[S-CG46] Hard-Rock Drilling Science: From Continental to Deep Sea Drilling, and Oman Project

[SCG46-05] Peridotites from 139°E Ridge, the southwestern tip of the Mariana forearc as a backarc origin

★Invited Papers

*Katsuyoshi Michibayashi^1,3, Yasuhiko Ohara^1,2,3 (1.Department of Earth and Planetary Sciences, Nagoya University, 2.Hydrographic and Oceanographic Department of Japan, 3.Research Institute for Marine Geodynamics, JAMSTEC)

Keywords:Southern Mariana Trench, peridotite, melt–rock interaction

Peridotites have been discovered on the southwestern slope of 139°E Ridge (11°12N 139°15E), where is a small ridge in the southwestern tip of the Mariana forearc near the junction with the Yap trench and Parece Vela Basin, from 5,999 and 3,430 m water depth using the submersible Shinkai6500 (6K1397 and 6K1398). The peridotites consisting of residual harzburgite and dunite show various textures with respect to their exposed depth in the tectonic ridge. Peridotites with the coarse-grainedclose (>1 mm) texture were sampled from the shallowest part (3,705-4,042 m) in the dive area, whereas peridotites with the small-grained (<0.5 mm) textures were sampled from the deepest part (5,996 m). Olivine fabrics vary associated with textures: (010)[100] A type pattern for the coarse grained textures, {0kl}[100] D type pattern for the fine-grained textures, and various indistinct patterns for the heterogeneous textures. The fine-grained textures with D type olivine fabrics would result from deformation under higher flow stress, suggesting that a ductile shear zone in the lithospheric mantle could occur in the deepest part of 139°E Ridge. Spinel compositions range from relatively low values (as low as 0.30) to moderately high values (up to 0.54), where the increase of Cr# appears to be correlated with Ti contents (0.03-0.49 wt.%), indicating melt-rock interaction under shallow lithospheric mantle condition, which is almost identical to those of Parece Vela Basin peridotites. It implies that Parece Vela Basin mantle may be exposed on the inner trench slope in the westernmost Mariana trench.