Mercury isotope fractionation during photoreduction in natural water is controlled by its Hg/DOC ratio

Photoreduction of Hg in natural water plays a crucial role in the production of elemental Hg and its biogeochemical cycle. Solar irradiation and dissolved organic carbon (DOC) in water are considered to be the major factors inducing Hg photoreduction. We investigated Hg isotope fractionation during photoreduction and its relationship with Hg/DOC ratios. Both mass dependent (MDF) and mass independent fractionation (MIF) was observed. MIF enriched ¹⁹⁹Hg and ²⁰¹Hg in the reactant Hg(II) and thus, significantly enhanced the fractionation between odd and even isotopes. This direction of MIF is consistent with magnetic isotope effect as the underlying cause for the odd isotope enrichment in reactants. MIF also occurred in dark controls. But in the absence of light, ¹⁹⁹Hg and ²⁰¹Hg were enriched in the product Hg(0), which is not explained by magnetic isotope effects. We propose that nuclear volume effect dominated Hg isotope fractionation under these conditions. The reduction kinetics and isotope fractionation during photoreduction strongly correlated to Hg/DOC concentration ratios. Although different reduction kinetics and fractionation factors were measured at different Hg/DOC ratios, the same Hg/DOC ratios led to almost identical results. The degree of MIF for the two odd isotopes was also affected by Hg/DOC ratios. For this reason, it is critical to study Hg photoreduction at a near-natural Hg/DOC ratio in order to better simulate natural conditions. We suggest that differences in Hg-DOC binding, which varies with Hg/DOC ratios, may be responsible for the relationship between Hg/DOC ratios and Hg photoreduction.     

Equilibrium thallium isotope fractionation and its constraint on Earth’s late veneer

Equilibrium isotope fractionation of thallium (Tl) includes the traditional mass-dependent isotope fractionation effect and the nuclear volume effect (NVE). The NVE dominates the overall isotope fractionation, especially at high temperatures. Heavy Tl isotopes tend to be enriched in oxidized Tl³⁺-bearing species. Our NVE fractionation results of oxidizing Tl⁺ to Tl³⁺ can explain the positive enrichments observed in ferromanganese sediments. Experimental results indicate that there could be 0.2–0.3 ε-unit fractionation between sulfides and silicates at 1650 °C. It is consistent with our calculation results, which are in the range of 0.17–0.38 ε-unit. Importantly, Tl’s concentration in the bulk silicate Earth (BSE) can be used to constrain the amount of materials delivered to Earth during the late veneer accretion stage. Because the Tl concentration in BSE is very low and its Tl isotope composition is similar with that of chondrites, suggesting either no Tl isotope fractionation occurred during numerous evaporation events, or the Tl in current BSE was totally delivered by late veneer. If it is the latter, the Tl-content-based estimation could challenge the magnitude of late veneer which had been constrained by the amount of highly siderophile elements in BSE. Our results show that the late-accreted mass is at least five-times larger than the previously suggested magnitude, i.e., 0.5 wt% of current Earth’s mass. The slightly lighter ²⁰⁵Tl composition of BSE relative to chondrites is probable a sign of occurrence of Tl-bearing sulfides, which probably were removed from the mantle in the last accretion stage of the Earth.

     

Nuclear field shift in natural environments

The nuclear field shift (NFS) is an isotope shift in atomic energy levels caused by a combination of differences in nuclear size and shape and electron densities at the nucleus. The effect of NFS in isotope fractionation was theoretically established by Bigeleisen in 1996 [Bigeleisen J. (1996) J. Am. Chem. Soc. 118:3676–3680] and has been analytically measured in laboratory chemical exchange reactions. More recently, some isotopic variations of heavy elements (Hg, Tl, U) measured in natural systems as well as isotopic anomalies measured for lower-mass elements in meteorites have been attributed to the NFS effect. These isotopic variations open up new and exciting fields of investigations in Earth sciences. In this paper, we review the different natural systems in which NFS has been proposed to be the origin of isotopic variations.     

