Rheological studies of olivine under high pressure and temperature

The rheological properties of San Carlos olivine and forsterite were studied under simultaneous high pressure and temperature conditions on the cubic anvil high-pressure apparatus. Using the synchrotron x-ray diffraction method, elastic strain, stress and strain rate information were extracted, and the flow laws on olivine were derived by fitting the experimental data. The microstructures of samples quenched under high pressure at different temperatures were observed using transmission electron microscopy (TEM).;The rheology of lab-dry San Carlos olivine was studied up to 9 GPa and 800°C. Our results show that lab-dry San Carlos olivine reaches yield point around 5.4 GPa differential stress at room temperature. The yield stress increases with the loading pressure and decreases with the increasing temperature. A dramatic decrease in strength occurs around 430--500°C that reflects a change of the flow mechanism. Two different flow laws at 9 GPa at different temperature regimes have been derived based on the experimental data. A 3&d2; -sigma-T deformation map was constructed using the derived flow laws. TEM observations reveal that c dislocation glide and recrystallization are the active deformation processes responsible for the stress instability above 400°C. It is believed that this stress instability can provide experimental constraints for the possible existence of a run-away plastic instability process that may be the cause of the deep earthquakes.;The variation of the differential stress with temperature and pressure was studied on both lab-dry and dehydrated forsterite samples up to 9 GPa confining pressure and 1000°C. Lab-dry forsterite exhibits identical rheological behaviors at similar pressure and temperature conditions as that of San Carlos olivine. The dehydrated forsterite sample behaves quite similarly to the lab-dry sample at room temperature, and appears to be stronger at high temperature. The water content in the sample can affect the critical temperature where the stress instability would be operative.