Activation volume of Si diffusion in San Carlos olivine:: Implications for upper mantle rheology

The effect of pressure on silicon diffusion in San Carlos olivine has been determined using a uniaxial split-sphere apparatus (USSA-2000) and the nuclear reaction analysis technique (NRA) on the Si-30 isotope. Experiments were performed at high temperature, T = 1763 K, and pressures between 4 and 9 GPa. The specimens were inserted into a pure Fe capsule, which is very effective in maintaining the oxygen fugacity within the stability field of olivine, as well as providing a soft medium to mechanically protect the crystals. Diffusion profiles along the (b) over right arrow crystallographic axis and of characteristic length of the order of 50 nm were obtained after annealing; the olivine samples between 1 and 4 hours. We find the activation volume for silicon diffusion in San Carlos olivine to be V-Si = +(0.7 +/- 2.3) x 10(-6) m(3)/mol after a correction for oxygen fugacity which is pressure-dependent. This result demonstrates that pressure has practically no effect upon silicon diffusion under our temperature and pressure conditions. Extrapolation of our high-pressure measurements to 1 atm gives a Si diffusion coefficient, log(D-Si) = - -18.9 +/- 1.0 (with D-Si in m(2)/s). Finally, our experiments show that according to the point-defect model of Jaoul [1990], the activation volume for creep of olivine at high temperature is dominated by the effect of pressure on the Mg defect concentration and has an activation volume close to 6 x 10(-6) m(3)/mol.