Localization of submicron inclusion re-equilibration at healed fractures in host garnet

Abstract

Microstructures in Permian inclusion-bearing metapegmatite garnets from the Koralpe (Eastern Alps, Austria) reveal re-equilibration by coarsening of abundant submicron-sized inclusions (1 μm–2 nm diameter) at the site of healed brittle cracks. The microstructures developed during Cretaceous eclogite-facies deformation and the related overprinting of the host–inclusion system. Trails of coarsened inclusions (1–10 μm diameter) crosscut the garnet, defining traces of former fractures with occasional en-echelon overlaps. Trails are flanked by 10- to 100-μm-wide ‘bleaching zones’ characterized by the absence of ≤1-μm-sized inclusions in optical and SE images. FEG-microprobe data show that trails and bleaching zones can form isochemically, although some trails exhibit non-isochemical coarsening. Cross-correlation-based EBSD analysis reveals garnet lattice rotation of up to 0.45°, spatially correlated with bleaching zones. The garnet lattice in the center of trails is misoriented around different axes with respect to the lattice either side of the trail. Elevated dislocation density within bleaching zones is confirmed by TEM observations. Dislocations represent a plastic wake formed by crystal plastic deformation at the crack tip. Fracture enhanced diffusion rates in the lattice adjacent to crack planes by introducing dislocations, priming these areas to behave differently to the bulk of the garnet during Cretaceous metamorphism and facilitating localized coarsening of inclusions. Diffusion within the bleaching zone was enhanced by a minimum factor of 102. The partially closed host–inclusion system records the influence of deformation mechanisms on re-equilibration and contributes to understanding of the interaction between deformation and chemical reaction during metamorphism.