Controlling Factors on REY Enrichments in Basins From the Pacific Ocean: Early Diagenesis and Local Constraints

Abstract

AbstractIncreasing interest in high‐tech metals (e.g., rare earth elements and yttrium, REY) has triggered extensive research on the enrichment of these metals in pelagic sediments. In the sediments collected worldwide, Ca‐phosphate and Fe‐Mn (oxyhydr)oxides are targeted as the two most important REY carriers based on multiple geochemical and mineralogical analyses. However, the formation of the REY‐rich stratum remains enigmatic as the influences of diagenesis and the local restrictions of different oceanic basins are complex, which require further investigation. In our study, we analyzed four sediment cores from the Western, Central North, and Eastern South Pacific (WP, CNP, and ESP), which are three of the most promising REY‐rich regions in global oceans. Sequential leaching quantifies the dominance of Ca‐phosphate by controlling ∼73.9%–93.3% of ΣREY and the secondary role of Fe‐Mn (oxyhydr)oxides by holding up to ∼97.8% of Ce and ∼4.82%–13.9% of ΣREY. The shallowly and deeply buried sediments show similar features regarding the REY proportions hosted by different phases, which indicates that the REY accumulation by Ca‐phosphate might majorly occur on the seawater‐sediment interface. The geochemistry of the studied sediment cores reveals that (a) REY enrichment in ESP is affected by hydrothermal Fe‐Mn (oxyhydr)oxides which can promote adsorptions of P and REY. (b) The Fe and Mn contents in WP sediments show an inverse relationship against REY enrichment which might be explained by the dust input from Central Asia. (c) The slow sedimentation rate of CNP enables the extraordinary REY‐uptake by Ca‐phosphate, but the low flux of phosphorus constrains the formation of high‐grade REY deposits.