Pu–Xe, U–Xe, U–Pb chronology and isotope systematics of ancient zircons from Western Australia

The presence of xenon isotopes from in-situ spontaneous fission of short-lived 244Pu has been confirmed in a suite of 16 Hadean detrital zircons from Western Australia. In order to investigate the effects of xenon loss on estimates of the inferred Pu/U ratio we have irradiated the zircons with thermal neutrons to generate Xe from 235U neutron fission. 131Xe/134Xe and 132Xe/134Xe ratios have been used to calculate the relative contributions from spontaneous fission of 244Pu and 238U and neutron fission of 235U and hence compare nominal Pu/U ratios and xenon retention ages. U–Xe ages are typically lower than the Pb–Pb ages, indicating that xenon loss is common. We show how the ternary mixing diagram can be used to place constraints on the timing and extent of this loss and to generate a corresponding Pu–U–Xe isochron. Although the zircons investigated in this study were extracted from the same metasedimentary unit, the timing of xenon loss is variable. This suggests that the loss may be the result of variable degrees of metamictization from grain to grain. Inferred (Pu/U)o ratios show a general decrease with the discordancy between Pb–Pb and U–Xe ages. For the least discordant samples we infer (Pu/U)o ∼ 0.008 which is close to the widely adopted chondritic value. While we cannot completely exclude the effects of Pu/U fractionation in magmatic and other processes between formation of the Earth and crystallisation of the zircons we conclude that they have been relatively small (< factor 2?) and reflect the compatible behaviour of Pu and U in the 4+ valence state.