Compositional controls on hydrogen generation during serpentinization of ultramafic rocks

Where ultramafic rocks are exposed to water at temperatures <400 degrees C, they inevitably undergo serpentinization reactions to form serpentine +/- brucite +/- talc +/- magnetite (in addition to minor or trace phase like chlorite, tremolite, secondary diopside, garnet, Ni-Fe sulfides, alloys). In many circumstances, this process releases substantial amounts of hydrogen. Since the compositional controls of the primary lithology on the secondary mineralogy, fluid composition, Fe-distribution, and H-2 formation are not well established, we used thermodynamic computations to examine the equilibrium mineral assemblages, mineral compositions and the chemistry of fluids during serpentinization of 21 different ultramafic rock compositions and 10 distinct compositions of olivine between 25 degrees C and 400 degrees C at 50 MPa. Our models predict some systematic differences between serpentinization of olivine-dominated lithologies (i.e. peridotite) and of orthopyroxene-dominated lithologies (i.e. pyroxenite). Most notably, it is predicted that serpentinization of peridotite causes the formation of serpentine having elevated Fe+3/(Fe+3 + Fe+2) values, Fe-bearing brucite (at temperatures <= ca. 320 degrees C), and magnetite (at temperatures >ca. 200 degrees 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 (aH(2(aq))), dissolved silica (aSiO(2(aq))), as well as the pH vary accordingly. At temperatures <= ca. 350 degrees C fluids interacting with peridotite are more reducing, have lower aSiO(2(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 aH(2(aq)) is apparent from our calculations. In contrast to forsterite-rich olivine, fayalite-rich olivine can be stable to temperatures as low as 180 degrees C in the presence of water. As a consequence, the predicted aH(2(aq)) for serpentinization of fayalite is maximal at temperatures <= 180 degrees C. (C) 2013 Elsevier B.V. All rights reserved.