[S-CG52] Oceanic plate as inputs to subduction zone: evolution process of the oceanic plate

[SCG52-09] Olivine slip-systems of petit-spot peridotite xenoliths and insights on upper mantle deformatio

*Dyuti Prakash Sarkar¹, Norikatsu Akizawa², Jun-ichi Ando¹, Masako Yoshikawa¹ (1.Hirosima University, Earth and Planetary Systems Science, Higashi-Hiroshima, Japan, 2.University of Tokyo, Atmosphere and Ocean Research Institute, Kashiwa, Japan)

Keywords:Olivine slip systems, Petit-spot volcano, Sub-grain boundary analysis, Lithospheric mantle

Petit-spots are small knolls (~1 km³ volume) that were first identified (Hirano et al. 2006) occurring in the ocean floor of the northwestern Pacific Plate and were considered to be formed as a result of plate flexure along the outer rise. The peridotite xenoliths from the petit spots thus provide a window into the upper mantle, and insights on the visco-elastic and petrological structures of oceanic plates. This research consists of the peridotite xenoliths collected using the deep-submergence research vehicle Shinkai6500 during the YK20-14S and YK21-07S scientific cruises. The xenoliths were obtained by dredging the petit-spot volcanic fields (Sites A and B) located on the seaward slope of the Northern Japan Trench. The peridotite xenoliths are hosted by highly vesicular alkaline basalts with the modal compositions ranging from harzburgitic to lherzolitic. The olivine compositions indicate forsterite number [(Mg/(Mg+Fe) atomic ratio) x 100] ranging 91–92, and the pyroxenes are enstatitic or diopsidic compositions. The reaction rims of the xenolithic samples are very limited indicating pristine preservation of the mantle rocks and limited reaction with the host basalts. The olivine grains of xenoliths show development of sub-grain boundaries, however, number of grains showing sub-grains vary between the different xenoliths. In absence of conspicuous foliation or elongation direction of the peridotite samples, the slip systems developed in olivine were determined by Electron Backscatter Diffraction (EBSD) mapping of subgrains and subgrain boundary trace analysis. The results aim to elucidate our understanding on the deformation mechanisms of the mantle in the northwestern Pacific region.