Intergranular diffusion kinetics of Fe and Mg during retrograde metamorphism of a pelitic gneiss from the Adirondack Mountains

Pelitic granulite gneiss from the Irving Pond Formation in the southern Adirondack Highlands, New York, contains chemical zoning in Fe and Mg in both individual mineral grains and over a distance of millimeters in the polycrystalline matrix. Garnet porphyroblasts are zoned with increasing XAlm and decreasing XPrp towards crystal rims. Compositions of individual biotite grains in the matrix vary systematically with distance from the garnet porphyroblasts up to about 1 mm, with the highest Mg concentrations in grains adjacent to the garnet and progressive Fe-enrichment in grains further away. e constructed numerical models of the distribution of Fe and Mg components obtained during the retrograde reaction and diffusion history of this rock based on the mineral assemblage, modes, textural information, and the preserved zoning trends. These simulations indicate that the effective rate of intergranular transport of Fe and Mg components through the polycrystalline matrix was only one order of magnitude faster than the rate of volume diffusion in garnet. The relatively slow intergranular kinetics indicate that grain boundaries did not provide rapid diffusion pathways. Absence of an interconnected fluid phase along grain edges and grain boundary annealing likely inhibited intergranular diffusivities during cooling. dels also suggest that the mineral assemblage experienced metamorphic temperatures in excess of 700°C, followed by rapid cooling. This T-t evolution is quite different than determined for the ca. 1060-1000 Ma regional granulite-facies metamorphism in the Adirondack Highlands. FeMg distribution in the gneiss likely reflects and earlier metamorphic episode.