Diffusion-controlled corona reaction and overstepping of equilibrium in a garnet granulite, Yenisey Ridge, Siberia

Diffusion modelling is applied to layered garnet-pyroxeue-quartz coronas, formed by a pressure-induced reaction between plagioclase and primary pyroxene in a metabasic granulite. The reconstructed reaction involves some change in composition of reactant minerals. The distribution of minerals between layers is satisfactorily explained by diffusion-controlled reaction with local equilibrium, in which the diffusion coefficient for Al was smaller than those for Fe, Mg and Ca by a factor of approximately four. Diffusion of Mg towards plagioclase implies a chemical-potential gradient for MgO component in a direction opposite to the changing Mg content of garnet; this is explained by the influence of Al2O3 on the chemical potential of the pyrope end-member. Grain-boundary diffusion is suggested to have operated, possibly with composition gradients different from those in the bulk minerals. Chemical-potential differences across the corona are estimated from the variation in garnet composition, enabling affinity (the free energy change driving the reaction) to be estimated as 6.9+/-1.8 kJ per 24-oxygen mole of garnet produced. This implies that the pressure for equilibrium among the minerals was overstepped by 1.4+/-0.4 kbar. The probable P-T conditions of reaction were in the range 650-790 degrees C, 8-10 kbar. Assuming a timescale of reaction between 10(6) and 10(8) years, estimated diffusion coefficients for Fe, Mg and Ca are in the range 9 x 10(-23) to 5 x 10(-20) m(2) s(-1). These are consistent with experimental values in the literature for solid-state diffusion; including grain-boundary diffusion.