Geochemical characteristics of a ferromanganese nodule with a tooth nucleus from the northwestern Pacific: Implications for element migration between Fe-Mn (oxyhydr)oxide and biogenic apatite

Biogenic apatite (fish teeth/bones) is considered the primary host mineral of rare earth elements and Y (REY) in deep-sea REY-rich sediments. Meanwhile, Fe-Mn (oxyhydr)oxides in the form of ferromanganese nodules and micronodules are also enriched in REY. Although studies have suggested that micronodules might release REY to pore water, eventually migrating into biogenic apatites during early diagenesis, robust evidence and mechanism for such migration remain elusive. Here, we present findings from a ferromanganese nodule with a fossil tooth nucleus discovered in surface sediments from the northwestern Pacific. Detailed geochemical characteristics of different parts of the ferromanganese nodule are provided, along with new analysis of modern shark teeth from the study area. The Fe-Mn (oxyhydr)oxides from the outer crust of the nodule are typically hydrogenetic, with lower Mn/Fe ratios (mean = 1.35, n = 33), in contrast to those filled in the tooth nucleus (mean Mn/Fe = 3.83, n = 20). Combined with compiled data on (micro)nodules and biogenic apatites in deep-sea sediments, we have identified the initial stage (Mn/Fe = 2.5 – 5) of early diagenesis of Fe-Mn (oxyhydr)oxides characterized by preferential release of light and middle rare earth elements (except Ce) compared to Fe-Mn (oxyhydr)oxides (micronodules) with higher degree of diagenesis. We demonstrated that biogenic apatites can inherit distinct fractionations of REY released from Fe-Mn (oxyhydr)oxides during different stages of diagenesis. Additionally, gradual elemental variations and substitutions of Sr2+ and Zn2+ by Na+ and REY3+ are observed in the enameloid of the fossil tooth, and substitution dominates the REY migration from Fe-Mn (oxyhydr)oxides to biogenic apatites. Thus, the studied nodule with directly contacting Fe-Mn (oxyhydr)oxides and biogenic apatite (enameloid), provides substantial evidence for potential element migration between Fe-Mn (oxyhydr)oxide and biogenic apatite and gives new insights into the mechanism for REY-enrichment in biogenic apatite.