Crystal preferred orientations of garnet: comparison between numerical simulations and electron back- scattered diffraction (EBSD) measurements in naturally deformed eclogites

摘要

Observations of dislocations, sub-grains and elongated crystal shapes support plastic deformation of garnet in laboratory experiments and naturally deformed eclogites. To evaluate the crystal preferred orientations (CPO) of garnet formed in axial shortening, pure shear and simple shear, we performed numerical simulations of CPO development during plastic flow using the visco-plastic self-consistent model. As input for the models we use the slip systems determined by transmission electron microscopy using experimentally deformed specimens. Although in garnet 66 slip systems are available, slip on the < 111 >{110} system provides over 86% of the total strain in the simulations. Characteristic CPO distributions are produced for the three deformation paths, with the CPO being strongest for axial shortening and weakest for simple shear. Compared with low-symmetry minerals, the pole figure densities of garnet, which has cubic symmetry, are weak. (100) axes tend to align with the shortening direction in all three deformation modes. The simulations are compared with CPO of naturally deformed garnet from nine eclogite samples from the Alps, Norway, and Mali, which contain 20-40% garnet. All samples show weak garnet CPO. Only two samples have a CPO pattern similar to the simulations for simple shear, no samples are similar to simulations for axial strain or pure shear. The presence of other weaker minerals, such as omphacite and quartz, with volume fractions higher than garnet, probably prevented garnet from becoming highly strained and developing characteristic CPOs in these eclogites. Higher volume fractions of garnet and higher temperature conditions may, however, allow the development of garnet CPO in the mantle transition zone, particularly within subducted oceanic material (MORB).