Geochemical constraints on the formation mechanisms of manganese nodules from Cipangu Basin

In this study, a total of 15 manganese nodule samples were collected from Cipangu Basin in North Pacific Ocean. The morphological structure, microscopic structure, mineral composition, contents of trace elements and rare earth elements, and the differences among samples from different regions were studied systematically. The genetic types and growth mechanism of the manganese nodules were discussed. The manganese nodules are elliptic and intergrowth body with smooth surface. The major minerals are vernadite, quartz, albite, and halite. The REE distribution pattern shows obvious positive Ce anomaly, negative Y anomaly, and LREE enrichment characteristics. The Mn/Fe ratios of the samples from three sites of the study areas are all less than 2.5, which are positively correlated with Ni and Cu; and Mn is highly positively correlated with Cu, Co, and Ni, indicating that Cu, Ni, and Co are more likely to occupy the δ-MnO₂ enrichment at the surface of manganese minerals. The microlayers of manganese nodules can be divided into two types, Type Ⅰ and Type Ⅱ. Type Ⅰ is laminated, columnar and laminated with dense and high reflectance structure, and Type Ⅱ is hybrid and ring with loose and low reflectance structure. The Mn/Fe, Ni, Ni + Cu, and Co in Type Ⅰ were greater than those of Type Ⅱ. The Type Ⅰ area has loose manganese nodule in structure reflecting great changes in sedimentary environment with great growth rate of nodule, while the Type Ⅱ zone has small changes in sedimentary environment with small growth rate. The manganese nodule samples are formed by metal precipitation from the ambient seawater (hydrogenetic) and the age was estimated to be 1.48 Ma based on the ordinary trace element test data. At last, according to the micro-analysis data, the growth age was 2.44 Ma by estimation.