Distribution of Rare Earth Elements and Yttrium in Sediments From the Clarion-Clipperton Fracture Zone, Northeastern Pacific Ocean

Deep-sea sediments enriched in rare earth elements and yttrium (REY) are attracting scientific and exploration attention as a new REY resource. In the Clarion-Clipperton Fracture Zone (CCFZ) of the northeastern Pacific, sedimentary units with high REY contents (average REY content >700 ppm) occur sporadically at only a few decimeters beneath the sediment surface covered with polymetallic nodules. Here we report the results of bulk chemical and sequential extraction analyses of sediment cores containing these units, collected from the southeastern part of the CCFZ. Correlations between element contents and shale-normalized REY patterns suggest that the REY are associated primarily with calcium phosphates. Sequential extraction to determine the contribution from different sedimentary components confirms that calcium phosphates contain >70% of trivalent REY in REY-enriched units. Despite the study location being favorable for nodule formation and subject to distal hydrothermal plume fallout, only subordinate amounts of REY are hosted by Mn- and Fe-(oxyhydr)oxides. Hydrothermal Fe-(oxyhydr)oxides may have played a more important role in accumulating REY, as evidence suggest that large amounts of Fe-(oxyhydr)oxides have been transformed into Fe-rich smectite during early diagenesis of REY-enriched units.