Renelle Dubosq , David Schneider, Alfredo Camacho, Baptiste Gault

Garnet is a high-strength mineral and preserves structures that can consequently be used to understand the flow strength and evolution of stress within the lower crust. Yet, the deformation mechanisms at the brittle¬-ductile transition of garnet remain ambiguous. Here, we study garnet porphyroclasts from an eclogite facies mylonite (central Australia) to investigate the mechanisms by which garnet is deformed under relatively dry, lower crustal conditions. Electron backscatter diffraction analysis reveals bands of small, relatively strain-free garnet with scattered orientations, outlined by polygonal to lobate high-angle grain boundaries cross-cutting the garnet porphyroclasts. Atom probe tomography of a high-angle grain boundary shows Fe enrichment in the form of planar and equally spaced arrays of Fe-rich nanoclusters. Our experiments demonstrate Fe segregation along grain boundaries of garnet, resulting in the nucleation of Fe-rich nanoclusters that can act as barriers for migrating dislocations which leads to strain-hardening that facilitates mechanical failure. The occurrence of strain-hardening in garnet potentially contributes to crustal strengthening that can lead to seismicity at depths.