The seismic anisotropy of glaucophane aggregates experimentally deformed at subduction zone pressure and temperature conditions

In subduction zone rocks, particularly mafic blueschists, the alignment of glaucophane amphiboles by deformation is observed to be a primary cause of bulk seismic anisotropy. We explore the effects of varied subduction zone P/T and deformation conditions (e.g., stress, strain rate) on the anisotropy of deformed glaucophane crystals. We deformed three aggregate samples of 10-20 μm diameter glaucophane crystals at different subduction zone temperatures (650-700℃) and strain rates (10-4-10-3 s-1). A confining pressure of 1.5 GPa was used for all samples. We used the MTEX Matlab Toolbox to analyze the crystallographic preferred orientation (CPO) of these deformed glaucophane samples using electron backscattered diffraction (EBSD) data. All samples display a CPO that has a strong c-axis pole parallel to the stretching direction within the shear plane and a strong a-axis pole perpendicular to the shear plane. Samples that were deformed at higher T conditions exhibited a stronger CPO. We used CPO data from each sample to calculate the anisotropy of P-wave velocity (AVp) using the Voigt-Reuss-Hill equation: the low-T/fast-strain sample has the lowest anisotropy (AVp = 14.4%); the high-T/slow-strain sample was calculated to be more anisotropic than the previous sample (AVp = 17.5%); and the high-T/fast-strain sample has the highest anisotropy (AVp = 18.3%). These results show a positive correlation between deformation temperature and strain rate and AVp in deformed glaucophane. The similar AVp values of the two high-T samples suggest that temperature is influential on AVp, but further work is needed to explore this relationship. Additional EBSD imaging of sample microstructures shows evidence of subgrain boundary formation and misorientation gradients. These microstructural data indicate that dislocation creep may have acted as a mechanism for glaucophane deformation in our samples, leading to a strong CPO and resultant strong seismic anisotropy.