Abstract
Methane clumped isotope signatures of abiogenesis may be diagnostic of the origin of methane on Earth and other planetary bodies. On Earth, identifying methane synthesis pathways has proven challenging because of the lack of unambiguous signatures of the provenance of methane molecules. The abundances of mass-18 isotopologues of methane,13CH3D and12CH2D2, could help identify occurrences of abiotic methane in nature. We performed synthesis of abiogenic methane in the laboratory and determined δ13C, δD, Δ13CH3D and Δ12CH2D2. We carried out a set of experiments in hydrothermal conditions between 130 and 300 °C. The experiments were performed by heating water in the presence of Fe0 powder and CO. The reduction of water on metallic iron led to the formation of H2. CO was reacted with both H2 and H2O, generating both CH4 and CO2. We systematically extracted and quantified methane, and determined δ13C, δD Δ12CH2D2, and Δ13CH3D of the methane using a Panorama gas-source mass spectrometer (Nu, Ametek). We observe δ13C and δD of methane to be isotopically depleted relative to the starting materials. The δ13C data indirectly suggest isotopic equilibrium may have been reached for carbon isotopes between methane, carbon monoxide and carbon dioxide in our experiments. In contrast, D/H ratios are inconsistent with equilibrium isotopic fractionation. This suggests that under our experimental conditions, hydrogen additions to carbon are governed by kinetics, and that subsequent D/H re-equilibration was limited. While Δ13CH3D values approximately track experimental temperature, Δ12CH2D2 values are displaced far from equilibrium. We find exclusively negative Δ12CH2D2 values, showing deficits down to 40‰ relative to thermodynamic equilibrium. We interpret the data as evidence for distinct, kinetically induced D/H pools contributing to methane assembly (a combinatorial effect). The cumulative D/H fractionations associated with CO (or CO2in nature) hydrogenation likely explain the direction and magnitude of Δ12CH2D2 values during abiotic methane formation. We suggest that near equilibrium Δ13CH3D with negative Δ12CH2D2 signatures will help identify methane formed abiotically in nature.