Diffusion data for clinopyroxenes from homogenization and self-diffusion experiments

Abstract

Kinetic experiments involving the homogenization of fine-scale, coherent (001) pigeonite lamellae in a sub-calcic diopside have been used to constrain Ca-Mg interdifusion coefficients for clinopyroxenes. At 1150'-1250'c and 25 kbar, average Ca-Mg effective binary interdiffusion coefficients are described by D : (f .SS x l0-7) exp(-360.87 kJIRT) (m2isec) (D : (3.89 x l0-3) exp(-86.25 kcattRl) (cm2lsec)) with an uncertainty of a factor of 2. Because of the large thermodynamic effect on diffusion near a solvus, actual interdiffusion coefficients should vary by an order of magnitude or more with composition for the conditions of these experiments. Attempts to obtain Ca and Fe self-difttsion coefficients in natural diopsides at I atm using a5Ca and 57Fe tracers and thin-fiIm, autoradiographic techniques were unsuccessful. These negative results place upper bounds on the Ca and Fe self-ditrusion coefficients in diopside that are consistent with the results of the homogenization experiments.