Variabilities and enrichment mechanisms of the dispersed elements in marine Fe–Mn deposits from the Pacific Ocean

Abstract

The eight elements Ga, Ge, Se, Cd, In, Te, Re and Tl are usually referred to as “dispersed elements”, and they have a lower average abundance in the Earth’s crust. Marine Fe–Mn crusts and nodules are potential resources for many of the metals used in emerging high- and green-technology applications. This study examines the concentrations of the dispersed elements in marine Fe–Mn deposits at different locations from the Pacific Ocean, ascertains the genesis and dispersed elements enrichment characteristics of marine Fe-Mn deposits, discusses the variations of the dispersed elements in marine Fe–Mn deposits profiles and variations with the dependency of genesis and water depth, and elucidates their incorporation mechanisms in marine Fe–Mn deposits. Nodules from the Mid-Pacific Mountains (CAD19 and CXD05) and Magellan Seamounts (MID18) formed under hydrogenetic conditions, nodules from the CCZ (WD1101, WT0301 and ET0301) formed under a mixture of hydrogenetic and diagenetic conditions, and the Fe–Mn crust MKD12 from the Magellan Seamounts formed under hydrogenetic condition throughout its growth history. Compared with the Earth’s crust and the pelagic clay, the marine Fe–Mn deposits are highly enriched in Te, Tl, Se and Cd, not enriched in Re, Ga, Ge, In. Elemental relationships show that there are distinct differences between the enrichment mechanisms of the dispersed elements in different types of Fe–Mn deposits. Cd, Se, Te and Tl in Fe-Mn nodules and crust profiles show certain regular variations, which mainly related to the changes of the relative contents of Mn minerals and Fe minerals, for the phosphatized crusts, the concentrations of dispersed elements would be influenced by phosphatization. There are obvious differences of Cd, Se, Te and Tl concentrations in different genetic types of Fe–Mn deposits, which probably attributed to the differences of sources, deposition rates and mineral compositions. The concentrations of Se, Cd, Te and Tl in Fe-Mn deposits from the Pacific Ocean mainly distribute in the ranges of 1000 ~ 2500 m and 4000 ~ 6000 m water depth. The water depth dependence of Se, Cd, Te and Tl concentrations in hydrogenetic Fe-Mn nodules and crusts display different variation patterns, which might be controlled by several factors such as the Mn-phase and Fe-phase system, sedimentation rates, biological productivity, diagenesis, as well as the geochemical behaviors of the elements themselves. Under the conditions of hydrogenetic and hydrogenetic-diagenetic, Cd, Se, Te and Tl are incorporated in marine Fe-Mn deposits mainly by adsorption onto the Mn-oxide or/and Fe oxyhydroxide particles. The processes of coupled redox and coprecipitation on Fe-Mn(oxyhydr)oxides could generate Te enrichment.