Effects of deformation history on texture evolution in experimentally deformed Åheim dunite

Seismic anisotropy in Earth's mantle is mainly attributed to the lattice preferred orientation (LPO) of olivine, its most abundant mineral. Experimental studies have shown that olivine LPO forms and evolves during deformation, with patterns that depend on the conditions and kinematics of deformation. Past studies have focused mainly on single crystals or polycrystals with initially random LPO. We have conducted experiments to evaluate the effect of a pre-existing texture on subsequent LPO evolution. The purpose of these experiments is to determine how seismic anisotropy is generated in rocks with complex deformation histories. The starting material for these experiments is the Åheim dunite, which is foliated and exhibits a moderately strong pre-existing LPO. Triaxial experiments were performed using a Griggs-type, solid medium apparatus at T = 1200°C, P ~1 GPa, to strains ranging from 0.18 - 0.77. Three different sample orientations were investigated, with the angle between the pole to the initial foliation plane and the experimental axis of compression varying from 0-90 degrees. Texture analysis was performed using electron backscatter diffraction (EBSD). Deformed samples reveal large areas of recrystallized grains with a significant reduction in grain-size. In general, the recrystallized grains have a weaker LPO than the starting material. Samples deformed perpendicularly to the foliation plane show a weakened LPO with no significantly modification in strength or symmetry with increasing strain. Samples with the foliation initially 45 degrees from the shortening axis show a similar initial LPO weakening but strengthen with subsequent strain (up to the initial LPO value). In addition, the [010] point maxima rotate from their initial 45 degree orientation to become parallel to the axis of shortening at a strain of ~0.7. Samples with the foliation plane parallel to the axis of shortening show much weaker LPO strength than the two other orientations. In these samples, the [010] maxima do not rotate toward the shortening axis. Rather, the [010] axes are distributed in a weak girdle that stretches from the initial [010] maximum towards the shortening axis. Predicted steady state textures are not observed in experiments with this initial orientation. These results emphasize the importance of pre-existing texture on the reorientation of LPO towards a new steady-state orientation.