不同检测方法对氢氧同位素分馏的影响

氢氧同位素的检测方法由最初的离线双路进样同位素比质谱法(Dual-inlet IRMS),发展到自动化程度较高的连续流水平衡法(Gasbench-IRMS)检测方法以及现阶段正在研究使用的热转换元素分析同位素比质谱法(TC/EA-IRMS)。为了探讨不同检测方法对氢氧同位素分馏的影响以及各方法的优缺点,文章应用Dual-inlet IRMS、GasbenchⅡ-IRMS、TC/EA-IRMS三种检测方法对四种不同水样的氢氧同位素进行检测,并用国际标准和国家标准对检测结果进行校正。结果表明,Dual-inlet IRMS法检测氢同位素的精密度高,重现性好;Gasbench-IRMS法检测氢同位素的结果重现性较差;Dual-inlet IRMS和Gasbench-IRMS法检测氧同位素要比TC/EA-IRMS法的精密度高,重现性好。用TC/EA-IRMS法检测氢氧同位素,分别用国际标准和国家标准校正,δD值的最大绝对偏差为1.13‰,δ18O值的最大绝对偏差为0.27‰。测定不同水样的氢氧同位素时,连续流GasbenchⅡ-IRMS测定氧同位素较有优势,而TC/EA-IRMS测定氢同位素比较有优势。样品测试过程中选用的校正标准不同,检测结果也存在一定的误差。     

一种新的、快速的碳、氮、硫同位素测定手段──EA－IRMS连线分析技术

一种新的、快速的碳、氮、硫同位素测定手段──ＥＡ－ＩＲＭＳ连线分析技术储雪蕾（中国科学院地质研究所，北京１０００２９）关键词元素分析仪－同位素比值质谱仪，连线分析碳，氮，硫同位素碳、氮、硫三种元素的稳定同位素在地学、生态学、环境科学等领域受到了重视，...     

三种方法测试岩溶水样氢氧同位素的对比研究

通过高温热转换元素-同位素比值质谱法（TC/EA-IRMS）、多用途气体制备仪-同位素比值质谱法（GasbenchⅡ-IRMS）以及激光光谱法对岩溶水样进行对比检测,其结果显示:对于氢同位素,TC/EA-IRMS的精密度达到0.3 ‰,激光光谱法的精密度达到0.1 ‰,均优于GasbenchⅡ-IRMS的精密度1.4 ‰;对于氧同位素,GasbenchⅡ-IRMS的精密度达到0.02 ‰,激光光谱法的精密度达到0.04 ‰,优于TC/EA-IRMS的精密度0.16 ‰。使用激光光谱法测定岩溶水样的氢氧同位素,所需要的样品量少,精密度高,能够满足岩溶区样品的高精度测试要求。      

元素分析仪-气体同位素质谱法分析硫酸钙样品的硫同位素组成

硫酸盐硫同位素的常规分析方法是将硫酸盐转化为硫酸钡后搭配双路进样SO2法,该法易于操作、数据稳定,但样品用量大、费时费力,需要繁杂的前处理,无法满足微量分析发展方向的需求。本文以石膏为例,以元素分析仪-气体同位素质谱法(EA-IRMS)直接测定硫酸钙样品硫同位素比值,对同一样品分别采用:①硫酸钙与V2O5混合后包裹于锡杯中密封,直接进行元素分析仪-气体同位素质谱分析;②硫酸钙充分溶于去离子水中,向溶有硫酸钙样品的液体中加入沉淀试剂BaCl2,将生成的硫酸钡沉淀滤出后,用去离子水清洗2~3遍,烘干后与V2O5混合包裹于锡杯中密封再进行质谱测定。实验选取了13件δ34S值变化范围介于-20‰^+30‰之间的天然石膏样品,将获得的硫同位素比值进行对比,二者δ34SV-CDT绝对差值在0.00‰~0.24‰,表明同一样品的硫同位素比值结果在误差范围内基本一致。与常规分析方法相比,本文建立的直接在线分析时无需任何化学前处理,只需直接加入适量的V2O5,V2O5和氧气中的外部氧在瞬间燃烧的过程中替代了硫酸钙本身的氧,生成的SO2气体的氧是均一的,其硫同位素比值能代表样品的硫同位素组成,无需进行氧同位素的校正。经过验证表明,硫酸钙样品的直接在线分析是完全可行的。     

