Microstructures and lattice preferred orientations in experimentally deformed clinopyroxene aggregates

Abstract

Microstructures and lattice preferred orientations (LPO) were analysed on experimentally deformed natural and hot-pressed clinopyroxene aggregates in order to understand the relationship between deformation processes and evolving microstructures. The LPO was measured using electron backscatter diffraction techniques in the scanning electron microscope (SEM). Microstructures were observed by polarized light microscopy and by orientation contrast in the SEM. Natural samples (Sleaford Bay pyroxenite) were deformed in axial compression stepping tests up to 16% shortening. These samples deformed mainly by twinning and dislocation glide with very little recrystallization. No clear LPO evolution apart from the initial LPO could be attributed to deformation. Synthetic clinopyroxenite samples were hot-pressed from powders of the same material with three different grain size ranges, and deformed in compression stepping experiments up to 28% shortening. In the samples with coarse (30 μm) and intermediate (20 μm) grain sizes, deformation was dominated by dislocation creep accommodated by subgrain rotation recrystallization. The recrystallized grains have sizes up to 8 μm and represent 15–25% of the sampled area. The sample with the finest initial grain size (5 μm) deformed dominantly by diffusion creep accompanied by grain boundary migration. All of the hot-pressed samples deformed in compression have a similar texture, consisting of a girdle of c[001] axes normal to compression and a point maximum of b[010] axes and a*(100) poles parallel to the compression direction. The LPO of recrystallized grains was separated on spatially resolved orientation maps and shows an S-type fabric with b[010] parallel to the compression axis and a random a*(100) distribution. The bulk fabric was largely present in the hot-pressed starting material and is interpreted as the result of compaction during cold-pressing. At moderate strains in axial compression, the initial compaction fabric has been weakly overprinted by the recrystallization fabric. One hot-pressed sample (coarse grain size) was deformed in torsion to a shear strain of γ=0.1. The microstructure indicates dominant dislocation creep and contains about 20% recrystallized grains. The texture evolved from the rotationally symmetric compaction LPO to an oblique fabric.