稀土元素在成岩型海洋铁锰结核中的富集特征及机制

Enrichment mechanism of rare earth element in marine diagenetic ferromanganese nodule

借助相分析中的偏提取方法对取自东太平洋深海平原上的成岩型铁锰结核进行了选择性提取实验以研究稀土元素在其中各矿物或氧化物相中的分布模式以及铁氧化物和锰氧化物对稀土元素的吸附机制。结果显示,尽管1 nm-水锰矿相对无定形铁的氧化物/氢氧化物而言是成岩型结核中的优势矿物,但是稀土元素主要富集在无定形铁的氧化物/氢氧化物中。虽然稀土元素在海水中主要是以碳酸盐络合物的形式存在,但是无定形铁的氧化物/氢氧化物则是从海水中获得自由稀土元素离子来络合,而1 nm-水锰矿则直接吸附稀土元素的碳酸盐络合物。因此,无定形铁的氧化物/氢氧化物对稀土元素来说具有比1 nm-水锰矿更强的络合能力。成岩型结核的Ce负异常是由于成岩型结核形成于缺氧微环境中,该环境不能把可溶性的Ce3+氧化成不溶性的Ce4+发生沉淀。

A series of selective dissolution experiments was conducted on marine diagenetic ferromanganese nodule to investigate the distributions of rare earth elements （REE） and the mechanism of combination between REE and the marine diagenetic ferromanganese nodule. The enrichments of REE are mainly attributed to amorphous ferric oxide/hydroxide rather than to 1 nm-manganates. The REE in seawater are more strongly combined by amorphous ferric oxide/hydroxide than by 1 nm-manganates. Amorphous ferric oxide/hydroxide preferentially combines free REE^3＋ in modern seawater, while 1 nm-manganates combine the REE-carbonate complexes. The slightly negative Ce anomalies in both amorphous ferric oxide/hydroxide and 1 nm-manganates indicate that Ce^3＋ can not be oxidized to Ce^4＋ to precipitate due to the reducing or oxygen depletion zone in the bottom sediments in which the diagenetic nodules were formed.