ABSTRACT

We analyzed 1338 deep-sea sediment samples from 19 Deep Sea Drilling Project/Ocean Drilling Program sites covering a large portion of the Indian Ocean, and constructed a new and comprehensive data set of their bulk chemical compositions, including rare-earth, major, and trace elements. The bulk-sediment rare-earth and yttrium (REY) composition of the REY-enriched samples, characterized by relatively small negative Ce anomalies, almost no Y anomalies, and enrichment in heavy rare-earth elements, can be interpreted as the superposition of the REY compositions of apatite and hydrogenous Fe-Mn oxides. Although the hydrothermal component is a key factor in the formation of REY-rich mud in the Pacific Ocean, it is less important in the Indian Ocean, probably because there is less hydrothermal input of Fe-oxyhydroxides from seafloor hydrothermal vents there. The relationships among Fe₂O₃, MnO, P₂O₅, Co, and total REY contents suggest that a primary factor controlling REY enrichment in deep-sea sediments is the sedimentation rate. A low sedimentation rate allows both fish debris apatite and hydrogenous Fe-Mn (oxyhydr)oxides to accumulate in the surface sediments. On the basis of these results, we identified two potential areas in the Indian Ocean where REY-rich mud may be present in surface sediments: the south-southeastern Wharton Basin and the southern Central Indian Ocean Basin. The resource potential of the latter area might be particularly high if the distributions of REY-rich mud and Fe-Mn nodule fields broadly overlap.