Oxygen isotopic composition of nano-scale uraninite at the Oklo-Okelobondo natural fission reactors, Gabon

摘要

High spatial resolution (10–30 µm), in situ oxygen isotopic analyses by secondary ion mass spectrometry (SIMS), coupled with high-resolution transmission electron microscopy (HRTEM), were used to show that uraninite from the Oklo-Okélobondo natural fission reactors that occur in near surface environments, have low 18O values and nanotextures that are consistent with interaction with ground water. These low 18O values (–14.4 to –8.5) suggest that the minerals exchanged with meteoric groundwater. In contrast, reactor zones that occur at depth have largely retained their original O isotopic composition (–10.2 to –5.6) and uraninites are well-crystallized and essentially defect-free. These observations clearly demonstrate that by combining both HRTEM and in situ O isotopic analyses by SIMS, it is possible to characterize the nano-scale porosity and post-depositional alteration of U-bearing phases.