Geochronology of the Oklo and Bangombé fossil natural fission reactors : tracing the effects of geological events

Sammanfattning: Thesis to be publicly defended for the degree of Doctor of Philosophy, in lecture room De Geersalen, Svante Arrhenius väg 8a, Stockholm, Friday February 1, 2002 at 10.00 am. Two billion years ago, natural fission reactions took place in the Oklo and Bangombé uranium ores in Gabon, West Equatorial Africa. Uraninite (UO 2+x) in the reactor zones is studied as a natural analogue to spent nuclear fuel. Since uraninite accumulates radiogenic Pb, U-Pb isotope systematics may be used to establish when uraninite alteration, and consequently migration of elements contained in the uraninite, occurred. The aim of this study is to establish when uraninite alteration and related radiogenic Pb loss occurred in the Oklo and Bangombé natural fission reactors.The isotopic compositions of U, Pb and S in uraninite, galena (PbS) and zircon (ZrSiO 4) were analysed using an ion microprobe. The ion microprobe technique requires the use of standards of the same mineral as the sample in order to determine instrumental fractionation. One part of this study involved describing and evaluating potential uraninite and galena standards for U and Pb isotopic analysis by ion microprobe. The galena standard analyses showed that Pb isotopic fractionation during galena analyses was ca 0.5% per atomic mass unit (a.m.u.) in favour of the lighter isotope. The uraninite standard study showed that U isotopic fractionation was ca 1.4% per a.m.u. in favour of the lighter isotope and that uraninite ion microprobe analysis may be influenced by significant amounts of hydrides. Hydride interferences during Pb isotopic analysis of uraninite were found to be of greater importance than Pb isotopic fractionation.A dolerite dyke swarm in the Francevillian basin, where the Oklo and Bangombé U deposits are located, was previously dated to sometime between ca 1000 and 700 Ma. In this study, zircon U-Pb ion microprobe analyses of one of the dolerite dykes yielded an age of 860±39 Ma (2). This age is in accordance with single-stage Pb-Pb model ages of large, mm-sized galena crystals that formed during a major Pb loss from uraninite in the reactor zones. Smaller galena crystals, ca 10 µm in diameter, are common in reactor zone 16 at Oklo. Variable Pb isotopic compositions of these crystals indicate that they formed sometime after the ca 860 Ma old major Pb loss from uraninite. An age of ca. 500 Ma is suggested for this second event of Pb loss and galena crystallisation. The existence of a second event of radiogenic Pb loss from uraninite is also indicated by the alteration of dolerite dykes that cross-cut the Oklo deposit. One of these altered dykes contains galena with Pb originating from the uranium ore as well as S from the surrounding sediment. Hence, the galena crystals in the dyke are not magmatic but a result of a later element migration. Pb-Pb and U-Pb systematics of uraninite also indicate that recrystallisation of uraninite occurred sometime between 600 to 500 Ma ago at both Oklo and Bangombé. Thus, this study has shown that at least two episodes of Pb loss from uraninite occurred after the formation of fission reactors; one at ca 860 Ma and one 600-500 Ma ago. Either one of these events might be related to regional Pb mobility in the Francevillian basin.