Water-Limited Serpentinization Drives Abiotic Hydrocarbons Production in Ultramafic Environments

Abiotic methane and light hydrocarbons generated through serpentinization have attracted growing attention as potential energy sources for the deep biosphere and as pathfinders of natural H₂ production. Although Fischer–Tropsch-type (FTT) reactions are central to the abiotic reduction of oxidized carbon by H₂, key uncertainties persist regarding catalytic mechanisms and the influence of water. Here, we show that ultramafic rocks (chromitite and harzburgite) readily produce CH₄, and minor gaseous hydrocarbons upon reaction with H₂ and CO₂ at 200–250 °C under dry conditions. Kinetic analyses, mass balance calculations, and Raman microspectroscopy indicate that solid carbonaceous compounds constitute the dominant products, while CO acts as a short-lived intermediate. Additional tests reveal that water vapor inhibits CO₂ hydrogenation, implying that efficient FTT processes require spatial or temporal decoupling from active serpentinization. Yet serpentinization consumes water and generates H₂, ultimately sustaining hydrocarbon formation and pointing to a dynamic interplay between hydration reactions and abiotic organic synthesis in ultramafic settings.