Ar-39 recoil losses and presolar ages in Allende inclusions

Argon analyses were performed for five coarse-grained, Ca-Al-rich inclusions from the Allende meteorite. The samples were neutron-irradiated in evacuated ampuoles, and the Ar in the ampuoles and in the samples was analyzed. Up to 60 percent of Ar-39 was lost from the samples into the ampuoles due to recoil during neutron-irradiation; this loss resulted in substantial increases in the apparent Ar-40-Ar-39 ages of the samples. Substantial amounts of trapped Ar-36 of unknown origin were found in the inclusions, and the presence of trapped Ar-40 could not be ruled out. It is inferred that the interpretation of high Ar-40/K-40 ratios as very high (presolar) ages may be subject to question. It is noted that the degrees of Ar-39 recoil loss reported is comparable to recoil loss in terrestrial rocks and cannot be attributed to KCl contamination of the samples.