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1、稳定同位素地球化学,储雪蕾Institute of Geology and Geophysics Chinese Academy of Sciences(联系电话：82998417；E-mail：),（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：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 Thermody
2、namic Properties of 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
3、，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
4、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(Isotope effect),同位素比值（Isotope ratio):R=重同位素丰度/轻同位素丰度,同位素分馏系数（Isotope fractionation factor):A-B=RA/R B 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（Isotope fractionation)：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
5、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）X1000 R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/1H SMOW：Standard mean of ocean water(标准平均大洋水)18O/16O SMOW：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：Belem
6、nitella Americana（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,D/H13C/12C15N/14N18O/16O34S/32S,dDd13Cd15Nd18Od34S,VSMOWVPDBAIRVSMOW,VPDBVCDT,1.5575 x 10-41.1237 x 10-23.677 x 10-32.0052 x 10-3,2.0672 x 10-34.500
7、5 x 10-2,NIST: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
8、应等等。,自然界存在三种类型的同位素分馏:平衡分馏（equilibrium fractionation）动力（学）分馏（kinetic fractionation）非质量依赖分馏（mass-independent fractionation),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
9、 高。,平衡分馏(Equilibrium fractionation),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。,zero point energy,平衡分馏的温度依赖性,harmonic oscilllator model,harmonic oscilllator model,data,data,
10、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。,（T200 C）,（T200 C）,因此，在较低的温度上会有更严重的同位素分馏。,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 ti
11、ghtly bonded to small,highly charged C4+,vs.weaker H+-so D18Ocal-water=d18Ocarb-d18Owater=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:D18OBa
12、-cal-water=25(vs.30 for Ca-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
13、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“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)13C
14、16O2(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
15、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-and open-system fractionation,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
16、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-independent fractionation),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=0.516 d18O,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
17、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)ref 1.9103,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）X 1000 同位素分馏系数与值的关系：103 ln A-B A-B=D A-B 即ln A-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：103 ln=a/T2+b/T+c（T
18、: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：103 ln=b/T+c 2）在高温下，b/T可以忽略，简化：103 ln=a/T2+c,4 同位素样品制备与质谱分析,A vacuum system for SO2 preparation,Sulfide minerals:such as pyrite,galena,sphalerite,etc.oxidizing agent:CuO,Cu2O,or V2O5 temperature:900 to 1100 CSulfate minerals:such as barite,gypsum,anhydrite oxidi
19、zing agent:Cu2O,or V2O5+SiO2 cover:Cu temperature:1100 to 1200 C,Ceramic boat,Iron ring,Sulfur isoiopic analysis for SO2 gas,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 cause
20、s separation 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 II,MS+EA,TC/EA,Geochemistry of Stable Isotopes,On-line sulfur isotope determ
21、ination using EA-IRMS-A new method of rapid analyses,This mothod is very useful in investigations 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:l
22、ower analytical precision:0.2-0.5 for d34S,II.硫同位素地球化学,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,The d34S distributi
23、on in the natureThe d34S secular variations of marine evaporites,1.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 substances,like crude
24、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 sources for Cu,Pb,Zn,Ag,and oth
25、er 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 geochemistry.,The d34S distribution in
26、nature,The common reference reservoirs 1)Meteoritic sulfur:0,such as Canon Diablo troilite Meteorites approximately have the same d34S values of the Earths bulk.The iron meteorites have an average isotope composition of 0.20.2.The average d34S value of mid-ocean ridge basalts is 0.30.5.2)Sea-water s
27、ulfate:21,in modern 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 reduct
28、ion produces species that are depleted in 34S.But,great isotope fractionations are related closely to a biological process,i.e.,bacterial sulfate reduction.,The d34S of sulfate in ancient oceans as recorded by marine evaporite sequences(Claypool et al.1980)has varied from a low of approximately 10 d
29、uring Permian 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 compos
30、ition of marine sulfate through geologic time.,The d34S secular variations of marine evaporites,The d34S secular variationsof marine evaporites,Changes in the d34S of marine sulfate during the geologic past may be caused by major changes in the budget between the individual reservoirs:during periods
31、 of high biological sulfate reduction(),which should take place under favorable paleogeographic conditions,the d34S of ocean water should increase.In contrast,periods of extended weathering()introduce additional light continental sulfur into the ocean which decreases the d34S value of ocean sulfate.
