High-Resolution Analysis of Fe-Mn Oxyhydroxide in Ferromanganese Nodules from the Northwestern Pacific Ocean and Insights on Element Mobility

Due to the multiple and poorly-crystallized phases in ferromanganese (Fe-Mn) nodules, research on the variability of micro-layers in nodules is limited and the accumulation of various elements with the growth of micro-layers have not been well documented. To address this gap, we examined the spatial distributions of elements in cross-sections of nodules from the Northwestern Pacific Ocean using in-situ electron microprobe analyses coupled with backscattered electron imaging and high-resolution X-ray elemental intensity mapping. Results suggest their significant selective enrichment of metal elements is similar to that of typical hydrogenetic nodules and diagenetic nodules. Moreover, WMn+Fe of Fe-Mn oxyhydroxide is equal to 50% after normalization and Co and Ni show positive relationships with Mn/Fe ratio, suggesting Fe oxyhydroxide would serve as a diluter at exposed surface during nodule growth. In suboxic conditions, Mn, Ni, and Co start to release, and the remaining Fe oxyhydroxide may react with the surrounding sediment to form Si-rich layers. Our studies reveal the diverse growth processes and show a much larger chemical heterogeneity of individual layers, which extend the models about the mechanisms of chemical precipitation and environmental conditions that existed during nodule growth.