Effect of Fluid Salinity on Reaction Rate and Molecular Hydrogen (H₂) Formation During Peridotite Serpentinization at 300°C

Abstract We conducted hydrothermal experiments at 300°C and at pressure varying from 2.2 to 3.4 kbar to study the effect of fluid salinity on the coupling between molecular hydrogen (H 2 ) formation and olivine serpentinization, where peridotite and olivine with 25–50 μm of starting grain sizes were reacted with pure H 2 O and saline solutions (0.5, 1.5, and 3.3 M NaCl). Serpentine, the main hydrous mineral in most experiments, was quantified according to calibration curves based on Fourier‐transformed infrared spectroscopy and X‐ray diffraction analyses. Compared to pure H 2 O, saline solutions promote the hydrothermal alteration of olivine and peridotite. For experiments with peridotite and pure H 2 O, 67% of reaction extent was achieved after 14 days, which increased to 89% in experiments with medium‐salinity solutions (1.5 M NaCl) over the same period. Medium‐ and high‐salinity solutions inhibit H 2 formation during serpentinization, which is associated with the serpentinization of pyroxene especially clinopyroxene. The redox conditions were constrained according to the equilibrium H 2,aq = H 2,g , and very reducing conditions were achieved during the serpentinization of olivine and peridotite. This study is the first to show iowaite formation directly from peridotite serpentinization, indicating alkaline solutions. Thermodynamic calculations suggest that the hydrolysis of NaCl (NaCl + H 2 O = HCl + NaOH) may yield alkaline solutions, due to higher dissociation constants of NaOH compared to HCl. This study suggests that chlorine greatly influences the serpentinization of olivine and peridotite in natural geological settings. It also indicates that iowaite formation may not require oxidizing conditions as previously thought.