Modeling Zinc Isotope Fractionations

In this study quantum mechanical models are used to estimate equilibrium zinc-isotope fractionations between zinc oxide and sulfide minerals, as well as aqueous complexes. A major goal in stable isotope geochemistry and biogeochemistry is the determination of equilibrium fractionations between different phases present in typical geochemical systems. The search for reliable biosignatures, in particular, will rely on careful studies contrasting biological fractionation processes, abiological kinetic fractionations, and equilibrium isotope effects. For many stable isotope systems, however, the determination of equilibrium fractionations is hampered by the difficulty of achieving isotopic equilibrium in a reasonable laboratory timescale, particularly at the low temperatures relevant to biogeochemistry. The equilibrium stable isotope geochemistry of heavy, biologically important elements like zinc and iron is particularly poorly known, because accurate measurements of their isotopic compositions have only recently become possible. Theoretical estimates of equilibrium stable isotope fractionations can provide a useful framework for understanding natural fractionation processes, and for extrapolating sparse experimental results to lower temperatures. Equilibrium stable isotope fractionations are mainly caused by isotopic effects on vibrational energies, so it is necessary to measure or model isotopic effects on vibrational frequencies before fractionations can be estimated. Quantum mechanical modeling using density functional theory (DFT) is a powerful technique for determining unknown properties of minerals, molecules, and aqueous complexes. Here DFT is used to estimate vibrational frequencies and zinc-isotope (⁶⁸Zn/⁶⁶Zn) fractionations in isotopically substituted ZnO (zincite) and two polymorphs of ZnS (sphalerite and wurtzite), as well as aqueous complexes like [Zn(H₂O)₆]²⁺ and [ZnCl₄]^2-. The results predict that sulfide minerals will have lower ⁶⁸Zn/⁶⁶Zn ratios than coexisting ZnO, by ~1\permil at room temperature, while fractionations between coexisting wurtzite and sphalerite are very small. ⁶⁸Zn/⁶⁶Zn in aqueous [Zn(H₂O)₆]²⁺ will be intermediate between ZnO and sulfides, while [ZnCl₄]^2- is similar to sulfides. A general depletion of heavy isotopes in sulfides equilibrated with oxide minerals and aqueous solutions is consistent with Fe-isotope measurements of pyrite in banded iron formations (Johnson et al., 2003, Contrib. Mineral. Petrol., v. 144, p. 523-547). The geochemistries of stable zinc, chromium, and iron isotope fractionations are in qualitative agreement, and suggest 1) that chloro-complexes will tend to concentrate the light isotopes of metallic elements, and 2) that complexes and minerals with 4-fold metal ion coordination (i.e., ZnO-zincite, [FeCl₄]^-) will tend to concentrate heavy isotopes relative to analogous materials with 6-fold coordination ([Zn(H₂O)₆]²⁺, [FeCl₆]^3-).