Serpentinization produces molecular hydrogen (H₂) that is capable of supporting communities of microorganisms in hydrothermal fields, which suggests that serpentinization may be closely related to the origin of life at the early history of the Earth and possibly other planets. In this study, serpentinization experiments were performed at 300 °C and 3.0 kbar with natural olivine and peridotite as starting reactants to quantify the influence of acidic and alkaline solutions on the processes of serpentinization. The results reveal that acidic and alkaline solutions greatly influence molecular hydrogen (H₂) generation and the rates of serpentinization. Acidic (pH = 2.50) and alkaline solutions (pH = 13.5) increased H₂ production and the rates of peridotite serpentinization. Highly acidic solutions (2 M HCl), however, decreased the production of H₂ after peridotite serpentinization by around two orders of magnitude. The decrease in H₂ production was associated with a sharp decline in the rates of reaction; e.g., when peridotite was reacted with neutral solutions (0.5 M NaCl), 88% of reaction progress was achieved after an experimental duration of 27 days, and the reaction extent decreased by ~50% for experiments with highly acidic solutions (2 M HCl) over the same period. In contrast, for experiments with solely olivine, highly acidic solutions (2 M HCl) promoted the rates of olivine serpentinization and H₂ production. The contrasting effect of highly acidic solutions (2 M HCl) on the processes of olivine and peridotite serpentinization may reflect the influence of pyroxene minerals, which could release SiO₂ during peridotite serpentinization and, consequently, hydrogen generation and reaction rates may decrease. The experimental results of this study suggest that H₂ production and the rates of serpentinization can be greatly influenced by acidic and alkaline solutions and co-existing minerals (e.g., pyroxene).

To understand Ti, Nb, and Ta mobility in supercritical fluids and Nb/Ta fractionation in subduction zones, we conducted piston cylinder experiments to determine rutile (TiO₂) solubility and Nb and Ta partition coefficients (Dru/sf Nb and Dru/sf Ta) in rutile (ru)‐silica‐rich supercritical fluid (sf) systems at 1.5–2.5 GPa and 920–1150 °C over variable fluid chemistries including solute (SiO₂ ± albite component), H₂O, Cl, and F contents. Under the investigated conditions, TiO₂ solubility in the supercritical fluids varies from 761 ± 107 to 9795 ± 448 ppm; Dru/sf Nb and Dru/sf Ta vary from 12 ± 1 to 208 ± 30 and 34 ± 5 to 2464 ± 140, respectively. Higher solute, Cl, and F contents in the systems and higher temperatures result in higher TiO₂ solubilities and lower Dru/sf Nb and Dru/sf Ta, and thus, fluid chemistry and temperature exert main controls on Ti, Nb, and Ta mobility. In all cases, Dru/sf Nb/Dru/sf Ta < 0.70, suggesting that supercritical fluids released from subducting slabs are higher in Nb/Ta relative to their protoliths. Therefore, such fluids are expected to result in higher Nb/Ta ratios in mantle wedges and arc magmas. Although Ti, Nb, and Ta could be mobile in supercritical fluids, Nb/Ta ratios in primitive arc basalts compared to those in mid‐ocean ridge basalts show that only ~10% of arc basalts are possibly disturbed by solute‐rich supercritical fluids during their generation. Therefore, if the fluids released from subducting slabs are dominantly supercritical, then most of them must be stable only in narrow P‐T spaces and hence too short‐lived to ascend very far into mantle wedges.

Hydrothermal experiments were performed at 311°C and 3.0 kbar on natural olivine and peridotite to investigate the kinetics of serpentinization. The results show that the rates of reaction strongly depend on grain sizes of solid reactants, with smaller grain sizes resulting in faster kinetics. After 27 days of reaction, the reaction extent was 99% for peridotite with grain sizes of <30 μm, and the reaction extent was 28% for grain sizes of 100–177 μm. Compared to peridotite, olivine is serpentinized at much slower rates, e.g., 5.3% of reaction extent was achieved for olivine with grain sizes of 100–177 μm after 27 days, approximately five times lower than that reached during peridotite serpentinization. Such contrasting results are due to the presence of pyroxene and spinel, an interpretation which is supported by a marked increase in reaction extents for experiments with the addition of pyroxene and spinel. The reaction extent achieved in experiments with 3 wt % spinel greatly increased to 98% after 27 days, much higher than that achieved during olivine serpentinization. These results appear to be related to pyroxene and spinel releasing Al and Cr during serpentinization. As indicated by compositions of serpentine, orthopyroxene lost ~60% of Al at a reaction extent of 59%. Influence of Al and Cr is suggested by a dramatic increase in reaction extents with the addition of Al₂O₃ and Cr₂O₃ powders. Olivine in natural geological settings is commonly associated with pyroxene and spinel; consequently, serpentinization kinetics may be much faster than previously thought.