微量水氢氧同位素在线同时测试技术——热转换元素分析同位素比质谱法

水中氢氧同位素组成的时空差异性,为水循环、古气候环境反演、水源识别等研究提供了一种非常有效的技术手段。文章采用热转换元素分析同位素比质谱法(TC/EA-IRMS),实现了在线单次分析过程中同时测定微量水的δD和1δ8O,用样量仅需0.2μL;δD和1δ8O的分析精度分别为0.81‰和0.06‰。通过分类测试(δD值相差小于50‰的水样和标准水列为同一类进行测试)等措施可消除记忆效应,同时实现对测试结果的精确校正,而无需准确标定参考气和测定H3+因子。     

Geochemistry of О, Н, C,S, and Sr isotopes in the water and sediments of the Aral basin

The paper presents original authors' data on the O, H, C, S, and Sr isotopic composition of water and sediments from the basins into which the Aral Sea split after its catastrophic shoaling: Chernyshev Bay (CB), the basin of the Great Aral in the north, Lake Tshchebas (LT), and Minor Sea (MS). The data indicate that the δ¹⁸О, δD, δ¹³C, and δ³⁴S of the water correlate with the mineralization (S) of the basins (as of 2014): for CB, S = 135.6‰, δ¹⁸О = 4.8 ± 0.1‰, δD = 5 ± 2‰, δ¹³C (dissolved inorganic carbon, DIC) = 3.5 ± 0.1‰, δ³⁴S = 14.5‰; for LT, S = 83.8‰, δ¹⁸О = 2.0 ± 0.1‰, δD =–13.5 ± 1.5‰, δ¹³C = 2.0 ± 0.1‰, δ³⁴S = 14.2‰; and for MS, S = 9.2‰, δ¹⁸О =–2.0 ± 0.1‰, δD =–29 ± 1‰, δ¹³C =–0.5 ± 0.5‰, δ³⁴S = 13.1‰. The oxygen and hydrogen isotopic composition of the groundwaters are similar to those in MS and principally different from the artesian waters fed by atmospheric precipitation. The mineralization, δ¹³С, and δ³⁴S of the groundwaters broadly vary, reflecting interaction with the host rocks. The average δ¹³С values of the shell and detrital carbonates sampled at the modern dried off zones of the basins are similar: 0.8 ± 0.8‰ for CB, 0.8 ± 1.4‰ for LT, and –0.4 ± 0.3‰ for MS. The oxygen isotopic composition of the carbonates varies much more broadly, and the average values are as follows: 34.2 ± 0.2‰ for CB, 32.0 ± 2.2‰ for LT, and 28.2 ± 0.9‰ for MS. These values correlate with the δ¹⁸O of the water of the corresponding basins. The carbonate cement of the Late Eocene sandstone of the Chengan Formation, which makes up the wave-cut terrace at CB, has anomalously low δ¹³С up to –38.5‰, suggesting origin near a submarine methane seep. The δ³⁴S of the mirabilite and gypsum (11.0 to 16.6‰) from the bottom sediments and young dried off zone also decrease from CB to MS in response to increasing content of sulfates brought by the Syr-Darya River (δ³⁴S = 9.1 to 9.9‰) and weakening sulfate reduction. The ⁸⁷Sr/⁸⁶Sr ratio in the water and carbonates of the Aral basins do not differ, within the analytical error, and is 0.70914 ± 0.00003 on average. This value indicate that the dominant Sr source of the Aral Sea is Mesozoic–Cenozoic carbonate rocks. The Rb–Sr systems of the silicate component of the bottom silt (which is likely dominated by eolian sediments) of MS and LT plot on the Т = 160 ± 5 Ma, I₀ = 0.7091 ± 0.0001, pseudochron. The Rb–Sr systems of CB are less ordered, and the silt is likely a mixture of eolian and alluvial sediments.     

