FeMg interdiffusion in olivine up to 9 GPa at T = 600–900°C; experimental data and comparison with defect calculations

Abstract

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°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 (RBS). The results yield a pre-exponential factor D 0 = 7.7 × 10 −8cm 2s −1 (7.7 × 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 O 2 1 6 correction is performed), corresponding to the expression D = D 0 exp( −E ∗ RT ) exp(ϵX Fe)exp ( −PV ∗ RT ) . Our data are fitted best with ϵ = 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 p O 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°C or more) under intrinsic conditions allow us to deduce that the migration activation volume for point defects is V M ∗ ≈ 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 D 0 now permit precise estimation of the closure temperatures of geothermometers based on the FeMg exchange between olivine and spinel.