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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, a rock 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 are both capable of fueling microbial activity, and wet ultramafic environments are therefore 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 100-350C, where H₂O, CO₂, light hydrocarbons, ammonia, sulfate, B, and alkali elements are driven off the down going 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 (pH25C = 12.5) springs. These are probably 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 I have received funding from NSF, in addition to that from NASA-NAI, to look at this problem from a theoretical standpoint, in collaboration with Dr. Tom McCollom at the University of Colorado. Together we are modeling the processes that generate this extreme environment, including 1) dehydration and dissolution of subducted organic and inorganic carbon from the subducting oceanic plate with increasing temperature and pressure; 2) ascent of this water and dissolved carbon and its input into the ultramafic rock of the overriding plate; 3) serpentinization of this rock and accompanying generation of H₂; 4) reaction of H₂ with carbon to make methane that ultimately fuels extremophilic (high pH) microbial communities in the shallow subseafloor. This methane is abiogenic, so this process may be analogous to what may be happening on Mars.