Study on the deformation microstructures of olivine and pyroxene in peridotite, Spitsbergen, Svalbard and implications for seismic anisotropy in the upper mantle

Abstract

Seismic anisotropy in the upper mantle can be explained mainly by lattice perferred orientation (LPO) of olivine and pyroxene. To understand deformation mechanism and seismic anisotropy in the upper mantle under Spitsbergen, Svalbard near Arctic, deformation microstructures of olivine and pyroxene in peridotite were studied. Samples are well oliated and LPOs of olivine and pyroxene were determined using EBSD with HKL Channel 5 software. Dislocation microstructures of olivine were observed using SEM after oxygen decoration of speciments. Water contents of olivine and pyroxene were measured using a Nicolet 6700 FTIR with Continuum FTIR microscope. Eight specimens out of tehn showed that [100] axis of olivine is aligned sybparalled to the lineation and (010) plane is subparalled to the foliation, which is a type-A LPO. Tow specimens showed that [100] axis of olivine is aligned subparalled to the lineation and both [010] and [001] axis are distributed in a girdle nearly perpendicular to the lineation, which is type-D LPO. Dislocation density of olivine in the samples showing type-D LPO was higher thatn that in the samples showing type-A LPO. Observation of strong LPOs and dislocations in the specimen indicate that the peridotites studied were deformed by dislocation creep. Seismic anisotropy calculated from the LPOs of olivine can be used well to explain seismic anisotropy of shear wave splitting in the lithospheric mantle under Spitsbergen, Svalbard