Abiotic methane formation during experimental serpentinization of olivine

Fluids circulating through actively serpentinizing systems are often highly enriched in methane (CH₄). In many cases, the CH₄ in these fluids is thought to derive from abiotic reduction of inorganic carbon, but the conditions under which this process can occur in natural systems remain unclear. In recent years, several studies have reported abiotic formation of CH₄ during experimental serpentinization of olivine at temperatures at or below 200 °C. However, these results seem to contradict studies conducted at higher temperatures (300 °C to 400 °C), where substantial kinetic barriers to CH₄ synthesis have been observed. Here, the potential for abiotic formation of CH₄ from dissolved inorganic carbon during olivine serpentinization is reevaluated in a series of laboratory experiments conducted at 200 °C to 320 °C. A ¹³C-labeled inorganic carbon source was used to unambiguously determine the origin of CH₄ generated in the experiments. Consistent with previous high-temperature studies, the results indicate that abiotic formation of CH₄ from reduction of dissolved inorganic carbon during the experiments is extremely limited, with nearly all of the observed CH₄ derived from background sources. The results indicate that the potential for abiotic synthesis of CH₄ in low-temperature serpentinizing environments may be much more limited than some recent studies have suggested. However, more extensive production of CH₄ was observed in one experiment performed under conditions that allowed an H₂-rich vapor phase to form, suggesting that shallow serpentinization environments where a separate gas phase is present may be more favorable for abiotic synthesis of CH₄.