Geochemical fixation of rare earth elements into secondary minerals in sandstones beneath a natural fission reactor at Bangombe, Gabon

摘要

To study geochemical processes for migration and fixation of fissiogenic rare earth elements (REE) in association with uranium dissolution. in situ isotopic analyses using an ion microprobe were performed on U- and REE-bearing secondary minerals, such as coffinite, francoisite, uraniferous goethite, and uraninite found in a sandstone layer 30 to 110 cm beneath a natural fission reactor at Bangombe, Gabon. Phosphate minerals such as phosphatian coffinite and francoisite with depleted U-235(U-235/U-238 = 0.00609 to 0.00638) contained large amount of fissiogenic light R E. while micro-sized uraninite grains in a solid bitumen aggregate have normal U isotopic values (U-235/U-238 = 0.00725) and small amount of fissiogenic REE components. The proportions of fissiogenic and non-fissiogenic REE components in four samples from the core of BAX03 vary in depth ranging from 30 cm to 130 cm beneath the reactor. which suggests mixing between fissiogenic isotopes from the reactor and non-fissiogenic isotopes from original minerals in the sandstone. Significant chemical fractionation was observed between Cc and the other REE in the secondary minerals, which shows evidence of air oxidizing atmosphere during their formation. Pb-isotopic analyses of individual minerals do not directly provide chronological information because of the disturbance of U-Pb decay system due to recent geologic alteration. However, systematic Pb-isotopic results from all of the minerals reveal the mobilization of fissiogenic isotopes, Ph and U from the reactor in association with dolerite dyke intrusion similar to0.798 Ga ago and the formation of the secondary minerals by mixing event between 2.05 Ga-old original minerals and reactor materials due to recent alteration. Copyright (C) 2005 Elsevier Ltd.