[M-IS27_2PM1] 海底マンガン鉱床の生成・環境・起源

[MIS27-02] 拓洋第５海山におけるマンガンクラスト表層と近傍海水の希土類濃度の比較

*得丸 絢加¹、朱 彦北²、野崎 達生³、高谷 雄太郎⁴、後藤 孝介⁵、鈴木 勝彦³、常 青³、木村 純一³、加藤 泰浩⁴、臼井 朗⁶、浦辺 徹郎⁷、鈴木 庸平¹ (1.東大・地惑、2.AIST・NIST、3.JAMSTEC・IFREE、4.東大・工、5.AIST・GSJ、6.高知大・理、7.国際資源開発研修センター)

キーワード：鉄マンガンクラスト, 海水, オスミウム同位体，, 希土類元素, 地球化学

Hydrogenetic ferromanganese (Fe-Mn) crusts are strongly enriched relative to the Earth's lithosphere in many rare and precious metals, including Co, Te, Mo, Bi, Pt, W, Zr, Nb, Y, and rare earth elements (REEs) (e.g. Hein et al., 2013). Accumulation of these trace metals from seawater is generally controlled by sorption (Koschinsky and Halbach, 1995; Koschinsky and Hein, 2003). REEs, except for Ce, behave as a dissolved trivalent cation in seawater and concentrated in the solid phase by adsorption (Nakada et al., 2013). Although a set of REE concentrations in Fe-Mn crusts is frequently used as a proxy to distinguish formation processes (e.g. Usui and Someya, 1997; Hein et al., 2000), it is not certain if REE concentrations in Fe-Mn crusts are correlated to those in surrounding seawater. In order to verify the correlation, REE concentrations in the outermost surface of Fe-Mn crusts and the surrounding seawater were directly compared at various depths (957-2987 m) on the Takuyo Daigo Seamount. We normalized the REE concentrations in the outermost surface of Fe-Mn crusts with its formation age, as the relative age of Fe-Mn crusts can be determined using osmium isotopic ratio (e.g. Klemm et al., 2005; 2008). For measurement of REE concentrations in seawater, inductively coupled plasma mass spectrometry (ICP-MS) was applied and solid phase extraction (SPE) techniques using chelating resins were conducted as pretreatment (Zhu et al., 2013). In this study, we compare depth profiles of the normalized REE concentrations in the outermost surface of Fe-Mn crusts and the REE concentrations in surrounding seawater.