189 research outputs found

    Non-traditional stable isotope behaviors in immiscible silica-melts in a mafic magma chamber

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    Non-traditional stable isotopes have increasingly been applied to studies of igneous processes including planetary differentiation. Equilibrium isotope fractionation of these elements in silicates is expected to be negligible at magmatic temperatures (δ57 Fe difference often less than 0.2 per mil). However, an increasing number of data has revealed a puzzling observation, e.g., the δ57 Fe for silicic magmas ranges from 0‰ up to 0.6‰, with the most positive δ57 Fe almost exclusively found in A-type granitoids. Several interpretations have been proposed by different research groups, but these have so far failed to explain some aspects of the observations. Here we propose a dynamic, diffusion-induced isotope fractionation model that assumes Si-melts are growing and ascending immiscibly in a Fe-rich bulk magma chamber. Our model offers predictions on the behavior of non-traditional stable isotope such as Fe, Mg, Si, and Li that are consistent with observations from many A-type granitoids, especially those associated with layered intrusions. Diffusion-induced isotope fractionation may be more commonly preserved in magmatic rocks than was originally predicted

    Mass Dependency of Isotope Fractionation of Gases Under Thermal Gradient and Its Possible Implications for Planetary Atmosphere Escaping Process

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    Physical processes that unmix elements/isotopes of gas molecules involve phase changes, diffusion (chemical or thermal), effusion and gravitational settling. Some of those play significant roles for the evolution of chemical and isotopic compositions of gases in planetary bodies which lead to better understanding of surface paleoclimatic conditions, e.g. gas bubbles in Antarctic ice, and planetary evolution, e.g. the solar-wind erosion induced gas escaping from exosphere on terrestrial planets.. A mass dependent relationship is always expected for the kinetic isotope fractionations during these simple physical processes, according to the kinetic theory of gases by Chapman, Enskog and others [3-5]. For O-bearing (O16, -O17, -O18) molecules the alpha O-17/ alpha O-18 is expected at 0.5 to 0.515, and for S-bearing (S32,-S33. -S34, -S36) molecules, the alpha S-33/ lpha S-34 is expected at 0.5 to 0.508, where alpha is the isotope fractionation factor associated with unmixing processes. Thus, one isotope pair is generally proxied to yield all the information for the physical history of the gases. However, we recently] reported the violation of mass law for isotope fractionation among isotope pairs of multiple isotope system during gas diffusion or convection under thermal gradient (Thermal Gradient Induced Non-Mass Dependent effect, TGI-NMD). The mechanism(s) that is responsible to such striking observation remains unanswered. In our past studies, we investigated polyatomic molecules, O2 and SF6, and we suggested that nuclear spin effect could be responsible to the observed NMD effect in a way of changing diffusion coefficients of certain molecules, owing to the fact of negligible delta S-36 anomaly for SF6.. On the other hand, our results also showed that for both diffusion and convection under thermal gradient, this NMD effect is increased by lower gas pressure, bigger temperature gradient and lower average temperature, which indicate that the nuclear spin effect may not be the significant contributor as the energies involved in the hyperfine effect are much smaller than those with molecular collisions, especially under convective conditions

    Diffusional fractionation of helium isotopes in silicate melts

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    Estimating Helium (He) concentration and isotope composition of the mantle requires quantifying He loss during magma degassing. The knowledge of diffusional He isotope fractionation in silicate melts may be essential to constrain the He loss. Isotopic mass dependence of He diffusion can be empirically expressed as D3He/D4He = (4/3)^β, where D is the diffusivity of a He isotope. However, no studies have reported any β values for He in silicate melts due to technical challenges in both experiments and computations. Here, molecular dynamics simulations based on deep neural network potentials trained by ab initio data show that β for He in albite melt decreases from 0.355 ± 0.012 at 3000 K to 0.322 ± 0.019 at 1700 K. β in model basalt melt takes a smaller value from 0.322 ± 0.025 to 0.274 ± 0.027 over the same temperature range. Based on our results, we suggest using D3He/D4He values of 1.097 ± 0.006 and 1.082 ± 0.008 in natural rhyolite and basalt melt, respectively, to interpret measured He concentration and isotope composition of natural samples

    First-principles computation of diffusional Mg isotope fractionation in silicate melts

