Reexamination of anomalous I-Xe ages: Orgueil and murchison magnetites and allegan feldspar

The extremely old I-Xe ages of Orgueil and Murchison magnetites, difficult to integrate into conventional meteorite evolutionary models, are not confirmed. New measurements indicate that Orgueil magnetite is 3 Ma younger than the Shallowater reference sample rather than 7 Ma older as previously reported by Lewis and Anders (1975). If these differences are due to difficulties with the irradiation monitors in the earlier studies, there are wider implications because the same KI monitor was used to establish the I-Xe age of Murchison magnetite, which is used, in turn, as a reference sample in many 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 of Allegan feldspar, reported by Brazzle et al. (1999) may also be an artifact of shock disturbance.