Hydrogen Isotope Fractionation in the Talc–Serpentine–Brucite–Water System: Theoretical Studies and Implications

Abstract

We have carried out DFT-based first-principles calculations of hydrogen isotope fractionation among serpentine, talc, brucite, and water as a function of temperature. The results for the fractionation factor (α) for the talc–water pair are in excellent agreement with experimental data; those for the serpentine–water pair agree very well with two sets of experimental data at 250–450 °C and empirical data based on δD data of oceanic serpentines at 25–235 °C but are widely different from a third set of experimental data. Using all calculated and experimental data except for the discrepant set, we present optimized ln α versus 1/T polynomial relations for the serpentine/talc–water(liquid) systems. The results for serpentine–water system strongly suggest that serpentinization of the oceanic peridotites had taken place predominantly by interaction with ocean water, with little or no involvement of magmatic water. For brucite–water(gas) system, our data follows the nonlinear trend (with a minimum) of the ln α versus 1/T2 described by the experimental data at 25–625 °C but falls below the latter by 5–13‰, depending on the temperature. The discrepancy with the experimental data has opposite sign but smaller magnitude compared to an earlier first-principles calculation. For this system, we present an expression for the temperature dependence of α at 1 bar pressure based on a selected set of experimental data. The mineral–water fractionation data have been combined to yield thermometric expressions based on hydrogen isotope fractionation in the talc/brucite–serpentine systems. The system of these and mineral–water hydrogen isotope fractionation relations versus temperature would enable simultaneous solutions for the temperature and source of water in the serpentinization processes in nature.