2012 年 62 巻 3 号 p. 197-209
We report bulk-sediment compositions of rare-earth elements and yttrium (REY), major and trace elements for “REY-rich mud” from 6 drill cores obtained by Ocean Drilling Program (Site 1215A, 1216A, 1217A, 1218A, 1220A and 1222A) off the southeastern coast of Hawaiian Islands. The REY-rich mud in study area has high REY contents, 400-1,178 ppm total REY (ΣREY), which are comparable to those of ion-absorption-type REY deposits in southern China. The core profiles indicate that the REY-rich mud has accumulated to thicknesses of 25-70 m. Some portions of REY-rich mud are typically enriched in Fe2O3, and their post-Archean average Australian shale (PAAS)-normalized REE patterns show strikingly negative Ce anomalies with the same magnitude of seawater. These geochemical characteristics are similar to those of modern hydrothermal Fe-rich sediments near the East Pacific Rise. This indicates that one of the main hosts for REY in the mud is a Fe-oxyhydroxide precipitate from hydrothermal plumes that has taken up REY from ambient seawater. On the other hand, the correlation diagram between ΣREY and Fe2O3 shows another ΣREY enrichment trend characterized by relatively low Fe contents, which infers an existence of the second REY-rich component. As the REY-rich samples on this trend contain significant amounts of phillipsite, the second REY-rich component is considered to be phillipsite. Here we estimate that a volume of 1 km2 × Maximum Cumulated Concentration Depth (MCCD; in metric unit), which represents an appropriate mining depth, at Site 1222A holds nearly 12,200 t (14,000 tREO), which could provide one-tenth of the current annual world consumption of REY. We conclude that deep-sea REY-rich mud off the southeastern coast of Hawaiian Islands has a potential as a new mineral resource for REY.