GRAIN-BOUNDARY DIFFUSION IN ENSTATITE

We have investigated grain boundary diffusion rates in enstatite by he ating single crystals of quartz packed in powdered San Carlos olivine (Mg0.90Fe0.10)(2)SiO4 at controlled oxygen fugacities in the range 10( -5.7) to 10(-8.7) atm and temperatures from 1350 degrees to 1450 degre es C for times from 5 to 100 h at 1 arm total pressure. Following the experiments, the thickness of the coherent polycrystalline reaction ri m of pyroxene that had formed between the quartz and olivine was measu red using backscatter scanning imaging in the electron microprobe. Qua ntitative microprobe analysis indicated that the composition of this r eaction phase is (Mg0.92Fe0.08)(2)Si2O6. The rate of growth of the pyr oxene increases with increasing temperature, is independent of the oxy gen fugacity, and is consistent with a parabolic rate law, indicating that the growth rate is controlled by ionic diffusion through the pyro xene rim. Microstructural observations and platinum marker experiments suggest that the reaction phase is formed at the olivine-pyroxene int erface, and is therefore controlled by the diffusion of silicon and ox ygen. The parabolic rate constants determined from the experiments wer e analyzed in terms of the oxide activity gradient across the rim to y ield mean effective diffusivities for the rate-limiting ionic species, assuming bulk transport through the pyroxene layer. These effective d iffusivities are faster than the lattice diffusivities for the slowest species (silicon) calculated from creep experiments, but slower than measured lattice diffusivities for oxygen in enstatite Thus silicon gr ain boundary diffusion is most likely to be the rate-limiting process in the growth of the pyroxene rims. Also, as oxygen transport through the pyroxene rims must be faster than silicon transport, diffusion of oxygen along the grain boundaries must be faster than through the latt ice. The grain boundary diffusivity for silicon in orthopyroxenite is then given by (D) over bar(Si)(gb) delta = (3.3+/-3.0) x 10(-9) f(O2)( 0.0) e(-400+/-65/RT) m(3)s(-1) where the activation energy for diffusi on is in kJ/mol, and delta is the grain boundary width in m. Calculate d growth rates for enstatite under these conditions are significantly slower than predicted by an extrapolation from similar experiments per formed at 1000 degrees C under high pressure (hydrous) conditions by Y und and Tullis (1992), perhaps due to water-enhancement of diffusion i n their experiments.