, C, O, and S are the greatest interest elements in sta

9、bleisotope geochemistry.For these elements, the common characteristics are :(1) They have low atomic mass.(2) The relative mass difference between their isotopes islarge.(3) They form bonds with a high degree of covalentcharacter.(4) The elements exist in more than one oxidation state,such as C and

10、S, form a wide variety of compounds, or areimportant constituents of naturally occurring solids andfluids, such as H and O. (5) The abundance of the rare isotope is sufficiently high(generally at least tenths of a percent) to facilitate analysis.Isotope effect is defined that the differences in chem

11、ical andphysical properties of an element arise from differences in atomicmass through isotopic substitution.同位素效应同位素效应(Isotope effect)Table Characteristic constants of H2O, D2O, and H218OConstantsH216OD2160 H218ODensity (20 C, in g cm-3)0.99791.1051 1.1106Temperature of greatest density (C)3.9811.2

12、44.30Melting point (760 Torr, in C)0.003.81 0.28Boiling point (760 Torr, in C)100.00101.42 100.14Vapor pressure (at 100 C, in Torr)760,00721.60Viscosity (at 20 C, in centipoise)1.0021.247 1.056同位素比值（同位素比值（Isotope ratio):R = 重同位素丰度重同位素丰度/轻同位素丰度轻同位素丰度同位素分馏系数（同位素分馏系数（ Isotope fractionation factor): A-B

13、 = RA/R B 即即 值，值，表示某元素的同位素在两种物质表示某元素的同位素在两种物质（A和和B）之间的分馏的程度。）之间的分馏的程度。 同位素分馏（同位素分馏（Isotope fractionation)： 同位素在不同物质或不同物相间分配比例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。不同的现象称之为同位素分馏。 值：值： 样品的同位素比值相对于标准样品同位素比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的千分偏差 （）= （R样样 R标标）/ R标标）X1000 = （R样样/R标标）- 1）X1000 R样样：样品的同位素比值：样品的同位素比值 R标标：

14、标准的同位素比值：标准的同位素比值 0 表明样品相对标准富集重同位素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表明样品与标准同位素比值相同稳定同位素标准稳定同位素标准2D/1H ：Standard mean of ocean water (标准平均大洋水)18O /16O ：Standard mean of ocean water (标准平均大洋水)PDB：Belemnitella Americana（美国北卡罗来纳州白垩系Pee Dee建造美洲似箭石）13C/12C PDB：Belemnitella Ame

15、ricana（美国北卡罗来纳州白垩系Pee Dee建造美洲似箭石）34S/32S CDT：美国亚利桑那州Canyon Diablo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：稳定同位素标准稳定同位素标准D/H13C/12C15N/14N18O/16O34S/32SD13C15N18O34SVSMOWVPDBAIRVSMOW, VPDBVCDT1.5575 x 10-41.1237 x 10-23.677 x 10-32.0052 x 10-3, 2.0672 x 10-34.5005 x 10-2NIST:

16、 National Institute of Standards and TechnologyIAEA: International Atomic Energy Agency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（即式发表，但推荐用现标准（即V标准）发表。标准）发表。2 同位素分馏机理同位素分馏机理从严格意义上讲，在周期表中所有元从严格意义上讲，在周期表中所有元素的不同种同位素由于其质量

17、上存在差别，素的不同种同位素由于其质量上存在差别，在自然界的各种在自然界的各种物理物理，化学化学和和生物生物的反应的反应和过程中都会发生同位素分馏。这些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。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: 平衡分馏平衡分馏（equilibrium fractionation） 动力（学）分馏动力（学）分馏（kinetic fractionation） 非质量依赖分馏非质量依赖分馏（mass-independent fractionati

18、on)同位素分馏的类型同位素分馏的类型- 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William Whites textbook(CornellUniv. )most imp.在 25C达到平衡时， CO2的18O/16O比值比 H2O 高。22221.0233COCOH OH ORR平衡分馏平衡分馏 (Equilibrium fractionation)这什么会出现平衡分馏？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

19、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。zero point energy平衡分馏的温度依赖性平衡分馏的温度依赖性harmonic oscilllator modelharmonic oscilllator modeldatadata简谐振荡模型给出简谐振荡模型给出ln 高温下高温下与与T T2 2成反比，成反比，低温下与低温下与T T成反比。成反比。11T（T20

