Insights on the formation of layered ferromanganese precipitates from the southern Mariana Arc, West Pacific, from micron-scale major, minor, and trace element variations

A combined laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and energy dispersive X-ray spectroscopy (EDS) study was used to map 49 elements in four FeMn precipitates produced from three different genetic processes (hydrogenetic, hydrothermal, and mixed-type hydrogenetic-hydrothermal) in samples obtained from the southern Mariana Arc. Results show Mn-oxide minerals are consistently found to be associated with Ba, Mo, Sb, V, Zn, and the rare earth elements and yttrium (REY), excluding Ce, whereas Fe-oxyhydroxide minerals are primarily associated with Ti, Co, Cr, Cu, Nb, and Pb. Element distributions in FeMn precipitates from the southern Mariana Arc differ from deposits formed in other environments; notably, there is a decoupling of As and Sb, and redox-sensitive elements are more variable than non-redox-sensitive elements. Samples previously classified as either hydrogenetic or hydrothermal in origin, based on bulk geochemical data, show characteristics of both at higher resolution. Combining high resolution in-situ data with genetic classification discrimination diagrams reveals more ambiguity in metal and metalloid origin than previously thought. Mariana Arc FeMn precipitates can be separated into two formation sub-types (hydrothermal or mixed hydrothermal/hydrogenetic) based on textural and compositional differences, e.g., differences in Ce/Ce and Y/Ho values and relative variations in (Zr + Y + Ce) versus (Co + Ni) versus (Mn + Fe). Improved classification and knowledge of how fast-growing hydrothermal FeMn precipitates form may help us identify a relatively renewable critical metal resource.