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    © 2020 Elsevier Ltd Diffusional isotope fractionation occurs in geochemical processes (such as magma mixing, bubble growth, and crystal growth), even at magmatic temperatures. Isotopic mass dependence of diffusion is commonly expressed as [Formula presented], where Di and Dj are diffusion coefficients of two isotopes whose masses are mi and mj. How the dimensionless empirical parameter β depends on temperature, pressure, and composition remains poorly constrained. Here, we conducted a series of first-principles molecular dynamics simulations to evaluate the β factor of Mg isotopes in MgSiO3 and Mg2SiO4 melts using pseudo-isotope method. In particular, we considered interactions between Mg isotopes by simultaneously putting pseudo-mass and normal-mass Mg atoms in a simulation supercell. The calculated β for Mg isotopes decreases linearly with decreasing temperature at zero pressure, from 0.158±0.004 at 4000 K to 0.121±0.017 at 2200 K for MgSiO3 melt and from 0.150±0.004 at 4000 K to 0.101±0.012 at 2200 K for Mg2SiO4 melt. Moreover, our simulations of compressed Mg2SiO4 melt along the 3000 K isotherm show that the β value decreases linearly from 0.130±0.006 at 0 GPa to 0.060±0.011 at 17 GPa. Based on our diffusivity results, the empirically established positive correlation between β and solvent-normalized diffusivity (Di/DSi) seems to be applicable only at constant temperatures or in narrow temperature ranges. Analysis of atomistic mechanisms suggests that the calculated β values are inversely correlated with force constants of Mg at a given temperature or pressure. Good agreement between our first principles results with available experimental data suggests that interactions between isotopes of major elements must be considered in calculating β for major elements in silicate melts. Also, we discuss diffusion-controlled crystal growth by considering our calculated β values

    An Update on the Non-Mass-Dependent Isotope Fractionation under Thermal Gradient

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    Mass flow and compositional gradient (elemental and isotope separation) occurs when flu-id(s) or gas(es) in an enclosure is subjected to a thermal gradient, and the phenomenon is named thermal diffusion. Gas phase thermal diffusion has been theoretically and experimentally studied for more than a century, although there has not been a satisfactory theory to date. Nevertheless, for isotopic system, the Chapman-Enskog theory predicts that the mass difference is the only term in the thermal diffusion separation factors that differs one isotope pair to another,with the assumptions that the molecules are spherical and systematic (monoatomic-like structure) and the particle collision is elastic. Our previous report indicates factors may be playing a role because the Non-Mass Dependent (NMD) effect is found for both symmetric and asymmetric, linear and spherical polyatomic molecules over a wide range of temperature (-196C to +237C). The observed NMD phenomenon in the simple thermal-diffusion experiments demands quantitative validation and theoretical explanation. Besides the pressure and temperature dependency illustrated in our previous reports, efforts are made in this study to address issues such as the role of convection or molecular structure and whether it is a transient, non-equilibrium effect only

    Sulfate oxygen-17 anomaly in an Oligocene ash bed in mid-North America: Was it the dry fogs?

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    The oxygen and sulfur isotope compositions of sulfate, particularly the oxygen-17 anomaly, provide clues to the origin of sulfate. We report a variable oxygen-17 anomaly for water- and acid-leached sulfate from an Oligocene volcanic ash bed in the northern High Plains, U.S.A. We find no sulfate O-17 anomaly, however, in freshly collected ashes from recent eruptions around the world. The Oligocene paleoclimatic and sedimentary evidence argues against a hyperarid condition in the High Plains, thus a long-term background atmospheric sulfate accumulation is less likely the origin. Combining sulfur isotope data, we suggest that the anomalous sulfate was associated with volcanic eruptions in the west, but was not directly carried by ash-falls. Extreme dry-fog (sulfate haze) events resulting from tropospheric oxidation of volcanic sulfur gases might provide a viable explanation for the sulfates in the ash bed that carry the oxygen-17 anomaly

    The Marinoan cap carbonate of Svalbard: Syngenetic marine dolomite with <sup>17</sup>O-anomalous carbonate‐associated sulphate

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    Two cap carbonates overlying Cryogenian panglacial deposits are found in North- East Svalbard of which the younger (635 Ma) forms the base of the Ediacaran Period. It is represented by a transgressive succession in which laminated do-lostone, typically around 20 m thick (Member D1), is succeeded transitionally by a similar thickness of impure carbonates (Member D2). In Spitsbergen, there is evidence of microbially influenced sediment stabilisation and carbonate precipitation in the lower part of D1, whilst the upper part of D1 and D2 show centimetre- decimetre- scale graded units with undulatory lamination interpreted as evidence of storm activity. Carbonate originated as possible freshwater whitings, as well as microbial precipitates. Exhumed and eroded hardgrounds display replacive 10– 30 μm dolomite crystals with cathodoluminescence characteris-tics consistent with early diagenetic manganese and iron reduction. Regionally, carbon isotope values consistently decrease by around 2‰ from around −3‰ over 30 m of section which is both a temporal and a bathymetric signal, but not a global one. An exponential decline in carbonate production predicted by box models is fitted by a semi- quantitative sedimentation model. A mass- anomalous 17O depletion in carbonate- associated sulphate in dolomite, inherited from pre-cursor calcite, decreases from −0.6 to −0.3‰ in the basal 15 m of section and then approaches background values. The post- glacial anomalous 17O depletion in carbonate- associated sulphate and barite elsewhere has been interpreted in terms of ultra- high pCO2 at the onset of deglaciation. Such anomalies, with larger amplitude, have been reported in Svalbard from underlying lacustrine and tufa-ceous limestones representing a hyperarid glacial environment. The anomalous sulphate could be produced contemporarily, or the internally drained landscape may have continued to release 17O-anomalous sulphate as it was transgressed during cap carbonate deposition. The late Cryogenian to earliest Ediacaran record in Svalbard provides the most complete record of the basal 17O - depletion event in the world
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