In SituDiamond-Anvil Cell Observations of Methanogenesis at High Pressures and Temperatures

Abstract

In situ high pressure−temperature (P−T) measurements on C−H−O fluids at lower crust and upper mantle conditions have been performed to assess the extent and limit of methane generation within the Earth. Experiments were conducted using diamond-anvil cells at temperatures from 250 to >1500 °C and pressures between 1 and 8 GPa, assuming conditions along a subduction-setting geotherm. In many experiments, methanogenesis is observed, in particular, in those experiments in which elemental carbon or reduced transition metals (e.g., Fe0) were present. The direct reduction of CO2 to methane was not observed under any set of conditions. Methanogenesis is sensitive to C−H−O fluid composition and, specifically, the activity of H2. Kinetic barriers to direct hydrogenation of CO persists over the range of conditions explored, and hydrogenated transition-metal carbonyl species likely play a critical role in methane formation. Surprisingly, the direct hydrogenation of graphite yields considerable methane. The observation of both CO2 and H2 generation during high T and P reactions of ferrous oxide, calcium carbonate, and water indicates that methanogenesis in the lower crust and upper mantle is plausible; however, the absolute yield of methane is strongly controlled by the activity of H2.