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