ON THE MECHANISM OF RESORPTION ZONING IN METAMORPHIC GARNET

Abstract

An analytical electron microscope study of almandine garnet from a metamorphosed Al-Fe-rich rock revealed detailed composition profiles and defect microstructures of resorption zoning along fluid-infiltrated veins and even into the garnet/ilmenite (inclusion) interface. This indicates a limited volume diffusion for the cations in substitution (mainly Ca and Fe) and an interface-controlled partition for the extension of a composition-invariant margin. A corrugated interface between the Ca-rich margin/zone and the almandine garnet core is characterized by dislocation arrays and recovery texture further suggesting a resorption process facilitated by diffusion-induced recrystallization, diffusion-induced dislocation migration and diffusion-induced grain boundary migration. Integrated microstructural and chemical studies are essential for understanding the underlying mechanisms of processes such as garnet zoning and its modification. Without this understanding, it will not be possible to reliably use garnet compositions for thermobarometry and other applications that rely on garnet chemical information.