The Influence of CO ₂ on Hydrogen Generation and Reaction Kinetics During Olivine and Peridotite Serpentinization

To investigate the influence of carbon dioxide (CO 2 ) on hydrogen (H 2 ) generation and reaction rates during serpentinization, we performed experiments at 300°C and 2.0–3.4 kbar by reacting peridotite and olivine (25–50 μm of grain sizes) individually with NaCl‐KHCO 3 solutions. These included 0.5 M NaCl combined with 0.1 M, 0.5 M, and 0.9 M KHCO 3 , corresponding to a range of CO 2 concentrations. Hydrogen was quantified by gas chromatography, and serpentinization extent was determined using standard curves based on infrared spectroscopy. The results demonstrate that KHCO 3 concentration strongly influences H 2 generation and serpentinization kinetics. In peridotite experiments, 0.1 M KHCO 3 reduced H 2 production and slowed the serpentinization rate, whereas higher KHCO 3 concentrations (0.5 and 0.9 M) enhanced serpentinization. Despite the accelerated reaction, H 2 yield decreased slightly at 0.5 M KHCO 3 and dropped by an order of magnitude at 0.9 M KHCO 3 , with minor carbonates (<5 wt.%) observed after prolonged durations. In olivine experiments, CO 2 consistently suppressed the reaction rates and H 2 formation, and olivine reacted more slowly and produced significantly less H 2 than peridotite in CO 2 ‐bearing fluids. This study indicates that low CO 2 concentrations retard serpentinization in both peridotite and olivine, whereas higher CO 2 levels promote the reaction specifically in peridotite while suppressing H 2 generation. These findings suggest that the CO 2 content of geological fluids is a critical factor controlling H 2 generation that is a key energy source for deep subsurface life.