FE-MG INTERDIFFUSION IN OLIVINE UP TO 9 GPA AT T=600-900-DEGREES-C - EXPERIMENTAL-DATA AND COMPARISON WITH DEFECT CALCULATIONS

Interdiffusion data at high pressures are needed to provide good estimates of the closure temperatures of geothermometers based on Fe-Mg exchange in olivine in contact with other phases and to understand the pressure dependence of high-temperature deformation in olivine. We report here measurements of the interdiffusion coefficient of Fe/Mg in San Carlos olivine of mean composition Mg/(Mg + Fe) = 0.90, in the range of temperatures T = 600-900 degrees C and pressures P = 0.5-9 GPa. The measurements were performed by preparing olivine samples covered by a thin layer of fayalite. These specimens were annealed in a uniaxial split-sphere apparatus (USSA-2000). The Fe-Mg interdiffusion profiles were analysed by Rutherford back-scattering (RES). The results yield a pre-exponential factor D-o = 7.7 x 10(-8) cm(2)s(-1) (7.7 x 10(-12) m(2) s(-1)), and an activation energy E(*) = 147 +/- 58 kJmol(-1) (or 62 +/- 58 (2 SD) kJ mol(-1) if a p(2)(1/6) correction is performed), corresponding to the expression D = D(o)exp(-E(*)/RT)exp(epsilon X(Fe))exp(-PV*/RT). Our data are fitted best with epsilon = 3; X(Fe) = Fe/(Fe + Mg) is between zero and one (in the present experiments X(Fe) is around 0.1), P is the pressure and V-* is found to be -0.5 +/- 0.6 cm(3) mol(-1) (or 1 +/- 0.9 cm(3) mol(-1) if a pO(2)(1/6) correction is performed), i.e. about zero in the P and T range investigated. Comparisons between the present results, obtained at relatively low temperature under extrinsic conditions for diffusion, with other Fe-Mg interdiffusion data at high temperature (1125 degrees C or more) under intrinsic conditions allow us to deduce that the migration activation volume for point defects is V-M(*) approximate to 0 whereas that for formation is V-F(*) = 5.5 cm(3) mol(-1). This has important implications for the creep sensitivity to pressure. Numerical simulations involving energy minimizations and performed with the CASCADE and PARAPOCS codes confirm these conclusions: one finds V-M(*) = 0 and V-F(*) = 4.8 cm(3) mol(-1). The activation energy E(*) deduced from the experiments as well as the pre-exponential factor Do now permit precise estimation of the closure temperatures of geothermometers based on the Fe-Mg exchange between olivine and spinel.