Shock compression experiment of forsterite: pulverization and frictional melting in a shear regime

Abstract:

Seismic waves may be generated by a rapid slip accompanied by a rapid stress drop at or near the running rapture tip. To study the detailed processes and the material change occurring at the fracture tip, we performed a series of shock compression experiments using a keyed powder gun and a single crystal of forsterite Fo 94. A olivine disket (diameter 12 mm, thickness 3 mm, perpendicular to the c-axis) is mounted in a steel capsule of diameter 2 cm and length 4 cm. Flyer speed was 1.5 km/s; applied pressure was ca. 31 GPa; and shock wave velocity and particle velocity were ca. 7 km/s and ca. 1 km/s, respectively. After the shock experiment the sample was recovered and examined by optical microscopy, SEM and TEM. Although the olivine largely remained to be a single crystal, it shows wavy extinctions and many parallel shear planes diagonal to the compression axis. Shear planes (i.e., fault) are macroscopically sharp and show displacement of up to 0.5 mm. The TEM observation of the fault wall revealed that the wall has a zonal structure. Well inside the wall are developed densely spaced and tangles [001] screw dislocations. Outer 2-5 micron zone is polycrystalline olivine of grain size 200-300 nm. The outermost zone is an aggregate of semi-rounded small olivine particles (ca. 200 nanometers) mounted in a matrix of glass of olivine composition. It is inferred from these microstructures that polygonization and pulverization predated the melting. Such pulverization is possible at a running fracture tip, where stress and strain rate are the highest. Moreover, very thin injection veins (ca. 50-350 nanometer thick) filled with olivine glasses locally occurred in the fault wall, suggesting fracturing and crack opening had occurred while olivine melt was present. The fracturing was probably propelled by the rapid sweep of shock waves running through the crystal. Melting probably took place by a rapid collision and frictions among the olivine nanoparticles. It is suggested that the whole process took place in a very short duration of the order of 0.5 microsecond. Apart from the role of the shock waves, the situation is considered to be analogous to natural earthquakes.