Hydrogen generation from serpentinization of iron-rich olivine on Mars, icy moons, and other planetary bodies

Abstract

Serpentinization of olivine-rich ultramafic rocks is increasingly recognized to have been widespread in the solar system throughout its history. This process has gained particular attention among planetary scientists because it generates molecular hydrogen (H2) and methane (CH4), compounds that can supply metabolic energy to biological communities and contribute to greenhouse warming of planetary atmospheres. While serpentinization of olivine has been the subject of numerous experimental and theoretical studies over the years, this work has focused almost exclusively on the magnesium-rich olivine that is characteristic of the terrestrial mantle. In contrast, very few studies have examined serpentinization of the more iron-enriched olivine compositions that may be more common in other rocky planetary bodies in the solar system. Accordingly, a series of thermodynamic models were constructed to investigate secondary mineral formation and H2 generation during serpentinization of olivine as a function of Fe content and temperature. The results show that serpentinization of Fe-rich olivine is capable of generating substantially greater amounts of H2 per mole than is observed for serpentinization of Mg-rich olivine, by a factor of two to ten depending on temperature and olivine composition. For all olivine compositions, H2 is generated from ferric Fe incorporated into both magnetite and serpentine at higher temperatures; however, it is derived exclusively from precipitation of serpentine at lower temperatures as brucite replaces magnetite in the equilibrium secondary mineral assemblage. Serpentine and brucite formed during serpentinization are predicted to become increasingly enriched in Fe with greater Fe content of the original olivine, although the serpentine has proportionally lower Fe contents than the olivine while brucite has somewhat higher Fe contents. Between 10% and 40% of the Fe partitioned into serpentine occurs in the ferric state (FeIII), which accounts for a substantial fraction of H2 production for most conditions. In addition, the maximum temperature at which olivine undergoes serpentinization decreases with increasing Fe content of the olivine, so that Fe-enriched olivine can remain stable to lower temperatures when compared with more Mg-rich olivine compositions. Overall, the results indicate that serpentinization on other planetary bodies may have a substantially greater capacity to supply H2 to support biological communities and enhance atmospheric greenhouse warming than analogous processes on Earth.