Low‐temperature catalytic CO 2 hydrogenation with geological quantities of ruthenium: a possible abiotic CH 4 source in chromitite‐rich serpentinized rocks

Metal‐catalysed CO 2 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 simulation experiments to obtain methane at temperatures >200°C. However, land‐based present‐day serpentinization and abiotic gas apparently develop below 100°C, down to approximately 40–50°C. Here, we document considerable methane production in thirteen CO 2 hydrogenation experiments performed in a closed dry system, from 20 to 90°C and atmospheric pressure, over 0.9–122 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 0.4–760 kg of chromitite). Methane production increased with time and temperature, reaching approximately 87 mg CH 4 per gram of Ru after 30 days (2.9 mg CH 4 /g ru /day) at 90°C. At room temperature, CH 4 production rate was approximately three orders of magnitude lower (0.003 mg CH 4 /g ru /day). We report the first stable carbon and hydrogen isotope ratios of abiotic CH 4 generated below 100°C. Using initial δ 13 C CO 2 of ‐40‰, we obtained room temperature δ 13 C CH 4 values as 13 C depleted as −142‰. With time and temperature, the C‐isotope separation between CO 2 and CH 4 decreased significantly and the final δ 13 C CH 4 values approached that of initial δ 13 C CO 2 . The presence of minor amounts of C 2 ‐C 6 hydrocarbons is consistent with observations in natural settings. Comparative experiments at the same temperatures with iron and nichel catalysts did not generate CH 4 . Ru‐enriched chromitites could potentially generate methane at low temperatures on Earth and on other planets.