Genesis of natural hydrogen: New insights from thermodynamic simulations

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The present work revises hypotheses to explain the molecular hydrogen genesis in Earth's crust based on iron cycle under water and sulfur environment. Indeed, known reactions are usually associated to minerals reacting with water under oxidizing conditions, such as the serpentinization. From geological observations, other gases have been identified in mid-oceanic ridges, in ophiolitic sites and in Proterozoic continental rocks (more than 550 million years) such as CH4, N2 and He in various proportions. We use thermodynamic simulations to predict the direction of a range of possible metamorphic reactions regarding Gibbs free energies as a function of depth in Earth's crust, temperatures up to 1500 °C, pressure up to 10 kbar and including the relative partial pressures of the gases expressed by the reaction quotient Qr. While the oxidation of ferrous into ferric iron forming molecular hydrogen has been already extensively studied through the serpentinization and siderite decomposition, the role of pyritization has been significantly underestimated. We show that iron-based oxide precursors under hydrogen sulfide lead to H2 formation and pyrite FeS2 down to the middle level of the crust (down to 12 km in the crust-using the thermal gradient of 30 °C, at ΔG = ΔG0, Qr = 1). As the crust can be considered as a semi-open system (with H2 leaking), its genesis is likely to appear also at deeper levels. The slow cooling of continental rocks through time in cratonic area is also compatible with the enhancement of pyritization reactions. The serpentinization is favored at lower temperatures. Those complementary processes can co-exist in black and white smokers and are key indices for hydrogen prospection as they involve redox reactions, impacting pH, with magnetic changes (through magnetite formation), thermal gradient changes and pyrite formation. The ‘olivinization’ of serpentinites (without hydrogen involved in the reaction), i.e. the inversion of serpentinization in order to close the iron cycle in the mantle, is evaluated to occur from 10 to 12 km using a normal thermal gradient. We find then that the iron cycle can be completed as the ferrous ion can be regenerated from the ferric iron via different mechanisms. It implies that natural hydrogen appears as a renewable raw matter on Earth, with clear interest in terms of industrial valorization.