6. Conclusion
We performed synthesis of abiogenic methane in the laboratory and determined δ13C, δD, Δ13CH3D and Δ12CH2D2 using a Panorama gas-source mass spectrometer. We carried out a set of 9 experiments in hydrothermal conditions, at temperatures between 130 and 300 °C. The experiments were performed by heating an aqueous solution in the presence of Fe0 powder and gaseous CO. We find δ13C and δD of methane to be isotopically depleted relative to the starting materials, consistent with equilibrium carbon isotope fractionation between CO2 produced in the experiment and the product CH4, and kinetic isotope D/H fractionations between H2 and product CH4. We find most of the product methane Δ13CH3D values trace the temperature of abiotic methane synthesis within a permil. At the same time, we observe Δ12CH2D2deficits of up to 40‰ relative to equilibrium attributable to a D/H combinatorial effect occurring during the methane assembly. The combinatorial effect results from our inability to determine D/H ratios of distinctive pools of hydrogen that contribute to individual hydrogens on the methane molecule. The D/H fractionation associated with the various steps of hydrogen addition to carbon can explain the direction and the magnitude of the Δ12CH2D2 signatures of methane that would otherwise be at bond-ordering equilibrium. In instances of some potentially abiotic methane found on Earth,, particularly under high T, Δ12CH2D2 deficits may have been generated during methane synthesis, but were subsequently erased by D/H reordering. In contrast, Δ12CH2D2 deficits generated in our experiments are seen on Earth at sites where the C-H bond activation was too slow to re-equilibrate Δ12CH2D2 to environmental temperatures. As such these dramatic deficits, when combined with near-equilibrium Δ13CH3D values, might be an avenue for the identification of abiogenesis on other worlds.