Zinc Isotopic Compositions of NIST SRM 683 and Whole‐Rock Reference Materials

In this study the homogeneity of the zinc isotopic composition in the NIST SRM 683 reference material was examined by measuring the Zn isotopic signature in microdrilled sample powders from two metal nuggets. Zinc was purified using AG MP‐1M resin and then measured by MC‐ICP‐MS. Instrumental mass bias was corrected using the “sample‐standard bracketing” method and empirical external normalisation with Cu doping. After evaluating the potential effects of varying acid mass fractions and different matrices, high‐precision Zn isotope data were obtained with an intermediate measurement precision better than ± 0.05‰ (δ⁶⁶Zn, 2s) over a period of 5 months. The δ⁶⁶Zn_JMC‐Lyon mean values of eighty‐four and fourteen drilled powders from two nuggets were 0.11 ± 0.02‰ and 0.12 ± 0.02‰, respectively, indicating that NIST SRM 683 is a good isotopic reference material with homogeneous Zn isotopes. The Zn isotopic compositions of seventeen rock reference materials were also determined, and their δ⁶⁶Zn values were in agreement with most previously published data within 2s. The δ⁶⁶Zn values of most of the rock reference materials analysed were in the range 0.22–0.36‰, except for GSP‐2 (1.07 ± 0.06‰, n = 12), NOD‐A‐1 (0.96 ± 0.03‰, n = 6) and NOD‐P‐1 (0.78 ± 0.03‰, n = 6). These comprehensive data should serve as reference values for quality assurance and interlaboratory calibration exercises.     

Off‐Mount Calibration and One New Potential Pyrrhotite Reference Material for Sulfur Isotope Measurement by Secondary Ion Mass Spectrometry

Sulfur isotope measurements in three sulfide (two pyrite and one pyrrhotite) samples on two epoxy mounts showed that the mount‐to‐mount variation of raw δ³⁴S values was negligible when secondary ion mass spectrometry (SIMS) analytical settings remained stable. In consequence, an off‐mount calibration procedure for SIMS sulfur isotope analysis was applied in this study. YP136 is a pyrrhotite sample collected from northern Finland. Examination of thin sections with a polarising microscope, backscattered electron image analyses and wavelength dispersive spectrometry mapping showed that the sample grains display no internal growth or other zoning. A total of 318 sulfur isotope (spot) measurements conducted on more than 100 randomly selected grains yielded highly consistent sulfur isotope ratios. The repeatability of all the analytical results of ³⁴S/³²S was 0.3‰ (2s, n = 318), which is the same as that of the well‐characterised pyrite reference materials PPP‐1 and UWPy‐1. Its δ³⁴S value determined by gas mass spectrometry was 1.5 ± 0.1‰ (2s, n = 11), which agrees with the SIMS data (1.5 ± 0.3‰, 2s) calibrated by pyrrhotite reference material Po‐10. Therefore, YP136 pyrrhotite is considered a candidate reference material for in situ sulfur isotope determination.     

