Deformation microstructure of amphibole peridotite from Aheim, Norway and its implication for the seismic anisotropy of the mantle wedge

The microstructures of amphibole peridotites from the Åheim, Norway were studied to understand the evolution of microstructures of olivine through the Scandian Orogeny and the subsequent exhumation process. The Western Gneiss Region, Norway had undergone UHP metamorphism and subsequent retrogression associated with the Scandian Orogeny. The Åheim amphibole peridotite shows clear porphyroclastic texture, abundant hydrous minerals such as tremolite or chlorite, and much evidence of localized deformation. LPOs of olivine and amphibole were determined by using electron back-scattered diffraction (EBSD) system attached to the scanning electron microscope (SEM).

Detailed microstructural analysis on the Åheim amphibole peridotites revealed the evidence of the multiple stages of deformation during the Scandian Orogeny. The coarse grains of olivine including porphyroclasts showed the A-type LPO of olivine (Jung & Karato, 2001), which is interpreted as an initial stage of deformation. The recrystallized-fine grains of olivine showed the B-type LPO of olivine (Jung & Karato, 2001), which is interpreted as a late-stage deformation in amphibolite facies condition. Observation of abundant hydrous minerals, hydrous inclusions in olivine, as well as high dislocation density of olivine in the fine-grained olivines suggest that fabric transition of olivine from the A-type to B-type LPO was resulted from the deformation in a water-rich condition during the exhumation process. The B-type LPO of olivine is important because it is the one of the possible mechanisms for causing the trench-parallel seismic anisotropy in the mantle wedge. A partial fabric transition from the A-type to the B-type LPO of olivine associated with the localized deformation in a water-rich condition might explain a weak seismic anisotropy observed in NE Japan or Mexico. Amphiboles in the amphibole-rich layer showed the Type-III LPO of amphibole (Ko & Jung, 2015). It is found that strong fabric strength and the resultant seismic anisotropy of amphibole can perform a similar role as other hydrous minerals such as serpentine or chlorite on the trench-parallel seismic anisotropy with the flow dipping along the subducting slab in the mantle wedge.

 

Jung, H., Karato, S., 2001, Science, 293, 1460-1463.

Ko, B., Jung, H., 2015, Nature Communications, 6: 6586.