同位素講座-1-2010

1、穩(wěn)定同位素地球化學(xué)穩(wěn)定同位素地球化學(xué)儲(chǔ)雪蕾（第一講）（第一講）I. I. 穩(wěn)定同位素基本原理穩(wěn)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. 穩(wěn)定同位素地球化學(xué)的誕生、發(fā)展離不開上個(gè)世紀(jì)3040年代兩位著名的科學(xué)家：Harold Urey(Univ. of Chicago)和 Alfred Nier(Univ. of Minnesota)的貢獻(xiàn)。 1934年諾貝爾化學(xué)獎(jiǎng)獲得者Urey奠定了同位素取代的物理化學(xué)性質(zhì)變化的理論基礎(chǔ)，并把它用于地球科學(xué)。1946年他在英國皇家學(xué)會(huì)上發(fā)表了“The Thermodynamic Properties of

6、Isotopic Substances”，并理論上預(yù)示CaCO3和H2O的氧同位素比值（18O/16O）只依賴于溫度的變化，提出了在海洋古溫度上的應(yīng)用。 他與Epstein、McCrea建立了第一個(gè)碳酸鹽的氧同位素實(shí)驗(yàn)室。 實(shí)現(xiàn)同位素分析始于質(zhì)譜儀的發(fā)明與設(shè)計(jì)，Nier的貢獻(xiàn)是最顯著的。 他設(shè)計(jì)和改進(jìn)的Nier-型質(zhì)譜儀一直是測定原子量的主要工具，也是測定重元素同位素的儀器，用于放射性同位素地質(zhì)年代學(xué)和地球化學(xué)的研究。 在他和他的同事測定輕元素的同位素組成時(shí)，發(fā)現(xiàn)了較大的變化。他們所測的灰?guī)r比海水富集18O約3%，與Urey通過統(tǒng)計(jì)力學(xué)計(jì)算的分餾系數(shù)一致。因此，一門基于理論、實(shí)驗(yàn)和質(zhì)譜分析技術(shù)

7、的新學(xué)科穩(wěn)定同位素地球化學(xué)誕生了。 穩(wěn)定同位素地球化學(xué)在地球科學(xué)中的應(yīng)用：1）同位素地質(zhì)溫度計(jì)；2）示蹤劑（包括確定物質(zhì)來源，物理化學(xué)條件與地質(zhì)過程機(jī)制，等）。測定穩(wěn)定同位素比值主要用氣體離子源的同位素質(zhì)譜儀。采用雙進(jìn)樣同位素比值質(zhì)譜儀，由于屬大型儀器、貴重，只有國家級(jí)科研院、所的實(shí)驗(yàn)室從事這方面測試與研究。本課程的內(nèi)容主要是介紹穩(wěn)本課程的內(nèi)容主要是介紹穩(wěn)定同位素地球化學(xué)原理與應(yīng)用，定同位素地球化學(xué)原理與應(yīng)用，重點(diǎn)介紹重點(diǎn)介紹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類同位素的分類： (1) 放射性同位素：原子核不穩(wěn)定，能自發(fā)進(jìn)放射性同

8、位素：原子核不穩(wěn)定，能自發(fā)進(jìn)行放射性衰變或核裂變，而轉(zhuǎn)變?yōu)槠渌活惡怂匦蟹派湫运プ兓蚝肆炎?，而轉(zhuǎn)變?yōu)槠渌活惡怂氐耐凰胤Q為放射性同位素。的同位素稱為放射性同位素。 (2) 穩(wěn)定同位素：原子核穩(wěn)定，其本身不會(huì)自穩(wěn)定同位素：原子核穩(wěn)定，其本身不會(huì)自發(fā)進(jìn)行放射性衰變或核裂變的同位素。發(fā)進(jìn)行放射性衰變或核裂變的同位素。 同位素的定義同位素的定義 同位素定義：核內(nèi)質(zhì)子數(shù)相同而中子數(shù)不同的同同位素定義：核內(nèi)質(zhì)子數(shù)相同而中子數(shù)不同的同一類原子。一類原子。傳統(tǒng)的穩(wěn)定同位素非傳統(tǒng)的穩(wěn)定同位素本課程 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.同位素效應(yīng)同位素效應(yīng)(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 = 重同位素豐度重同位素豐度/輕同位素豐度輕同位素豐度同位素分餾系數(shù)（同位素分餾系數(shù)（ Isotope fractionation factor): A-B

