Oxygen Isotopes and U, Pb, REES Data on Clays and Uraninites of the Oklo Natural Reactors: Temperatures, Water Radiolysis and Weathering Effects

L. Pourcelot Centre de Géochimie de la Surface-CNRS 1, rue Blessig 67084 Strasbourg, France


F. Gauthier-lafaye Centre de Géochimie de la Surface-CNRS 1, rue Blessig 67084 Strasbourg, France

Two billion years ago the Uranium ore deposit of Oklo underwent extensive hydrothermal alteration due to the important heat when natural fission reactions of 235U occurred. The cores of the reactors (up to sixteen) are caracterized by a strong uraninite ore (60% U) and abnormal U and REEs isotopic compositions. Hydrothermal fluids due to fission reactions altered the hosted sandstones and allowed the precipitation of a pure clay matrix (mostly chlorites) embedding the core. One billion years after fission reactions stopped doleritic dikes intruded the basin deposits, but the reactors remained preserved till today. The aim of this study is to determine the temperatures of the reactors during criticality by using oxygen isotopic composition of clay fractions and uraninites. Chlorites were analyzed by using ICP and electron microprobe analysis in order to estimate oxygen isotope fractionations using theoretical bond-type equations. Detections of abnormal isotopic compositions of Sm and Nd (fission products) in clay fractions were performed by ICP-MS. U, Pb and Th contents of uraninites were measured by using electron microprobe.

Reactor 10 is considered to be a typical well preserved reactor. Clay minerals are mainly sudoites at the edge and Mg-chlorites in the core. d18O of sudoites and Mg-chlorites range from 0 to 2 and from 2 to 11,7” SMOW respectivelly. This yields sudoite precipitation temperatures of 400°C for sudoites and of 200 to 350°C for Mg-chlorites. Ferric/ferrous iron ratios of chlorites increase from the edge to the core of the reactor. This oxydation can be related to the oxydo-reduction reaction occuring during UO2 precipitation. Samples located close to the reactorr were affected by radiolysis of water. This produce oxidizing agents and consequently increased ferric/ferrous iron ratios. Anomalous Sm and Nd isotopic compositions are restricted to the core of the reactor and its direct vicinity. This suggests little migration of the fission products after fission reactions. Uraninite of the core have high Pb/U ratio (Pb/U=0,18) indicating little loss of lead. d18O of uraninites (-4” SMOW) equilibrates with interstitial waters (5” SMOW).

Reactor 9 is considered to be a typical weathered reactor. Clays around the core are intergrade Al-chlorites. d18O of those chlorites are homogeneous and surprisingly high (17-18” SMOW) whereas d18O recorded by uraninites of the core are low (-13” SMOW). This corresponds to low temperature late meteoric fluid influence which caused
chlorite weathering and O-isotope exchange. Higher ferric/ferrous iron ratios of clays, in this reactor, is possibly caused by oxydant products made during radiolysis of water. This process induces weakness of clays making easier their alteration and oxygen isotopic exchange with meteoric water. We also attribute to this event the migration of Sm and Nd in the clay matrix.

Values of d18O (-10” SMOW) and Pb/U (0,09) of uraninites of reactors 2, 13, 16 and Bangombé are intermediate between those of reactors 9 (-13” SMOW, Pb/U=0,08) and 10 (-4” SMOW, Pb/U=0,18). We assume that Pb/U reset is associated with the thermal event of dolerite intrusion and that d18O shift of uraninites is caused by meteoric fluid exchange. A plot of Pb/U vs d18O allows to precise the respective effects of these events on the behavior of fission products and actinides.