High-resolution LA-ICP-MS mapping of deep-sea polymetallic micronodules and its implications on element mobility

Enrichments of REY (rare earth + yttrium) and other trace metals (Co and Ni) in deep-sea ferromanganese (Fe–Mn) micronodules have received increasing attention in both deep-sea research and mineral exploration. Due to the presence of multiple, easily-crushed and poorly-crystallized phases in micronodules, the genesis of micronodules and their adsorption of various trace elements are poorly understood. To address this gap, we examined the spatial distributions of elements in cross-sections of micronodules from the western tropical North Pacific Ocean using high-resolution (HR) LA-ICP-MS raster mapping coupled with laser Raman and X-ray photoelectron spectroscopy (XPS). The ferromanganese micronodules we studied are dominated by Fe and Mn oxides with minor carbonate minerals, such as siderite, rhodochrosite and calcite. LA-ICP-MS maps show that these micronodules consist of a Mn-rich core and a Fe-rich rim. The Fe-enriched rim is enriched in As and surrounds a Mg, Mn, Cu, Co and Ni concreted core. Laser Raman maps show that the micronodule core contains more birnessite, an important scavenger of trace metals in deep sea sediments, than the rim. The birnessite filled core of these micronodules does not have elevated REY. Indeed, birnessite line channels may feed metal-rich fluid containing REY to adjacent minerals, including well-crystallized bio-apatite and zeolite, as high Ce and Y levels are spatially correlated with these minerals. The observed element profiles and XPS observations showing the coexistence of multiple oxidation states of Mn (+2, +3 and +4), Fe (+2 and +3) and Ce (+3, +4) demonstrate that the Fe–Mn phases of these micronodules are of a diagenetic origin and that they are sites of redox-driven metal enrichment in deep-sea sediment.