RedOx gradient as the main driver for magnetite formation during serpentinization: implications for natural H2 production

Serpentinization reaction is known as one of the main sources of natural H₂ at the Earth’s surface. Estimates of H₂ production during this reaction require an in-depth understanding of the mineralogical processes leading to iron oxidation. The study of serpentinized peridotites collected at 13 localities at mid-ocean ridges, in ophiolites and ultramafic bodies reveals the development of an alteration sequence during reaction.  At the olivine contact, a first reaction zone is composed of a fine-grained mixture of serpentine, Fe-brucite and awaruite (Reaction Zone 1). Thermodynamic modelling with the latest data for the Fe(OH)₂ endmember indicates that awaruite formation limits H₂ production with H₂ concentrations comprised between 10^-3 and 10^-2 mol/kg. These values are consistent with the maximum values measured in fluids expelled at ultramafic-hosted hydrothermal sites. At the mesh rim, a second alteration zone composed of Ni-bearing magnetite, serpentine and Mg-brucite is found (Reaction Zone 2). Serpentine and Mg-brucite display a porous symplectite microtexture, indicating formation after Reaction Zone 1 by a dissolution-precipitation process. Magnetite formation in Reaction Zone 2 could not be reproduced with thermodynamic modelling by modifying, as previously thought, temperature or water to rock ratio. However, removing H₂ from the system was found to reproduce both the mineralogy and the composition of Reaction Zone 2. This indicates that H₂ diffusion is the main driver for magnetite formation during serpentinization. The H_{2, aq} concentrations at the equilibrium with Reaction Zone 2 fall in the 10^-7 - 10^-3 mol/kg range. Based on the mineralogical observations and thermodynamic modelling performed here, two regimes for H₂ production during olivine serpentinization can be proposed. If H₂ diffusion is limited, the serpentinizing fluid contains between 10^-3 and 10^-2 mol/kg of H₂ but the overall H₂ production is one order of magnitude smaller than previous estimates. If H₂ diffusion proceeds rapidly, the overall H₂ production is comparable to previous estimates but the expected H₂ concentration in the serpentinization fluid at the equilibrium with the reaction products is extremely low (10^-7 to 10^-3 mol/kg).