Fe-Mg Interdiffusion in Olivine and Magnesiowüstite Under Hydrous Conditions: Implications for Mantle Processes

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KohlstedtEAR-0106981Support is requested to complete our experimental investigation designed to quantify the dependence of the kinetics of Fe-Mg interdiffusion in olivine and magnesiowustite on water/hydroxyl concentration is proposed. For many nominally anhydrous minerals, a small amount of 'water' significantly enhances many, if not all, diffusion-dependent properties including creep rate, dislocation recovery kinetics, electrical conductivity and ionic diffusion. In most cases, however, these observations are only qualitative. Does the presence of water enhance diffusivity or rate of creep ... yes or no? We have just recently determined a quantitative relationship between diffusivity and water/hydroxyl content (water fugacity) for olivine; none exists for magnesiowustite. To utilize observations determined from laboratory experiments to develop models of the geochemical evolution and geodynamical behavior of the mantle, it is critical to quantify the dependence of kinetic properties on water content. Therefore, a laboratory study is proposed to extend our investigation of the dependence of Fe-Mg interdiffusivity on water concentration in olivine and to include magnesiowustite. High-temperature, high-pressure diffusion experiments are being carried out on single crystals at controlled water fugacities at pressures in the range 0.05 to 15 GPa. Rutherford backscattering spectroscopy (RBS) and electron probe microanalysis (EPMA) are employed to measure the resulting diffusion profiles. Fourier transform infrared (FTIR) analyses are used to determine the hydroxyl concentration. In addition, transmission electron microscopy (TEM) and x-ray diffraction analyses are employed to check for the presence of humite-like layers. For olivine, experiments are needed to determine the dependence of diffusivity on temperature and pressure as well as to verify our initial measurement of the dependence of diffusivity on water fugacity. For magnesiowustite, all of these experiments are needed. An important goal of our experiments is to determine quantitative relationships between cation (Fe-Mg) diffusivity and water/hydroxyl concentration for two important mantle minerals under well-defined thermochemical conditions. The proposed research is designed to use these results in the context of point defect thermodynamics to gain a fundamental understanding of the mechanism(s) by which water influences transport properties such as ionic diffusion and electrical conductivity. The resulting theoretical framework is essential for reliably extrapolating laboratory results to mantle environments and for utilizing experimental observations to develop models of the dynamic behavior of the Earth's interior.