Rare-Earth Elements in Deep-Sea Sediments in the South Pacific Gyre: Source Materials and Resource Potentials

Abstract

Deep-sea sediments enriched in rare-earth elements and yttrium (REY) plus scandium (Sc), termed “REY-rich mud,” have attracted attention as a possible resource for these critical industrial elements. Examples have been reported from the western North Pacific, central Pacific, low-latitude South Pacific, and eastern South Pacific. Although previous studies of pelagic clay have reported the existence of highly REY-rich mud in the ultraoligotrophic South Pacific Gyre, neither the source materials nor the resource potentials for REY and Sc of the sediment have been quantified. We analyzed the major- and trace-element contents of bulk sediments in Integrated Ocean Drilling Program Holes U1365A to U1370D, drilled in the South Pacific Gyre. The elemental relationships suggest that the enrichment in REY and Sc reflects the accumulation of biogenic Ca phosphate in an environment with low sedimentation rates as well as the tectonic transition of depositional environments from hydrothermally influenced sites near the mid-ocean ridge to distal basins far from hydrothermal vents. The maximum total REY content of 4,662 ppm at Hole U1366C is the highest value yet reported from the South Pacific Ocean. Although the REY- and Sc-enriched sediment layers of most cores from the South Pacific Gyre are located deeper beneath the seafloor than those elsewhere in the Pacific Ocean, Hole U1367B demonstrates large resource potentials (1.21 × 104 t/km2 of REY oxides and 102 t/km2 of Sc) in the uppermost 6.5-m interval, making it the most promising site for REY and Sc yet found in the South Pacific Gyre.