Accurate and High‐Precision Determination of Boron Isotopic Ratios at Low Concentration by MC‐ICP‐MS (Neptune)

We report an approach for the accurate and reproducible measurement of boron isotope ratios in natural waters using an MC‐ICP‐MS (Neptune) after wet chemistry sample purification. The sample matrix can induce a drastic shift in the isotopic ratio by changing the mass bias. It is shown that, if no purification is carried out, the direct measurement of a seawater diluted one hundred times will induce an offset of ?7‰ in the isotopic ratio, and that, for the same concentration, the greater the atomic mass of the matrix element, the greater the bias induced. Whatever the sample, it is thus necessary to remove the matrix. We propose a method adapted to water samples allowing purification of 100 ng of boron with a direct recovery of boron in 2 ml of 3% v/v HNO₃, which was our working solution. Boron from the International Atomic Energy Agency IAEA‐B1 seawater reference material and from the two groundwater reference materials IAEA‐B2 and IAEA‐B3, was chemically purified, as well as boron from the certified reference material NIST SRM 951 as a test. The reproducibility of the whole procedure (wet chemistry and MC‐ICP‐MS measurement) was ± 0.4‰ (2s). Accuracy was verified by comparison with positive‐TIMS values and with recommended values. Seawater, being homogeneous for boron isotope ratios, is presently the only natural water material that is commonly analysed for testing accuracy worldwide. We propose that the three IAEA natural waters could be used as reference samples for boron isotopes, allowing a better knowledge of their isotopic ratios, thus contributing to the certification of methods and improving the quality of the boron isotopic ratio measurements for all laboratories.     

Boron Isotope Analysis of Silicate Glass with Very Low Boron Concentrations by Secondary Ion Mass Spectrometry

We present an improved method for the determination of the boron isotopic composition of volcanic glasses with boron concentrations of as low as 0.4–2.5 μg g^?1, as is typical for mid‐ocean ridge basalt glasses. The analyses were completed by secondary ion mass spectrometry using a Cameca 1280 large‐radius ion microprobe. Transmission and stability of the instrument and analytical protocol were optimised, which led to an improvement of precision and reduction in surface contamination and analysis time compared with earlier studies. Accuracy, reproducibility (0.4–2.3‰, 2 RSD), measurement repeatability (2 RSE = 2.5–4.0‰ for a single spot with [B] = 1 μg g^?1), matrix effects (? 0.5‰ among komatiitic, dacitic and rhyolitic glass), machine drift (no internal drift; long‐term drift: ~ 0.1‰ hr^?1), contamination (~ 3–8 ng g^?1) and machine background (0.093 s^?1) were quantified and their influence on samples with low B concentrations was determined. The newly developed set‐up was capable of determining the B isotopic composition of basaltic glass with 1 μg g^?1 B with a precision and accuracy of ± 1.5‰ (2 RSE) by completing 4–5 consecutive spot analyses with a spatial resolution of 30 μm × 30 μm. Samples with slightly higher concentrations (≥ 2.5 μg g^?1) could be analysed with a precision of better than ± 2‰ (internal 2 RSE) with a single spot analysis, which took 32 min.     

Sulphur isotopic investigation of vein lead-zinc mineralization at Tyndrum,Scotland

The Tyndrum Pb+Zn veins, hosted by late Proterozoic quartzites, were probably generated in the Tournaisian (360 Ma). By determination of sulphur isotopic ratios of vein minerals three aspects of the Tyndrum mineralization were addressed, (i) sulphate sulphur sources; (ii) reduced sulphur source; (iii) isotopic equilibrium in the vein system including geothermometry. Twelve galenas have δ³⁴S values ranging from +3.55 ‰ to +6.38 ‰ (this excludes one value of +11.21 ‰ from a large but nearly barren quartz vein). Other sulphides are enriched or depleted in ³⁴S in the sense expected for isotopic equilibrium although there is no evidence for isotopic equilibrium between the vein minerals. The sulphide sulphur source was probably in the Dalradian metasediments where disseminated pyrite averages +6 ‰. Baryte had δ³⁴S values averaging 14 ‰ and was therefore not in isotopic equilibrium with sulphides: a continental groundwater source is most likely.     

