High production and fluxes of H2 and CH4 and evidence of abiotic hydrocarbon synthesis by serpentinization in ultramafic-hosted hydrothermal systems on the Mid-Atlantic Ridge

Between 12° and 40°N on the Mid-Atlantic Ridge (MAR), high- or low-temperature hydrothermal activity and mantle degassing are indicative of ongoing serpentinization process. Chemical composition of fluids from ultramafic-hosted hydrothermal fields (Rainbow, 36°14' N; Lost City, 30°N Logatchev I and II, 14°45' N; Ashadze I and II, 12°58' N), all located along the MAR is compared here. The uniformity in end-member major, minor element concentrations, and gas contents suggest that, at each ultramafic field, all vented fluids are issued from a nearly identical source. In all cases, vent fluids show low H₂S content (<1 mmol kg^-1), extraordinary high H₂ (10 to 26 mmol kg^-1) end-member concentrations compared to basalt-hosted fluids. Hydrogen production represents between 40% to 80% of the total extracted gas volume. The total hydrogen discharge Φ_H2 is found to be between 2.5 to 7.5 million standard cubic meters per year for the Rainbow single site. Based on Rainbow H₂/³He and ³He/heat ratios, a global H₂ flux for slow spreading ridges of 2 × 10⁹ m³ STP a^-1 is estimated. Like basalt-hosted fluids, the ultramafic-hosted fluids are controlled by phase separation. But everywhere, H₂ content is extraordinarily enriched in low or high chlorinity phases, demonstrating that the serpentinization process is mainly responsible for hydrogen production. As a consequence of the high reducing power of these systems, isotopic measurements of light hydrocarbons show that abiogenic hydrocarbons are generated by Fischer-Tropsch type reaction. The serpentinization of ultramafics is a common feature occurring along the MAR and strongly contributes to the whole budget of hydrogen and abiogenic methane on Earth.