Evaluating the contribution of hydrothermal fluids and clay minerals to the enrichment of rare earth elements and yttrium (REY) in deep-sea sediments

Abstract

Deep-sea REY-rich sediments have attracted considerable attention in recent studies because of their extremely high REY resource potential. Previous studies highlighted that hydrothermal precipitates (e.g., Fe-Mn (oxyhydr)oxides) have played an important role in REY enrichment in deep-sea sediments by scavenging REY from seawater. However, the exact extent of the contribution of hydrothermal scavenging to the REY accumulation has not yet been evaluated. In addition, REY enrichment generally occurred in the pelagic clays, and whether the clays have played a role in the REY accumulation is poorly constrained. We studied bulk-sediment geochemistry and mineral fractions of sediments from two sites in the Eastern South Pacific Ocean. The results show that both cores contain REY-rich sediments layers possessing average ΣREY contents of 989 ppm and 1563 ppm, respectively. The calculated REY resource amounts in the 1 km × 1 km × optimal recovery depth (m) of the two sites are summed to 4690 t-REO. Carbonate compensation depth is one of the factors controlling the REY contents in the sediments in our study area. The shale-normalized REY patterns and calculated simulation results suggest that direct input of REY from the hydrothermal sources to the sediments was not significant. The evaluation of the contributions of REY scavenged by hydrothermal Fe-Mn (oxyhydr)oxides suggests that for the metalliferous sediments in which Fe-Mn (oxyhydr)oxides are hydrothermal-dominated, the Fe2O3 and ΣREY contents show a pronounced positive correlation for all cores, implying hydrothermal scavenging of REY significantly contributed to the accumulation of REY. However, with the continuous addition of hydrothermal Fe-Mn (oxyhydr)oxides in the pelagic clay sediments, the ΣREY contents in the REY-rich sediments firstly exhibited an increase, and then when the total Fe2O3 components achieved a certain value (∼25 wt% in this study) they started to decline. This finding suggests the appropriate input of hydrothermal Fe-Mn (oxyhydr)oxides into the sediments certainly increases the ΣREY contents through a scavenging mechanism, but that excessive addition of them would, in turn, decrease the ΣREY contents of the sediments. Additionally, the amount of clay minerals exhibited a significant positive correlation with ΣREY contents, therefore, we inferred that clay minerals probably played an important role in REY accumulation in the sediments.