On the Origin of Xe-Isotope Anomalies: A New Approach Based on the 131Xe/134Xe vs 132Xe/129Xe Diagrams

Abstract

The previous discussion by Shukolyukov et al., (1994) of the origin of anomalous Xe isotope compositions that were observed in natural gases, meteorites, and the Oklo uranium deposit did not include the 12gxe, 13~ and 136Xe excesses. It is a commonly accepted that the excess of 136Xe relative to the atmosphere results from Uranium fission where 136I with a very short half-life (86 sec.) in 136I(~ ,7)136Xe is the only 136Xe precursor. The excesses of 13~ and 128Xe can exclusively be explained by 1313-decay of 13~ and 128Te respectively. However, to account for the excesses-relative to their atmospheric abundance-of the remaining isotopes, 129Xe, 131Xe, 132Xe, and 134Xe, presents a major problem. The possible origins of the 129Xe and 131Xe excesses might be different from that of 132Xe and 134Xe. On one hand, all four isotopes can be produced by the 13-decay of Te, Sb and I precursors after their injection into the crystal lattice after mineral formation. Following Meshik (1988) and, Xe produced by this process is termed Chemical Fractionated Fission Xe (CFF-Xe). On the other hand, however, the excessive abundances of 129Xe and 131Xe could result from nuclear reactions following neutron-capture and/or resonance neutron capture by ~28Te and 13~ respectively (Browne and Berman, 1973): 12STe(n,7)129Xe, 13~ Laboratory of Isotope Geochemistry, Vemadsky Institute of RAS, Moscow, 117975, Russia