Magnesium Isotope Compositions of Natural Reference Materials

This study presents a chemical protocol for the separation of Mg that is particularly adapted to alkali‐rich samples (granite, soil, plants). This protocol was based on a combination of two pre‐existing methods: transition metals were first removed from the sample using an AG‐MP1 anion‐exchange resin, followed by the separation of alkalis (Na, K) and bivalent cations (Ca²⁺, Mn²⁺ and Sr²⁺) using a AG50W‐X12 cation‐exchange resin. This procedure allowed Mg recovery of ～ 10 0 ± 8%. The [Σcations]/[Mg] molar ratios in all of the final Mg fractions were lower than 0.05. The Mg isotope ratios of eleven reference materials were analysed using two different MC‐ICP‐MS instruments (Isoprobe and Nu Plasma). The long‐term reproducibility, assessed by repeated measurements of Mg standard solutions and natural reference materials, was 0.14‰. The basalt (BE‐N), limestone (Cal‐S) and seawater (BCR‐403) reference materials analysed in this study yielded δ²⁶Mg mean values of ?0.28 ± 0.08‰, ?4.37 ± 0.11‰ and ?0.89 ± 0.10‰ respectively, in agreement with published data. The two continental rocks analysed, diorite (DR‐N) and granite (GA), yielded δ²⁶Mg mean values of ?0.50 ± 0.08‰ and ?0.75 ± 0.14‰, respectively. The weathering products, soil (TILL‐1) and river water (NIST SRM 1640), gave δ²⁶Mg values of ?0.40 ± 0.07‰ and ?1.27 ± 0.14‰, respectively. We also present, for the first time, the Mg isotope composition of bulk plant and organic matter. Rye flour (BCR‐381), sea lettuce (Ulva lactuva) (BCR‐279), natural hairgrass (Deschampsia flexuosa) and lichen (BCR‐482) reference materials gave δ²⁶Mg values of ?1.10 ± 0.14‰, ?0.90 ± 0.19‰, ?0.50 ± 0.22‰ and ?1.15 ± 0.27‰ respectively. Plant δ²⁶Mg values fell within the range defined by published data for chlorophylls.     

小秦岭镰子沟金矿床地质特征、黄铁矿原位硫同位素组成及成因意义

镰子沟金矿床位于华北陆块南缘小秦岭金矿矿集区西部,矿体赋存于太华群上部岩层内,受断裂构造和石英脉控制,带状钾长石化是矿床典型的围岩蚀变,矿物组合为石英-黄铁矿-方铅矿-黄铜矿±重晶石±磁铁矿。为了探讨成矿物质来源及矿床成因,采用LA-MC-ICP-MS技术对镰子沟金矿床黄铁矿进行原位微区硫同位素分析,获得单颗粒黄铁矿的硫同位素变化范围为-15.27‰~-11.98‰,平均-13.35‰,小于共生重晶石硫同位素值9.8‰~12.4‰。根据硫化物与共生硫酸盐矿物,估算成矿热源总硫值为-3.6‰,与新太古界太华群和燕山期华山花岗岩均不同。综合矿化蚀变、地球化学及同位素组成,认为太华群对金矿床成矿物质来源的影响较华山花岗岩明显,但非主要来源,金矿床成矿与深部流体或隐伏岩体有关,矿床受深部流体和构造控制,深部仍有寻找构造蚀变型或微细浸染型金矿体的潜力,但规模有限。     

元素分析仪-同位素比值质谱仪测定海洋沉积物有机碳稳定同位素方法初探

初步建立了利用元素分析仪-同位素比值质谱仪(EA-IRMS)联用技术测定海洋沉积物中有机碳稳定同位素的方法。标定了标准工作参考气CO2(δ13CvsPDB为-32.053‰);验证了仪器的稳定性,标准偏差为0.009‰;当离子流强度范围为0.4～8.7 V时,总体线性为0.0337‰/V,小于仪器线性指标0.06‰/V的要求;同时测定了国家标准物质GBW 04408、国际标准物质Urea和海洋沉积物样品M01的精密度和准确度,标准偏差在0.04‰～0.13‰范围内;并在三家实验室进行了测量比对实验,标准偏差小于0.20‰,满足地质样品再现性0.5‰的要求。所选样品区域δ13Corg范围为-25.29‰～-22.30‰,表明该海域总有机碳是陆源和海源两种来源的混合物。     

