CHEMICAL DIFFUSION IN MELTS ON THE CAMGSI2O6-CAAL2SI2O8 JOIN UNDER HIGH-PRESSURES

This study was designed to obtain data on the rates of chemical diffusion between melt couples of diopside to Di40An60 composition and to determine changes in diffusion rates with temperature and pressure. Chemical diffusion in melts along the join diopside-anorthite can be described as binary (i.e., compositions remain on the join in the diffusion zone) up to a difference of 20 wt% between melt couple compositions. For larger concentration differences, multicomponent diffusion matrices should be calculated. The diagonal elements of these matrices are similar to the effective binary diffusion coefficients determined from the binary diffusion equation, but the off-diagonal terms are of the same order of magnitude and cannot be neglected. Pressures up to 20 kbars do not change the diffusion coefficients significantly, although a diffusivity maximum may exist near 10 kbars. Activation energies for MgO and Al2O3 diffusion are consistent with a mechanism of individual ionic species diffusing through the melt. © 1990.