高温下非传统稳定同位素分馏

过去十几年来,非传统稳定同位素地球化学在高温地质过程的研究中取得了的重大进展。多接收诱导耦合等离子质谱(MC-ICP-MS)的应用引发了稳定同位素分析方法的重大突破,使得精确测定重元素的同位素比值成为可能。本文总结了以Li、Fe和Mg同位素为代表的非传统稳定同位素在岩石地球化学研究中的应用。Li同位素目前被广泛地用于地幔地球化学、俯冲带物质再循环和变质作用的研究中,可以用来示踪岩浆的源区性质和扩散等动力学过程。不同价态的Fe在矿物熔体相之间的分配可以产生Fe同位素分馏,可以发生在地幔交代、部分熔融、分离结晶等过程中。岩浆岩的Mg同位素则大致反映其源区的特征,地幔的Mg同位素组成比较均一,这为研究低温地球化学过程中Mg同位素的分馏提供一个均一的背景。此外,Cl,Si,Cu,Ca,U等等同位素体系也具有广阔的应用前景。对同位素分馏机制的实验研究和理论模拟为理解非传统稳定同位素数据提供了必要的指导。实验表明,高温下具有不同的迁移速度的轻、重同位素可以产生显著的动力学同位素分馏,这一分馏可以在化学扩散、蒸发和凝华等过程中发生;同位素在矿物和熔体以及流体相中化学环境的差异使得不同相之间可以发生平衡分馏。而最近的硅酸盐岩浆的热扩散和热迁移实验则揭示了一种"新"的岩浆分异和同位素分馏机制。沿着温度梯度,硅酸盐岩浆可以发生显著的元素和同位素分异,湿的安山岩可以通过这种方式演变成花岗质成分,因此这个过程可能对陆壳的产生和演化有重大影响。如果温度梯度在岩浆作用中能长期存在,热扩散就可以产生稳定同位素的分馏,这一机制有别于传统的平衡和动力学同位素分馏。 而多个稳定同位素体系的正相关关系是示踪热迁移过程的最有力证据。在热扩散过程中,流体承载的物质的浓度和它的索瑞系数有关。但是这个系数对体系的很多参数非常敏感,变化极大,因此对热扩散效应的研究产生极大的困难。对热扩散实验的镁、钙和铁同位素测量表明,同位素比值的变化与体系的化学组成以及总温度无关,只和温度变化的幅度有关,这意味着即使元素的索瑞系数变化多端,某一元素的同位素之间的索瑞系数的差别总为常数。这一发现有助于简化对热扩散和索瑞系数这一基础物理问题的研究 。     

The Role of Isotopic Reference Materials for the Analysis of "Non-Traditional" Stable Isotopes

The advent of multiple collector-inductively coupled plasma-mass spectrometry has provided an impetus to the study of isotope abundance variations in natural materials. In particular, the study of "non-traditional" stable isotopes has revealed isotope fractionation variations caused by a range of physiochemical and biological mechanisms. The magnitude of these variations may be < 1 per mil per mass unit, but are significant in terms of the experimental uncertainties involved, provided rigorous mass spectrometric protocols are followed. The double spike technique can be used effectively to evaluate isotope fractionation effects for both multiple collector-inductively coupled plasma-mass spectrometry and thermal ionisation mass spectrometry. The demanding nature of this research implies the need for internationally-accepted reference materials so that interlaboratory comparisons can be made with confidence. At present, isotopically certified reference materials are unavailable for many elements, including Cu, Zn, Mo and Cd, and it is important that this situation be rectified as soon as practicable. Until such time as isotopically certified reference materials become available for every element, stable isotope geochemists should adopt a common reference material as the standard for each element so that rigorous interlaboratory comparisons can be made.     

干法灰化和湿法消解植物样品的铜锌铁同位素测定对比研究

测定有机样品中元素含量和同位素时,处理样品常用的方法有干灰化法和湿法两种.目前,测定样品中Cu、Zn、Fe同位素时,多采用湿法处理样品.相对于湿法处理样品,干灰化法溶样迅速、耗酸量少,适合处理大量样品,但高温灼烧过程可能会导致样品中Cu、Zn、Fe同位素的分馏.本研究利用海州香薷植株,以湿法为基础,对干灰化法和湿法处理...     

