Reduction of CO₂ during serpentinization of olivine at 300°C and 500 bar

CO₂ reduction processes occurring during experimental serpentinization of olivine at 300°C and 500 bar confirm that ultramafic rocks can play an important role in the generation of abiogenic hydrocarbon gas. Data reveal that conversion of Fe(II) in olivine to Fe(III) in magnetite during serpentinization leads to production of H2 and conversion of dissolved CO₂ to reduced-C species including methane, ethane, propane, and an amorphous carbonaceous phase. Hydrocarbon gases generated in the process fit a Schulz-Flory distribution consistent with catalysis by mineral reactants or products. Magnetite is inferred to be the catalyst for methanization during serpentinization, because it has been previously shown to accelerate Fischer-Tropsch synthesis of methane in industrial applications involving mixtures of H₂ and CO₂. The carbonaceous phase was predominantly aliphatic, but had a significant aromatic component. Although this phase should ultimately be converted to hydrocarbon gases and graphite, if full thermodynamic equilibrium were established, its formation in these experiments indicates that the pathway for reduction of CO₂ during serpentinization processes is complex and involves a series of metastable intermediates.