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Project Progress

Ultramafic rocks of the mantle comprise two-thirds of Earth’s mass and 83% of its volume, and are abundantly represented within other bodies in the inner solar system. When H₂O is added to these Mg- and Fe(II)-rich silicates the product is serpentinite, typically composed of serpentine, Mg₃Si₂O₅(OH)₄, brucite, Mg(OH)₂, and magnetite, Fe₃O₄. The resulting oxidation of Fe(II) by H₂O produces H₂, which can then react with CO₂ to produce methane. Abiogenic methane and H₂ can both fuel microbial activity, and wet ultramafic environments are a promising place to look for extraterrestrial life. We have been studying such an environment within the seafloor, in the outer forearc of the Mariana subduction system, where Pacific lithosphere subducts beneath the Philippine plate. Sediment and altered Pacific crust devolatilize progressively as they subduct, especially between 10 and 30 km depth at 100-350°C, where H₂O, CO₂, light hydrocarbons, ammonia, sulfate, B, and alkali elements are driven off the downgoing plate. H₂O converts the overlying mantle to serpentinite that, because of its lower density, rises buoyantly along fractures in the forearc to form large (50 km across and 2 km high) serpentinite mud volcanoes on the seafloor. Excess water exits from serpentinite at their summits as highly alkaline (pH25ºC = 12.5) springs. These are perhaps the highest pH waters ever found on Earth, yet these methane-rich upwelling waters support abundant Archaea that anaerobically oxidize methane (AOM) to carbonate while reducing sulfate, all at an in-situ pH of 13.1. Such a high pH cannot be generated by serpentinization alone. In the past year we have gained significant insight into the processes that generate this extreme environment. They involve input of dissolved inorganic carbon from dissolution of CaCO₃, triggered by a mineralogical transition from lawsonite, a hydrous Ca-silicate stable up to ~225ºC, to epidote, a Ca-silicate stable at higher temperatures.