Kinetics of low-temperature H2 production in ultramafic rocks by ferroan brucite oxidation

Ferroan brucite, (Mg,Fe)(OH)₂, is among the potential mineral candidates for low temperature (<423 K) abiotic H₂ production in ultramafic rocks. To verify this assumption, synthetic ferroan brucite with grain size similar to that observed in natural samples (40–100 nm) was reacted with pure water at temperatures ranging from 348 to 573 K. Experimental products are consistent with the reaction 3 Fe(OH)₂^brucite = Fe₃O₄ + H₂ + 2 H₂O. This reaction reached completion in ∼2 months at 378 K and is thermally activated with an activation energy of 145 ± 1 kJ/mol. The standard state formation enthalpy and the third law entropy of amakinite, Fe(OH)₂, were refined from the experimental dataset. The new thermodynamic parameters imply that ferroan brucite is stable at significantly lower hydrogen activity than previously calculated. The alteration of Fe-brucite produces H₂ at rates compatible with present day observations of H₂ emissions in natural settings (ophiolite and mid-oceanic ridges). However, efficient fluid renewal is required, as opposed to H₂ production through olivine serpentinisation, which can proceed in static hydraulic conditions.