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 nucl ear reaction analysis technique (NRA) on the Si-30 isotope. Experiments wer e 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 effe ctive in maintaining the oxygen fugacity within the stability field of oliv ine, as well as providing a soft medium to mechanically protect the crystal s. Diffusion profiles along the (b) over right arrow crystallographic axis and of characteristic length of the order of 50 nm were obtained after anne aling; the olivine samples between 1 and 4 hours. We find the activation vo lume 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 pressur e-dependent. This result demonstrates that pressure has practically no effe ct upon silicon diffusion under our temperature and pressure conditions. Ex trapolation 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, o ur experiments show that according to the point-defect model of Jaoul [1990 ], the activation volume for creep of olivine at high temperature is domina ted by the effect of pressure on the Mg defect concentration and has an act ivation volume close to 6 x 10(-6) m(3)/mol.