Study on the Lattice Preferred Orientation (LPO) of Olivine, Amphibole, and Chlorite in Amphibole Peridotites from SW Norway

Abstract:

Hydrous phases such as amphibole and chlorite are elastically very anisotropic. 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 lattice-preferred orientation (LPO) that can significantly affect the seismic anisotropy. Therefore, we studied the LPOs of olivine, amphibole, and chlorite in amphibole peridotites from southwestern Norway. To determine the LPOs of minerals, we used SEM/EBSD (Electron Backscattered Diffraction) technique. The samples are fine-grained and well-foliated rocks with a porphyroclastic texture. They are mostly composed of olivine(60–90%), amphibole(10–40%), and chlorite(0.2–10%) and a minor enstatite and spinel.

In this study, a gradual change in olivine LPO was identified. 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. We calculated seismic velocity and seismic anisotropy of the studied rock samples. The P- and S-wave anisotropies of amphibole were up to 15.2% and 11.9%, respectively. The P- and S-wave anisotropies 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. Our data suggest that the existence of the LPO of hydrous minerals, such as amphibole and chlorite, in a peridotite enhances the seismic anisotropy of the whole rock.