Evaluation of effects of partial pressure of H2O on power law creep for ultrabasic rocks.

The numerical parameters in the flow equation of the power law creep for ultrabasic rocks are evaluated based upon a critical evaluation of the published experimental data on the steady state deformation. The power law creep controls the deformation of ultrabasic rocks under the condition of (deviatoric stress/shear modulus)<2×10-3, and the result is expressed byε=6.3×103σ2.7exp(-(1.2×105-1.15PH2O)/RT)where ε is the strain rate in sec-1, σis the deviatoric stress in bar, and PH2O is the partial pressure of H2O in bar. The value of exponent n=2.7, which is considerably smaller than that accepted up to the present, is found to be reasonable for ultrabasic rocks. Quantity V*H2O(=-1.15cal/bar·mole= -48cm3/mole) is regarded as the activation volume for H2O pressure, and it is named hydrolytic activation volume. Based on the idea that the activation energy of creep is linearly related to the melting temperature of the material, the above value of the hydrolytic activation volume is shown to be consistent with that calculated by using a value of the reduction of the solidus of olivine due to PH2O.