Deformation microstructure of amphibolite at high pressure and implications for seismic anisotropy in the crust

Abstract:

Amphibolite is one of the major constituent rocks in the middle to lower crust. Amphibole is elastically anisotropic and the CPO of amphibole has a large effect on seismic anisotropy in the crust. In many previous studies on natural amphibolites, four types of CPOs of amphibole were observed. The [100] axes of three types of amphibole CPOs (Type – I, II, III) are aligned subnormal to the foliation while those of the type – IV CPO are aligned as a girdle subnormal to the lineation. Type – I, II and III CPOs were found in previous experimental study at the pressure of 1GPa, but genesis of type – IV CPO has been unknown. Previous studies on natural rocks showing the type – IV CPO indicate that high temperature, small grain size and/or high strain may affect the formation of type – IV CPO, but it has not been clearly revealed. Therefore, in this study, shear deformation experiments were carried out at the pressure (0.5GPa) and high temperature (500 – 700℃) to find the origin of the type – IV CPO. At the pressure of 0.5 GPa, the type – IV CPO was found at high-strain (γ > 3.0) at all temperature. It is found that the [100] axes girdle, which is the unique character of the type – IV, is produced by mixing of different sizes of amphiboles. We also calculated the seismic anisotropy of hornblende by using the CPO of amphibole. The fast and slow direction of the P-wave and the fast S-wave polarization direction are similar for all four CPO types of amphibole. However, due to the [100] axes girdle, type – IV CPO amphiboles show smaller seismic anisotropy than those of other amphibole CPO types. Therefore, if the type – IV CPO is taken into account, the seismic anisotropy which is produced by the CPO of amphibole in the crust may be lower than that from previous study.

Kim, J., and Jung, H., 2019, New crystal preferred orientation of amphibole experimentally found in simple shear. Geophysical Research Letters, 46. https://doi.org/10.1029/2019GL085189