Lattice preferred orientation (LPO) of olivine, amphibole, and chlorite found in hydrated mantle peridotites from Bjørkedalen, southwestern Norway and implications for seismic anisotropy

Abstract

Understanding lattice preferred orientations (LPOs) of olivine is important in the study of mantle flow and seismic anisotropy of the upper mantle of the Earth. Hydrous phases such as amphibole and chlorite, on the other hand, are elastically very anisotropic compared to olivine. Even though olivine is the major mineral that constitutes the upper mantle of the Earth, both amphibole and chlorite in a deformed peridotite under water-rich condition may develop LPO that can significantly affect the seismic anisotropy. Therefore, LPOs of olivine, amphibole, and chlorite in amphibole peridotites from Bjørkedalen, southwestern Norway were studied using SEM/EBSD (Electron Backscattered Diffraction) technique. Seismic velocity and seismic anisotropy were calculated using LPOs of studied minerals. In this study, a gradual change in olivine LPO was observed. One sample showed that [100] axes are aligned subparallel to lineation and [010] axes subnormal to foliation, which is known as type-A LPO. The other sample showed that [010] axes are aligned subnormal to foliation and [001] axes parallel to lineation, which is known as type-B LPO. Other samples showed intermediate type between type-A and type-B LPO. Amphibole LPOs showed that [001] axes are aligned subparallel to lineation and [100] axes subnormal to foliation. Chlorite LPOs showed that [100] axes are aligned subparallel to lineation and [001] axes subnormal to foliation. The P- and S-wave anisotropies of amphibole were up to 15.2% and 11.9%, respectively, while those of chlorite were up to 25.2% and 46.2%, respectively. It turned out that seismic anisotropy of both amphibole and chlorite is much higher than that of olivine. In addition, it was found that amphibole and chlorite contribute to strong trench-parallel seismic anisotropy depending on flow geometry. This study suggests that the existence of the LPO of amphibole and chlorite in a hydrated peridotite changes the seismic anisotropy of the whole rock significantly.