^(39)Ar recoil losses and presolar ages in Allende inclusions

Abstract

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 ^(39)Ar from ^(39)K (n, p) reactions was lost out of the inclusions into the ampoules. The ^(39)Ar losses resulted in substantial increases in the apparent ^(40)Ar-^(39)Ar ages of the samples. ^(39)Ar recoil loss during neutron-irradiation is a major effect and must be accounted for in ^(40)Ar-^(39)Ar dating. All of the Allende inclusions studied contained substantial trapped ^(36)Ar. The origin of the trapped ^(36)Ar is unknown, and the possible presence of trapped ^(40)Ar cannot be excluded. Ar measurements on Allende inclusions which have yielded anomalously old ages must be re-examined in the context of ^(39)Ar recoil loss and possible contributions of trapped ^(40)Ar. Allende inclusions appear on both accounts to be poor candidates to search for relicts of presolar materials with well-defined K/Ar ages.

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