3:15 PM - 3:30 PM
[SIT04-10] Grain boundary sliding as the major deformation mechanism of olivine in Earth's upper mantle
We conducted uniaxial deformation experiments on olivine aggregates with a composition of Mg1.8Fe0.2SiO4 at pressures 1.5-6.7 GPa and at temperatures 1273-1473 K with strain rates of 0.3-7.2*10-5 s-1 using a deformation-DIA apparatus. The averaged values of stress exponent, activation energy, and activation volume were obtained to be 3.0, 423 kJ/mol, and 17.7 cm3/mol, respectively. The obtained parameters supports the deformation of water-poor olivine controlled by DisGBS. A significant water-fugacity dependency of creep strength of olivine on water fugacity was observed, and the water fugacity exponent was obtained to be 1.25. The dependency of creep strength of olivine controlled by DisGBS on pressure is weak due to competing the pressure-hardening effect of activation volume and the pressure-softening effect of water fugacity. Because creep strength of olivine controlled by the grain boundary sliding is insensitive to pressure, the estimated viscosity of water-poor olivine is independent of depth and is in a range of 1020-1021.5 Pa s throughout the upper mantle, which is consistent with geophysically observed viscosity profiles. Viscosity of the deep upper mantle would be overestimated by ~10-104 times if we assume the conventional dislocation creep mechanism for water-poor olivine.