Serpentinization-Driven H2 Production From Continental Break-Up to Mid-Ocean Ridge Spreading: Unexpected High Rates at the West Iberia Margin

Molecular hydrogen (H 2 ) released during serpentinization of mantle rocks is one the main fuels for chemosynthetic life. Processes H production at slow-spreading mid-ocean ridges (MORs) have received much attention in past. Less well understood passive continental margins where different rock types are involved (lherzolite instead harzburgite/dunite MORs) and alteration temperatures tend to be lower (<200°C vs. >200°C). To help closing this knowledge gap we investigated drill core samples from West Iberia margin. Lherzolitic compositions spinel geochemistry indicate that exhumed peridotites resemble sub-continental lithospheric mantle. The strongly serpentinized, mainly consist serpentine with little magnetite, generally brucite-free. Serpentine can uncommonly Fe-rich, X Mg = Mg/(Mg + Fe) < 0.8, shows distinct compositional trends toward a cronstedtite endmember. Bulk silicate fraction Fe(III)/?Fe ratios 0.6–0.92 0.58–0.8, respectively; our data show 2/3 ferric Fe accounted by Fe(III)-serpentine. Mass balance thermodynamic calculations suggest sample’s initial produced ?120 >300 mmol per kg rock. cold, late-stage weathering serpentinites seafloor caused additional formation. These results generation potential evolves transition break-up ultraslow and, eventually, slow MOR spreading. Metamorphic phase assemblages systematically vary between these settings, which has consequences yields serpentinization. At magma-poor rifted ultraslow-spreading MORs, hosts most Fe(III). Hydrogen 120 50–150 rock, respectively, may expected <200°C. contrast, produce 200–350 , related magnetite formation >200°C. Since, comparison geothermal gradients MORs lower, larger volumes low-temperature serpentinite should form settings. Serpentinization lherzolitic particularly high amounts under conditions within habitable zone. Magma-poor hence more relevant environments hydrogenotrophic microbial life than previously thought.