Nanoscale study of Fe-Mn micronodules in deep-sea sediment from the Western Pacific Ocean

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• Nanoscale scanning reveals layered growth from core to edge, clarifying evolution; • Mineral phases and PbI 2 show distinct REE partitioning patterns; • Nanoparticles shape early micronodule formation and elemental enrichment. This study collected early-stage Fe-Mn micronodules from the seafloor of the Western Pacific and analyzed their structural characteristics and elemental distribution using Scanning Electron Microscopy (SEM), Focused Ion Beam (FIB) and Transmission Electron Microscopy-Energy Dispersive X-ray Spectroscopy (TEM-EDS). The results indicate that the micronodule exhibits stratified growth profiles, including laminated structures, lamellar structures and porous loose layers. Fe and Ti are concentrated in the core region of the micronodule, followed by a clay mineral layer enriched in Al, Mg, Si and K, while the outermost layer is dominated by Mn, forming a dense layer enriched with I and Pb at the edge. The distribution of Fe and Mn within the micronodule shows a clear negative correlation. Further analysis revealed that the main mineral phases in the micronodule (ferrihydrite and montmorillonite), as well as the titanium oxides, manganese oxides and lead iodide located at the edge of the micronodule, all exist as nanoparticles. Ferrihydrite is positively correlated with the enrichment of heavy rare earth elements (HREEs) such as Dy, Tb, Er and Tm, while manganese oxides play a significant role in the selective removal or enrichment of specific light rare earth element (LREE) like Eu, as well as certain HREEs such as Er, Tm and Gd. Additionally, the presence of Ti, clay minerals, I and Pb may have a critical influence on the enrichment of Tm, Er, Gd, Yb, Ho, Ce, Pr, La, Nd, Sm, K and Mg in the regional marine environment. These findings enhance our understanding of the mechanisms governing the occurrence and distribution of rare earth elements (REEs) in Fe-Mn micronodules and highlight the pivotal role of nanoparticles in the early-stage growth and elemental enrichment of micronodules, providing new insights into the genesis and evolution of deep-sea Fe-Mn micronodules.