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Twinning induced reduction of seismic anisotropy in lawsonite blueschist
Seungsoon Choi,Olivier Fabbri,Gultekin Topuz,Aral I. Okay,Haemyeong Jung 대한지질학회 2021 대한지질학회 학술대회 Vol.2021 No.10
Lawsonite is an important mineral to understand seismic anisotropy in subducting oceanic crust because of its large elastic anisotropy and prevalence in cold subduction zones. However, there is a lack of knowledge on how lawsonite twinning affects seismic anisotropy despite previous reports showing the existence of twins in lawsonite. We thus investigated the effect of twins in lawsonite on lattice preferred orientation (LPO), fabric strength, and seismic anisotropy of lawsonite using the lawsonite blueschists from Alpine Corsica (France) and Sivrihisar Massif (Turkey). LPOs of minerals were measured by using the electron backscattered diffraction (EBSD) facility attached to scanning electron microscope. The EBSD analyses of lawsonite revealed that {110} twinning in lawsonite is developed and [001] axes are strongly aligned subnormal to the foliation and both [100] and [010] axes are aligned subparallel to the foliation. It is found that the existence of twins in lawsonite could induce a large reduction of seismic anisotropy, especially for the maximum S-wave anisotropy up to 3.67% in lawsonite and 1.46% in the whole rocks. Therefore, lawsonite twinning needs to be considered in the interpretation of seismic anisotropy in the subducting oceanic crust in cold subduction zones.