Dynamics of coupled olivine dissolution and serpentine precipitation revealed by hydrothermal flow-through experiments at 260 °C–300 °C

Abstract

Serpentinization is an important process that influences rock rheology, global fluid circulation, and microbiological activity in the deep sea, and is induced by hydrothermal fluid flow. Elemental olivine dissolution and serpentine precipitation govern the overall dynamics of serpentinization, and its prediction and modeling require an understanding of the reaction mechanism including the rate-limiting process. In this study, we examined the initial stage of coupled olivine dissolution and serpentine precipitation at 260 °C–300 °C and 50 MPa using a hydrothermal flow-through experimental apparatus. Saline water (0.5 m NaCl) was continuously injected into olivine powder at a constant flow rate (3.3 × 10−5 L s−1). Petrologic observation of solid products after experimental runs revealed that serpentine is produced from olivine. The hydration flux of serpentinization in this experiment is similar to estimates made using closed-system experiments, indicating that fluid flow does not affect the hydration rate at the flow rates adopted. Moreover, mass-balance calculations based on outlet fluid compositions during experiments suggest that both the overall serpentine precipitation and olivine dissolution rates decrease over time, and the serpentine precipitation rate is lower than the olivine dissolution rate during serpentinization. Although a comparison of speciated solutions observed in this experiment with results obtained by kinetic reaction modeling did not constrain the relationship between serpentine precipitation and olivine dissolution, a comparison with other experimental data and kinetic modeling results indicates that serpentine precipitation occurs more slowly than olivine dissolution at 170 °C. These results suggest that the relationship between serpentine precipitation and olivine dissolution kinetics varies with temperature and time, with such dynamic behavior governing serpentinization processes in the oceanic crust.