32、Such periods of extended weathering are geologically plausible in periods of high tectonic,mountain-building activity.,Sulfur cycle in nature,While the partial cycle between ocean and evaporites only involves sulfate transfer from one reservoir to the other,bacterial sulfate reduction,as well as the
33、 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 of organic compounds or of free atmospheric oxygen are needed.Especially in the latter case,oxygen consump
34、tion during weathering is appreciable.,2.Factors controlling sulfur isotope fractionation,Isotope equilibrium fractionation:equilibrium fractionation factor and isotope geothermometerIsotope kinetic fractionationIsotope fractionation during bacterial sulfate reductionRayleigh isotope fractionation,T
35、he fractionation factor（a）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:aA-B=RA/RB where R is 34S/32S.This equation can be recast in terms of d values as aA-B=(1+dA/1000)/(1+dB/1000)=(1000+dA)/
36、(1000+dB),Values of a are typically near unity,with variations normally in the third decimal place(1.00 x).The value Da-b is defined as Da-b=dA-dB Because 1000ln(1.00 x)is approximately equal to x,Da-b 1000 ln aA-B.,Example:For an isotope exchange reaction 32SO42-+H234S=34SO42-+H232Sthe equilibrium
37、fractionation factor between sulfate and sulfide(i.e.,asulfate-sulfide)is about 1.075 at 25 C(Tudge and Thode 1950).,How to obtain equilibrium isotopic fractionation factor(three ways):,(1)experimental determination;(2)theoretical estimation using calculated bond strengths or statistical mechanical
38、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 temperature;2)when in equilibrium,
39、sulfur species of higher valence(i.e.,more oxidized)trend to be more enriched in the heavier isotopes,such that d34SSO4(and sulfate minerals)d34S SO2 d34S S SH2S(and sulfide minerals)3)the fractionation factors between sulfate minerals and SO42-are quite small,but those among some sulfide minerals a
40、nd aqueous sulfides are very significant.,Sulfur isotopic features of equilibrium fractionation:,Sulfur isotope geothermometry,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 t
41、he temperature recorded by a coexisting pair of barite(Ba)and pyrite(Py)can derived as follows:1000 ln aBa-Py D Ba-Py=d34SBa-d34SPy(1)Thus,D Ba-Py 1000 ln aBa-H2S-1000 ln aPy-H2S(2)Substituting from the above Table yieldsD Ba-Py=(6.463x106)/T2+0.56(0.40 x106)/T2=(6.063x106)/T2+0.56(3)with T in K.Sol
42、ving for T,and converting to C yields:T(C)=(6.063x106/(D Ba-Py-0.56)1/2-273.15(4)For example,for a mineral pair with d34SBa=21.0 and d34SPy=5.1,a temperatue of 356 C is calculated using Equation(4).,Isotope kinetic fractionation,During nonequilibrium,unidirectional chemical reactions,the fractionati
43、on of sulfur isotopes arises from the fact that chemical reaction rates are mass dependent and that one isotopic 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 li
44、ghter isotope.For example,oxidation of sulfide to sulfate can be considered as two separate reactions with 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,a=k1/k2.,Sulfur isotope k
45、inetic fractionation,1)Low-temperature oxidative alteration of sulfide minerals to sulfate minerals:Isotope kinetic effect is commonly negligible,i.e.,d34Sproduct(sulfate)d34Sreactant(sulfide)2)Thermochemical reduction of sulfate due to interaction with organic matter:The kinetic fractionation was l
46、ess 10 during this reduction.,Bacterial sulfate reduction-isotope kinetic effects,The fractionation of sulfur isotopes between sulfate and sulfide during bacterial sulfate reduction is a kinetically controlled process in which 34S is enriched in the sulfate relative to the sulfide.The sulfate-reduci
47、ng bacteria more readily metabolize 32S relative to 34S.Thus,the d34S of the residual aqueous sulfate increase during the reaction progress.The fractionation associated with bacterial sulfate reduction(1000ln aSO4-H2S)typically ranges from 15 to 60(Goldhaber and Kaplan 1975)in marine settings,compar
48、ed to an equilibrium,abiotic fractionation of approximately 73 at 25C.The magnitude of the fractionation has been shown to be a function of the rate of sulfate reduction,which can be related to sedimentation rates.The smaller fractionations(15)correspond to faster rates of sulfate reduction and sedi
49、mentation,whereas the larger fractionations(60)correspond to slower rates of sulfate reduction and sedimentation(Goldhaber and Kaplan 1975).,Recently,Canfield et al.(1998,2001)have developed an argument based on the S isotopic composition of biogenic sedimentary sulfides,which reflect SO42-availabil
50、ity and redox conditions at their time of formation.When the availability of SO42-is strongly limited(SO42-concentration 45 between sedimentary sulfides and sulfates may indicate increased oxygenation of the environment.,Canfield and Thamdrup(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
