Carbonate fluorapatite pellets in pelagic sediments: Implications for REY enrichment and Nd isotopic signatures in abyssal environments

Deep-sea sediments enriched in rare-earth elements and yttrium (REY) have attracted global attention due to their strategic importance for cutting-edge technologies. While bioapatite fossils are considered the primary REY reservoir in marine deposits, REY-rich sediments predominantly occur in pelagic oligotrophic regions where bioapatite abundance is insufficient to account for the observed REY and phosphorus inventories. Here, we report peculiar pellets of carbonate fluorapatite (CFA) as a REY host in pelagic sediments from the Pacific Ocean. These authigenic pellets likely form through precipitation within microenvironments (e.g., foraminifera tests) at the sediment-seawater interface, utilizing dissolved constituents derived from organic matter degradation and carbonate shell dissolution. Additionally, the higher abundance of CFA pellets in sediments near the East Pacific Rise suggests that hydrothermal venting may promote CFA formation, because particulate Fe from hydrothermal plumes can efficiently bind to dissolved P and thus promote P burial into sediments. During formation, CFA pellets incorporate REY from ambient fluids and reach exceptional enrichment (∼23,000 μg/g), thereby serving as an important REY reservoir. The pellets' extreme REY enrichment (∼10⁸ times pore fluid concentrations) and lattice-hosted incorporation appear to preserve pristine REY signatures (including Nd isotopes), indicating a potential proxy for archiving bottom water Nd isotopic signatures.