Geochemical characteristics and genesis of the polymetallic nodules in the Pacific survey area

The polymetallic nodules in China's polymetallic nodules exploration contract area in the East Pacific Basin (EP nodules) are largely buried or semi-buried in a sedimentary environment dominated by clay and siliceous components. The manganese mineral phases of the EP nodules mainly include vernadite and todorokite, with high REY and high Cu and Ni contents and Mn/Fe ratio. These characteristics indicate the nodules are affected by interstitial water thus falling into the range of diagenesis and hydrogenesis. In comparison, the polymetallic nodules in the Northwest Pacific Basin (WP nodules) are mostly exposed to seawater, and the surrounding sediments are mainly deep-sea clay. The manganese mineral phases of the WP nodules are almost all vernadite, with high REY, high Co content and low Mn/Fe ratio. It shows that the WP nodules are of typical hydrogenic form. The NASC-normalized REY patterns of the polymetallic nodules in both regions show positive Ce and negative Y anomalies and no/weak Eu anomaly, while the REY patterns of ambient seawater show obvious negative Ce and positive Y anomalies and no obvious Eu anomaly. This inverse coupling relationship implies that the REY patterns of the polymetallic nodules are the result of selective enrichment from seawater. The WP nodules have higher Ce content and δCe than the EP nodules. Both Co and Ti correlate positively with Ce, which may collectively affect the selection of Ce and Y in seawater by polymetallic nodules. In this study, dissolved oxygen in seawater was not a factor controlling the level of positive Ce anomaly observed in the polymetallic nodules. Rather, elements such as Co and Ti and their related components can cause strong fractionation of Ce and other REY, which may be the controlling factor. The surface samples of polymetallic nodules and cobalt-rich crusts in the study area, with εNd values ranging from -6.6 to -2.5, represent the most radioactive ferromanganese samples of Nd in the global sea area. Combining the εNd result with REY patterns and Eu anomaly, we suggest that REY in polymetallic nodules mainly come from the surrounding continental crust with relatively high εNd and can be transported to the ocean via rivers or atmospheric deposition. However, the radioactive Nd isotopes released from basalt in the Northwest Pacific Basin have only weak effect on the seawater. © 2021, Editorial Office of Earth Science Frontiers. All right reserved.