Investigation on deformation of olivine at high pressure and low temperature

Olivine is the dominant mineral in the upper mantle. Several deformation experiments of polycrystalline powdered San Carlos olivine at subduction zone conditions (pressures of 3-5 GPa and temperatures of 25-1100°C) have been performed on a deformation DIA (D-DIA) apparatus, SAM85, at X17B2, National Synchrotron Light Source (NSLS). Enstatite (MgSiO3) (3-5% total quality of sample) is used as buffer to control the activity of silica. Ni foil is used in some experiments to buffer the oxygen fugacity. Water content is confirmed by IR spectra of the recovered samples. The Total (plastic and elastic) strains (macroscopic) are derived from the direct measurements of the images taken by X-ray radiograph technique. Differential stresses are measured at constant strain rate (∼10-5-10-7s -1) and at different pressures and temperatures with synchrotron x-ray. It can be concluded that in the regime of 25-400°C, there is a small increase stress at steady state along with the temperature drop; in the regime of 400°C to transition temperature, the differential stress at steady state is a constant (∼3 GPa) and is relatively insensitive to the changes of temperature and strain rate; however, it drastically decreases to about 1 GPa and becomes temperature-dependent above the transition temperature and thereafter. The transition temperature is between 700°C and 900°C. No significant difference of differential stress and strain is observed between the samples with/without Ni foil buffered below 700°C. Two regimes with different deformation mechanisms for olivine have been determined in this study: regime of low temperature plasticity at low temperature (below transition temperature) and regime of power law creep at high (above the transition temperature). The annealing process can obviously shift up the transition temperature between regimes of temperature insensitive and sensitive. Grain size affects the rheological properties of olivine in the low temperature dislocation regime. Existence of water obviously decreases the transition temperature of the boundary between the regimes of low temperature plasticity and paw-law creep. (100)[001] and {hk0}[001] (h>k) are the predominant active slip systems in the deformation at low temperature. The instability of olivine could be the mechanism for the deep-focus earthquake happened in the subduction zone slab.