Oxygen three-isotope fractionation lines in terrestrial silicate minerals: An inter-laboratory comparison of hydrothermal quartz and eclogitic garnet
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δ18O
Mass-independent fractionation
Equilibrium fractionation
Research on mass independent oxygen isotope fractionation sheds a new sight on many important geologic events.Based on a simple introduction of mass independent isotope fractionation,this paper summarizes the progress in research on mass independent oxygen isotope fractionation,including the genetic mechanism of mass independent oxygen isotope fractionation,the establishment of methods to precisely determine oxygen isotopic composition,and its application in the field of geology and environment.Then,the future trend in research on mass independent oxygen isotope fractionation is also discussed.
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Isotope Analysis
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Mercury (Hg) stable isotope fractionation has been widely used to trace Hg sources and transformations in the environment, although many important fractionation processes remain unknown. Here, we describe Hg isotope fractionation during the abiotic dark oxidation of dissolved elemental Hg(0) in the presence of thiol compounds and natural humic acid. We observe equilibrium mass-dependent fractionation (MDF) with enrichment of heavier isotopes in the oxidized Hg(II) and a small negative mass-independent fractionation (MIF) owing to nuclear volume effects. The measured enrichment factors for MDF and MIF (ε202Hg and E199Hg) ranged from 1.10‰ to 1.56‰ and from -0.16‰ to -0.18‰, respectively, and agreed well with theoretically predicted values for equilibrium fractionation between Hg(0) and thiol-bound Hg(II). We suggest that the observed equilibrium fractionation was likely controlled by isotope exchange between Hg(0) and Hg(II) following the production of the Hg(II)-thiol complex. However, significantly attenuated isotope fractionation was observed during the initial stage of Hg(0) oxidation by humic acid and attributed to the kinetic isotope effect (KIE). This research provides additional experimental constraints on interpreting Hg isotope signatures with important implications for the use of Hg isotope fractionation as a tracer of the Hg biogeochemical cycle.
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To utilize stable Hg isotopes as a tracer for Hg cycling and pollution sources in the environment, it is imperative that fractionation factors for important biogeochemical processes involving Hg are determined. Here, we report experimental results on Hg isotope fractionation during precipitation of metacinnabar (β-HgS) and montroydite (HgO). In both systems, we observed mass-dependent enrichments of light Hg isotopes in the precipitates relative to the dissolved Hg. Precipitation of β-HgS appeared to follow equilibrium isotope fractionation with an enrichment factor ε202Hgprecipitate–supernatant of −0.63‰. Precipitation of HgO resulted in kinetic isotope fractionation, which was described by a Rayleigh model with an enrichment factor of −0.32‰. Small mass-independent fractionation was observed in the HgS system, presumably related to nuclear volume fractionation. We propose that Hg isotope fractionation in the HgS system occurred in solution during the transition of O- to S-coordination of Hg(II), consistent with theoretical predictions. In the HgO system, fractionation was presumably caused by the faster precipitation of light Hg isotopes, and no isotopic exchange between solid and solution was observed on the timescale investigated. The results of this work emphasize the importance of Hg solution speciation and suggest that bonding partners of Hg in solution complexes may control the overall isotope fractionation. The determined fractionation factor and mechanistic insights will have implications for the interpretation of Hg isotope signatures and their use as an environmental tracer.
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Stable isotope analyses of CaCO 3 minerals are very important for investigations of paleoclimate and sedimentary environments For this reason, the experimental calibration of oxygen isotope fractionation factors in the system of CaCO 3 H 2O has been an attractive subject in stable isotope geochemistry since its birth However, there are considerably large differences in the oxygen isotope fractionation factors between CaCO 3 and H 2O measured by different investigators, due to CaCO 3 polymorphs in nature ( i e calcite, aragonite and vaterite) As a result, different temperatures are yielded when these different fractionation factors are applied to isotope geothermometry Thus it is of critical importance in low temperature and environmental geochemistry to make the correct and reasonable choice of oxygen isotope fractionation equations in the calcite water and aragonite water systems In this paper the experimental calibration history, approaches and results of oxygen isotope fractionations in the CaCO 3 H 2O system are systematically summarized and reviewed, and the different expressions about the oxygen isotope fractionation between CaCO 3 and H 2O are unified Salt effect and kinetic effect on oxygen isotope fractionation as well as oxygen isotope inheritance in the processes of polymorph transformation are discussed as well The equilibrium equation of oxygen isotope fractionation between calcite and water is recommended on the basis of reprocessing a large number of the known experimental data and comparing them with theoretical calculations However, the theoretical calculation results concerning the aragonite water system has to be confirmed by further experiments
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Equilibrium fractionation
Methylmercury
Mass-independent fractionation
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Biogeochemical Cycle
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Copper shows limited isotopic variation in equilibrated mantle-derived silicate rocks, but large isotopic fractionation during kinetic processes. For example, lunar and terrestrial samples that have experienced evaporation were found to have an isotopic fractionation of up to 12.5‰ in their 65Cu/63Cu ratios, while komatiites, lherzolites, mid-ocean ridge and ocean island basalts show negligible Cu isotope fractionation as a result of equilibrium partial melting and crystal fractionation. The contrast between the observed magnitudes of equilibrium and kinetic isotope fractionation for Cu calls for a better understanding of kinetic Cu isotope fractionation. One of the mechanisms for creating large kinetic isotopic fractionation even at magmatic temperatures is diffusion. In this study, we performed Cu isotopic measurements on Cu diffusion couple experiments to constrain the beta factor for Cu isotopic fractionation by diffusion. We demonstrate a Monte Carlo approach for the regression and error estimation of the measured isotope profiles, which yielded beta values of 0.16 ± 0.03 and 0.18 ± 0.03 for the two experimental charges measured. Our results are subsequently applied to a quantitative model for the evaporation of a molten sphere to discuss the role of diffusion in affecting the bulk Cu isotopic fractionation between liquid and vapor during evaporation. We apply the model to Cu evaporation experiments and tektite data to show that convection primarily governs mass transport for evaporation during tektite formation. In addition, we show that Cu isotopes can be used as a tool to test the role of kinetics during various magmatic processes such as magmatic sulfide ore deposit formation, porphyry-type ore deposit formation, and fluid-rock interactions.
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Radiogenic nuclide
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Minerals and rocks exhibit various isotope compositions depending on their origins and histories. In interpreting their isotopic variations, the equilibrium isotope fractionation factor is a key because it depends on the environment parameters such as temperature. Recent studies have shown that the effect of pressure on the isotope fractionation, which was considered negligible compared to temperature, is significant under the conditions of the Earth's interior. In this article we review recent advances in experimental studies to determine the isotope fractionation of iron and hydrogen at high pressure over several GPa, discussing their issues and future perspectives.
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Equilibrium fractionation
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