不同地质储库中的镁同位素组成及碳酸盐矿物形成过程中的镁同位素分馏控制因素

镁同位素在低温地球化学过程中显著的分馏效应,是其示踪地球表生环境演化及物质循环的基础。本文在前人研究的基础上,对地球上不同地质储库中的镁同位素组成及碳酸盐矿物形成过程中的镁同位素分馏控制因素进行了总结:火成岩的镁同位素组成较均一;风化产物总体富集重的镁同位素,且变化较大;碳酸盐岩中灰岩相对白云岩富集轻的镁同位素,但总体上富集轻的镁同位素;岩石类型、风化强度以及植被等因素对河流地表水的镁同位素组成影响较大,导致地表水的镁同位素组成总体变化较大;海水的镁同位素组成均一,平均值约为-0.83‰;低温条件下,控制碳酸盐矿物无机成因过程中镁同位素分馏的因素有矿物相、沉淀速率和温度,其中矿物相是主要控制因素;生物成因碳酸盐矿物镁同位素组成与生物体对含镁碳酸盐矿物的利用形式有关,除了需考虑与无机碳酸盐沉淀类似的控制因素外,还需考虑不同物种对轻、重镁同位素的选择性吸收能力;因生物成因海相碳酸盐矿物几乎都是由最初的无定形相碳酸盐转变而来,故生物成因海相碳酸盐矿物的镁同位素特征不能代表生成无定形相碳酸盐的流体的镁同位素特征。镁同位素在低温条件下具有良好的分馏效应,随着分析测试技术的发展及不同地质储库中镁同位素组成数据的积累和完善,有关表生环境中镁同位素分馏机制的许多问题将逐步得到解决,镁同位素在揭示地球表生环境演化及物质循环方面将发挥更大的作用。     

铀"稳定"同位素分馏及其在地球科学中的应用

铀“稳定”同位素（238U/235U,通常记为δ238U）目前已经成为非传统稳定同位素领域的研究热点.20世纪人们曾经认为铀同位素不存在分馏,因而铀同位素研究发展缓慢.然而随着分析技术的发展,人们发现自然界中铀同位素238U和235U存在显著的分馏,可以作为良好的示踪工具.迄今为止,已经有大量铀同位素作为古氧化还原指标的研究发表,比如用铀同位素示踪地球近地表环境氧含量随时间的演化以及生物大灭绝与海洋氧化还原状态之间的潜在关系.尽管铀同位素在水圈和生物圈协同演化领域取得了丰硕的研究成果,但仍有不少问题亟待深入解决.例如,生物和非生物还原高价铀的微观反应过程对铀同位素分馏的影响,以及铀同位素如何示踪铀矿物质来源等.系统总结了铀同位素地球化学最近十年的研究进展,希望将来铀同位素在铀多金属矿床成因和高温地球化学领域能有所突破.      

Some Key Problems on the Petrogenesis of Seafloor Basalts,Abyssal Peridotites and Geodynamics—A Non-traditional Isotope Approach

This paper discusses some major research to be carried out in the next five years in the newly established Laboratory of Ocean Lithosphere and Mantle Geodynamics. By using our existing sample collections of global mid-ocean ridge basalts, gabbros and abyssal peridotites from the Pacific, Atlantic and Indian oceans, the research includes: ①Using Ti-Zr-Hf stable isotope methods to test the hypothesis that the observed huge Nb-Ta and Zr-Hf fractionations result from mass-dependent fractionation under mantle magmatic conditions; ②Using a MORB sample suite of uniform ratios of incompatible elements and Sr-Nd-Pb isotopes with large major element compositional variation to test the common hypothesis of iron isotope fractionation, i.e, the affinity of heavy Fe with ferric Fe (Fe³⁺), and both heavy Fe and ferric Fe (Fe³⁺) being more incompatible than light Fe and ferrous Fe (Fe²⁺) during magma evolution; while using an incompatible trace element and Sr-Nd-Pb isotope highly variable MORB suite to test the same hypothesis during low-degree mantle melting (i.e, the effect of mantle metasomatism); ③Proposing and testing the hypothesis that the high oxygen fugacity of the Earth’s mantle is a consequence of plate tectonics by subducting partially serpentinized oceanic mantle lithosphere with abundant ferric Fe (e.g. Fe³⁺/SFe>2); ④The recent work by Andersen et al. (Nature, 2015) is a milestone contribution by using U isotope variation in oceanic basalts to hypothesize that the O₂-rich atmosphere since the late Archean (abont 2.4 Ga) mobilized the water soluble U (6+ vs. 4+) from continents, transported to the ocean and subducted with sediments to the upper mantle, which explains the low Th/U in MORB (<2.5) and the high Th/U (>3.5) ocean island basalts (OIB) do not see such U addition effect probably because OIB source materials are all ancient (> abont 2.4 Ga) if there were subducted component. The Cenozoic alkali basalts from eastern China are ideal materials for evaluating the significance of the subducted seafloor materials for the petrogenesis of OIB and enriched MORB by using the U isotope approach, which is expected to revise and improve the Andersen et al hypothesis.     

