Migration and exchange of rare earth elements between phosphate and iron manganese phase: A case study from fish tooth-bearing nodules

Deep-sea sediments that are widely distributed in the Pacific Ocean could be an important future resource for rare earth element (REE) and yttrium (REY) due to their high concentrations of REY. Bioapatite fossils as well as Fe-Mn (oxyhydr) oxides (e.g., micronodules) are recognized as significant carriers of REY in deep-sea sediments. Notably, bioapatite fossils, account for more than 60% of the total REY content in bulk sediments, underscoring their significance as primary host for REY. However, the mechanisms of REY enrichments, variations in Ce anomalies, and REY substitution in bioapatite fossils remain under explained. In this study, we performed detailed in situ geochemical, isotope geochemical and chemical imaging analyses on fish tooth-bearing nodules. Our results indicate that fish teeth are mainly composed of Ca, P, F and Na, which accounting for over 95 % of the total composition of these teeth. On the one hand, the concentrations of Ca, P and F exhibit strong positive correlations with each other: their contents increase from the inside to the outside of the dentine shell. On the other hand, Na concentration display a different trend that the highest concentration is found in the center part of shark teeth. Despite the theoretical nearly absence of seawater as potential transport media, REYs migrated from the ferromanganese oxides phase to phosphate phase, and their contents decreased from inside layers to outside layers of dentine. During the long-term substitution processes in geological time, a portion of Ce3+ would be oxidized into immobile Ce4+ under oxic conditions, resulting in less incorporation of Ce3+ into the outer layers of dentine compared to other REY3+. As a consequence, stronger negative Ce anomalies were observed in the inside layers than in outside layers of dentine. The laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) mapping together with the in-situ geochemistry results indicate the negligible participation of Si in REY substitution, whereas Na exhibits a pronounced preference for substituting Ca2+ with REY3+. Therefore, we propose the following substitution scheme for incorporating REY into the crystal lattice of fish teeth: REY3+ + Na+<-> 2Ca2+. Our study clarified the fraction of REY during their migration from the Fe-Mn phase to the phosphate phase, and also contributes to a deeper understanding of the REY enrichment mechanism in pelagic sediments.