Investigation on Deformation of Olivine at Subduction Zone Conditions

Deep focus earthquakes are dependent on the rheology of the host material. Several deformation experiments of San Carlos olivine at subduction zone conditions (pressures of 3-5 GPa and temperatures of 25-900°C) have been carried out on a deformation DIA (D-DIA) apparatus, Sam85, at X17B2, NSLS. Powder samples are used in these experiments. 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. Samples are compressed at room temperature and are then annealed at 1200°C for at least 2 hours before deformation. 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 the differential stress at steady state is stable (~3 GPa) and is relatively insensitive to the changes of strain rate and the temperature between 25-700°C; however, it drastically decreases to about 1 GPa and becomes temperature-dependent 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. The results place constraint on the plastic instability model as a mechanism of deep focus earthquake.