20、0 C ）211T因此，在较低的温度上因此，在较低的温度上会有更严重的同位素分会有更严重的同位素分馏。馏。General rule of thumb: the heavy isotope will be concentrated in the phase in whichit is most strongly bound (or lowest energy state). Solidliquidgas, covalentionic, etc.Ex: 18O in carbonates- heavily enriched in carbonate because O tightly bonded

21、 to small, highly charged C4+, vs. weaker H+- so D18Ocal-water = 18Ocarb-18Owater = 30Ex: quartz (SiO2) most enriched mineralLattice configuration (aragonite vs. calcite) plays a secondary role (D18Oarag-cal = 0.5)Chemical substitutions in the lattice (ie. Ba instead of Ca) also have a small effect:

22、D18OBa-cal-water = 25 (vs. 30 for Ca-cal)富集规律（平衡分馏）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，

23、是同位素地质温度计的理论依据。动力分馏动力分馏 (Kinetic fractionation)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，因此它相对由于轻同位素取代具有相对高的势能，因此它相对“活泼活泼”，优先反应。，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

24、，细菌还原产生的硫化物贫34S。212kEmv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:221122AABBm vm v1/21/2451.01144ABBAvmvm如此，如此， 12C16O2 比比13C16O2 扩散的速度要快扩散的速度要快 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

25、 一些物理一些物理-化学（如蒸发、扩散、单向或未完成的化学化学（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。这些过程往往受素动力分馏。这些过程往往受化学反应动力学化学反应动力学控制，其控制，其造成的同位素分馏受造成的同位素分馏受扩散速度扩散速度或或反应速度反应速度控制，依赖于控制，依赖于路径路径、时间时间与与速度速度。 生物参与的化学过程，一般同位素动力分馏明显，这生物参与的化学过程，一般同位素动力分馏明

26、显，这在在C和和S同位素分馏的研究中占有重要位置。同位素分馏的研究中占有重要位置。Closed- and open-system fractionation瑞利同位素分馏瑞利同位素分馏(Rayleigh isotope fractionation)推导：Thiemens and Heidenreich, 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

27、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。非质量依赖分馏非质量依赖分馏 (Mass-independent fractionation)质量相关定则质量相关定则 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 1717O/O/ 1818O O (1/32 - 1/33)/(1/32 - 1/34) (1/32 - 1/33)/(1/32 - 1/34) = 0.516 = 0.516 即即

28、 1717O = 0.516O = 0.516 1818O O地球样品普地球样品普遍满足遍满足质量相质量相关分馏线关分馏线或或质质量分馏线量分馏线。质量分馏线质量分馏线的的斜率在斜率在0.5000.500到到0.5260.526范围内。范围内。质量分馏线质量分馏线D D33S和和D D36S定义定义D D33S = (33S/32S)sample/(33S/32S)ref (34S/32S)sample /(34S/32S)ref0.515103D D36S = (36S/32S)sample/(36S/32S)ref (34S/32S)sample /(34S/32S)ref 1.9103硫

29、的质量相关和非质量相关同位素分馏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同位素地质温度计原理 值值： （）= (R样样/ R标标）- 1）X 1000 同位素分馏系数同位素分馏系数 与与 值的关系：值的关系： 103 ln A-B A - B = D D A-B 即即ln A-B与与A, B两种物质的两种物质的 值之差相关。值之差相关。 同位素平衡分馏系数与温度的关系同位素平衡分馏系数与温度的关系： 103 ln = a/T2 + b/T + c （T: K) 其中其中a，b，c 分别为常数。分别为常数。 1）在一般低温下，）在一般低温下，a/T2可以忽略，简化：可以忽略，简

30、化： 103 ln = b/T + c 2）在高温下，）在高温下，b/T可以忽略，简化：可以忽略，简化： 103 ln = a/T2 + c 4 同位素样品制备与质谱分析同位素样品制备与质谱分析Conventional methods for SO2 preparation The amount of sample required variesamong laboratories but typically ranges from 5 to20 mg of pure mineral separate, such as sulfides(pyrite, galena, sphalerite,

