Isotopic evidence for the retention of ⁹⁰Sr inferred from excess ⁹⁰Zr in the Oklo natural fission reactors: Implication for geochemical behaviour of fissiogenic Rb, Sr, Cs and Ba

In order to investigate the mobility of fissiogenic ⁹⁰Sr in the geological environment, the Zr isotopic compositions of seven samples from one of the newly formed Oklo natural reactor zones (i.e., reactor core and adjacent rocks (10, SF84)) in the Republic of Gabon were determined with an inductively coupled plasma mass spectrometer (ICP-MS). Zr isotopes in uraninite grains from different reactor zones were also measured by secondary ion mass spectrometry (SIMS). Fissiogenic Zr isotopic abundances of three samples from the reactor core have excess ⁹⁰Zr, which has never before been formed in previous Oklo samples. In this paper, the geochemical behaviour of ⁹⁰Zr is discussed by making use of the relative retentivity inferred from the isotopic abundance of Sr. The excess in ⁹⁰Zr suggests dependence on the degree of retention/migration of ⁹⁰Sr, the precursor of ⁹⁰Zr in the fission chain. In the aqueous phase, chemical fractionation between Sr and Zr could have occurred before radioactive ⁹⁰Sr decayed. Considering the halflife of ⁹⁰Sr (t _{1 2} = 29.1 y), considerable amounts of the latter have been produced during criticality. Sr and Zr (including ⁹⁰Zr) could have been redistributed between the reactor core and its vicinity. The retentivity of fissiogenic ⁹⁰Zr in reactor core 10 is not homogeneous. In addition, the distributions of Rb, Cs and Ba is also heterogeneous.