硼同位素分馏的实验理论认识和矿床地球化学研究进展

硼是一种中等挥发性元素,具有¹¹B和¹⁰B两个稳定同位素。两个同位素间高达10%的相对质量差使其在地质过程中引起高达-70‰至+75‰的硼同位素变化。硼在自然界主要与氧键合形成三配位(BO₃)和四配位(BO₄)结构,因而¹¹B和¹⁰B间同位素分馏主要受控于三配体(BO₃)和四面体(BO₄)间配分。本文综述了低温和高温地质过程的硼同位素分馏的理论和实验研究进展。在溶液中B(OH)₃和${B(OH)^{-}_{4}}$间硼同位素分馏受pH和热力学p-T条件控制,实验和理论表征获得常温常压条件下的B(OH)₃和$B(OH)^{-}_{4}$间同位素分馏系数(α_3-4)变化范围为1.019 4至1.033 3。低温条件下矿物(如碳酸盐、黏土矿物(蒙脱石和伊利石)、针铁矿、水锰矿、硼酸盐)与溶液间硼同位素分馏行为除了受p-T-pH影响外,矿物表面吸附引起的分馏效应十分显著。在中高温过程(蒙脱石伊利石化、富硼电气石和白云母矿物与热液流体,以及硅酸盐熔体与流体)中硼同位素分馏行为受到硼配位构型、化学成分以及物理化学条件的控制。随着硼同位素分馏机理研究的深入以及越来越完善的地质储库硼同位素端员特征表征,硼同位素地球化学指标可以灵敏示踪成矿物质来源、探究成矿作用与成因模式和重建成矿过程物理化学条件。目前矿床硼同位素地球化学研究的难点在于实现不同赋存相(如流体、矿物和熔体)中硼配位键合结构和硼同位素组成的精细化表征。     

Ion-exchange fractionation of copper and zinc isotopes

Whether transition element isotopes can be fractionated at equilibrium in nature is still uncertain. Standard solutions of Cu and Zn were eluted on an anion-exchange resin, and the isotopic compositions of Cu (with respect to Zn) of the eluted fractions were measured by multiple-collector inductively coupled plasma mass spectrometry. It was found that for pure Cu solutions, the elution curves are consistent with a ⁶³Cu/⁶⁵Cu mass fractionation coefficient of 0.46‰ in 7 mol/L HCl and 0.67‰ in 3 mol/L HCl between the resin and the solution. Batch fractionation experiments confirm that equilibrium fractionation of Cu between resin and 7 mol/L HCl is ∼0.4‰ and therefore indicates that there is no need to invoke kinetic fractionation during the elution. Zn isotope fractionation is an order of magnitude smaller, with a ⁶⁶Zn/⁶⁸Zn fractionation factor of 0.02‰ in 12 mol/L HCl. Cu isotope fractionation results determined from a chalcopyrite solution in 7 mol/L HCl give a fractionation factor of 0.58‰, which indicates that Fe may interfere with Cu fractionation.Comparison of Cu and Zn results suggests that the extent of Cu isotopic fractionation may signal the presence of so far unidentified polynuclear complexes in solution. In contrast, we see no compelling reason to ascribe isotope fractionation to the coexistence of different oxidation states. We further suggest that published evidence for iron isotopic fractionation in nature and in laboratory experiments may indicate the distortion of low-spin Fe tetrahedral complexes.The isotope geochemistry of transition elements may shed new light on their coordination chemistry. Their isotopic fractionation in the natural environment may be interpreted using models of thermodynamic fractionation.     

