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.
Abstrsct In a granulite from the Kokchetav massif, a complex mineral assemblage and intricate textures have resulted from a combination of unusual rock composition and two–stage metamorphic history. The second, contact metamorphism produced mainly cordierite and anthophyllite, reflecting a bulk composition attributed to pre–metamorphic alteration of basic igneous rock. From the first, highpressure metamorphism, garnet relics persist while another mineral has been completely pseudomorphed. The garnet is partly replaced by a symplectite of three minerals: orthopyroxene vermicules in a coarser intergrowth of cordierite and calcic plagioclase. Despite variable proportions of cordierite and plagioclase, the Al:Si ratio of the symplectite is almost constant, because the proportion of orthopyroxene is smaller where the dominant aluminous mineral is cordierite (Al:Si ≈ 0.8) than where the even more aluminous plagioclase (Al:Si ≈ 0.89) is prominent. The bulk Al:Si ratio of this symplectite, approximately 0.69, is very close to that of reactant garnet (0.66), indicating that Al and Si have been retained almost completely during the local reaction, while other elements were more mobile. In the pseudomorphs, aluminous cores (with Al:Si ratios 1.61–1.93) indicate that the mineral which has been completely replaced was probably kyanite. These cores comprise plagioclase, zoisite, corundum and spinel, and are surrounded by layers of plagioclase and cordierite. Fe, Mg, and Ca have diffused to the core, through layers with low bulk concentrations of these elements, probably by grainboundary diffusion in the solid state.
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