Nanostructural and chemical evolution of garnet during high-strain deformation

Garnet is a common high-pressure mineral in the Earth’s lithosphere. Considered a high-strength mineral stable across a wide range of pressure and temperature conditions, it is generally accepted that garnets can retain their microstructures and chemical composition during deformation and metamorphism. Therefore, the trace and major element compositions of garnet are commonly used for geothermobarometers and geochronometers to provide the conditions and timing of metamorphic events. We combine electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI) and atom probe tomography (APT) on garnet from an eclogite facies mylonite in the Musgrave Province (central Australia) to investigate the mechanisms of element mobility during high-strain deformation under dry (<0.002 wt% H₂O), lower crustal conditions. Previous investigations suggest that mylonitization occurred at 1.2 GPa and 650°C. Herein, we focus on two garnet clasts that are located within one common high-strain zone. EBSD and ECCI data from garnet clast 1 reveal a micro-shear zone characterized by recrystallized strain-free garnet crystals. APT analysis of one of the recrystallized high-angle grain boundaries shows Fe enrichment in the form of  equally spaced (4.5–6.5 nm), planar arrays of Fe-rich nanoclusters (3.0–7.5 nm). The combined data suggests that these nanoclusters formed as a result of enhanced Fe diffusion along high-angle grain boundaries of recrystallized garnet during high-strain deformation. Garnet clast 2 evinces crystal-plasticity associated with brittle deformation in the form of heterogeneous misorientation patterns and low-angle grain boundary development at the rim of the garnet porphyroclast. APT analysis of a low-angle grain boundary within the highly-strained clast shows Ca enrichment and Mg depletion along dislocations, suggesting crystal-plasticity enhances element mobility via 'pipe' diffusion, with dislocations acting as high-diffusivity pathways. Our data reveal the interaction of chemical and mechanical processes at the nanoscale through deformation-induced enhanced element diffusivity. Consequently, caution should be exercised when using deformed garnets as petrological tools because of this enhanced major element mobility during high-strain deformation. To evaluate this hypothesis, we modelled the Ca diffusion profiles across undeformed and deformed garnet rims within clast 2. Simulations based on the measured Ca concentration profiles of the undeformed rims yield time estimates of 0.8 to 1 Ma for the duration of high-strain deformation at eclogite facies along the shear zone, whereas simulations across the deformed rims yield longer time estimates of 5 to 22 Ma. Our study thus highlights the importance of conducting a thorough microstructural and geochemical analysis on garnet prior to utilizing the mineral for petrological applications.