ALUMINUM-26 IN METEORITES .6. ACHONDRITES

Cosmogonic Al26 was measured by non-destructive γ-γ coincidence counting in 21 achondrites and 6 chondrites. A weighted regression analysis of these and earlier data gave the following mean production rates of Al26, in dpm per kg of target element: Al 476 ± 54, Si 310 ± 10, S 38 ± 44. These figures are based on the following assumed or calculated production rates: Mg 0, Ca 7, (Fe + Ni) 2·2 dpm/kg. Five additional sets of production rates were calculated on the basis of various alternative assumptions. Fifteen of the meteorites had Al26 contents more than two standard deviations below the mean saturation value; three of these low values are due to short radiation ages, and most of the remaining ones, to shielding. Ureilites and diogenites, in particular, tend to have abnormally low Al26 contents. A review of Al26 data for chondrites shows that those determined by non-destructive γ-γ coincidence counting show little dispersion; only 11 of 71 cases differ by more than ±15% from the calculated saturation value. The mean Al26 content is 0.94 of saturation value, and shows virtually no dependence on recovered mass in the range 1-700 kg. Measurements by other methods often are substantially lower, apparently due to systematic errors. The eucrite Serra de Magé has an Al26 age of 0.065 ± 0.018 m.y., very much shorter than its He3, Ne21, Ar38 age of 1·9 m.y. Apparently it has suffered a two-stage irradiation; first for ≥32 m.y. shielded by 1·5-5 m of overlying material, and then for {precedes above single-line equals sign}0·10 m.y. as an unshielded, decimeter-sized fragment. A similar history is indicated for the Cl chondrite Ivuna. © 1969.