钛同位素地球化学综述

随着分析技术的进步,非传统稳定同位素体系在地球化学、天体化学和生物地球化学等研究领域的应用日益广泛。钛(Ti)是一个非常重要的过渡族金属元素,在地球和其他类地球行星中广泛存在。但是由于Ti是一种难熔的、流体不活动性元素,高温地质过程中Ti同位素分馏很小。人们对Ti同位素体系的地球化学应用的关注相对其他非传统稳定同位非常有限。而近年来,随着化学纯化方案的优化以及双稀释剂方法的改进和仪器质谱性能的提高,Ti同位素组成的高精度测试已经能够实现。天然样品中Ti同位素组成的变化随之得以发现,使得学者们能够利用这一新的稳定同位素体系来解决与高温和低温地球化学相关的问题。很快Ti同位素体系地球化学研究成为当前国际地质学界的前沿研究课题和新的发展方向之一。本文首先在简要介绍Ti元素和Ti同位素体地球化学性质的基础上,介绍了Ti元素化学分离和Ti同位素分析方法。随后笔者总结了已有的不同类型球粒陨石和地球样品的质量相关Ti同位素组成研究结果,对硅酸盐地球的Ti同位素组成做了初步评估。前人对高温地质样品的Ti同位素组成研究初步探明Ti同位素在岩浆演化过程,例如部分熔融和结晶分异等重要地质过程中的分馏行为。笔者在此基础上探讨了结晶分异过程中引起Ti同位素分馏的主要控制因素,指出Ti同位素是潜在的研究岩浆演化过程的新工具。最后笔者探讨了Ti同位素地球化学未来的发展方向,以加速我国在Ti同位素地球化学方面的应用研究。     

镉稳定同位素研究进展

Cd具有挥发性和亲硫性,在海洋环境中Cd为微量营养元素,而在生态环境及农业土壤环境中Cd为有毒元素.因此,镉同位素被应用于海洋科学、地球科学、环境科学及农业科学研究,并展现出巨大的应用潜力.本文总结了近年来富含有机质的环境样品、植物样品和生物样品的消解方法,以及Cd分离纯化及双稀释剂校正方法的研究进展.采用微波、高压灰...     

Optical and Mössbauer spectroscopy of iron in rock-forming silicates

The inner nebula out to ~3 A.U. was depleted in volatile elements that included potassium and manganese at a very early stage of solar-system history. The inner planets and many meteorites inherited this element signature, the cause of which probably was early violent solar activity. Because of this evidence for elemental depletions correlated with volatility, one might also expect to find examples of fractionation, particularly among lower mass elements. Here we discuss the search for such effects among the isotopes of K, Mg, Si, and Ca in a wide variety of terrestrial, lunar, and meteoritic samples. We examine examples of vaporization without isotope fractionation, and a comparison of the effects expected between distillation and condensation. Effects attributable both to evaporation and condensation are observed in refractory inclusions (CAIs) in meteorites and reflect localized events in the early nebula. However, the lack of isotopic fractionation that is observed among a wider variety of presolar-system materials rules out the general operation of Rayleigh-type fractionation on primitive solar-nebular material. We conclude with a discussion of volatileelement behavior during the giant Moon-forming impact that shows that the material in the Moon was not subjected to Rayleigh-type distillation.     

General equations for modeling fluid/rock interaction using trace elements and isotopes

General equations for modeling open and closed system fluid/rock interaction using trace elements and isotopes are presented. Taylor's (1977) open system equation for stable isotopes is extended to account for a change in the Δ value between rock and fluid during interaction. It is shown that hydrothermal systems in which fractionation factors vary during fluid/rock interaction will significantly alter the isotopic ratios of rocks compared to equivalent systems with invariant fractionation factors. The equations for modeling trace element exchange are analogous to the stable isotope equations and permit the calculation of fluid/rock ratios using changes in trace element concentrations in the rock. Similarly to stable isotope exchange, the change of the trace element concentration in a rock during infiltration is a function of the trace element concentration in the fluid, the fluid/rock partition coefficient and the fluid/rock ratio rather than simply the concentration of the trace element in the fluid.     

