Deep-sea rare earth element-rich sediments: A review of distribution, carriers and petrogenesis

Rare earth elements and yttrium (REY) enriched deep-sea sediments are promising mineral resources that are associated with apatite and Fe-Mn (hydro)oxides. With terrestrial REY resources dwindling, attention has shifted towards these essential elements within the deep-sea sediment. Here, we review existing literatures on the REY data from deep-sea sediments of all oceans. Based on the average REY content exceeding 700 μg/g, there are five major REY enrichment belts located in the Western, Central, Northeast, Southeast Pacific and the Central Indian Ocean, respectively. The water depth of five belts range from 3380 m to 5792 m, with the vast majority of depths exceeding 4000 m. The Western Pacific REY-rich layers mainly occurred at ∼5–6 m below seafloor (mbsf), with one to three REY-rich layers occurring in the topmost 10 mbsf. The deepest layer recognized in the Western Pacific occurred at around 171–172 mbsf. As such, this belt is probably the best-endowed REY-rich sedimentary region in the oceans. The REY contents of deep-sea sediment in Central Pacific are lower than that in Western Pacific, and most REY-rich layers extend to depths greater than 30 m. The Northeast Pacific REY-rich sediment lies within the Clarion-Clipperton Fault Zone (CCFZ), which is featured by Fe-Mn nodules in surface sediment and REY-rich sediment at greater depths. The REY-rich sediment in Southeast Pacific shows evidence of submarine hydrothermal origin with the depth of the layers varying significantly from 0.26 to 69.5 mbsf. The Central Indian Ocean exhibits similarities in REY content and thickness to the Pacific. The REY-rich sediment layers are shallow (mostly 0–5 mbsf) and occur near the surface of Central Indian Ocean Basin. Whereas, in the Wharton Basin, the layers tend to be deeper and are frequently found below 100 mbsf. Although the bioapatite, Fe-Mn (hydr)oxides, zeolites and clay minerals are the primary mineral components of REY-rich sediment, REY might be mainly hosted by apatite. Fe-Mn (hydro)oxide is a temporary carrier for REY that scavenges REY from seawater, and then transports REY to bioapatite during the early diagenesis. The genesis of these REY-rich deep-sea sediments can be traced to hydrogenetic, diagenetic and hydrothermal processes. Effective utilization of REY-rich deep-sea sediments requires systematic characterization of their spatial distribution, carrier phases, formation chronology and petrogenesis, as well as reconciling economic viability with ecological preservation.