Simple shear deformation of olivine aggregates

Simple shear deformation of synthetic olivine aggregates to high strains sh ows that dynamic recrystallisation has strong effects on the mechanical beh aviour and the development of lattice preferred orientations. At 1473 K, a pronounced strain hardening is observed associated with grain elongation an d limited dynamic recrystallisation along grain boundaries. The lattice pre ferred orientations of relict olivine grains generally follow the strain el lipsoid up to a shear strain of 1.1. Both [100] and [001] axes have peaks p arallel to the maximum grain elongation direction. At 1573 K and shear stra ins >0.6, a moderate strain softening was associated with the development o f significant dynamic recrystallisation. The lattice preferred orientation of relict olivine grains is characterised by point maxima with [100] axes p arallel to the shear direction, [010] axes perpendicular to the shear plane , and [001] axes within the shear plane and perpendicular to the shear dire ction. 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, l eading 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 measurem ents reveal a bimodal pattern of [100] axes: one parallel to the shear dire ction and the other perpendicular to the maximum principal compressive stre ss. Analysis of the results shows that the development of the stress-contro lled orientations is closely associated with grain boundary migration proce sses during recrystallisation and growth. As a consequence, the direction o f the fastest seismic velocity would not be parallel to the shear direction for olivine aggregates when grain boundary migration has a strong influenc e on the fabric. (C) 2000 Elsevier Science B.V. All rights reserved.