Lattice Preferred Orientation (LPO) of Olivine, Amphibole, and Chlorite Found in Hydrated Mantle Peridotites from Bjørkedalen, Southwestern Norway and Implications for Seismic Anisotropy

Abstract:

Lattice preferred orientation (LPO) of olivine is one of the key to interpret seismic anisotropy and to deduce flow pattern of the upper mantle. Although olivine is thought to account for most of seismic anisotropy of the upper mantle, a deformed peridotite under water-rich environment may include hydrous phases that develop LPO that can considerably contribute to the seismic anisotropy. Hydrous phases such as amphibole and chlorite are known to be elastically very anisotropic. Thus, LPOs of olivine as well as amphibole and chlorite in hydrated mantle peridotites from Bjørkedalen, southwestern Norway were investigated using SEM/EBSD system. Seismic velocity and seismic anisotropy of the three minerals were calculated on the basis of their measured LPOs. Olivine showed type-A and -B LPO, and a mixed LPO between type-A and type-B. 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. Numerous hydrous inclusions found in olivine and orthopyroxene indicate that olivine fabric may have changed from type-A to type-B due to high water content under high stress. It is found that seismic anisotropy of both amphibole and chlorite is much higher than that of olivine. In addition, it is also found that amphibole and chlorite can lead to strong trench-parallel seismic anisotropy depending on flow geometry. Amphibole produced trench-parallel polarization direction of the fast S-wave from the dipping angle of 45° while chlorite showed the same characteristics from the dipping angle of 50°. This study suggests that the existence of the LPO of amphibole and chlorite in a hydrated peridotite can change the seismic anisotropy of the whole rock significantly.