铬同位素在高温岩浆过程中的研究进展

铬(Cr)属于氧化还原敏感元素,在岩浆过程中是一种中度相容和轻度亲铁元素。Cr在硅酸盐地球中主要有三种价态:Cr²⁺、Cr³⁺和Cr⁶⁺。Cr存在于不同来源的矿物和岩石中,其氧化还原状态和同位素组成可以为其成因、氧化还原条件和相关成矿历史提供有价值的信息。近年来,铬同位素越来越多地应用到现代环境、古环境、行星的演化以及高温地质过程等领域中,而高温地质过程中储库的铬同位素及其分馏机理研究是其他工作的基础。尤其是随着质谱技术的发展,Cr同位素在高温环境中的分馏机制及行为也引起了更多的关注。本文主要介绍不同储库的Cr同位素组成及高温岩浆过程中Cr同位素研究的最新进展。     

Germanium isotope measurements of high-temperature geothermal fluids using double-spike hydride generation MC-ICP-MS

We present a double-spike isotope dilution MC-ICP-MS technique for the determination of germanium (Ge) isotope fractionation. Using this technique we determined Ge isotope compositions of geothermal spring fluids, a Columbia River Basalt sample, and an in-house diatom standard. Our technique uses a ⁷³Ge/⁷⁰Ge double spike in combination with hydride generation for Ge extraction from the sample matrix. Fractionation is determined on the ⁷⁴Ge/⁷²Ge mass ratio. The double spike allows us to effectively correct analytical isotope fractionation. Our external standard reproducibility is 0.4‰ (2 SD) over the course of several months. The minimum quantity of Ge needed for isotope analysis is approximately 2 ng. Consistent with previous work on geothermal fluids, Ge in the geothermal spring samples presented here is enriched over Si as compared to low temperature weathering signatures. This observation is typically interpreted as Ge exclusion during silicate mineral precipitation (e.g., quartz). Our isotope results indicate that the analyzed high temperature fluids fractionate Ge isotopes with a range in δ⁷⁴Ge between −0.4‰ and −1.4‰ relative to a Columbia River basalt. We cautiously interpret the observed fractionation as preferential removal of heavy Ge isotopes out of solution during cooling of the hydrothermal fluid and subsequent precipitation of quartz.     

Mass dependent stable isotope fractionation of mercury during mer mediated microbial degradation of monomethylmercury

Controlling bioaccumulation of toxic monomethylmercury (MMHg) in aquatic food chains requires differentiation between biotic and abiotic pathways that lead to its production and degradation. Recent mercury (Hg) stable isotope measurements of natural samples suggest that Hg isotope ratios can be a powerful proxy for tracing dominant Hg transforming pathways in aquatic ecosystems. Specifically, it has been shown that photo-degradation of MMHg causes both mass dependent (MDF) and mass independent fractionation (MIF) of Hg isotopes. Because the extent of MDF and MIF observed in natural samples (e.g., fish, soil and sediments) can potentially be used to determine the relative importance of pathways leading to MMHg accumulation, it is important to determine the potential role of microbial pathways in contributing to the fractionation, especially MIF, observed in these samples. This study reports the extent of fractionation of Hg stable isotopes during degradation of MMHg to volatile elemental Hg and methane via the microbial Hg resistance (mer) pathway in Escherichia coli carrying a mercury resistance (mer) genetic system on a multi-copy plasmid. During experimental microbial degradation of MMHg, MMHg remaining in reactors became progressively heavier (increasing δ²⁰²Hg) with time and underwent mass dependent Rayleigh fractionation with a fractionation factor α_202/198 = 1.0004 ± 0.0002 (2SD). However, MIF was not observed in any of the microbial MMHg degradation experiments indicating that the isotopic signature left by mer mediated MMHg degradation is significantly different from fractionation observed during DOC mediated photo-degradation of MMHg. Additionally, a clear suppression of Hg isotope fractionation, both during reduction of Hg(II) and degradation of MMHg, was observed when the cell densities increased, possibly due to a reduction in substrate bioavailability. We propose a multi-step framework for understanding the extent of fractionation seen in our MMHg degradation experiments and, based on estimates of the rates of the various steps involved in this mer mediated pathway, suggest which steps in the process could contribute towards the observed extent of fractionation. This framework suggests that at lower cell densities catalysis by MerB was the rate limiting step while at higher cell densities transport into the cell, which does not cause fractionation, became the rate limiting step. In addition to presenting evidence for absence of MIF during mer mediated Hg transformations, based on the nature of Hg compounds and microbe-Hg interactions, we suggest that the nuclear spin dependent MIF (i.e., the magnetic isotope effect) is also unlikely to occur during other non mer mediated ‘dark’ microbial Hg transformations (e.g., formation of MMHg and oxidative degradation of MMHg). Because of the important implications of the absence of MIF during biological processes on Hg isotope systematics, we discuss theoretical considerations and experimental strategies that could be used to confirm this suggestion.     

