Abiotic Methane Generation via CO₂ Hydrogenation With Natural Chromitite Under Hydrothermal Conditions

<jats:title>Abstract</jats:title><jats:p>In modern continental serpentinization systems and ultramafic rock‐hosted sub‐seafloor hydrothermal systems, it is believed that chromitite plays an important role in abiotic hydrocarbon generation. Previous experiments have suggested that chromite acts as a catalyst of CH<jats:sub>4</jats:sub> generation, but the composition of chromite used in the previous experiment is unrealistic on Earth. On the other hand, other studies have suggested that natural chromite including Mg and Al cannot function as a catalyst for CH<jats:sub>4</jats:sub> generation. Consequently, it still remains uncertain whether naturally occurring Cr‐rich minerals promote CH<jats:sub>4</jats:sub> generation. We monitored the reaction between naturally occurring chromitite and CO<jats:sub>2</jats:sub>‐rich fluid at 300°C, 500 bars. We performed two experiments in different initial CO<jats:sub>2</jats:sub>/H<jats:sub>2</jats:sub> ratios. In both experiments, CH<jats:sub>4</jats:sub> was generated immediately after the beginning of experiments. When CO<jats:sub>2</jats:sub> was more abundant than H<jats:sub>2</jats:sub>, the CH<jats:sub>4</jats:sub> concentration in the fluid decreased below the detection limit value. On the other hand, when H<jats:sub>2</jats:sub> was more abundant than CO<jats:sub>2</jats:sub>, the CH<jats:sub>4</jats:sub> concentration in the fluid was maintained above 0.01 mmol/kg. This is the first report to demonstrate that naturally occurring Cr‐rich minerals act as a promotor of CH<jats:sub>4</jats:sub> generation. Cr‐rich minerals such as Cr spinel are common accessory mineral in ultramafic rock. Therefore, on the early Earth, a certain level (on the order of 0.01 mmol/kg) of CH<jats:sub>4</jats:sub> was likely produced through reactions between ultramafic rock and CO<jats:sub>2</jats:sub>‐bearing fluid. To produce more abundant CH<jats:sub>4</jats:sub>, more favorable conditions featuring greater quantities of Cr spinel and much higher H<jats:sub>2</jats:sub> concentration (H<jats:sub>2</jats:sub>/CO<jats:sub>2</jats:sub> ratio) must be necessary.</jats:p>