The influence of silica on reaction rates and molecular hydrogen (H2) generation during olivine hydrothermal alteration

Hydrothermal alteration of olivine greatly influences geodynamics and the recycling of volatiles (such as water and carbon) in subduction zones. Silica is an important component of geological fluids, and its influence on the hydrothermal alteration of olivine remains poorly constrained. In this study, we performed experiments at 300–515°C and 3.0 kbar (1 bar=10⁵ Pa) by reacting well homogenized mixtures of olivine and silica powders with saline solutions (0.5 mol L⁻¹ NaCl). Silica greatly influences the reaction pathways, reaction rates, and molecular hydrogen (H₂) formation during olivine hydrothermal alteration. In experiments at 300°C and 3.0 kbar with mixtures of olivine and 10 wt% silica, olivine was replaced by serpentine and talc. The proportions of serpentine and talc were determined according to standard curves based on infrared spectroscopy analyses. Around 6.5% serpentine and 12% talc were produced after an experimental duration of 7 days, which had no change after a longer period (14 days). Compared to the kinetics in silica-free systems, the rates of olivine hydrothermal alteration in experiments with 10 wt% silica are much lower. The overall reaction is: 4.5Forsterite+5.5SiO_2,aq+4H₂O=Serpentine+2Talc. With the addition of more silica (20 wt% and 40 wt%), olivine was transformed into talc. The rates of reaction were much faster, e.g., for experiments with olivine and 20 wt% silica, 43% of talc was produced after 14 days, which increased to 77% for experiments with 40 wt% silica over the same period. The overall reaction is: 3Forsterite+5SiO_2,aq+2H₂O=2Talc. In experiments at 400–505°C and 3.0 kbar, the promoting effect of silica on olivine hydrothermal alteration was also observed, which is closely associated with a decrease in Gibbs free energies of olivine hydrothermal alteration. At 300°C and 3.0 kbar, silica decreased H₂ formed during olivine hydrothermal alteration by around an order of magnitude, resulting in an increase in oxygen fugacity. Based on measured H₂, we calibrated oxygen fugacities, ranging from 0.96 to 3.41 log units below FMQ (fayalite-magnetite-quartz buffer assemblage). This study suggests that the infiltration of SiO₂-bearing fluids into peridotites greatly influences redox conditions and the rates of olivine hydrothermal alteration.