First-Principles Investigations Of Equilibrium O,Si,Ca Isotope Fractionations Between Common Minerals,V Isotope Fractionation In Solutions

Stable Isotope Geochemistry is an important branch of Geochemistry. As the equi-librium inter-mineral isotope fractionation factor of certain isotope is the function of T between two coexisting minerals(phases), the stable isotopes can be used as Geother-mometer.Three main methods are adopted to determine the stable isotope fractionation fac-tor for isotope exchange reactions, there are direct experimental determinations in the laboratory, empirical or semi-empirical calibrations and theoretical calculations. Every method has its own advantages and disadvantages. Using two or more methods at the same time is helpful in obtaining accurate isotope fractionation factors. To those sys-tems, where the rate of isotope exchange is limited, or the experimental P-T conditions are too high to achieve, theoretical calculations becomes useful.First principles calculations allow us to investigate the fractionation mechanism at atomic level, such as how the bond length of the certain atoms, concentration of elements in crystal, chemical composition and valence would influence the fractionation process. The methods we used here is first principles calculations based on density functional theory (DFT).In this study, we investigate the Ca isotope fractionation factor between doped OPX and pure CPX. Our calculations reveal that, besides temperature, the equilibrium inter-mineral Ca isotope fractionation factor between OPX and CPX also has a strong dependence on the Ca content of OPX. Specifically, the fractionation increases substan-tially with decreasing Ca/Mg(atomic ratio) of OPX when Ca/Mg in OPX is lower than1/15. Such a compositional effect on inter-mineral Ca isotope fractionation provides a convincing interpretation to the different degrees of isotope fractionations between co-existing OPX and CPX observed in Kilbourne Hole and San Carlos peridotites. Lower Ca/Mg in OPX leads to a shorter Ca-0bond length and thus enrichment of heavy Ca isotopes in OPX.We also investigate the Si isotope fractionation factor among the most abundant silicate minerals in crust and mantle. Our calculation reveals the3o Si-enrichment or-der follows a sequence of quartz> albite> anorthite> olivine≈zircon> enstatite> diopside. The fractionation factors among silicate minerals allow us to evaluate Si iso-tope fractionation between minerals and melt based on magma composition and CIPW normative mineral modes. Fractional crystallization of olivines from basaltic magmas will shift the residual melts toward heavier Si isotopic compositions, whereas crystal-lization of quartz and feldspar from Si-rich magmas has little effect on the Si isotopic compositions of the residual melts. Hence, the Si isotopic compositions of granites are mainly controlled by those of their crustal sources.Investigations on the equilibrium V isotope fractionation factors between different V species in solution reveal that the V species of high valence state are enriched in heavy V isotopes. In addition, the simulation of V adsorption on FeOOH surface reveal that the adsorbed V species are enriched in light V isotopes, leaving heavy V isotopes in the remaining solution. These results indicate that low-temperature oxidation-reduction processes and adsorption processes could significantly fractionate V isotopes.