Rare earth elements and Yttrium (REY) abundances and distribution characteristics depending on lithofacies in the South Pacific sediment

Since rare earth elements and Yttrium (REY) were considered critical resources in modern technological and economic industries, deep-sea sediments in the world oceans have started to gain attention as an essential source of REY. In particular, recent studies have discovered that deep-sea sediments in the western North Pacific Ocean near Minamitorishima island have the highest REY concentrations (over 5,000 ppm of REY). In this study, our goals are to identify the existence potential of REY-rich sediment in the South Pacific Ocean and investigate REY abundance and distribution characteristics depending on lithofacies. We acquired the sediment samples from seven sites (U1365, U1366, U1367, U1368, U1369, U1370, U1371) recovered from Integrated Ocean Drilling Project (IODP) Expedition 329. The sediment samples were analyzed for bulk chemical composition, mineral composition, and sedimentary facies. The results indicate that REY concentrations ranged from 53 to 4,177 ppm. U1365 and U1366 sediments showed extremely high REY abundances over 2,000 ppm. On the other hand, U1368 and U1371 sediments showed the lowest contents of REY, less than 200 ppm. Based on the geochemical results, the sediments were divided into six lithofacies: Bioapatite-rich clay, Fe or Mn-rich clay, zeolitic clay, pelagic clay, siliceous ooze, and calcareous ooze. Bioapatite-rich clay with high P₂O₅ content contained the highest REY peak layers (1,141 ~ 4,177 ppm). We observed abundant fish teeth debris in the sediments composed of biogenic calcium phosphate. Fe or Mn-rich clay contained an average of 1,073 ppm of REY, indicating the second-highest abundance among the six lithofacies. XRD analysis and wet sieving results suggested that Fe or Mn originated mainly from goethite derived from hydrogenous and hydrothermal origins. Zeolitic clay and pelagic clay contained an average of 729 ppm and 344 ppm of REY, respectively. In addition, siliceous ooze and calcareous ooze showed an average of 152 ppm and 130 ppm of REY, respectively. These results imply that clay deposits are expected to have high REY contents than biogenic ooze. In addition, it implies that the main host phases of REY from deep-sea clay in the South Pacific are bioapatite and Fe or Mn (oxyhydr)oxides.