[S-IT21] 核-マントルの相互作用と共進化

[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.