Abstract
The microstructure of amphibole peridotites from Åheim, Norway were analyzed to understand the evolution of the lattice-preferred orientation (LPO) of olivine throughout the Scandian Orogeny and its implication for the seismic anisotropy of the subduction zone. The Åheim peridotites had a porphyroclastic texture and some samples contained an abundant amount of hydrous minerals such as tremolite. Detailed microstructural analysis on the Åheim peridotites revealed multiple stages of deformation. The coarse grains showed an A-type LPO of olivine, which can be interpreted as the initial stage of deformation. The spinel-bearing samples showed a mixture of B-type and C-type LPOs of olivine, which is considered to represent the deformation under water-rich conditions. The recrystallized fine-grained olivine displays a B-type LPO, which can be interpreted as the final stage of deformation. Microstructures and water content of olivine indicate that the dominant deformation mechanism of olivine showing a B-type LPO is a dislocation creep under water-rich condition. The observation of the B-type LPO of olivine is important for an interpretation of trench-parallel seismic anisotropy in the mantle wedge. The calculated seismic anisotropy of the tremolite showed that tremolite can contribute to the trench-parallel seismic anisotropy in the mantle wedge.
Highlights
The deformation behavior of olivine is key to understanding the mantle flow and seismic anisotropy in the upper mantle [1,2,3,4,5]
Deformation was associated with the infiltration of fluid and enhanced dynamic recrystallization under water-rich conditions that resulted in the fabric transition of the olivine from an A-type to C- or B-type lattice-preferred orientation (LPO)
Olivines in the spinel-bearing assemblage showed a C-type LPO of olivine which can be interpreted as a result of deformation under water-rich conditions after granulite facies metamorphism
Summary
The deformation behavior of olivine is key to understanding the mantle flow and seismic anisotropy in the upper mantle [1,2,3,4,5]. Many experimental studies concerning the deformation of olivine have reported that it has various types of lattice-preferred orientations (LPOs), depending on the physicochemical conditions during its deformation, and that the different LPOs of olivine may influence the seismic anisotropy of the upper mantle [3,6,7,8,9,10,11]. The fabric transition of olivine in the mantle wedge from an A-type to a B-type LPO of olivine is proposed as a possible mechanism for the change in the shear wave splitting pattern observed in the subduction zone [3,7,8,12,13,14]. The exact mechanism for the fabric transition of olivine in the mantle wedge is still under debate
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