Iron Isotopic Compositions of Geological Reference Materials and Chondrites

High‐precision iron isotopic compositions for Fe‐bearing geological reference materials and chondrites with a wide range of matrices (e.g., silicates, oxides, organic‐bearing materials) are reported. This comprehensive data set should serve as a reference for iron isotopic studies across a range of geological and biological disciplines for both quality assurance and inter‐laboratory calibration. Where comparison is available, the iron isotopic compositions of most geological reference materials measured in this study were in agreement with previously published data within quoted uncertainties. Recommendations for the reporting of future iron isotopic data and associated uncertainties are also presented. Long‐term repeat analyses of all samples indicate that highly reproducible iron isotopic measurements are now obtainable (± 0.03‰ and ± 0.05‰ for δ⁵⁶Fe and δ⁵⁷Fe, respectively).     

Isotopic compositions of C,O, Sr,and S and problem of ages of the Katera and Uakit Groups,western Transbaikal region

The age of the Katera Group, which occupies a large area in the western North Muya Range and occurs 100–150 km east of the Uakit Group, is a debatable issue. Based on geological correlations with reference sections of the Baikal Group and Patom Complex, the Katera and Uakit groups were previously considered nearly coeval units and assigned to Late Precambrian (Khomentovskii and Postnikov, 2002; Salop, 1964). This was supported partly by the Sm–Nd model datings (Rytsk et al., 2007, 2009, 2011). Finds of the Paleozoic flora substantiated the revision of age of the Uakit Group and its assignment to the Late Devonian–Early Carboniferous (Gordienko et al., 2010; Minina, 2003, 2012, 2014). We have established that Sr and C isotopic compositions in carbonates of these groups differ drastically, as suggested by their different ages. Sediments of the Nyandoni Formation (Katera Group), which contains carbonates characterized by minimum values of ⁸⁷Sr/⁸⁶Sr = 0.7056 and maximum values of δ¹³C = 4.9‰, were accumulated in the first half of Late Riphean (800–850 Ma ago), whereas the overlying Barguzin Formation (⁸⁷Sr/⁸⁶Sr_min = 0.70715, δ¹³C_max= 10.5‰) was deposited at the end of Late Riphean (700–750 Ma). Judging from the isotope data, the Nerunda Formation (Uakit Group), which contains carbonates with characteristics matching the most rigorous criteria of fitness for the chemostratigraphic correlation (Sr content up to 4390 μg/g, Mn/Sr < 0.1, δ¹⁸O = 23.0 ± 1.8‰), was deposited at the end of Vendian ~550–540 Ma ago). The sequence includes thick typical carbonate horizons with very contrast carbon isotopic compositions: the lower unit has anomalous high δ¹³C values (5.8 ± 1.0‰); the upper unit, by anomalous low δ¹³C values (–5.2 ± 0.5‰]). Their Sr isotopic composition is relatively homogeneous (⁸⁷Sr/⁸⁶Sr = 0.7084 ± 0.0001) that is typical of the Late Vendian ocean. The S isotopic composition of pyrites from the Nyandoni Formation (Katera Group) (δ³⁴S = 14.1 ± 6.8‰) and pyrites from the Mukhtunny Formation (Uakit Group) (δ³⁴S = 0.7 ± 1.4‰) does not contradict the C and Sr isotopic stratigraphic data.     

