Deformation microstructures of olivine and chlorite in chlorite peridotites from Almklovdalen in the Western Gneiss Region, SW Norway and implications for seismic anisotropy

Abstract

Chlorite peridotites from Almklovdalen in SW Norway were studied to understand the deformation processes and seismicanisotropy in the upper mantle. The lattice preferred orientation (LPO) of olivine and chlorite was determined using electronbackscattered diffraction (EBSD)/scanning electron microscope. A sample with abundant garnet showed [100] axes ofolivine aligned sub-parallel to lineation, and [010] axes aligned subnormal to foliation: A-type LPO. Samples rich in chloriteshowed different olivine LPOs. Two samples showed [001] axes aligned sub-parallel to lineation, and [010] axes alignedsubnormal to foliation: B-type LPO. Two other samples showed [100] axes aligned sub-parallel to lineation, and [001] axesaligned subnormal to foliation: E-type LPO. Chlorite showed a strong LPO characterized by [001] axes aligned subnormalto foliation with a weak girdle subnormal to lineation. Fourier transform infrared (FTIR) spectroscopy of the specimensrevealed that the olivines with A-type LPO contain a small amount (170 ppm H/Si) of water. In contrast, the olivines withB-type LPOs contain a large amount (340 ppm H/Si) of water.The seismic anisotropy of the olivine and chlorite was calculated. Olivine showed Vp anisotropy of up to 3.8% and amaximum Vs anisotropy of up to 2.7%. However, the chlorite showed a much stronger Vp anisotropy, up to 21.1%, and amaximum Vs anisotropy of up to 31.7%. A sample with a mixture of 25% of olivine and 75% of chlorite can produce a Vpanisotropy of 14.2% and a maximum Vs anisotropy of 22.9%. Because chlorite has a wide stability field at high pressureand high temperature in subduction zone, the strong LPO of chlorite can be a source of the observed trench-normal ortrench-parallel seismic anisotropy in the mantle wedge as well as in subducting slabs depending on the dipping angle of slabin a subduction zone where chlorite is stable.