Subducted hydrogen is crucial in controlling chemical equilibria and influencing physical properties of Earth's interior. While water and hydroxyl (OH–) groups are traditionally considered the dominant hydrogen species at depth, emerging evidence suggests that molecular hydrogen (H2) can be incorporated in significant concentrations within various rock-forming silicates under high-pressure, high-temperature conditions.

To investigate the factors promoting H2 incorporation in silicates in silicates, we conducted double-capsule, piston-cylinder and multi-anvil experiments (1–9.6 GPa, 500–950 °C, ΔFMQ ≈ –5.7 to +0.4) on a model system composed of crystalline SiO2 and water. Analysis of experimental fluids coexisting with SiO2, obtained using a capsule-piercing device connected to a quadrupole mass spectrometer, revealed an excess of H2 in most runs compared to thermodynamic models and reference blank experiments. This discrepancy is attributed to H2 absorption into quartz (and coesite) during the experiment, followed by rapid release upon quench and depressurization. Estimated absorbed H2 concentrations range from ~100 to ~2100 µg H2 per gram SiO2, increasing with pressures and under more reducing conditions, but decreasing with temperature. Based on this data, we developed an empirical model to quantify H2 absorption as a function of pressure and temperature at redox conditions controlled by the wüstite-magnetite buffer.

Our findings indicate that H2 absorption in silicates is most pronounced in the forearc-to-subarc regions of subduction zones, potentially representing a non-negligible contribution to the deep hydrogen cycle. Furthermore, due to the substantial reversibility of this process, variations in pressure, temperature and redox conditions can induce H2 release, potentially altering the redox state of the surrounding environment. Desorption may ultimately produce H2-rich fluids, which can foster the formation of reduced species and minerals or contribute to hydrous redox melting processes.