13、 = RA/R B 即即 值，值，表示某元素的同位素在兩種物質(zhì)表示某元素的同位素在兩種物質(zhì)（A和和B）之間的分餾的程度。）之間的分餾的程度。 同位素分餾（同位素分餾（Isotope fractionation)： 同位素在不同物質(zhì)或不同物相間分配比例同位素在不同物質(zhì)或不同物相間分配比例不同的現(xiàn)象稱之為同位素分餾。不同的現(xiàn)象稱之為同位素分餾。 值：值： 樣品的同位素比值相對(duì)于標(biāo)準(zhǔn)樣品同位素比值樣品的同位素比值相對(duì)于標(biāo)準(zhǔn)樣品同位素比值的千分偏差的千分偏差 （）= （R樣樣 R標(biāo)標(biāo)）/ R標(biāo)標(biāo)）X1000 = （R樣樣/R標(biāo)標(biāo)）- 1）X1000 R樣樣：樣品的同位素比值：樣品的同位素比值 R標(biāo)標(biāo)：

14、標(biāo)準(zhǔn)的同位素比值：標(biāo)準(zhǔn)的同位素比值 0 表明樣品相對(duì)標(biāo)準(zhǔn)富集重同位素表明樣品相對(duì)標(biāo)準(zhǔn)富集重同位素 0 表明樣品相對(duì)標(biāo)準(zhǔn)虧損重同位素表明樣品相對(duì)標(biāo)準(zhǔn)虧損重同位素 = 0 表明樣品與標(biāo)準(zhǔn)同位素比值相同表明樣品與標(biāo)準(zhǔn)同位素比值相同穩(wěn)定同位素標(biāo)準(zhǔn)穩(wěn)定同位素標(biāo)準(zhǔn)2D/1H ：Standard mean of ocean water (標(biāo)準(zhǔn)平均大洋水)18O /16O ：Standard mean of ocean water (標(biāo)準(zhǔn)平均大洋水)PDB：Belemnitella Americana（美國北卡羅來納州白堊系Pee Dee建造美洲似箭石）13C/12C PDB：Belemnitella Ame

15、ricana（美國北卡羅來納州白堊系Pee Dee建造美洲似箭石）34S/32S CDT：美國亞利桑那州Canyon Diablo鐵隕石中的隕硫鐵(FeS)樣品的值的計(jì)算需要引入一個(gè)標(biāo)準(zhǔn)。在對(duì)于樣品的同位素組成進(jìn)行比較時(shí)，必須采用同一的標(biāo)準(zhǔn)。國際選定的標(biāo)準(zhǔn)如下：穩(wěn)定同位素標(biāo)準(zhǔn)穩(wěn)定同位素標(biāo)準(zhǔn)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同位素比值參考標(biāo)準(zhǔn)豐度比值鑒于原有的國際標(biāo)準(zhǔn)已用盡，國際原子能機(jī)構(gòu)制做了下鑒于原有的國際標(biāo)準(zhǔn)已用盡，國際原子能機(jī)構(gòu)制做了下述標(biāo)準(zhǔn)供使用。目前，發(fā)表論文可用原標(biāo)準(zhǔn)和現(xiàn)標(biāo)準(zhǔn)兩種方述標(biāo)準(zhǔn)供使用。目前，發(fā)表論文可用原標(biāo)準(zhǔn)和現(xiàn)標(biāo)準(zhǔn)兩種方式發(fā)表，但推薦用現(xiàn)標(biāo)準(zhǔn)（即式發(fā)表，但推薦用現(xiàn)標(biāo)準(zhǔn)（即V標(biāo)準(zhǔn)）發(fā)表。標(biāo)準(zhǔn)）發(fā)表。2 同位素分餾機(jī)理同位素分餾機(jī)理從嚴(yán)格意義上講，在周期表中所有元從嚴(yán)格意義上講，在周期表中所有元素的不同種同位素由于其質(zhì)量

17、上存在差別，素的不同種同位素由于其質(zhì)量上存在差別，在自然界的各種在自然界的各種物理物理，化學(xué)化學(xué)和和生物生物的反應(yīng)的反應(yīng)和過程中都會(huì)發(fā)生同位素分餾。這些反應(yīng)和過程中都會(huì)發(fā)生同位素分餾。這些反應(yīng)和過程包括：蒸發(fā)作用，擴(kuò)散作用，吸附和過程包括：蒸發(fā)作用，擴(kuò)散作用，吸附作用，化學(xué)反應(yīng)，生物化學(xué)反應(yīng)等等。作用，化學(xué)反應(yīng)，生物化學(xué)反應(yīng)等等。自然界存在三種類型的同位素分餾: 平衡分餾平衡分餾（equilibrium fractionation） 動(dòng)力（學(xué)）分餾動(dòng)力（學(xué)）分餾（kinetic fractionation） 非質(zhì)量依賴分餾非質(zhì)量依賴分餾（mass-independent fractionati

