Abiotic methane formation during experimental serpentinization of olivine

Significance

Abiotic methane discharged from serpentinizing rocks supplies metabolic energy to chemosynthetic microbial communities and may have done so since the earliest lifeforms evolved on Earth. Several recent reports have claimed observation of abiotic formation of methane during low-temperature serpentinization of olivine-rich rocks during laboratory experiments. However, using ¹³C-labeled carbon sources, this study shows that the methane observed in such experiments is predominantly derived from background sources rather than abiotic synthesis. Conversely, more rapid production of methane is observed when an H ₂-rich vapor phase is present within the reaction vessel. Overall, the results indicate that in situ abiotic synthesis may contribute less methane to near-surface serpentinites than some recent studies have suggested.

Abstract

Fluids circulating through actively serpentinizing systems are often highly enriched in methane (CH ₄). In many cases, the CH ₄ 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 CH ₄ 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 CH ₄ synthesis have been observed. Here, the potential for abiotic formation of CH ₄ 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 CH ₄ generated in the experiments. Consistent with previous high-temperature studies, the results indicate that abiotic formation of CH ₄ from reduction of dissolved inorganic carbon during the experiments is extremely limited, with nearly all of the observed CH ₄ derived from background sources. The results indicate that the potential for abiotic synthesis of CH ₄ in low-temperature serpentinizing environments may be much more limited than some recent studies have suggested. However, more extensive production of CH ₄ was observed in one experiment performed under conditions that allowed an H ₂-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 CH ₄.