Reexamination of anomalous I-Xe ages: Orgueil and Murchison magnetites andAllegan feldspar

The extremely old I-Xe ages of Orgueil and Murchison magnetites, difficult to integrate into conventional meteorite evolutionary models, are not confi rmed. New measurements indicate that Orgueil magnetite is 3 Ma younger than the Shallowater reference sample rather than 7 Ma older as previously repo rted by Lewis and Anders (1975). If these differences are due to difficulti es with the irradiation monitors in the earlier studies, there are wider im plications because the same KI monitor was used to establish the I-Xe age o f Murchison magnetite, which is used, in turn, as a reference sample in man y subsequent I-Xe studies. Confirmation of these new results and a careful review of existing I-Xe age data are therefore in order. The old I-Xe age o f Allegan feldspar, reported by Brazzle et al. (1999) may also be an artifa ct of shock disturbance. Copyright (C) 2000 Elsevier Science Ltd.