18、on)同位素分餾的類型同位素分餾的類型- 主要由同位素取代所造成的氣體、液體的分子和固體晶格中原子的振動(dòng)能的差異造成- 動(dòng)能的差異與質(zhì)量有關(guān)- 體系趨向能態(tài)最低- 共價(jià)鍵具有大的平衡分餾，而離子鍵平衡分餾小，通?？珊雎岳纾阂訵illiam Whites textbook(CornellUniv. )most imp.在 25C達(dá)到平衡時(shí)， CO2的18O/16O比值比 H2O 高。22221.0233COCOH OH ORR平衡分餾平衡分餾 (Equilibrium fractionation)這什么會(huì)出現(xiàn)平衡分餾？這什么會(huì)出現(xiàn)平衡分餾？哪個(gè)化學(xué)鍵容易被打破？-重同位素的分子具有比輕同位素

19、的分子低的零點(diǎn)能。-勢(shì)能越高越容易脫離勢(shì)阱，結(jié)合的鍵也越容易破裂。-重同位素具有比輕同位素更強(qiáng)的結(jié)合能，即化學(xué)鍵能大，或鍵強(qiáng)度高。為什么與溫度有關(guān)?-輕、重同位素分子零點(diǎn)能差異隨溫度增加而減少。-鍵能在非常高的溫度下趨近一致，所以同位素分餾系數(shù)將會(huì)趨近于1，即不產(chǎn)生分餾。zero point energy平衡分餾的溫度依賴性平衡分餾的溫度依賴性harmonic oscilllator modelharmonic oscilllator modeldatadata簡諧振蕩模型給出簡諧振蕩模型給出ln 高溫下高溫下與與T T2 2成反比，成反比，低溫下與低溫下與T T成反比。成反比。11T（T20

20、0 C ）211T因此，在較低的溫度上因此，在較低的溫度上會(huì)有更嚴(yán)重的同位素分會(huì)有更嚴(yán)重的同位素分餾。餾。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)富集規(guī)律（平衡分餾）富集規(guī)律（平衡分餾）規(guī)律：重同位素相對(duì)富集在化學(xué)鍵強(qiáng)或能態(tài)最低的物相中。同位素平衡分餾小結(jié) 不同物質(zhì)或物相間的同位素比值達(dá)到恒定不變時(shí)，即達(dá)到了同位素平衡狀態(tài)，這種狀態(tài)的分餾稱為同位素平衡分餾。 一旦同位素平衡狀態(tài)建立后，只要體系的物理化學(xué)性質(zhì)不變化，則在不同礦物或物相中同位素組成就維持不變，這是同位素平衡分餾的特點(diǎn)。 同位素平衡分餾與路徑、同位素交換速率、壓力等都無關(guān)，而僅與溫度有關(guān)。同位素平衡分餾的研究只考慮過程的始態(tài)與終態(tài)，對(duì)其演化過程及時(shí)間不予考慮。因此，同位素平衡分餾又稱熱力學(xué)分餾，

23、是同位素地質(zhì)溫度計(jì)的理論依據(jù)。動(dòng)力分餾動(dòng)力分餾 (Kinetic fractionation)起因：由速度、單向、不完全的反應(yīng)或過程引起（包括生物為媒介的反應(yīng)或過程）。例如：伴隨著蒸發(fā)過程、擴(kuò)散過程、分解反應(yīng)過程，及光合 過程等等發(fā)生的同位素分餾都屬于動(dòng)力分餾。由于輕同位素取代具有相對(duì)高的勢(shì)能，因此它相對(duì)由于輕同位素取代具有相對(duì)高的勢(shì)能，因此它相對(duì)“活潑活潑”，優(yōu)先反應(yīng)。，優(yōu)先反應(yīng)。-例如，C-H鍵比C-D鍵容易破裂，它容易反應(yīng)。-反應(yīng)沒有達(dá)到平衡時(shí)，輕同位素相對(duì)富集在產(chǎn)物中，而重同位素則在反應(yīng)物中相對(duì)富集。-通常生物為媒介的氧化還原反應(yīng)中會(huì)產(chǎn)生大的動(dòng)力分餾，例如：光合作用生成的有機(jī)體貧13C

24、，細(xì)菌還原產(chǎn)生的硫化物貧34S。212kEmv考慮兩個(gè) CO2分子: 12C16O2 (質(zhì)量數(shù) = 12 + 2*16 = 44) 13C16O2 (質(zhì)量數(shù)= 13 + 2*16 = 45)假定為理想氣體，動(dòng)能相同時(shí)則:它們的速度比:221122AABBm vm v1/21/2451.01144ABBAvmvm如此，如此， 12C16O2 比比13C16O2 擴(kuò)散的速度要快擴(kuò)散的速度要快 1.1% 。不是理想氣體，由于氣體的碰撞使這兩種分子運(yùn)動(dòng)速度的差異減小，分餾減小。氣體分子的速度差異- 理想氣體的動(dòng)能是相同的。- 因此，重同位素與輕同位素的質(zhì)量之不同是通過速度來補(bǔ)嘗的，即同位素動(dòng)力分餾小結(jié)