31、etc.) and sulfates(gypsum, anhydrite, and barite) for 34S and thetypical analytical uncertainties (1) forconventional techniques are 0.2 for 34S. Sulfide minerals: such as pyrite, galena, sphalerite, etc. oxidizing agent: CuO, Cu2O, or V2O5 temperature: 900 to 1100 CSulfate minerals: such as barite,

32、 gypsum, anhydrite oxidizing agent: Cu2O, or V2O5 + SiO2 cover: Cu temperature: 1100 to 1200 CCeramic boat Iron ring In ion source, SO2 gas is ionized to positively charged particles, which are accelerated through a voltage gradient. The SO2+ ion beam passes through a magnetic field, which causes se

33、paration of various masses such as 64 (32S16O2) and 66 (34S16O2, 34S18O 16O).The beam currents are measured in Faraday cups and can be related to the isotopic ratio when the sample and standard gases are compared.GasBench IIMS + EATC/EAGeochemistry of Stable Isotopes This mothod is very useful in in

34、vestigations on environment, ecology and mineral resources.Advantages:1) impurity: whole rock, such as black shale;2) small amount of sample: 1 mg ( 10 mg S in sample);3) rapidly, continuously, and automaticallyDisadvantage:lower analytical precision: 0.2-0.5 for 34S II. II. 硫同位素地球化学硫同位素地球化学Sulfur i

35、sotope should be the most complex in all the thoseisotope system because of variable valence states in nature:S+6O4 (sulfate) S+4O2 S0 FeS-12 H2S-2 (sulfide)Significant equilibrium isotopic fractionations occur between eachof these valence states. Each of these valence states forms a varietyof compo

36、unds, and fractionations can occur between these as well. In addition, sulfur is important in biological processes andfractionations in biologically mediated oxidations and reductionsare often different from fractionations in the abiologicalequivalents.The partitioning of isotopes between two substa

37、nces withdifferent isotope rations is called isotope fractionation. Three processes cause the isotope fractionation between two substances in nature: Isotope exchange reactions; Kinetic processes during a chemical reaction or physical process, such as freeze, evaporation, etc.; Biological processes

38、The 34S distribution in the nature The 34S secular variations of marine evaporites1.Sulfur isotope variations in geological systems Sulfur is present in nearly all natural environments: as a minor component in igneous and metamorphic rocks, mostly as sulfides; in the biosphere and related organic su

39、bstances, like crude oil and coal; in ocean water as sulfate and in marine sediments as both sulfide and sulfate. It may be a major component in ore deposits, where it is the dominant non-metal as sulfates in evaporites. In addition, various sulfide ore deposits are economically very important sourc

40、es for Cu, Pb, Zn, Ag, and other metals. These occurrences cover the whole temperature range of geologic interest. Sulfur is bound in various oxidation states, from sulfides to elemental sulfur, to sulfates. From these facts it is quite clear that sulfur is of special interest in stable isotope geoc

41、hemistry. 1) Meteoritic sulfur: 0 , such as Canon Diablo troilite Meteorites approximately have the same 34S values of the Earths bulk. The iron meteorites have an average isotope composition of 0.20.2. The average 34S value of mid-ocean ridge basalts is 0.30.5 . 2) Sea-water sulfate: 21 , in modern

42、 ocean Geochemical processes, the most notable of which are oxidation and reduction, profoundly fractionate sulfur isotopes away from bulk-Earth values in geological systems. Oxidation processes produce species that are enriched in 34S relative to the starting material, whereas reduction produces sp

43、ecies that are depleted in 34S. But, great isotope fractionations are related closely to a biological process, i.e., bacterial sulfate reduction. The 34S of sulfate in ancient oceans as recorded by marine evaporite sequences (Claypool et al. 1980) has varied from a low of approximately 10 during Per

44、mian and Triassic time to a high of 35 during Cambrian time. Because the isotope fractionation between the sulfate-containing evaporite and the sulfate in ocean water is almost negligible, the observed trend in evaporite sulfate should closely reflect fluctuations in the sulfur isotope composition o

45、f marine sulfate through geologic time. Changes in the 34S of marine sulfate during the geologic past may be caused by major changes in the budget between the individual reservoirs: during periods of high (), which should take place under favorable paleogeographic conditions, the 34S of ocean water

46、should increase. In contrast, periods of extended () introduce additional light continental sulfur into the ocean which decreases the 34S value of ocean sulfate. Such periods of extended weathering are geologically plausible in periods of . While the partial cycle between ocean and evaporites only i

