Molecular hydrogen can be produced naturally in a number of geologic settings, from within earth’s continental crust, along subducting slabs, to its deep interior. While the modern energy sector takes most interest in hydrogen accumulation and preservation at near-surface conditions, we focus our attention to the preceding migration of hydrogen through mantle and crustal rocks. With hydrogen immiscibility driving preferential migration through a largely oxidized lithosphere, hydrogen must react with oxygen-bearing minerals, leading to potential back transformation into water. This work investigates reactions of molecular hydrogen with the most abundant mineral in earth’s upper mantle, olivine, and as well as a simple oxide, wustite, at mantle lithosphere conditions.

Here we present a series of single-stage piston-cylinder double capsule experiments: five H₂-olivine(Fo89) experiments and two H₂-wustite experiments. Pressure-temperature conditions ranged from 1-2 GPa and 400-800°C, while experiment durations ranged from seven days to six weeks. Quadrupole mass spectrometry data of all experiments show production of redox water, with up to ~3.5 µmol in the H₂-olivine experiments and up to ~55 µmol in the H₂-wustite experiments. Native iron products observed in reacted samples support a viable mechanism of Fe²⁺ reduction and subsequent H₂ oxidation into redox water. Together, results show that reduction potential of iron is significantly less in silicates than in oxides, that reaction kinetics vary significantly depending on both temperature and the phase in which iron is bound, and most importantly, that H₂ can reduce iron in typical mantle olivine (Fo89) to generate redox water at mantle lithosphere conditions.

water,olivine,hydrogen,piston-cylinder