Geochemistry of rare earth elements in micro-and macronodules from the pacific bioproductive zone

Abstract

Concentrations and compositions of rare earth elements (REE) in three micronodule fractions (50–250, 250–500, and >500 µm), coexisting macronodules, and host sediments are examined. The samples were collected from three sites (Guatemala Basin, Peru Basin, and northern equatorial Pacific) located in elevated bioproductivity zones of the surficial water. The influence of micronodule size is dominant for REE compositions and subordinate for REE concentrations. For example, the Ce concentration inversely correlates with the micronodule fraction dimension and drops to the lowest value in macronodules and host sediments. The Ce decrease is generally accompanied by the Mn/Fe increase in micro- and macronodules. Hence, the role of diagenetic source of material directly correlates with the micronodule dimension. The contribution of diagenetic source is maximal for macronodules. The REE signature distinctions of micronodules and macronodules can be attributed to variations of hydrogenic iron oxyhydroxides and diagenetic (hydrothermal) iron hydroxophosphates that are the major REE carriers in ferromanganese ore deposits. The relationship and general trend in the chemistry of coexisting macronodules suggest that they can represent products of the initial stage of nodule formation.