Low-temperature nanocatalysis of abiogenic methane during peridotite alteration

Mantle peridotite alteration can catalyse abiogenic hydrocarbon formation, particularly CH4. This gas sustains the deep biosphere, promotes gas hydrate formation, and may form gas reservoirs. Although CH4 synthesis is known to occur in the presence of metal alloys or chromite during serpentinization at T>200 °C, little is known about the potential for massive CH4 catalysis at lowtemperatures. Here, we present first results of our investigations into the natural catalyst within the massive, low-temperature (~50 °C1) CH4 seep at the Chimaera site, Turkey. CH4 has been used at this site for >2000 years and the original reservoir is estimated to be thousands to millions of cubic meters making it the Earth’s largest onshore abiogenic CH4 seep. Our investigations show that the serpentinites are massively enriched in chromite. Using micro-analytics we show that chromite alters to a mesoporous network of nanocrystalline spinel phases. We argue that the crystallographic preferred pore wall orientation, high surface area, and trace metal content of these spinel phases facilitates low- T CH4 synthesis with significant production capacities over timescales of several hundred to thousands of years. Using paleo-tectonic reconstructions we estimate the amount of CH4 production since ophiolite emplacement.