High-pressure shock compression of olivine: Dynamic pulverization and frictional melting

In order to study seismo-mechanochemical processes, two kinds of shock compression experiments - the recovery and the in-situ VISAR (velocity interferometer system for any reflector) experiments - were performed, using a single crystal of forsteritic olivine and a single-stage powder propellant gun. The generated peak shock pressures were 31.1 GPa for both the recovery and the VISAR experiments. Many shear planes were generated in the olivine crystal. The VISAR experiment revealed detailed time profiles for the particle and shock wave velocities during the compression and yielded the Hugoniot elastic limit (HEL) of the olivine to be 7.44 GPa. We observed, in one recovered sample, a clear local melting occurring along a shear plane. The microstructural observations of the shear plane and its walls using a field-emission scanning electron microscopy (SEM) and an analytical transmission electron microscopy (ATEM) revealed that plastically-deformed olivine was pulverized into a few hundred-nanometer size particle assemblages within a few-micrometers narrow zone along the shear plane and, locally have partially melted. Moreover, injection veins filled with olivine melt were found in the wall of the same shear plane. It was shown from the estimate of the critical resolved shear stress (CRSS) along the shear planes and the inferred slip velocity using the Hugoniot elastic limit (HEL) data that the power generated by the friction was large enough for melting to occur in the olivine. The whole process occurred in a time scale <0.6 microsecond. Pulverization (comminution) before melting is considered to be a common phenomenon in the formation of pseudotachylytes and in dynamically expanding seismic faults upon earthquakes.