COMPARING TERRESTRIAL PLANET SERPENTINIZATION IN MELANGE TERRAINS

The planetary crusts of Mars and Earth have patchy distributions of mantle-derived ultramafic rock units, which alter in the presence of water to serpentinites, generating energy (via water-rock redox reactions, (Ehlmann et al., 2010) and carbon (largely CH₄ but other organics and condensed carbon phases are possible (Milesi et al., 2016)) that may fuel extreme life. Initial mineralogies in these parent rocks are dominated by pyroxene and olivine, and transform to serpentine, brucite, magnetite, and other minerals via hydration and other reactions. The primary minerals react with water at physicochemical conditions present in the lithosphere where planetary surface pressures and temperatures accommodate liquid water and the secondary minerals of interest are within their stability fields. In this study, we present modeling constraints on the formation of serpentinites in mélange terrains on Earth and Mars; both planets have documented serpentinites in a mélange terrain geological setting (Ehlmann et al., 2010). With geochemical data and pressure and temperature constraints derived from the mélange serpentinites of the Coast Range Ophiolite (CRO, northern CA, USA), we discuss geochemical changes associated with different vein-filling minerals in CRO serpentinites and infer how the chemistry of fluids passing through serpentinites changes over time. We place the observed data in a modeling context (using the REACT mode in the Geochemist’s Workbench) to understand the changing water-rock system responsible for the observed mineralogical and/or geochemical patterns. Modeled results will provide a foundation for inferring shifts in the endolithic habitability of serpentinite veins. Given that serpentinization involves the production of abiotic CH₄, H₂, and heat (via exothermic reactions) is a postulated driver of early life on Earth (Russell et al., 2010), this focused study of serpentinization in mélange terrains will clarify the habitable limits in this geologic setting.