Simple shear deformation of olivine aggregates

Abstract

Simple shear deformation of synthetic olivine aggregates to high strains shows that dynamic recrystallisation has strong effects on the mechanical behaviour and the development of lattice preferred orientations. At 1473 K, a pronounced strain hardening is observed associated with grain elongation and limited dynamic recrystallisation along grain boundaries. The lattice preferred orientations of relict olivine grains generally follow the strain ellipsoid up to a shear strain of 1.1. Both [100] and [001] axes have peaks parallel to the maximum grain elongation direction. At 1573 K and shear strains >0.6, a moderate strain softening was associated with the development of significant dynamic recrystallisation. The lattice preferred orientation of relict olivine grains is characterised by point maxima with [100] axes parallel to the shear direction, [010] axes perpendicular to the shear plane, and [001] axes within the shear plane and perpendicular to the shear direction. The results suggest that at 1473 K both the b =[100] dislocations and b =[001] dislocations contribute to plastic deformation; at 1573 K dynamic recrystallisation relaxed constraints on deformation at grain boundaries, leading to a situation where a single slip system with b =[100] dislocations controls the rheology and the fabric. The lattice orientations of dynamically recrystallised olivine grains were measured using the electron backscatter diffraction technique. The measurements reveal a bimodal pattern of [100] axes: one parallel to the shear direction and the other perpendicular to the maximum principal compressive stress. Analysis of the results shows that the development of the stress-controlled orientations is closely associated with grain boundary migration processes during recrystallisation and growth. As a consequence, the direction of the fastest seismic velocity would not be parallel to the shear direction for olivine aggregates when grain boundary migration has a strong influence on the fabric.