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 estim ates of the closure temperatures of geothermometers based on Fe-Mg exc hange in olivine in contact with other phases and to understand the pr essure dependence of high-temperature deformation in olivine. We repor t here measurements of the interdiffusion coefficient of Fe/Mg in San Carlos olivine of mean composition Mg/(Mg + Fe) = 0.90, in the range o f 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 sp lit-sphere apparatus (USSA-2000). The Fe-Mg interdiffusion profiles we re analysed by Rutherford back-scattering (RES). The results yield a p re-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), corr esponding 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 arou nd 0.1), P is the pressure and V- is found to be -0.5 +/- 0.6 cm(3) m ol(-1) (or 1 +/- 0.9 cm(3) mol(-1) if a pO(2)(1/6) correction is perfo rmed), i.e. about zero in the P and T range investigated. Comparisons between the present results, obtained at relatively low temperature un der extrinsic conditions for diffusion, with other Fe-Mg interdiffusio n data at high temperature (1125 degrees C or more) under intrinsic co nditions allow us to deduce that the migration activation volume for p oint 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 cr eep sensitivity to pressure. Numerical simulations involving energy mi nimizations 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 t he pre-exponential factor Do now permit precise estimation of the clos ure temperatures of geothermometers based on the Fe-Mg exchange betwee n olivine and spinel.