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American Mineralogist; October 2003; v. 88; no. 10; p. 1583-1590
© 2003 Mineralogical Society of America
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Oxygen isotopic composition of nano-scale uraninite at the Oklo-Okélobondo natural fission reactors, Gabon

Mostafa Fayek1,3,*, Satoshi Utsunomiya2, Rodney C. Ewing2, Lee R. Riciputi1,3 and Keld A. Jensen2,4

1 Geological Sciences, University of Tennessee, Knoxville, Tennessee 37996, U.S.A.
2 Nuclear Engineering and Radiological Sciences and Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-2104, U.S.A.
3 Chemical Sciences Division, Oak Ridge National Lab, Oak Ridge, Tennessee 37831, U.S.A.
4 National Institute of Occupational Health, Denmark, DK-2100 Copenhagen

Correspondence: * E-mail: mfayek{at}utk.edu

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 {delta}18O values and nanotextures that are consistent with interaction with ground water. These low {delta}18O values (–14.4 to –8.5{per thousand}) 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{per thousand}) 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.






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