Strain hardening induced by crystal plasticity: A new mechanism for brittle failure in garnets

Abstract

Garnet is a high-strength mineral stable across a wide range of pressure and temperature conditions and preserves structures that can consequently be used to understand the evolution of stress within the lower crust. Yet, the deformation mechanisms in the brittle-ductile regime 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 separating 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 nanoclusters. Our data demonstrate Fe segregation along grain boundaries of garnet, resulting in the nucleation of Fe-rich nanoclusters that act as barriers for migrating dislocations which could lead to strain-hardening and eventually facilitate mechanical failure.