47、nvolves sulfate transfer from one reservoir to the other, bacterial sulfate reduction, as well as the weathering of sulfides from argillaceous sediments, change the valence state of the sulfur. Therefore, during a period with increased rate of one of these two processes, appreciable amounts either o

48、f organic compounds or of are needed. Especially in the latter case, during weathering is appreciable.2.Factors controlling sulfur isotope fractionation Isotope equilibrium fractionation: equilibrium fractionation factor and isotope geothermometer Isotope kinetic fractionation Isotope fractionation

49、during bacterial sulfate reduction Rayleigh isotope fractionation The fractionation factor（）is defined as the ratio of the numbers of any two isotopes in one chemical compound A divided by the corresponding ratio for another chemical compound B: A-B = RA/RB where R is 34S/32S. This equation can be r

50、ecast in terms of values as A-B = (1+ A/1000)/(1+ B/1000) = (1000+ A)/(1000+ B) Values of are typically near unity, with variations normally in the third decimal place (1.00 x). The value D Da-b is defined as D Da-b = A - B Because 1000ln(1.00 x) is approximately equal to x, D Da-b 1000 ln A-B.Examp

51、le: For an isotope exchange reaction 32SO42- + H234S = 34SO42- + H232Sthe equilibrium fractionation factor between sulfate and sulfide (i.e., sulfate-sulfide) is about 1.075 at 25 C (Tudge and Thode 1950).(1) experimental determination;(2) theoretical estimation using calculated bond strengths or st

52、atistical mechanical calculations based on data on vibrational frequencies of compounds;(3) analysis of natural samples for which independent estimates of temperature are available. 1) the magnitude of fractionation factor depends primarily on temperature, becoming smaller with increasing temperatur

53、e; 2) when in equilibrium, sulfur species of higher valence (i.e., more oxidized) trend to be more enriched in the heavier isotopes, such that3) the fractionation factors between sulfate minerals and SO42- are quite small, but those among some sulfide minerals and aqueous sulfides are very significa

54、nt.Sulfur isotope geothermometry is typically based on the isotopic partitioning between two sulfur-bearing minerals, for an example, barite and pyrite. An equation to calculate the temperature recorded by a coexisting pair of barite (Ba) and pyrite (Py) can derived as follows:1000 ln Ba-Py D D Ba-P

55、y = 34SBa - 34SPy(1)Thus,D D Ba-Py 1000 ln Ba-H2S - 1000 ln Py-H2S(2)Substituting from the above Table yieldsD D Ba-Py = (6.463x106)/T2 + 0.56 (0.40 x106)/T2 = (6.063x106)/T2 +0.56 (3)with T in K. Solving for T, and converting to C yields:T ( C) = (6.063x106/(D D Ba-Py - 0.56)1/2-273.15 (4)For examp

56、le, for a mineral pair with 34SBa = 21.0 and 34SPy = 5.1, a temperatue of 356 C is calculated using Equation (4). During nonequilibrium, unidirectional chemical reactions, the fractionation of sulfur isotopes arises from the fact that chemical reaction rates are mass dependent and that one isotopic

57、species reacts more rapidly than another. In general, the molecules containing the lighter isotope will have the faster reaction rate. Consequently, the product tends to be enriched in the lighter isotope. For example, oxidation of sulfide to sulfate can be considered as two separate reactions with

58、different rate constants: k1H232S 32SO42- k2 H234S 34SO42- The ratio of two rate constants k1/k2 is equal to the kinetic isotopic effect, i.e., kinetic fractionation factor, = k1/k2 .1) Low-temperature oxidative alteration of sulfide minerals to sulfate minerals: Isotope kinetic effect is commonly n

59、egligible, i.e., 34Sproduct(sulfate) 34Sreactant(sulfide)2) Thermochemical reduction of sulfate due to interaction with organic matter: The kinetic fractionation was less 10 during this reduction. The fractionation of sulfur isotopes between sulfate and sulfide during bacterial sulfate reduction is

60、a kinetically controlled process in which 34S is enriched in the sulfate relative to the sulfide. The sulfate-reducing bacteria more readily metabolize 32S relative to 34S. Thus, the 34S of the residual aqueous sulfate increase during the reaction progress. The fractionation associated with bacterial su