Precise Determination of Silicon Isotopes in Silicate Rock Reference Materials by MC‐ICP‐MS

A HF‐free sample preparation method was used to purify silicon in twelve geological RMs. Silicon isotope compositions were determined using a Neptune instrument multi‐collector‐ICP‐MS in high‐resolution mode, which allowed separation of the silicon isotope plateaus from their interferences. A 1 μg g^‐1 Mg spike was added to each sample and standard solution for online mass bias drift correction. δ³⁰Si and δ²⁹Si values are expressed in per mil (‰), relative to the NIST SRM 8546 (NBS‐28) international isotopic RM. The total variation of δ³⁰Si in the geological reference samples analysed in this study ranged from ‐0.13‰ to ‐0.29‰. Comparison with δ²⁹Si values shows that these isotopic fractionations were mass dependent. IRMM‐17 yielded a δ³⁰Si value of ‐1.41 ± 0.07‰ (2s, n = 12) in agreement with previous data. The long‐term reproducibility for natural samples obtained on BHVO‐2 yielded δ³⁰Si = ‐0.27 ± 0.08‰ (2s, n = 42) on a 12 month time scale. An in‐house Si reference sample was produced to check for the long‐term reproducibility of a mono‐elemental sample solution; this yielded a comparable uncertainty of ± 0.07‰ (2s, n = 24) over 5 months.     

The Cu isotopic signature of granites from the Lachlan Fold Belt,SE Australia

This contribution reports our preliminary work to determine Cu isotope ratios for various granite rocks and examine the Cu isotope systematics within granite suites. A chemical procedure, modified from Maréchal [Maréchal, C.N., Télouk, P. and Albarède, F., 1999. Precise analysis of copper and zinc isotopic compositions by plasma-source mass spectrometry. Chemical Geology, 156(1–4): 251–273.], was used to separate Cu from rock matrix. Quantitative recovery (100.6 ± 1.6%), with a low total procedural blank (2.65 ± 0.66 ng) for Cu, has been achieved, allowing Cu isotopic measurements on samples with as little as 10 ppm Cu. The Cu isotope ratios (δ⁶⁵Cu relative to NIST SRM 976) of 32 rock samples, ranging from mafic to felsic compositions, from 3 batholiths (2 I-type, 1 S-type) from the Lachlan Fold Belt in southeastern Australia, vary from ? 0.46‰ to 1.51‰. Most of them cluster around zero, with mean values for the I-type and S-type granites of 0.03 ± 0.15‰ and ? 0.03 ± 0.42‰ (2 sigma) respectively. These data, together with Cu isotope ratios of two loess samples, provide preliminary evidence that the baseline Cu isotopic composition of the crystalline part of upper continental crust is close to zero. The tight clustering of Cu isotope ratios of rocks from the I-type suites suggests that high-temperature magmatic processes do not produce significant Cu isotope fractionation. However, two granites with abnormally heavy Cu isotope signatures (up to 1.51‰) appears to be the result of localized hydrothermal alteration. Measurable variation in Cu isotopic composition of the S-type granite may reflect isotopic heterogeneity in the sedimentary source region as a result of redox processes or may be due to hydrothermal overprinting. Thus, Cu isotope geochemistry may be a useful tracer for studying hydrothermal alteration and source heterogeneity of granitic rocks.     

Genesis of the Weiquan Ag–Polymetallic Deposit in East Tianshan, China: Evidence from Zircon U–Pb Geochronology and C–H–O–S–Pb Isotope Systematics

