Compositional controls on hydrogen generation during serpentinization of ultramafic rocks

Abstract Where ultramafic rocks are exposed to water at temperatures  ca. 200 °C), while serpentinization of pyroxenite does not produce magnetite, but instead forms Fe-rich serpentine with relative low Fe+ 3/(Fe+ 3 + Fe+ 2) values and Fe-poor talc. The predicted activities of dissolved hydrogen (aH2(aq)), dissolved silica (aSiO2(aq)), as well as the pH vary accordingly. At temperatures ≤ ca. 350 °C, fluids interacting with peridotite are more reducing, have lower aSiO2(aq) and higher pH than fluids interacting with pyroxenite. A direct correlation between the iron content of olivine, its stability relative to water, temperature and aH2(aq) is apparent from our calculations. In contrast to forsterite-rich olivine, fayalite-rich olivine can be stable to temperatures as low as 180 °C in the presence of water. As a consequence, the predicted aH2(aq) for serpentinization of fayalite is maximal at temperatures ≤ 180 °C.