Insights into molecular hydrogen solubility in SiO2 polymorphs from redox-buffered experiments at subduction conditions

The incorporation of hydrogen into various nominally anhydrous rock-forming minerals is well-established. While the focus on hydrogen solubility has dominantly centered on the formation of hydroxyl groups, emerging evidence suggests that the diffusion of molecular hydrogen (H2) also plays a significant role in the Earth's interior. Specifically, the mobility of H2 can impact the δD signature of mantle minerals and contribute to redox reactions, potentially influencing the equilibria among minerals, fluids, and melts. In this context, subduction zones emerge as a crucial region for hydrogen production. However, the processes of hydrogen incorporation and release, as well as the potential minerals involved, remain unclear. To address this gap, we explored the solubility of H2 in SiO2 polymorphs under a range of P-T-redox state conditions relevant to subduction settings (1–9.6 GPa, 500–900 °C, ∆FMQ ~0 to ~–5.7) running piston-cylinder and multi-anvil experiments buffered using the double capsule technique. In each experiment the inner capsule was filled with a mixture of powdered ultrapure quartz sand (Sigma-Aldrich), previously calcinated at 1000 °C, and ultrapure water, while the outer capsule contained water and a mineral assemblage capable of buffering the hydrogen fugacity. After each run, a capsule-piercing device directed the quenched fluids from the inner capsules to a quadrupole mass spectrometer. The quenched fluids exhibited significant H2 concentrations, reaching a maximum of ~33 mol% for quartz at 2 GPa, 700 °C, and ∆FMQ ~–5.7. Hydrogen concentration decreased under more oxidized conditions, higher temperatures, and lower pressures. No detectable hydrogen was found in the fluid from the experiment in the stishovite stability field. Remarkably, the measured H2 concentrations surpassed those predicted by thermodynamic modeling for an aqueous fluid at the investigated P-T-redox state conditions by up to two orders of magnitude. Using single quartz crystals as a starting material rather than powder did not alter the H2/SiO2 molar ratio, ruling out surface effects. Thus, we support the hypothesis that H2 is soluble in quartz and coesite crystal lattice (up to ~4000 µg H2/g SiO2), with bulk diffusion mechanisms consistent with observed reversible hydrogen release upon cooling and/or depressurization. The transition to a denser octahedrally coordinated structure appears to hamper this process. Our experiments suggest that, at conditions encountered in subduction zones, quartz and coesite can act as sinks for molecular hydrogen, with important amounts of H2 dissolved even at the relatively oxidized conditions of ∆FMQ ~–1.6. As the process is largely reversible, changes in P-T-redox state may induce a release of hydrogen at depth, potentially perturbing the redox state of subduction channel rocks and, ultimately, the mantle wedge.