Diffusion and diffusion creep in olivine and ultrabasic rocks.

Abstract

The applicability of Winchell's compensation law relating the activation energy to the pre-exponential constant is examined for diffusion in orthosilicate, and the diffusion coefficients of oxygen, silicon, magnesium and iron in olivine are estimated. Effects of high solid pressure and the partial pressure of H2O on diffusion are also evaluated by means of the compensation law and Liempt's empirical law between melting temperature and activation energy. The rate of diffusion creep in ultrabasic rocks is evaluated. The ratecontrolling process of creep changes with temperature; oxygen lattice diffusion is the rate-controlling process at high temperatures, while the oxygen grain boundary diffusion dominates at low temperatures. The strain rate by diffusion creep is so small that diffusion creep cannot be the dominant mechanism for flow at the actively deforming part in the upper mantle. The time and distance required to reach a state of chemical equilibrium between two geologic media in contact are also evaluated by using the values of the diffusion coefficients.