Rates and mechanisms of Fe-Mg interdiffusion in olivine at 980°-1300°C

Abstract

Fe-Mg interdiffusion rates have been measured in olivine solid solutions using the diffusion couple technique. Measured diffusion rates along [001] at fO2 = 10-12 bars and between 980°C and 1300°C are found to be slower by about 2 orders of magnitude, compared to previous studies with the exception of some isolated data points of Misener [1974]. There is no change in temperature dependence within this temperature interval for any composition studied. The temperature dependence at a composition of Fo86 is described by an activation energy of 226±18 kJ/mol and a preexponential factor of (5.38±0.89)× 10-9 m2/s. The data are consistent internally as well as with well-established theoretical models relating tracer and chemical diffusivities. It is shown that diffusion coefficients are a unique function of pressure, temperature, major element composition and oxygen fugacity for olivines and are not dependent on trace element contents of starting crystals. It is argued that a twofold classification of diffusion mechanisms into intrinsic and extrinsic is inadequate for Fe-bearing silicates, and at least three categories need to be defined. Consideration of the data in combination with the point defect chemistry indicates that the observed diffusion in olivines occurs within a "transition-metal extrinsic" regime. For low temperature extrapolations, significant changes in temperature dependence of diffusion rates are not anticipated, although the oxygen fugacity dependence may change. For calculations in natural systems the present data would yield higher closure temperatures, longer timescales, slower cooling rates, and shorter length scales of diffusion compared to those obtained using earlier diffusion data.