Low‐temperature catalytic CO2 hydrogenation with geological quantities of ruthenium: a possible abiotic CH4 source in chromitite‐rich serpentinized rocks

Metal‐catalysed 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 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 CO₂ 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 CH₄ per gram of Ru after 30 days (2.9 mgCH₄/g_ru/day) at 90°C. At room temperature, CH₄ production rate was approximately three orders of magnitude lower (0.003 mgCH₄/g_ru/day). We report the first stable carbon and hydrogen isotope ratios of abiotic CH₄ generated below 100°C. Using initial δ¹³C_CO2 of ‐40‰, we obtained room temperature δ¹³C_CH4 values as ¹³C depleted as ?142‰. With time and temperature, the C‐isotope separation between CO₂ and CH₄ decreased significantly and the final δ¹³C_CH4 values approached that of initial δ¹³C_CO2. 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.