The Weiquan Ag-polymetallic deposit is located on the southern margin of the Central Asian Orogenic Belt and in the western segment of the Aqishan-Yamansu arc belt in East Tianshan,northwestern China. Its orebodies, controlled by faults, occur in the lower Carboniferous volcanosedimentary rocks of the Yamansu Formation as irregular veins and lenses. Four stages of mineralization have been recognized on the basis of mineral assemblages, ore fabrics, and crosscutting relationships among the ore veins. Stage I is the skarn stage(garnet + pyroxene), Stage Ⅱ is the retrograde alteration stage(epidote + chlorite + magnetite ± hematite 士 actinolite ± quartz),Stage Ⅲ is the sulfide stage(Ag and Bi minerals + pyrite + chalcopyrite + galena + sphalerite + quartz ± calcite ± tetrahedrite),and Stage IV is the carbonate stage(quartz + calcite ± pyrite). Skarnization,silicification, carbonatization,epidotization,chloritization, sericitization, and actinolitization are the principal types of hydrothermal alteration. LAICP-MS U-Pb dating yielded ages of 326.5±4.5 and 298.5±1.5 Ma for zircons from the tuff and diorite porphyry, respectively. Given that the tuff is wall rock and that the orebodies are cut by a late diorite porphyry dike, the ages of the tuff and the diorite porphyry provide lower and upper time limits on the age of ore formation. The δ~(13)C values of the calcite samples range from-2.5‰ to 2.3‰, the δ~(18)O_(H2 O) and δD_(VSMOW) values of the sulfide stage(Stage Ⅲ) vary from 1.1‰ to 5.2‰ and-111.7‰ to-66.1‰, respectively,and the δ~(13)C, δ~(18)O_(H2 O) and δD_(V-SMOW) values of calcite in one Stage IV sample are 1.5‰,-0.3‰, and-115.6‰, respectively. Carbon, hydrogen, and oxygen isotopic compositions indicate that the ore-forming fluids evolved gradually from magmatic to meteoric sources. The δ~(34)S_(V-CDT) values of the sulfides have a large range from-6.9‰ to 1.4‰, with an average of-2.2‰, indicating a magmatic source, possibly with sedimentary contributions. The ~(206)Pb/~(204)Pb, ~(207)Pb/~(204)Pb, and ~(208)Pb/~(204)Pb ratios of the sulfides are 17.9848-18.2785,15.5188-15.6536, and 37.8125-38.4650, respectively, and one whole-rock sample at Weiquan yields~(206)Pb/~(204)Pb,~(207)Pb/~(204)Pb, and ~(208)Pb/~(204)Pb ratios of 18.2060, 15.5674, and 38.0511,respectively. Lead isotopic systems suggest that the ore-forming materials of the Weiquan deposit were derived from a mixed source involving mantle and crustal components. Based on geological features, zircon U-Pb dating, and C-H-OS-Pb isotopic data, it can be concluded that the Weiquan polymetallic deposit is a skarn type that formed in a tectonic setting spanning a period from subduction to post-collision. The ore materials were sourced from magmatic ore-forming fluids that mixed with components derived from host rocks during their ascent, and a gradual mixing with meteoric water took place in the later stages.     

Determination of the isotopic composition of molybdenum in the bottom sediments of freshwater basins

This paper presents the results of measurements of the Mo isotopic composition in the bottom sediments (BS) of freshwater basins. Mo isotopic ratios were measured using a multicollector inductively coupled plasma mass spectrometer (MC ICP MS). Efficient methods were used in this study for Mo separation from the elements of the sample matrix and correction for instrumental mass discrimination. This allowed us to achieve a high accuracy of 0.06, 0.08, and 0.14‰ (2 σ) for the measurement of ⁹⁷Mo/⁹⁵Mo, ⁹⁸Mo/⁹⁵Mo, and ¹⁰⁰Mo/⁹⁵Mo, respectively. The range of variations in Mo isotope ratios observed in the collected BS columns was ～2.2‰ in terms of δ⁹⁷Mo/⁹⁵Mo. The results obtained here suggest that geochemical processes occurring during Mo migration with land water can change the isotopic composition of Mo. It is pointed out that the potential use of Mo isotopic systematics for reconstructions of redox conditions in seawater over the geologic past requires the quantification of isotopic effects of Mo accompanying its migration on land and the extent of possible variations in the isotopic composition of Mo entering the ocean.