Constraining the ductile deformation mechanisms of garnet across pressure-temperature space

Abstract

Garnet is a common mineral at elevated pressures, and how it deforms plays an important role in the strength of lower crustal shear zones, subduction thrusts, and the mantle. Strong shape preferred orientations in elongate garnets at elevated temperatures and pressures attests to garnet's ability to deform through ductile mechanisms; however, how individual garnets deformed is frequently ambiguous or disputed. Garnet microstructures from the Morin shear zone and the Sulu ultra-high pressure terrane are revisited using fine-scale electron backscattered diffraction and wavelength dispersive spectroscopy mapping. The dominant deformation mechanism at each site is re-interpreted, and we show that garnet deformed through dissolution-precipitation creep at Sulu and through dislocation-assisted diffusion creep at Morin. These observations are integrated into a compilation of ductile deformation microstructures for garnet which reveals domains where dissolution-precipitation creep, recovery-accommodated dislocation creep (through subgrain rotation dynamic recrystallization), and dislocation-assisted diffusion creep dominate in P-T space. Garnet may deform through ductile deformation at temperatures as low as ~500 °C and deforms under low differential stresses in both eclogite and granulite facies conditions. The efficacy of deformation indicates that 1) garnet may not be the strongest phase in these environments, and 2) that at elevated temperatures, inclusion barometry may be affected by garnet's inability to maintain high differential stress.