Abstract
Nowadays, pelagic sediments are globally explored as a potential resource of rare earth elements and yttrium (termed as REY), and have received unprecedented attention. However, the formation mechanism of REY-rich deep-sea mud remains controversial. The Fe-Mn (oxyhydr)oxides are ubiquitous in oxic marine sediments and potentially beneficial for the REY enrichment. The research of them is important for understanding the ore-formation of REY-rich mud, and thus can help to evaluate the role of marine sediments in global REY cycles. In this contribution, using advanced in situ analytical techniques, we provide new results on the nanoscale mineralogy and geochemistry of Fe-Mn (oxyhydr)oxides that are recovered from REY-rich deep-sea sediments in central North Pacific Ocean. In situ X-ray diffraction analyzer and scanning electron microscope reveal that the Fe-Mn minerals (e.g., vernadite, birnessite, and todorokite) occur as overgrowths on the surface of nuclei to form Fe-Mn micronodules. Transmission electron microscope (TEM) imaging reveals that these Fe-Mn minerals are composed of nanocrystals with high intercrystalline porosity, indicating a potential strong adsorption capacity. The TEM observations also imply a mineral phase transformation from birnessite (vernadite) to todorokite during the oxic diagenesis, accompanied by corresponding changes in geochemical compositions. Laser ablation-inductively coupled plasma-mass spectrometry demonstrates that Fe-Mn (oxyhydr)oxides contain high REY concentrations (varying from 439 to 1,654 ppm, with an average of 790 ppm) and can be regarded as an important host of REY. Furthermore, the PAAS-normalized and seawater-normalized REY patterns both infer that the REY contents of Fe-Mn (oxyhydr)oxides are ultimately derived from seawater and characterized by preferential scavenging of Ce. Overall, Fe-Mn (oxyhydr)oxides not only contribute a moderate proportion of REY, but also provide specific information for understanding geochemical processes related to REY enrichment in deep-sea sediments.
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