Geochemistry and cosmochemistry of potassium stable isotopes

Stable potassium isotopes are one of the emerging non-traditional isotope systems enabled in recent years by the advance of Multi-Collector Inductively-Coupled-Plasma Mass-Spectrometry (MC-ICP-MS). In this review, we first summarize the geochemical and cosmochemical properties of K, its major reservoirs, and the analytical methods of K isotopes. Following this, we review recent literature on K isotope applications in the fields of geochemistry and cosmochemistry. Geochemically, K is a highly incompatible lithophile element, and a highly soluble, biophile element. The isotopic fractionation of K is relatively small during magmatic processes such as partial melting and fractional crystallization, whereas during low-temperature and biological processes fractionation is considerably larger. This resolvable fractionation has made K isotopes promising tracers for a variety of Earth and environmental processes, including chemical weathering, low-temperature alteration of igneous rocks, reverse weathering, and the recycling of sediments into the mantle during subduction. Sorption and interactions of aqueous K with different clay minerals during cation exchange and clay formation are likely to be of fundamental significance in generating much of the K isotope variability seen in samples from the Earth surface and samples carrying recycled surface materials from the deep Earth. The magnitude of this fractionation is process- and mineral-dependent. Comprehensive quantification of pertinent K isotope fractionation factors is currently lacking and urgently needed. Significant fractionation during biological activities, such as plant uptake, demonstrates the potential utility of K isotopes in the study of the nutrient cycle and its relation to the climate and various ecosystems, enabling new and largely unexplored avenues for future research.Of significant importance to the cosmochemistry community, K is a moderately volatile element with large variations in K/U ratio observed among chondrites and planetary materials. As this indicates different degrees of volatile depletion, it has become a fundamental chemical signature of both chondritic and planetary bodies. This volatile depletion has been attributed to various processes such as solar nebula condensation, mixing of volatile-rich and -poor reservoirs, planetary accretional volatilization via impacts, and/or magma ocean degassing. While K isotopes have the potential to distinguish these different processes, the current results are still highly debated. A good correlation between the K isotope compositions of four differentiated bodies (Earth, Mars, Moon, and Vesta) and their masses suggests a ubiquitous volatile depletion mechanism during the formation of the terrestrial planets. It is still unknown whether any of the K isotopic variation among chondrites and differentiated bodies can be attributed to inherited signatures of mass-independent isotopic anomalies.     

汞同位素地球化学研究及其在矿床学中的应用进展

汞作为一种重要的成矿元素,广泛分布于不同地质体中,并参与成岩成矿作用.随着质谱技术的飞跃发展,汞同位素地球化学研究取得引人瞩目的进展.汞同位素被广泛地应用于示踪地球表生生物地球化学过程及汞污染等.近年来,汞同位素又被应用于揭示行星的演化过程、识别地质历史时期大火成岩省及示踪矿床成矿物质来源等方面.本文在前人研究的基础上...