μXANES study of iron redox state in serpentine during oceanic serpentinization

Abstract

Serpentinization of ultramafic rocks at mid-ocean ridges generates significant amounts of H2, CH4, and supports specific biological communities. The abiotic H2 production is attributed to the reduction of H2O during serpentinization, which balances oxidation of ferrous iron contained in primary minerals (mainly olivines and pyroxenes) to ferric iron contained in secondary minerals (mainly serpentines and magnetite). Magnetite has thus far been considered as the sole Fe3+-carrier for estimating bulk H2 production, notably because the valence of iron in serpentine minerals and its relationship with both magnetite abundance and serpentinization degree are usually not determined. We show that the serpentine contribution to the Fe and Fe3+ budget has a significant effect on H2 production. We performed μ-XANES analysis at the Fe K-edge on thin sections of peridotites with various degrees of serpentinization from ODP Leg 153 (MARK region, 23°N). Fe3+/FeTot in oceanic serpentines is highly variable (from ~0.2 to 1) at the thin section scale, and it is related non-linearly to the local degree of serpentinization. A typical value of 0.7 is observed above 60% serpentinization. The highest values of Fe3+/FeTot observed within or close to late veins suggest that the Fe3+/FeTot in serpentine record the local water–rock (W/R) ratio, as previously proposed from thermodynamic modeling. We estimate that the (W/R) ratio increased from ~0.6 to 25 during serpentinization at MARK, and locally reached ~100 in veins. Mass balance calculations combining all mineral and bulk rock analyses provide the distribution of Fe and Fe3+ as serpentinization progresses. Serpentine dominates the Fe3+ budget of the rock over magnetite during the first 75% of serpentinization, contributing up to 80% of the total Fe3+. At later stages, serpentine contribution to the Fe3+ budget decreases down to ~20%, while magnetite formation exponentially increases. Iron transfer from serpentine to magnetite balances the bulk Fe3+ content of the rock that increases almost linearly with the advance of the reaction. Formation of serpentine accounts for the majority of Fe3+ and H2 production at early stages of serpentinization at a depth >2km at MARK where the concentration of H2 can reach more than 100mM according to the low W/R. H2 production values and depths can vary from one site to another, depending on the evolution of the temperature, W/R ratio, inlet fluid composition, and favored formation of serpentine vs. magnetite. At MARK, Fe3+ in serpentine represents 15–27% of the total Fe contained in a rock serpentinized to more than 80%, and accounts for 25% of the total H2 production that is estimated at 325–335mmol/kg of rock. The absence of magnetite does not necessarily mean a negligible H2 production, even at low T conditions (<150–200°C) under which the Fe- and Fe3+-richest serpentines have been observed. Serpentine minerals are important Fe3+-carrier in the altered ocean lithosphere, and may affect mantle redox state while dehydrating at depth in subduction zones.