Hydrogen (H2) production and coupled serpentinization kineticsduring peridotite hydrothermal alteration

Hydrogen (H₂) is an important clean energy source, and its geological production is closely linked to serpentinization. Serpentinization is a low-temperature (≤500 °C) hydrothermal alteration of ultramafic rocks (typically komatiites and peridotite), where olivine and pyroxene of ultramafic rocks are transformed into serpentine, (±) talc, and (±) brucite. Serpentinization occurs in diverse geological settings, including the ocean floor, mid-ocean ridges, and subduction zones. Critically, the process generates hydrogen, which sustains microbial communities in hydrothermal ecosystems. The H₂ originates from the reduction of water-derived H⁺, driven by the oxidation of ferrous iron (Fe²⁺) in olivine and pyroxene to ferric iron (Fe³⁺).

    In spite of its significance, hydrogen predicted based on thermodynamic models is around 1-3 orders of magnitude higher compared to hydrogen formed in experimental studies (McCollom and Bach, 2009; McCollom et al., 2016). This gap indicates that serpentinization kinetics may greatly influence H₂ formation. Consistently, most previous experiments were performed using olivine, with sluggish rates of serpentinization and very low H₂ concentrations, e.g., 2.8% of serpentine was produced at 300 °C and 500 bar after a reaction period of 111 days (McCollom et al., 2016). The concentrations of dissolved H₂ in fluids were only 11 mmol/kg, significantly lower than the maximum H₂ concentrations predicted under the same P-T conditions (~350 mmol/kg). The close association between serpentinization kinetics and H₂ formation has not been systematically investigated.

    This study involved serpentinization experiments on peridotite at 300 °C and 24-300 MPa under conditions of varying fluid pH, salinity, and the addition of N₂ and CO₂. The results demonstrate that H₂ yield is strongly controlled by the relative serpentinization rates of olivine versus pyroxene. Higher H₂ production correlates with conditions that accelerate olivine reaction relative to pyroxene, such as the addition of acidic, alkaline, or low-salinity (0.5 M NaCl) fluids. Conversely, in high-salinity fluids (1.5 M and 3.3 M NaCl), pyroxene reacts faster than olivine, and H₂ production is significantly suppressed (Huang et al., 2023). The addition of N₂ and CO₂ enhances the serpentinization of pyroxene but decreases H₂ production (Shang et al., 2023). We conclude that faster relative rates of olivine serpentinization enhance H₂ generation, whereas faster rates of pyroxene inhibit it. This inhibition is likely due to silica released during pyroxene alteration, which is known to suppress H₂ formation (Huang et al., 2024). Our study establishes the coupled kinetics of olivine and pyroxene as a key factor controlling the efficiency of natural hydrogen generation.

References:

Huang, R. F. et al. (2023). Journal of Geophysical Research: Solid Earth, https:doi.org/10.1029/2022JB025218.

Huang, R. F. et al. (2024) Science China: Earth Sciences, 67, 222-233.

McCollom, T. M. and Bach, W. (2009) Geochimica et Cosmochimica Acta, 73, 856-875.

McCollom, T. M. et al. (2016). Geochimica et Cosmochimica Acta, 181, 175-200.

Shang, X.Q et al. (2023) Science Bulletin, 68, 1109-1112