Effect of pressure on the kinetics of peridotite serpentinization

Abstract

The kinetics of serpentinization were investigated at 400–500 °C and 3.0–20 kbar using natural olivine and peridotite for run durations of 9–38 days. The serpentine varieties were lizardite for all the experiments. The serpentinization kinetics at 400 °C and 3.0 kbar and at 500 °C and 20 kbar displayed a sigmoidal kinetic behavior, with very sluggish reaction rates at the early stage of serpentinization, followed by a sudden increase in the reaction rates. They were 1–2 orders of magnitude faster than serpentinization kinetics commonly used for modeling serpentinization-related processes. Pressure greatly increases the rates of peridotite serpentinization at 500 °C, e.g., 19% of reaction extent was achieved within 20 days for experiments at 500 °C and 3.0 kbar with starting grain sizes of < 30 μm, which increased to 96% for experiments at 500 °C and 20 kbar over the same period. Compared to olivine, peridotite was serpentinized at much faster rates at 400–500 °C and 3.0 kbar, reflecting the influence of pyroxene and spinel. Pyroxene minerals released some of its SiO2 during serpentinization, which was involved in the serpentinization of olivine, supported by larger amounts of SiO2 in olivine-derived serpentine compared to those of primary olivine. Thermodynamic calculations suggest that Gibbs energy of olivine hydrolysis decreases greatly with increasing pressures, and it becomes negative at 400–500 oC and 3.0–20 kbar with the involvement of silica. The experimental results of this study may be applied to natural geological settings where abundant H2O is present.