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Cation diffusion in olivine to 1400°C and 35 KB Misener, Donald James


Thirty diffusion experiments were performed on crystalline samples of Fe-Mg olivine. Interdiffusion coefficients of Fe and Mg have been determined between 900 and 1100°C and from 1 atm. to 35 Kilo-bars using diffusion couples of fayalite from Rockport, Mass. and Fo₉₁Fag olivine from St. John Island, Red Sea, Egypt. The diffusion of cations is strongly dependent on olivine composition and crystallographic orientation. The diffusion coefficient varies with temperature and pressure according to an empirical Arrhenius relationship, with an activation enthalpy for diffusion of ΔH* = 49.83 + 9.05 (N₂) Kilocalories/mole where N₂ = Cation mole fraction Mg An average value of 5.50 cm³/mole was calculated for the activation volume of diffusion. Diffusion couples of Red Sea olivine-MgO powder and couples of Fog₉₃Fa₇ olivine-synthetic forsterite (FO₁₀₀) were used to determine the interdiffusion coefficient of Fe and Mg in olivine between 1200 and 1400° C. The interdiffusion coefficient increases with increasing Fe content and with temperature. Diffusion is faster parallel to the c, [001], axis than paralleltto either a, [100], or b, [010]. At 7 cation mole percent Fe in the olivine, the activation enthalpy [001] is 65.6 - 3.6 kcal/mole. Calculations of ionic electrical conductivity in olivine usingCresultsiofiathis investigation agree with observed conductivity measurements. The results indicate that at depths greater than 100 Km. in the mantle ionic conduction is the dominant mechanism of electrical conduction. Estimates of temperature versus depth are made using the derived donductivities in con-junction with conductivity-depth profiles calculated from published electromagnetic depth sounding results. Experimental and theoretical results of steady state creep studies suggest that the large scale deformation of the upper mantle is ultimately controlled by diffusion in the olivine lattice. Results of this investigation indicate that cationic diffusion is not the rate controlling process in the deformation of silicates. The investigation also indicates that the theories relating cation diffusion to melting in metals may be extended to include silicates.

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