TERRESTRIAL PLANET SERPENTINIZATION IN MELANGE SETTINGS

Mars and Earth have planetary crusts with exposures of mantle-derived ultramafic rocks dominated by pyroxene and olivine. These initial minerals transform to serpentine, brucite, magnetite, and other secondary phases via metamorphic hydration and other reactions. In this work, altered peridotites of the regionally extensive Coast Range Ophiolite (CRO), from localities in the UC-Davis McLaughlin Natural Reserve, Lower Lake, CA are compared and contrasted with serpentinites of the Nili Fossae, a mélange terrain located in the Syrtis Major quadrangle at approximately 22°N, 75°E, Mars. The habitability of serpentinizing systems is conveyed as a function of changing system parameters (such as temperature, Eh, pH, and activities of aqueous geochemical species), and provides insight into the biological prospects of serpentinization in mélange terrains in a general sense. Petrography of the serpentinized Jurassic age ultramafic unit in the Coast Range Ophiolite confirms the dominance of secondary phases (serpentine, other clays, carbonates, magnetite) and presence of relict primary minerals (olivine and pyroxene). Major element concentrations for crystals of olivine (from McL-239A and McL-329), show concentrations of MgO (avg: 48.9 wt%), SiO2 (avg: 40.8 wt%) and high FeO (avg: 10.2 wt%) that are high relative to San Carlos, Olivine (standard). Backscatter electron images (BSE) of such crystals in McL-239A and McL-329 show relict olivine surrounded by a serpentine-rich matrix. The crystal chemistry of these representative samples of olivine from the CRO serve as an analog for the olivine-rich protolith that underwent serpentinization observed in the Nili Fossae. Using CRO data to construct a model protolith reasonable for ultramafics of the Nili Fossae mélange (constrained by CRISM observations), I deduce evolving habitability as the model protolith reacts with feasible, co-occurring fluid chemistries. Major aqueous geochemical compositions are based on based on postulated planetary analog waters that are largely Na-Cl, Mg-Cl, or Ca-Cl solutions. The modeled water-rock reactions were performed at conditions associated with both CRO and the Nili Fossae mélange settings. Habitability was assessed using a Gibbs Free Energy minimization strategy, for serpentinization-driven methanogenesis (MG) and methanotrophy (MT). I show that the bioenergetic yields of fundamental methanogenetic and methanotrophic reactions progress favorably between -120 kJ/mol to a maximum of -400 kJ/mol as serpentinization progresses under different groundwater/hydrothermal conditions in mélange terrains