High silicon self-diffusion coefficient in dry forsterite

Silicon self-diffusion coefficients (D _Si) in dry synthetic forsterite single crystals were measured at temperatures of 1600 and 1800K, from ambient pressure up to 13GPa using an ambient pressure furnace and Kawai-type multi-anvil apparatus. The water contents in the samples were carefully controlled at <1μg/g. Diffusion profiles were obtained by secondary ion mass spectrometry (SIMS) in depth profiling mode. Small negative pressure dependence of D _Si is determined with an activation volume of 1.7±0.4cm ³/mol. The activation energy is found to be 410±30kJ/mol. LogD _Si values (D _Si in m ²/s) at 1600 and 1800K at ambient pressure are determined to be -19.7±0.4 and -18.1±0.3, respectively. These values are ~2.4 orders of magnitude higher than those reported by Jaoul et al. (1981). We speculate that their low D _Si might reflect the effects of a horizontal migration of the isotopically enriched thin films applied on the sample surfaces, which may inhibit diffusion into the substrate during annealing. Our results for D _Si resolve the inconsistency between D _Si measured in diffusion experiments and those deduced from creep rates measured in deformation experiments.