Lanthanide tetrad effect observed in the Oklo and ordinary uraninites and its implication for their forming processes

Abstract

The abundances of fourteen rare earth elements (REE) were precisely determined for hydrothermal and sedimentary uraninite (UO₂) ore samples, including those from the Oklo natural nuclear reactor. A lanthanide tetrad effect is recognized from the REE patterns of all uraninite samples, irrespective of their origin of occurrence and uranium contents. The REE patterns of Oklo uraninites are characterized by the shape of W-type tetrad effect, distinct from the patterns of hydrothermal uraninites with M-type effects. One uraninite ore in sandstone from the Orphan deposit, Arizona, U.S.A., shows M-type tetrad effect, indicating that the uraninite may be detrital, and not precipitated during or shortly after the deposition of the enclosing sediment. These results suggest that lanthanide tetrad effect in uraninites closely relates to conditions and processes of their formation.