Xenon in Archean barite: Weak decay of 130Ba, mass-dependent isotopic fractionation and implication for barite formation

In order to investigate radioactive decay of 130Ba and 132Ba which have half-lives on the order 1020–1021 a, the isotopic composition of xenon has been measured in 3.5 Ga barite of the Dresser Formation, Pilbara, Western Australia. The analyzed samples were collected at about 86 m depth from a diamond drill core (Pilbara Drilling Project). The fact that the sample has been shielded from modern cosmic ray exposure reduces the number of potentially interfering production pathways, simplifying interpretation of the Xe isotope spectrum. This spectrum is clearly distinct from that of either modern or ancient atmospheric Xe. A strong excess of 130Xe is identified, as well as other isotopic excursions which are attributed to mass-dependent isotopic fractionation and contributions from products of uranium fission. The mass-dependent fractionation, estimated at 2.1 ± 0.3% amu−1, can be accounted for by mutual diffusion and Rayleigh distillation during barite formation that is consistent with geological constraints. After correction for mass-dependent fractionation, the concentrations of fissiogenic Xe isotopes demonstrate that the U–Xe isotope system has remained closed over 3.5 Ga. From the excess of 130Xe, the two successive electron capture half life of this isotope is estimated at 6.0 ± 1.1 × 1020 a, which is 3.4 times faster than previously estimated (Meshik et al., 2001). We could not find evidence of 132Ba decay within our Xe isotope spectra.