The role of recrystallization in the preferred orientation of olivine

Abstract

The effects of recrystallization on the preferred orientation in olivine have been studied. The recrystallization mechanisms studied include grain growth, the deformation-induced grain boundary migration (DIGBM) and subgrain rotation (SGR). Grain boundary migration is involved in the first two, but not in SGR. Significant preferred orientation was found only in the latter two where dislocations are involved. The combined microstructural and fabric (preferred orientation) observations indicate that the grain boundary migration produces strong preferred orientation when the driving force is the dislocation energy but not when the grain boundary energy is the driving force. The nature of preferred orientation due to dislocation-related recrystallization (DIGBM or SGR) was found to depend on the mechanism of recrystallization. The DIGBM produces preferred orientation in which the low dislocation density grains with unfavourable orientation for easy slip dominate, thereby significantly altering the preferred orientation formed by dislocation glide. In contrast, SGR does not essentially alter the deformation fabric, and therefore the fabric associated with SGR recrystallization is related to the geometry of flow. The present results demonstrate that the existence of seismic anisotropy in the upper mantle is strong evidence for the dislocation mechanism(s) of deformation, and suggest that care must be exercised in applying the results of laboratory fabric studies made for a particular mechanism of recrystallization to the Earth's interior where the dominant mechanism(s) of dynamic recrystallization can be different.