39Ar recoil losses and presolar ages in Allende inclusions

Ar analyses are reported for five coarse-grained, Ca-Al-rich inclusions from the Allende meteorite. The samples were neutron-irradiated in individual evacuated ampoules, and the Ar gas in the ampoules as well as the samples was analyzed. A large fraction (up to 60%) of the 39Ar from 39K (n, p) reactions was lost out of the inclusions into the ampoules. The 39Ar lossses resulted in substantial increases in the apparent 40Ar-39Ar ages of the samples. 39Ar recoil loss during neutron-irradiation is a major effect and must be accounted for in 40Ar-39Ar dating. All of the Allende inclusions studied contained substantial trapped 36Ar. The origin of the trapped 36Ar is unknown, and the possible presence of trapped 40Ar cannot be excluded. Ar measurements on Allende inclusions which have yielded anomalously old ages must be re-examined in the context of 39Ar recoil loss and possible contributions of trapped 40Ar. Allende inclusions appear on both accounts to be poor candidates to search for relicts of presolar materials with well-defined K/Ar ages. (orig.)