Mechanisms of enrichment of the rarer elements in marine manganese nodules

Abstract

The rare-earth distribution has been determined in five manganese nodules from a range of marine environments. Manganese nodules from Loch Fyne, Scotland, and the Gulf of Aden are characterized by a low absolute abundance of the rare-earth elements and a low Ce/La ratio compared with deep-sea nodules. This difference is interpreted in terms of the diagenetic remobilization of manganese in the high sedimentation regimes leading to the effective fractionation of manganese from the rare-earth elements and the resultant depletion of the rare earths in nodules from these environments. The mechanism of incorporation of the rare-earth elements into manganese nodules is thought to be either the direct adsorption of these elements from seawater or the scavenging of the elements by colloidal iron oxide prior to their incorporation into the authigenic phase of nodules. There appears to be no evidence for the surface transfer of these elements from inorganic detritus into the authigenic phase of nodules as suggested by previous authors. The distribution of U and Th in the nodules is controlled by factors similar to those controlling the rare-earth abundance in nodules. The higher U/Th ratios in nodules from Loch Fyne and the Gulf of Aden is considered to be due to the preferential reduction of UO 2(CO 3) 3 4− to some lower-valency uranium species such as U 3O 8 in nodules from less oxidizing environments.