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

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.