Pore fluid pressure generated by dehydration of hydrous minerals can trigger deep and intermediate-depth earthquakes. To investigate its effect on frictional behaviors, we conducted friction experiments on simulated bassanite gouges and observed the stress drops and recurrence intervals of stick-slip events decreased over time under 200 MPa and 110 °C. Microstructural observations indicates that gouges deformed under room temperature shows the development of numerous shear bands including Riedel shears. On the other hand, a sample deformed under 200 MPa and 110 °C had few Riedel shear planes, indicative of the elevated pore fluid pressure suppressing the development of shear planes. We derived time function of dehydration-driven pore fluid pressure evolution using Avrami kinetics. Because of the mathematical similarity of Avrami equation and the fractal geometry, our result shows that the kinetics-driven pore fluid pressure evolution controlled the stick-slip behaviors of the basanite gouges under dehydration.

Olivine is the main constituent of the upper mantle, and its phase transformation affects the rheology of the subduction zone. It is crucial to reveal the kinetics of olivine (α)-spinel (γ) phase transformation under differential stress. To investigate the effect of microstructural properties on phase transformations such as grain boundary energy and plastic strain, we conducted a phase-field simulation using germanate olivine, an analog of silicate olivine. We conducted the simulations under various confining pressures of 1-5 GPa, temperatures of 1000 and 1200 K, with/without plastic strain, and various grain boundary energy. Under static conditions, the volume fraction of the γ phase increases as the overpressure increases because the chemical-free energy promoting grain growth dominates over the elastic strain energy, inhibiting grain growth. Under differential stress, at a slight overpressure, the volume fraction of the γ phase increases proportion to the chemical-free energy’s magnitude. Meanwhile, at a significant overpressure, the γ phase’s volume fraction decreases due to the sizeable elastic strain energy. Furthermore, the volume fraction of the γ phase under differential stress is more significant than under static conditions due to the considerable shear plastic strain. The grains of the γ phase under differential stress at low confining pressure are lens-shaped with a strong preferred orientation normal to the maximum compression direction because of the shear plastic strain. Meanwhile, the grains of the γ phase at a high confining pressure are ultra-thin because of the considerable elastic strain energy.