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

Abstract

Diffusion modelling is applied to layered garnet–pyroxene–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 Al 2 O 3 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 °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 9×10 −23 to 5×10 −20 m 2 s −1 . These are consistent with experimental values in the literature for solid‐state diffusion, including grain‐boundary diffusion.