日本地球惑星科学連合2015年大会

講演情報

口頭発表

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-CG64] 海洋底地球科学

2015年5月27日(水) 16:15 〜 18:00 A05 (アパホテル&リゾート 東京ベイ幕張)

コンビーナ:*沖野 郷子(東京大学大気海洋研究所)、田所 敬一(名古屋大学地震火山研究センター)、石塚 治(産業技術総合研究所活断層火山研究部門)、土岐 知弘(琉球大学理学部)、高橋 成実(海洋研究開発機構地震津波海域観測研究開発センター)、座長:石川 直史(海上保安庁海洋情報部)、生田 領野(静岡大学理学部)

17:36 〜 17:39

[SCG64-P07] プチスポット溶岩中のかんらん石捕獲結晶

ポスター講演3分口頭発表枠

*滝 嵐1平野 直人2山本 順司3町田 嗣樹4石井 輝秋5 (1.東北大学大学院理学研究科、2.東北大学東北アジア研究センター、3.北海道大学総合博物館、4.早稲田大学創造理工学部環境資源工学科、5.公益財団法人 深田地質研究所)

キーワード:プチスポット, かんらん石, 捕獲結晶

Petit-spot is a small volcano erupted on the seafloor. The magma comes from asthenosphere, just below oceanic lithosphere, through a crack in subducting plate. The petit-spot volcanoes appear globally on the seafloor where the plate is flexing (e.g., Japan and Chile Trenches). The petit-spot lavas and entrained mantle materials have been already reported from areas of Japan Trench oceanward slope (Sites A), and of NW Pacific (Site B). Although the discovery of the petit-spots has been anticipated from Site C (offshore of Fukushima, south of Site A), lava samples and entrained mantle materials have never been reported. To examine the activity of the petit-spot volcanoes, we conducted the nine submersible dives of the SHINKAI 6500 submersible during cruise YK14-05 of R/V Yokosuka at Site C in April 2014.
Alkaline pillow lavas were collected from the Site C during cruise YK14-05. Eruption age is at the time between 0.31 and 2.1 Ma estimated on the basis of the thickness of paragonite on quenched glass rind. The lavas are classified into basanite, and include large amount of olivine (>10% normative olivine). Large (1-5 mm) olivines have anhedral morphology. The large olivines show forsterite numbers (Fo) of 88-90 and NiO contents of 0.3-0.5 wt. %, corresponding to the composition of the primitive mantle peridotite. On the other hand, the small olivines surrounding the large olivines have similar range of compositions (Fo of 84-87, CaO contents of >0.1 wt. %) to those of groundmass olivines. These observations imply that large olivines are fragments of mantle peridotites, that is, these are mantle xenocrysts. If these are xenocrystic olivines, it tells us the cryptic aspects of an old oceanic lithosphere. Fo values of the present olivine xenocrysts are slightly lower than those of the mantle xenoliths reported from Site A and B (90-93). The chemically heterogeneous mantle might be existed in the subducting NW Pacific plate.