Fe–Mn micronodules born in the metalliferous sediments of two spreading centres: the East Pacific Rise and Mid-Atlantic Ridge

A geochemical study of Fe–Mn micronodules associated with the metalliferous sediments at two spreading centres has shown that their composition depends on the site of micronodule formation. Close to the hydrothermal mounds they exhibit significant variation in elemental content related to the type of hydrothermal discharge (low- or high-temperature), the nature of primary hydrothermal matter (plume fall-out, oxidised sulfides), and the extent of diagenesis. In this environment three types of micronodules can be distinguished although not observed as pure end-members: (1) diagenetic micronodules; (2) micronodules formed generally from the plume fall-out of oxyhydroxide matter; and (3) micronodules grown on the oxidised sulfide grains supplied to the sediments by slumping or fall-out of nearby buoyant plume. Away from the active spreading centre, the hydrothermal signatures of primary precipitates are gradually masked and hydrogenous/diagenetic processes lead the micronodule formation. Composition of micronodules becomes less variable. Well-pronounced, deep rift valleys confine the hydrothermal plume, which brings the hydrothermal suspension into contact with restricted volumes of seawater and, consequently, weakens the hydrogenous influence on the primary hydrothermal matter. Shallow rift valleys do not confine hydrothermal plumes, which are scattered over hundreds of kilometres by bottom currents. This brings the hydrothermal suspended matter into contact with large volumes of seawater. Extensive scavenging occurs, which masks the hydrothermal signal away from the spreading axis and enhances the hydrogenous one. Thus, the ridge crest morphology, defined by the spreading rate, is supposed to play a certain role, though indirect, in the chemical composition of the primary precipitates and, consequently, in the composition of the micronodules formed.