日本地球惑星科学連合2019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-IT 地球内部科学・地球惑星テクトニクス

[S-IT21] 核-マントルの相互作用と共進化

2019年5月27日(月) 15:30 〜 17:00 ポスター会場 (幕張メッセ国際展示場 8ホール)

コンビーナ:河合 研志(東京大学大学院理学系研究科地球惑星科学専攻)、飯塚 毅(東京大学)、太田 健二(東京工業大学大学院理工学研究科地球惑星科学専攻)、土屋 卓久(愛媛大学地球深部ダイナミクス研究センター)

[SIT21-P17] Ancient, highly depleted mantle constrained by Re-Os isotope and highly-siderophile element compositions of French Polynesian xenoliths

*秋澤 紀克1石川 晃2鈴木 勝彦3小木曽 哲4 (1.東京大学 大気海洋研究所、2.東京工業大学 理学院地球惑星科学系、3.JAMSTEC 海底資源研究開発センター、4.京都大学 大学院人間・環境学研究科)

キーワード:ハルツバーガイト、ダナイト、白金族元素、TIMS、ICP-MS、全岩化学組成

Mantle xenoliths, which are fragmented mantle material trapped by ascending magma, can provide lithological and geochemical information of the inaccessible mantle. In order to better understand the lithological and geochemical characteristics of the mantle, we conducted extensive sampling of mantle xenoliths throughout French Polynesia archipelago: Tahiti, Moorea, and Rurutu islands. We collected a total of 42 mantle xenoliths: 1 lherzolite, 14 harzburgites, 22 dunites, 3 wehrlites, and 2 orthopyroxenites. Here, we present whole-rock rhenium-osmium isotope and highly-siderophile element (Os, Ir, Ru, Pt, Pd, and Re) compositions in addition to whole-rock major-element compositions. Osmium isotope ratio (187Os/188Os) of the mantle xenoliths ranges from 0.1172 to 0.1464, where the harzburgites show a statistical bimodal distribution with unradiogenic (187Os/188Os < 0.125) and radiogenic (187Os/188Os > 0.125) compositions. The harzburgites with unradiogenic 187Os/188Os are depleted with regards to whole-rock major-element compositions (< 1.22 wt% Al2O3), and chondrite-normalized patterns of highly-siderophile elements. Considering the lithological variation of the mantle xenoliths used herein, the mantle is considerably heterogeneous beneath French Polynesia archipelago. In particular, the contrasting isotopic compositions of the harzburgites suggest different mantle reservoirs with unradiogenic and radiogenic 187Os/188Os compositions. We propose a possibility that the harzburgites with unradiogenic 187Os/188Os are impinged mantle material beneath normal oceanic lithosphere due probably to ascending plumes.