Determination of interdiffusivity in major rock-forming minerals

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In this research project, cation diffusion in olivine and Cr-Al spinel was examined by vacuum evaporation technique. The Fe-Mg interfiffusion and Ni tracer diffusion coefficients in olivine were determined at 1300〜1500℃ with gem-quality San Carlos olivine. The composition dependence and anisotropy were determined by applying a diffusion model with moving boundary. The diffusivity was found to be largest along the c-axis, intermediate along the b-axis, and smallest along the a-axis. At 1400℃, the interfifusion coefficient is estimated to be 8.7x10^<-15>±2.8(m^2/sec) along the c-axis. The anisotropy was also significant at 1500 and 1300℃, but there is a tendency that the diffusivities along a-and b-axis become similar at lower temperatures. Elemental diffusion in Cr-Al spinel was studied on octahedral chromian spinel from the Horokanai ophiolite at 1300〜1500℃. The spinel samples were evaporated incongruently developing outer reaction zone with low iron content and inner diffusion zone. By analyzing the compositional profile, the magnetite component was selectively evaporated through rapid diffusion promoted by point defects derived from reduction of ferric iron to ferrous iron. At 1300 and 1400℃, Al shows marked uphill diffusion, which clearly indicate that the Cr diffusivity is much slower than that of Al.