The Effect of the Garnet Content on Deformation Mechanisms and Weakening of Eclogite: Insights From Deformation Experiments and Numerical Simulations

Abstract

We performed deformation experiments on omphacite-garnet aggregates at a temperature of 1000°C, a confining pressure of 2.5 GPa, and a strain rate of 3 × 10−6 s−1 and complemented them by numerical simulations to gain insight into the role of garnet fraction for the deformation behavior of dry eclogite, with a focus on strain weakening mechanisms. We determined the spatial and temporal evolution of strain and strain rate by basing numerical simulations on experimentally derived microstructures, and thereby identified characteristic deformation mechanisms. Pure omphacite and garnet aggregates deform by two different mechanisms. Internally strained clasts and low-angle grain boundaries indicate crystal plasticity for omphacitite; the fracture dominated fabric of garnetite documents brittle deformation. Electron channeling contrast imaging, however, revealed low-angle grain boundaries and free dislocations in garnet crystals, suggesting that minor crystal plasticity accompanies the brittle failure. Eclogitic aggregates show varying deformation behavior between the two end-members shifting from crystal plastic toward brittle deformation with increasing garnet content. All samples exhibit strain weakening. The intensity of weakening shows a positive correlation with the garnet content. Our combined experimental, numerical, and microstructural investigations suggest that the majority of strain weakening is associated with crystal plastic processes in omphacite. Numerical simulations and experiments show that a garnet content above 25% enhances the activity of crystal plastic processes in omphacite and results in strain localization, which subsequently weakens the eclogite.