講演情報
[S3-P-01]Olivine microstructures from the petit-spot peridotite xenoliths along the Japan Trench: insights on deformation fabrics at the lithosphere-asthenosphere boundary
*Dyuti Prakash SARKAR1, Hanaya Okuda2, Yuto Sato2, Yuka Niwa3, Akira Ishikawa3, Katsuyoshi Michibayashi4, Jun-ichi Ando5, Masako Yoshikawa5, Shiki Machida6, Naoto Hirano7, Norikatsu Akizawa5 (1. Yamaguchi Univ. Sci., 2. Kochi Institute for Core Sample Research JAMSTEC, 3. Tokyo Tech., 4. Nagoya Univ. Sci., 5. Hiroshima Univ. Sci., 6. ORCeNG, Chiba Institute of Technology, 7. CNEAS, Tohoku Univ.)
キーワード：
Petit-spot volcano、Olivine slip systems、Sub-grain boundary analysis、LAB
Petit-spot volcanoes were first identified as small volcanic knolls (~1 km³ in volume) on the Pacific Plate, formed due to plate flexure along the outer rise near the Japan Trench (Hirano et al., 2006). These volcanoes erupt vesicular alkaline basalts containing fresh peridotite xenoliths and are transported trench-ward by plate subduction. In this study, we examine peridotite xenoliths collected by the Human Occupied Vehicle (HOV) Shinkai 6500 during scientific cruises YK20-14S, YK21-07S, and YK24-10S. The sampling areas for YK20-14S and YK24-10S are located at Site A, one of the petit-spot locations reported by Hirano et al. (2006). The xenoliths range in size of 1 cm ~ 5 cm, and modal their composition ranges from harzburgite to lherzolite. The contacts of the xenoliths with the host basalt suggest limited reaction along boundaries, with a few samples exhibiting spongy margins. Olivine compositions are characterized by high forsterite number (Fo ~92), and pyroxenes range from enstatitic orthopyroxene to diopsidic clinopyroxene. Both spinel- and garnet-bearing peridotites were identified among the samples. To investigate internal structures of the xenoliths, we employed X-ray computed tomography (CT). Observation of the xenolith thinsections in petrographic microscope, indicate presence of subgrain boundaries in olivine grains of the preserved coarser relict grains. Electron backscatter diffraction (EBSD) was also used to determine crystallographic preferred orientations (CPOs) and quantifying subgrain boundaries. For smaller xenoliths 1-3 cm, only single thinsections were prepared. However, in case of larger xenolith samples ~5 cm, two perpendicular sections were prepared for EBSD analysis. Subgrain boundary analysis allowed us to infer the dominant olivine slip systems even in samples with weak overall fabric. Additionally, Fourier-transform infrared (FTIR) spectroscopy using the protocol of Sato el al. (2023) indicate that olivine grains in these xenoliths are dry (5-13 wt. ppm H2O). The crystallographic data and water content measurements provide insight into the active slip systems in the xenoliths and their implications for deformation processes at the lithosphere–asthenosphere boundary (LAB) beneath the northwestern Pacific.