Rare earth and other element contents and speciations in samples from Core AKO26-35, Pacific Ocean, Southwest Basin

Ferromanganese micro- and macronodules in eupelagic clays at Site AKO26-35 in the Southwest Pacific Basin were studied in order to check REE distribution during ferromanganese ore formation in non-productive zones of the Pacific Ocean. Host sediments and their labile fraction, ferromanganese micronodules (in size fractions 50-100, 100-250, 250-500, and >500 ?m) from eupelagic clays (horizons 37-10, 105-110, 165-175, and 189-190 cm), and buried ferromanganese micronodules (horizons 64-68, 158-159, and 165-166 cm) were under study. Based on partition analysis data anomalous REE enrichment in eupelagic clays from Site AKO26-35 is related to accumulation of rare earth elements in iron hydroxophosphates. Concentration of Ce generally bound with manganese oxyhydroxides is governed by oxidation of Mn and Ce in ocean surface waters. Micronodules (with Mn/Fe from 0.7 to 1.6) inherit compositional features of the labile fraction of bottom sediments. Concentrations of Ce, Co, and Th depend on micronodule sizes. Enrichment of micronodules in hydrogenic or hydrothermal matter is governed by their sizes and by a dominant source of suspended oxyhydroxide material. The study of buried ferromanganese micronodules revealed general regularities in compositional evolution of oxyhydroxide matrices of ferromanganese micro- and macronodules. Compositional variation of micro- and macronodules relative to the labile fraction of sediments in the Pacific non-productive zone dramatically differs from the pattern in bioproductive zones where micronodule compositions in coarser fractions are similar to those in associated macronodules and labile fractions of host sediments due to more intense suboxidative diagenesis.

针对西南太平洋盆地AKO26-35站位远洋黏土（eupelagic clays）中的铁锰微结核与大结核（ferromanganese micro- and macronodules）开展研究，旨在探究太平洋非生产力带铁锰成矿过程中的稀土元素（Rare Earth Elements，REE）分布规律。本次研究对象涵盖赋存沉积物及其不稳定组分、采自远洋黏土层位（37~10 cm、105~110 cm、165~175 cm及189~190 cm）的铁锰微结核（粒级分别为50~100 μm、100~250 μm、250~500 μm及>500 μm），以及层位为64~68 cm、158~159 cm及165~166 cm的埋藏型铁锰微结核。基于分配分析数据，AKO26-35站位远洋黏土中异常的稀土元素富集现象，与稀土元素在羟基磷酸铁矿物中的富集有关。通常与羟基氧化锰结合的铈（Ce）浓度，受表层海水锰与铈的氧化过程控制。锰铁比值介于0.7~1.6之间的微结核，继承了海底沉积物不稳定组分的地球化学组成特征。铈、钴（Co）及钍（Th）的浓度随微结核粒径发生变化。微结核的水成或热液物质富集程度，受其粒径以及悬浮态羟基氧化物物质的主要来源共同控制。对埋藏型铁锰微结核的研究，揭示了铁锰微结核与大结核的羟基氧化物基质组成演化的一般规律。相较于生物生产力带，太平洋非生产力带中铁锰微结核与大结核相对于沉积物不稳定组分的组成变化模式存在显著差异：在生物生产力带中，由于亚氧化成岩作用更为强烈，较粗粒级的微结核组成与伴生的大结核及赋存沉积物的不稳定组分较为相似。