The record in the meteorites: 6. On the chronology of the early solar system

Abstract

An attempt is made to account for the differences between the isotopic composition of terrestrial and meteoritic xenon reported by Reynolds. The two chief mechanisms proposed (other than the decay of I129 to Xe129) are the production of Xe131–136 by spontaneous fission of extinct nuclides in the earth, and the production of Xe124–128 by nuclear spallation reactions in the early history of the solar nebula. About 9.6 per cent of the Xe136 in the earths's atmosphere appears to have arisen from the spontaneous fission of 76-million-year Pu244, as proposed by Kuroda. The Pu244-Xe136 decay interval of the earth is 290 m.y., and its I129-Xe129 decay interval may be estimated as ≥210 m.y. Thus, the earth appears to be 100–200 m.y. younger than the meteorites. Possible errors in these determinations are discussed. A dating method, similar to the Pb207-Pb206 method and based upon the two decay systems I129-Xe129 and Pu244-Xe136, is proposed and the appropriate equations are given. The initial solar-system ratios of I129/I127 and Pu244/U238, which can be determined by this method, would provide a crucial test of models of nucleosynthesis.