Rare Earth Elements Composition and Constraint on the Genesis of the Polymetallic Crusts and Nodules in the South China Sea

The rare earth elements (REE) composition of the polymetallic crusts and nodules obtained from the South China Sea (SCS) were analyzed through inductively coupled plasma mass spectrometry. Results revealed great differences in the REE abundances (ΣREE) of the SCS polymetallic crusts and nodules; the crusts show the highest ΣREE, whereas the nodules exhibit the lowest ΣREE. The similarity in their NASC–normalized patterns, the enriched light REE (LREE), the markedly positive Ce anomaly (δCe), and the non– or weakly positive Eu anomaly (δEu), suggest that the polymetallic crusts and nodules are of hydrogenetic origin. Moreover, the REE contents and their relevant parameters are quite different among the various layers of the crusts and nodules, which probably results from the different marginal sea environments and mineral assemblages of the samples. The growth profiles of the SCS polymetallic crusts and nodules reveal the tendency ΣREE and δCe to slightly increase from the outer to the inner layers, suggesting that the growth environments of these samples changed smoothly from an oxidizing to a relatively reducing environment; in addition, the crust ST1 may have experienced a regressive event (sea–level change) during its growth, although the REE composition of the seawater remained relatively stable. On the basis of the regional ΣREE distribution in the SCS crusts and nodules, the samples collected near the northern margin were influenced by terrigenous material more strongly compared with the other samples, and the REE contents are relatively low. Therefore, the special geotectonic environment is a significant factor influencing the abundance of elements, including REE and other trace elements. Compared with the oceanic seamount crusts and deep–sea nodules from other oceans, the SCS polymetallic crusts and nodules exhibit special REE compositions and shale–normalized patterns, implying that the samples are of marginal sea–type Fe–Mn sedimentary deposits, which are strongly affected by the epicontinental environment, and that they grew in a more oxidative seawater environment. This analysis indicates that the oxidized seawater environment and the special nano property of their Fe–Mn minerals enrich the REE adsorption.