Carbonate control of H₂ and CH₄ production in serpentinization systems at elevated P-Ts

Serpentinization of forsteritic olivine results in the inorganic synthesis of molecular hydrogen (H₂) in ultramafic hydrothermal systems (e.g., mid-ocean ridge and forearc environments). Inorganic carbon in those hydrothermal systems may react with H₂ to produce methane (CH₄) 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 Fe²⁺ being incorporated into carbonates more rapidly than Fe²⁺ oxidation (and concomitant H₂ formation) leading to diminished yields of H₂ and H₂-dependent CH₄. In addition, carbonate formation is temporally fast in carbonate oversaturated fluids. Our results demonstrate that carbonate chemistry ultimately modulates the abiotic synthesis of both H₂ and CH₄ in hydrothermal ultramafic systems and that ultramafic systems present great potential for CO₂-mineral sequestration.