Observational constraints on the process and products of Martian serpentinization

Abstract

The alteration of olivine-rich rocks to serpentine minerals, (hydr)oxides, and H2(aq) through serpentinization is long thought to have influenced the distribution of habitable environments on early Mars and the evolution of the early Martian hydrosphere and atmosphere. Nevertheless, the planetary importance of Martian serpentinization has remained a matter of debate; H2 production rates and mechanisms are poorly understood for serpentinization of Fe-rich Martian olivines, and orbital surveys suggest that serpentines are rarely present in the Martian crust. To constrain the process and products of Martian serpentinization, we acquired petrographic, geochemical, mineralogical, and synchrotron-based spectroscopic data on serpentinized Fe-rich olivines from the 1.1 Ga Duluth Complex (Minnesota, USA). These data indicate that serpentinized Fe-rich olivine would have been accompanied by a five-fold increase in H2 production relative to serpentinized terrestrial mantle peridotites. In contrast to previous expectations of mineral products, a vacancy-coupled Fe3+ substitution for Mg2+ in the serpentine octahedral site yields hisingerite (with minor magnetite) as the dominant Fe serpentine mineral at comparatively low temperature and pH, consistent with meteorite mineralogy and in-situ rover data. The widespread occurrence of Fe3+-phyllosilicates in highly magnetized regions of the Martian crust supports the hypothesis that serpentinization was more pervasive on early Mars than currently estimated. Our results provide the Mars 2020 Perseverance Rover mission with new and specific mineralogical and geochemical signatures of serpentinization and H2 production as it approaches the largest olivine-bearing lithological unit identified on the surface of Mars.