Peridotites outcropped in the southern Mariana Trench

The Izu-Bonin-Mariana(IBM) arc, which is a typical oceanic arc, has 3000 km in length from the north to the south. The southern Marianas from the south of Guam to Yap Trench junction shows characteristic morphology where the trench axis runs across both Mariana volcanic arc and backarc basin. Previous geological expeditions in the southern Mariana Trench on the eastern side of the Challenger Deep have found trench peridotites as shallow as 5800 m below the sea surface [1]. In this study, we report the geochemistry and fabric data for peridotites on the western side of Challenger Deep for the first time since Hawkins and Batiza (1977) [2] and attempt to present the full picture of mantle domain outcropped in the southern Mariana trench. We examined two tectonic ridges, which are in the southwestern tip Mariana forearc near the Yap Trench junction area (Site1:11°2'N 139°3'E) and the other area close to Challenger Deep (Site2: 11°1'N 140°3'E), in the western side of the Challenger Deep using the submersible Shinkai 6500(6K-1397,-1398 and -1429) and deep-tow camera(YKDT-170,-171) as part of R/V Yokosuka cruise in 2014 and 2015. 36 and 8 Peridotites were recovered from Site1 and Site2 ridges respectively. Site1 peridotites were classified as Lherzolite, Harzbugite, Pl-bearing Harzburgite and Dunite. Spinel compositions range from relatively low values (as low as 0.30) to moderately high values (up to 0.54). The increase of Cr# appears to be correlated with Ti contents (0.03-0.49 wt.%), indicating that melt-rock interaction under shallow lithospheric mantle conditions [3]. Furthermore, chemical compositions of Spinel Mg# and Cr# are almost identical to those of Parece Vela Basin peridotites. On the other hand, Site2 peridotites, which were classified Harzburgite and Dunite, have high Spinel Cr#(0.6-0.9) and low Ti contents(>0.1wt.%) similar to those from the northern Mariana forearc [4]. The above data suggests that two ridges in the western side of the Challenger Deep maybe originate from each different mantle domain in the Mariana subduction system in spite of only less than 130km away from each other. REFERENCES [1] Michibayashi et al., 2009, G3 [2] Hawkins and Batiza, 1977, Earth Planet. Sci. Latt. [3] Ohara et al.,2003, G3 [4] Ishii et al.,1992, Proc. Ocean Drill. Program Sci. Results