JpGU-AGU Joint Meeting 2017

講演情報

[JJ] ポスター発表

セッション記号 M (領域外・複数領域) » M-IS ジョイント

[M-IS17] [JJ] 海底マンガン鉱床の科学:基礎から応用まで

2017年5月21日(日) 13:45 〜 15:15 ポスター会場 (国際展示場 7ホール)

コンビーナ:臼井 朗(高知大学海洋コア総合研究センター)、高橋 嘉夫(東京大学大学院理学系研究科地球惑星科学専攻)、伊藤 孝(茨城大学教育学部)、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)

[MIS17-P01] 超深海における海水起源鉄マンガンクラストのREY組成

*浅見 慶志朗1平野 直人2町田 嗣樹3安川 和孝4,5加藤 泰浩4,5,3 (1.東北大学大学院理学研究科、2.東北アジア研究センター、3.海洋研究開発機構、4.東京大学大学院工学系研究科、5.千葉工業大学次世代海洋資源研究センター)

キーワード:鉄マンガンクラスト、希土類元素、プチスポット火山

Ferromanganese crusts, mainly composed of Iron and Manganese (oxhydro)oxide, occur regarding two kinds of their origins in submarine environment. One is hydrogenetic origin enriched in trace elements and rare earth elements (REE), the other is hydrothermal origin lacking those elements. The former is a useful tool to record oceanic paleo-environment, because they adsorb the elements from ambient seawater.
The relationships between chemical composition of ferromanganese crusts and water depth have already been studied to investigate the accumulation mechanism of elements. Most of ferromanganese crusts previously reported, however, have been only from shallower than 4 km depth of seamount. Here, the petit-spot volcanoes, monogenetic volcanoes occurred on bending oceanic plate, are possible to supply the ferromanganese crusts on abyssal plane covered with sediments of 5 to 6 km in water depth. The ferromanganese crusts from petit-spot volcanoes, therefore, are expected to record the chemical signatures of the deep-sea water in abyssal to hadal zones. We analyzed the samples off NE-Japan and Minamitorishima Island (5.2-6.0 km in depth), and those in Japan Trench (6.7-7.1 km in depth)
The data of them show low Mn/Fe ratios (0.63 to 1.5), high contents of Co, Ni, and REE and Y (REY), and positive Ce anomalies, all of which indicate hydrogenetic character. We used the ratios of two adjacent elements in REY to compare the compositions regardless of the contaminating detritus in ferromanganese crust. Shale-normalized REY patterns of the ferromanganese crusts shows characteristically lower LaSN/PrSN ratios than 1 although the typical crusts have LaSN/PrSN ratios systematically higher than 1. A strong correlation between the LaSN/PrSN ratios and water depths (correlation coefficient shows -0.9) suggests that the LaSN/PrSN in hydrogenetic crusts is a potential hydro-barometer of (paleo)ocean. Other hydrogenetic ferromanganese crusts from the Pacific Ocean also show the similar trend, but different from those in the Japan Trench. REY ratios of ferromanganese crusts from the Japan Trench (approximately 7km) are similar values to shallower crusts in this study. This would be due to remaining REY components on those of shallower portion prior to trench-oceanward slope on the plate motion (approximately 5.3 km in depth). The variations of REY ratios with water depth in ferromanganese crusts reflect REY compositions of ambient seawater.