A first-principles molecular dynamics study of molecular hydrogen diffusion in Fe-free olivine

Molecular hydrogen (H₂) may be an important form of water in nominally anhydrous minerals in the Earth’s mantle and plays a critical role in mantle water cycle, but the transport properties of H₂ remain unclear. Here, the diffusion of H₂ in Fe-free olivine lattice is investigated at pressures of 1–13 GPa and temperatures of 1300–1900 K by first-principles molecular dynamics. The activation energy and activation volume for H₂ diffusion in Fe-free olivine are determined to be 55 ± 8 kJ/mol and 3.6 ± 0.2 cm³/mol, respectively. H₂ diffusion in Fe-free olivine is faster than H⁺ by 1–4 orders of magnitude and therefore it is more favorable for hydrogen transportation under upper mantle conditions. H₂ can be carried to the mantle transition zone by subducting slabs without releasing to the surrounding mantle. The upper mantle may act as a lid, preventing the releasing of H₂ produced in the deep mantle to the surface.