25、 一些物理一些物理-化學(xué)（如蒸發(fā)、擴(kuò)散、單向或未完成的化學(xué)化學(xué)（如蒸發(fā)、擴(kuò)散、單向或未完成的化學(xué)反應(yīng)等）過程和生物（如光合作用、呼吸作用和細(xì)菌硫反應(yīng)等）過程和生物（如光合作用、呼吸作用和細(xì)菌硫酸鹽還原等）過程中伴隨發(fā)生的同位素分餾稱之為同位酸鹽還原等）過程中伴隨發(fā)生的同位素分餾稱之為同位素動(dòng)力分餾。這些過程往往受素動(dòng)力分餾。這些過程往往受化學(xué)反應(yīng)動(dòng)力學(xué)化學(xué)反應(yīng)動(dòng)力學(xué)控制，其控制，其造成的同位素分餾受造成的同位素分餾受擴(kuò)散速度擴(kuò)散速度或或反應(yīng)速度反應(yīng)速度控制，依賴于控制，依賴于路徑路徑、時(shí)間時(shí)間與與速度速度。 生物參與的化學(xué)過程，一般同位素動(dòng)力分餾明顯，這生物參與的化學(xué)過程，一般同位素動(dòng)力分餾明

26、顯，這在在C和和S同位素分餾的研究中占有重要位置。同位素分餾的研究中占有重要位置。Closed- and open-system fractionation瑞利同位素分餾瑞利同位素分餾(Rayleigh isotope fractionation)推導(dǎo)：Thiemens and Heidenreich, 1983; Theimens, 1999 (review)在隕石、大氣光化學(xué)反應(yīng)的產(chǎn)物中觀察到了非質(zhì)量依賴同位素分餾。非質(zhì)量依賴分餾要通過三個(gè)或三個(gè)以上同位素的體系研究來確定，如16O、17O和18O體系；32S、33S、34S和36S體系。機(jī)制是光子的量子效應(yīng)造成光化學(xué)反應(yīng),或自由基參與的化

27、學(xué)反應(yīng)。這些反應(yīng)與同位素的質(zhì)量無關(guān)。用途：天體化學(xué)、地球早期大氣氧的增加、大氣化學(xué)（如氣溶膠）等。非質(zhì)量依賴分餾非質(zhì)量依賴分餾 (Mass-independent fractionation)質(zhì)量相關(guān)定則質(zhì)量相關(guān)定則 對(duì)于小的同位素分餾（20）的三同位體系的同位素比值是各種同位素質(zhì)量倒數(shù)之差的函數(shù)。 如分子氧（氧氣）來講有三種穩(wěn)定同位素：16O16O、16O17O和16O18O，遵守質(zhì)量相關(guān)定則的地球上物質(zhì)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地球樣品普地球樣品普遍滿足遍滿足質(zhì)量相質(zhì)量相關(guān)分餾線關(guān)分餾線或或質(zhì)質(zhì)量分餾線量分餾線。質(zhì)量分餾線質(zhì)量分餾線的的斜率在斜率在0.5000.500到到0.5260.526范圍內(nèi)。范圍內(nèi)。質(zhì)量分餾線質(zhì)量分餾線D D33S和和D D36S定義定義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、的質(zhì)量相關(guān)和非質(zhì)量相關(guān)同位素分餾硫的質(zhì)量相關(guān)和非質(zhì)量相關(guān)同位素分餾3 同位素地質(zhì)溫度計(jì)原理同位素地質(zhì)溫度計(jì)原理 值值： （）= (R樣樣/ R標(biāo)標(biāo)）- 1）X 1000 同位素分餾系數(shù)同位素分餾系數(shù) 與與 值的關(guān)系：值的關(guān)系： 103 ln A-B A - B = D D A-B 即即ln A-B與與A, B兩種物質(zhì)的兩種物質(zhì)的 值之差相關(guān)。值之差相關(guān)。 同位素平衡分餾系數(shù)與溫度的關(guān)系同位素平衡分餾系數(shù)與溫度的關(guān)系： 103 ln = a/T2 + b/T + c （T: K) 其中其中a，b，c 分別為常數(shù)。分別為常數(shù)。 1）在一般低溫下，）在一般低溫下，a/T2可以忽略，簡化：可以忽略，簡

30、化： 103 ln = b/T + c 2）在高溫下，）在高溫下，b/T可以忽略，簡化：可以忽略，簡化： 103 ln = a/T2 + c 4 同位素樣品制備與質(zhì)譜分析同位素樣品制備與質(zhì)譜分析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. 硫同位素地球化學(xué)硫同位素地球化學(xué)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