2:00 PM - 2:15 PM
[BBC03-02] Experimental study on hydrogen generation during low temperature serpentinization by using natural samples
Keywords:serpentinization, serpentine, ultramafic rock, hydrogen, amorphous Mg-silicate mineral, thermodynamic calculation
The results of experiments show that samples with lower degrees of serpentinization tend to generate larger amounts of hydrogen, and that harzburgites tend to generate more hydrogen than dunite when they are at the same degree of serpentinization. However, some highly serpentinized samples also generated large amounts of hydrogen though these results werepooly reproduced. This may have stemmed from the oxidation of trace minerals (e.g., Ni-Fe alloy) or their catalytic effects, in addition to the dissolution of Fe2+-bearing primary minerals, because these minerals were heterogeneously observed by microscopy in the serpentinites. Experiments were also conducted with some additives (e.g., MgO, SiO2, and magnetite) to investigate the effect of solution chemistry and formation of secondary minerals on hydrogen generation. The results show that hydrogen was generated the most under Si-rich conditions. Solution chemistry suggests that, olivine and pyroxene were likely dissolved more rapidly with Si-rich solutions. This is consistent with the trend that harzburgites generate hydrogen more than dunite because harzburgite contains pyroxene, which dissolved much Si compared to olivine. Moreover, thermodynamic calculations suggest that the precipitation of amorphous Mg-silicate minerals (e.g., M-S-H), which are commonly precursors of serpentine controls the solution chemistry including decreasing pH. Because the dissolution rate of the primary minerals is greater at lower pH, the precipitation of Mg-silicate minerals may have promoted the dissolution cycle of the primary minerals and therefore generation of hydrogen, especially under Si-rich conditions.
Mayhew, L. E., Ellison, E. T., Miller, H. M., Kelemen, P. B., & Templeton, A. S. (2018). Iron transformations during low temperature alteration of variably serpentinized rocks from the Samail ophiolite, Oman. Geochimica et Cosmochimica Acta, 222, 704–728.