Determination of atomic diffusivity in silicates at high pressure and temperature

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To investigate the kinetics of diffusion of divalent cations in olivine at high temperature and pressure, the following experiments were conducted : (a)syntheses of olivine single crystals using the FZ and CZ methods and polycrystalline materials by multiple sintering and mixing, (b)annealing of olivine diffusion couples consisting of Mg_2SiO_4 and(Mg, M)_2SiO_4 [M= Fe, Co, Ni]at high temperatures of 1400-1600 C and high pressures of 5-10 GPa using an MA8-type apparatus, (c)electron microprobe analyses of recovered olivine samples to obtain diffusion profiles.Although the duration of annealing at high temperature and pressure was as short as 1-4 hours, cation diffusion has occurred more than several tens um in width. The diffusion profiles were analyzed by the Boltzmann-Metano's method to obtain the cation interdiffusion coefficients. The cation diffusion coefficients i were determined as a function of pressure, temperature and chemical composition. We found a pressure enhanced diffusion in our experimentally annealed samples. The enhancement is interpreted as a result of high deviatric stress in a sample assembly or associated diffusion of hydrogen and cation(Mg, Fe, Co, Ni)in olivine. We will therefore conclude cation in olivine can diffuse faster at high temperature and pressure in the deep upper mantle than previously proposed.Furthermore, microstructures of silicates in experimentally shocked specimens and meteorites were studied by the transmission electron microscopy(TEM)to investigate the diffusion processes under the high shock pressure. In many shocked meteorite specimens(e. g. ureilites, ordinary chondrites, lunar meteorites), we found diffusion induced phase transformations have occurred during the shock compression.