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 seismic anisotropy in the upper mantle. The lattice preferred orientation (LPO) of olivine and chlorite was determined using electron backscattered diffraction (EBSD)/scanning electron microscope. A sample with abundant garnet showed [100] axes of olivine aligned subparallel to lineation, and [010] axes aligned subnormal to foliation: A-type LPO. Samples rich in chlorite showed different olivine LPOs. Two samples showed [001] axes aligned subparallel to lineation, and [010] axes aligned subnormal to foliation: B-type LPO. Two other samples showed [100] axes aligned subparallel to lineation, and [001] axes aligned subnormal to foliation: E-type LPO. Chlorite showed a strong LPO characterized by [001] axes aligned subnormal to foliation with a weak girdle subnormal to lineation. FTIR spectroscopy of the specimens revealed that the olivines with A-type LPO contain a small amount (170 ppm H/Si) of water. In contrast, the olivines with B-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 a maximum Vs anisotropy of up to 2.7%. However, the chlorite showed a much stronger Vp anisotropy, up to 21.1% and a maximum Vs anisotropy of up to 31.7%. A sample with a mixture of 25% of olivine and 75% of chlorite can produce a Vp anisotropy of 14.2% and a maximum Vs anisotropy of 22.9%. Because chlorite has a wide stability field at high pressure and high temperature in subduction zone, the strong LPO of chlorite can be a source of the observed trench-normal or trench-parallel seismic anisotropy in the mantle wedge as well as in subducting slabs depending on the dipping angle of slab in a subduction zone where chlorite is stable.