[M-GI34] 海底マンガン鉱床の環境，開発，地球史

[MGI34-06] Mineralogical and geochemical characteristics of ferromanganese nodules in the Minamitorishima EEZ

*中村 謙太郎¹、堀之内 航一¹、下村 遼¹、町田 嗣樹²、安川 和孝^1,2、藤永 公一郎^1,2、加藤 泰浩^1,2 (1.東京大学大学院工学系研究科システム創成学専攻、2.千葉工業大学次世代海洋資源研究センター)

キーワード：マンガンノジュール、マンガン鉱物、全岩化学組成、形成環境、南鳥島EEZ

Ferromanganese nodules are spherical material mainly composed of manganese, iron, and other transition metal (oxyhydro)oxides, which are widely distributed on the deep seafloor around the world [1]. It has been widely accepted that ferromanganese nodules can be classified into three types: hydrogenetic, diagenetic and hydrothermal types [1]. Among them, hydrogenetic nodules are composed of Fe–Mn (oxyhydr)oxides that precipitated from seawater and thus, this type of nodules can be expected to provide proxy paleoceanographic records [2, 3, 4, 5].
In this study, we investigated mineralogy and geochemistry of ferromanganese nodules sampled from the Minamitorishima EEZ for the purpose of elucidating formation environment of the nodules. Both the ferromanganese mineral phase and bulk chemical compositions clearly show that these nodules are hydrogenetic in origin. Based on X-ray CT and micro X-ray mapping analyses, oxide layer of the nodules can be subdivided into five layers [6. 7]. Although ferromanganese mineral is common to all the oxide layers, we found that Mn/Fe and Ni content systematically changed from inner layer to outer layer. The result implies that bottom water of the Minamitorishima EEZ has gradually been oxygenated toward the present.