Unexpected high amounts of H2 produced during serpentinization at magma-poor rifted margins

At magma-poor rifted margins, serpentinization of lherzolitic mantle rocks releases molecular hydrogen (H2) that supports chemosynthesis-based deep life. Until now, however, H2 fluxes in these systems remain largely unquantified. To help closing this knowledge gap we investigated serpentinization and H2 production using drill core samples from the West Iberia margin (Ocean Drilling Program Leg 103, Hole 637A). The mostly lherzolitic samples are strongly serpentinized, consist of serpentine with little magnetite, and are generally brucite-free. Serpentine can be uncommonly Fe-rich, with XMg = Mg/(Mg+Fe) < 0.8, and exhibits distinct compositional trends towards a cronstedtite endmember. Bulk rock and silicate fraction Fe(III)/∑Fe ratios range from 0.6–0.92 and 0.58–0.8, respectively. Our data show that more than 2/3 of the ferric Fe is accounted for by Fe(III)-serpentine. Mass balance and thermodynamic calculations suggest that the initial serpentinization of the samples at temperatures of <200°C likely produced about 100–250 mmol H2 per kg rock, which is 2–3 times more than previously estimated. The cold, late-stage weathering of the serpentinites at the seafloor caused additional H2 formation. Owing to generally lower geothermal gradients, the amounts of H2 produced under conditions close to/within the habitable zone at magma-poor margins are likely larger than those at slow-spreading mid-ocean ridges. These settings may hence be particularly suitable environments for hydrogenotrophic microbial life.