Element migration of ferromanganese crust on the Western Pacific MC Seamount: insights from LA-ICP-MS mapping analysis

The ferromanganese crust is a widely distributed seafloor deposit that plays a crucial role in the geochemical cycle of critical metals such as Co and Ni. Despite numerous studies on the geochemical properties of ferromanganese crusts, the element migration of Fe-Mn crust under the influence of hydrogenic, hydrothermal, diagenetic, and phosphatized processes remain unclear. This study investigates the mineralogical and geochemical compositions of a seafloor ferromanganese sample and its carbonate substrate rocks using XRD, SEM, EPMA, and LA-ICP-MS spot and mapping analysis. The LA-ICP-MS mapping reveals that the hydrothermal ferromanganese sample has Mn-rich nuclei surrounded by a Fe-rich reaction interface. The Mn-rich nuclei consist of hollandite, todorokite, and rhodochrosite, with a high Mn/Fe ratio and low total REY content. The hydrogenic reaction interface between the Mn-rich nuclei and substrate rocks is mainly composed of CFA, buserite, and vernadite, with relatively low Mn/Fe and high ΣREY content. The non-phosphatized bedrock is primarily composed of calcite with low trace metal content. In contrast, the calcite in the phosphatized bedrock is partially replaced by apatite, resulting in high metal content such as Fe, Mn, and REY. Elemental maps show that the Mn-rich nuclei are enriched in Cu, Ba, and Sr, while the reaction interface is enriched in Co, Ni, Mo, Ti, Fe, Zr, Nb, Pb, and Bi. The geochemical and mineralogy characteristics of the reaction interface indicate that the hydrothermal ferromanganese sample undergoes hydrogen growth and phosphorylation after formation, and a variety of critical metals are transferred from seawater to the surface of the hydrothermal ferromanganese sample. In conclusion, this study provides insights into the element migration of Fe-Mn oxide under the influence of various processes and highlights the importance of understanding the geochemical properties of ferromanganese crusts in the context of critical metal cycles.