What caused rare earth element enrichment in early Cambrian phosphate nodules?

Rare earth elements (REEs, including Y) exhibit a high affinity for phosphate minerals and rocks, as demonstrated by the significant reserves of REEs found in marine sediments associated with apatite. The process of REE enrichment within marine sediments has garnered considerable attention due to its minimal environmental impact and substantial mining potential. However, the factors controlling the differences in REE enrichment in phosphorites with varying temporal and spatial distributions are still unclear. In this study, we investigated early Cambrian phosphate nodules from the Niutitang Formation, which developed along the northern and southern shelf margins of the Yangtze Block, South China, to understand the factors contributing to their REE enrichment. On a basin-wide scale, significant differences in the recorded εNd(t) values between the northern and southern nodules were observed, possibly due to variations in the Nd flux and isotope compositions of the weathered source materials. Overall, increased terrestrial inputs are associated with greater REE enrichment. Compared to bedded phosphorites, phosphate nodules exhibit higher concentrations of REEs. This may be attributed to their formation under a slower sedimentation rate and larger surface area to mass ratio. These factors could explain why phosphate nodules in the south-central section (Lishupo) show higher REE concentrations than those in other sections. However, the considerable variation of REEs in the southern region may also have been controlled by the redox environment during the formation of the phosphate nodules. There, stronger redox fluctuation facilitated REE enrichment, as indicated by the wider range of Fe isotope compositions (0.05‰−2.08‰) and higher REE concentrations in the Lishupo ore section. In comparison, phosphate nodules in other sections with a narrow range of Fe isotope compositions (−0.26‰ to +0.44‰) demonstrate weaker redox fluctuations and lower REE contents. In addition, the reducing environment promotes the release of more REEs from host shales and Fe-Mn oxides, which are subsequently incorporated into a more stable phosphate phase. Overall, during the diagenetic process, more dissolution of terrigenous clay minerals and frequent redox fluctuations can lead to an increase in the REE content in pore water, which is eventually incorporated into the stable phosphate phase. This may be the key factor controlling the differences in REE enrichment in phosphorites. This study offers a new perspective on the migration and enrichment mechanisms of REEs in ocean basins.