Carbonate control of H 2 and CH 4 production in serpentinization systems at elevated P‐Ts

Serpentinization of forsteritic olivine results in the inorganic synthesis of molecular hydrogen (H 2 ) in ultramafic hydrothermal systems (e.g., mid‐ocean ridge and forearc environments). Inorganic carbon in those hydrothermal systems may react with H 2 to produce methane (CH 4 ) and other hydrocarbons or react with dissolved metal ions to form carbonate minerals. Here, we report serpentinization experiments at 200°C and 300 bar demonstrating Fe 2+ being incorporated into carbonates more rapidly than Fe 2+ oxidation (and concomitant H 2 formation) leading to diminished yields of H 2 and H 2 ‐dependent CH 4 . In addition, carbonate formation is temporally fast in carbonate oversaturated fluids. Our results demonstrate that carbonate chemistry ultimately modulates the abiotic synthesis of both H 2 and CH 4 in hydrothermal ultramafic systems and that ultramafic systems present great potential for CO 2 ‐mineral sequestration.