日本地球惑星科学連合2022年大会

講演情報

[E] ポスター発表

セッション記号 S (固体地球科学) » S-CG 固体地球科学複合領域・一般

[S-CG46] ハードロック掘削科学〜陸上掘削から深海底掘削そしてオマーン〜

2022年6月1日(水) 11:00 〜 13:00 オンラインポスターZoom会場 (27) (Ch.27)

コンビーナ:Sayantani Chatterjee(Niigata University, Department of Geology, Faculty of Science)、コンビーナ:道林 克禎(名古屋大学 大学院環境学研究科 地球環境科学専攻 地質・地球生物学講座 岩石鉱物学研究室)、高澤 栄一(新潟大学理学部理学科地質科学科プログラム)、座長:Chatterjee Sayantani(Niigata University, Department of Geology, Faculty of Science)、道林 克禎(名古屋大学 大学院環境学研究科 地球環境科学専攻 地質・地球生物学講座 岩石鉱物学研究室)、高澤 栄一(新潟大学理学部理学科地質科学科プログラム)

11:00 〜 13:00

[SCG46-P04] Inversely depleted forearc mantle section records the subduction zone infancy: Umigame Seamount, Bonin Trench

*道林 克禎1,2、斎藤 すず8柿畑 優季1針金 由美子3岡本 敦4大柳 良介5畠山 航平6,7片山 郁夫6石塚 治3小野 重明2 (1.名古屋大学、2.海洋研究開発機構、3.産業総合研究所、4.東北大学、5.国士舘大学、6.広島大学、7.明星大学、8.静岡大学)

キーワード:小笠原海溝、海亀海山、マントル、カンラン岩

The Izu-Bonin-Mariana arc is a typical intra-oceanic arc system. Forearc basalts erupted by magmatic activity during the initial stages of subduction found in Izu-Bonin-Mariana arc provide key information on the initiation of magmatic activity and process of subduction during island arc formation. In recent years, many explorations and research with a focus on volcanic rocks have been conducted, so that models for the development of crustal structure in island arc have been proposed in detail. Since outcrops of plutonic rocks including peridotites in the forearc section expose commonly deeper than 6000 meters on the landward trench slope, few research has considered the forearc mantle by analyzing ultramafic rocks, although structural and petrological studies of forearc peridotites obtained from the landward slope of Izu-Bonin Trench help to understand structural development of the Izu-Bonin arc in the early subduction. More recently, investigations for the landward slope of Izu-Bonin Trench were conducted using Shinkai6500 (Dive 6K1505, 6K1506 and 6K1507) as a part of YK17-14 cruise by R/V Yokosuka in 2017. Shinkai6500 collected serpentinized peridotite, gabbro and basalt in Umigame Seamount (Dive 6K1507). Peridotites have preserved their primary information in mantle despite of their heavy serpentinization. Peridotites show coarse-grain textures with various degrees of serpentinization. Olivine-spinel compositions are in a range of the Olivine-Spinel Mantle Array of Arai (1994), where spinel compositions in harzburgites showed high Cr#(=Cr/(Cr+Al)) of 0.69 to 0.78, indicating that they are forearc peridotites after melting by the addition of water. Cr# for spinel in harzburgites is slightly higher than Cr# of the peridotites (Cr#=0.64-0.73; Harigane et al., 2013) occurred at about 1000 meters shallower than those in this study, indicating that more depleted peridotites have been developed at deeper levels. Olivine crystal-fabrics of harzburgite samples are mostly E-type, suggesting that E-type could be developed due to an input of slab-derived hydrous fluid to the forearc mantle during the initial stages of subduction as discussed by Harigane et al. (2013). In addition, a few harzburgite samples have A-type fabrics, indicating that A-type olivine fabric could be developed where high temperature mantle flow occurred in the mantle wedge during the subduction zone infancy before the input of slab-derived hydrous fluid. Consequently, Umigame Seamount owns a rare ultramafic body preserving information during the initial stages of subduction.