[S-CG51] Hard-Rock Drilling Science: Continental and Deep-Sea Drilling, and Ophiolite

[SCG51-02] Petrological characteristics and tectonic implications of the South Mariana Forearc Ridge peridotites, Mariana Trench

*Kana Miyata¹, Shigeki Uehara², Katsuyoshi Michibayashi^1,3, Yasuhiko Ohara^4,1,3 (1.Department of Earth and Planetary Sciences, GSES, Nagoya University, 2.Institute of Geosciences, Shizuoka University, 3.Volcanoes and Earth's Interior Research Center, IMG, JAMSTEC, 4.Hydrographic and Oceanographic Department of Japan)

Keywords:Mariana Trench, South Mariana Forearc Ridge, Challenger Deep, peridotites, amphibole

The Mariana Trench is the deepest in the world, with a depth exceeding 10,000 m^[1]. The southern Mariana Trench is characterized by a complex topography that crosses both volcanic arc and backarc basin, as the South Mariana Forearc Ridge separates the Mariana Trench from the Mariana Trough near the Challenger Deep^[2]. In this study, we aimed to explain the petrographic characteristics of the South Mariana Forearc Ridge peridotites. We conducted petrographic observations, crystallographic orientation analysis, and major element composition analysis. The samples used in this study were 55 peridotites from four dive sites (6K-1095, 6K-1232, 6K-1233, 6K-1234). They were collected during YK08-08 and YK10-12 aboard the R/V Yokosuka using the submersible Shinkai 6500. The samples were serpentinized harzburgites and dunites. The major minerals in the harzburgite were olivine and orthopyroxene, and minor clinopyroxene, spinel, amphibole and plagioclase. Regarding the major element composition, the harzburgites were residual mantle peridotites considering the Mg# (Mg/[Mg+Fe]) of olivine and spinel. The Mg# of olivine ranged from 0.89 to 0.92, the Cr# of spinel ranged from 0.32 to 0.99, the Mg# from 0.11 to 0.62, and the TiO₂ content ranged from 0.01 to 0.03 wt%. The amphibole K₂O/TiO₂ ratios ranged from 0.00 to 3.14. From the K₂O/TiO₂ ratio of amphibole, the peridotites in this study were classified into two groups, corresponding to the backarc (K₂O/TiO₂ = 0–0.50) and forearc (K₂O/TiO₂ = 1.00–3.14) amphibole. The forearc peridotites tended to have low TiO₂ values in spinel (< 0.05 wt%) and high Cr# (>0.6). EBSD analysis confirmed that amphibole contained needle-like spinel inclusions in 6K-1233-R13, which could be evidence of a hydrous melt reaction^[3]. The backarc peridotites had higher TiO₂ values in spinel (> 0.05 wt%) and lower Cr# (<0.6). Based on these results, the development of the South Mariana Forearc Ridge will be discussed.
[1] Greenaway et al. 2021 Deep-Sea Research Part I, [2] Stern et al. 2020 Island Arc, [3] Harano & Michibayashi 2024 Lithos.