日本地球惑星科学連合2019年大会

講演情報

[J] ポスター発表

セッション記号 S (固体地球科学) » S-GC 固体地球化学

[S-GC41] 固体地球化学・惑星化学

2019年5月29日(水) 17:15 〜 18:30 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:下田 玄(産業技術総合研究所地質調査総合センター)、鈴木 勝彦(国立研究開発法人海洋研究開発機構・海底資源研究開発センター)、山下 勝行(岡山大学大学院自然科学研究科)、石川 晃(東京工業大学理学院地球惑星科学系)

[SGC41-P04] オスミウム同位体と強親鉄性元素組成からみたハワイ諸島、カウラ島産マントル捕獲岩の起源

*藤田 遼1石川 晃1横山 哲也1Michael Garcia2 (1.東京工業大学、2.ハワイ大学)

キーワード:Os同位体、ハワイ、マントル捕獲岩

The Emperor Seamount-Hawaiian island chain is thought to originate from melting of a heterogeneous mantle plume, although the nature and extent of heterogeneity remain unclear. It has been suggested that some Hawaiian mantle xenoliths with highly unradiogenic Os isotope ratios (down to 187Os/188Os=0.1138) represent the depleted part of ascending Hawaiian plume, and that the depleted materials are more commonly distributed in mantle plumes than has been inferred from the compositions of erupted lavas. To better understand the lithological and geochemical variations of Hawaiian xenoliths and to evaluate the robustness of the above model, we conducted petrological and geochemical studies on 11 peridotite xenoliths (9 lherzolites, 1 dunite, and 1 wehlrite) collected from the island of Kaula, Hawaii. Here we present the results on the whole-rock Re-Os isotope systematics and highly siderophile element compositions in the xenoliths, together with the major element compositions of constituent minerals (olivine, clinopyroxene, orthopyroxene and spinel). The rock texture and mineral compositions indicate that the Kaula peridotites are samples from normal oceanic mantle and residues of variable degrees of melting. However, they have highly unradiogenic Os isotope ratios (down to 187Os/188Os=0.1187) yielding minimum rhenium-depletion ages of up to ∼1.3 Ga, which are similar to those of O’ahu peridotites. Such unradiogenic Os isotopic compositions are not found in Hawaiian lavas and in depleted mantles (e.g., mid-oceanic ridge basalts). Importantly, as is observed in the O’ahu peridotites, the 187Os/188Os ratios of Kaula peridotites correlate with the degree of depletion, suggesting that they are related to an ancient melting event. Therefore, the Kaula peridotites are possibly the fragments of a Hawaiian mantle plume that consists of ancient depleted and recycled mantle lithosphere.