High silicon self-diffusion coefficient in dry forsterite

Silicon self-diffusion coefficients (DSi) in dry synthetic forsterite single crystals were measured at temperatures of 1600 and 1800 K, from ambient pressure up to 13 GPa using an ambient pressure furnace and Kawai-type multi-anvil apparatus. The water contents in the samples were carefully controlled at <1 μg/g. Diffusion profiles were obtained by secondary ion mass spectrometry (SIMS) in depth profiling mode. Small negative pressure dependence of DSi is determined with an activation volume of 1.7±0.4 cm3/mol. The activation energy is found to be 410±30 kJ/mol. LogDSi values (DSi in m2/s) at 1600 and 1800 K 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 DSi 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 DSi resolve the inconsistency between DSi measured in diffusion experiments and those deduced from creep rates measured in deformation experiments.

► Small negative pressure dependence of Si diffusion coefficient in dry forsterite. ► Si diffusion coefficient is 2-3 orders of magnitude higher than previous studies. ► Present Si diffusion coefficient explains the high dislocation creep rates. ► Si diffusion coefficient slightly increases with depth in the upper mantle. ► Viscosity slightly decreases, or nearly constant with depth.