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 H 2 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(OHThis 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 H 2 at rates compatible with present day observations of H 2 emissions in natural settings (ophiolite and mid-oceanic ridges).However, efficient fluid renewal is required, as opposed to H 2 production through olivine serpentinisation, which can proceed in static hydraulic conditions.