Low‐temperature catalytic CO 2 hydrogenation with geological quantities of ruthenium: a possible abiotic CH 4 source in chromitite‐rich serpentinized rocks
Metal-catalyzed CO2 hydrogenation is considered a source of methane in serpentinized (hydrated) igneous rocks and a fundamental abiotic process germane to the origin of life. Iron, nickel, chromium, and cobalt are the catalysts typically employed in hydrothermal simulations experiments to obtain methane at temperatures >200°C. However, land-based present-day serpentinization and abiotic gas apparently develop below 100 °C, down to ~40-50°C. Here, we document considerable methane production in thirteen CO2 hydrogenation experiments performed in a closed dry system, from 20 to 90°C and atmospheric pressure, over 0.9 to 122 days, using concentrations of non-pretreated ruthenium equivalent to those occurring in chromitites in ophiolites or igneous complexes (from 0.4 to 76 mg of Ru, equivalent to the amount occurring approximately in 0.4 to 760 kg of chromitite). Methane production increased with time and temperature, reaching ~87 mg CH4 per gram of Ru after 30 days (2.9 mgCH4/gru/day) at 90°C. At room temperature, CH4 production rate was approximately three orders of magnitude lower (0.003 mgCH4/gru/day). We report the first stable carbon and hydrogen isotope ratios of abiotic CH4 generated below 100°C. Using initial δ13CCO2 of -40‰, we obtained room temperature δ13CCH4 values as 13C depleted as -142‰. With time and temperature the C-isotope separation between CO2 and CH4 decreased significantly and the final δ13CCH4 values approached that of initial δ13CCO2. The presence of minor amounts of C2-C6 hydrocarbons is consistent with observations in natural settings. Comparative experiments at the same temperatures with iron and nichel catalysts did not generate CH4. Ru-enriched chromitites could potentially generate methane at low temperatures on Earth and on other planets.This article is protected by copyright. All rights reserved.
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