Meteorites with short cosmic-ray exposure ages, as determined from their Al 26 content

Abstract

The Al 26 content was determined by γ-γ coincidence spectrometry in thirty-one stony meteorites: ten carbonaceous, four enstatite, three bronzite, five hypersthene and five amphoteric chondrites; one angrite, one eucrite, one ureilite and one diogenite. In the following eight meteorites, the Al 26 content was low enough to permit determination of a cosmic-ray exposure age (million years): Ivuna 0.18±0.05, Cold Bokkeveld 0.32± 0.04, Nogoya 0.14±0.08, Pollen 1.0±0.5, Grosnaja 1.5± 0.4, St. Marks 0.9 ± 0.2, Shaw 0.40 ± 0.07, Appley Bridge 1.5±0.5. Seres and Bath have gas contents suggesting ages <1 10 6 yr, but their Al 26 contents correspond to ages ≥1.5 and ≥ 1.9 10 6 yr. These two discordant cases appear to be due to diffusion losses and a sample mixup, respectively. The Goalpara ureilite has only about 60 % its expected Al 26content. If this result is attributed to shielding, Goalpara must have had a preatmospheric mass of ≥700 kg. The mean Al 26content found for sixteen samples of ordinary and enstatite chondrites is 63 dpm/kg, appreciably higher than the value of 47 dpm/kg determined by means of γ-ray spectrometry (Rowe et al., 1963). The discrepancy may be due to a calibration error in the latter method, and to sampling statistics. The present data scatter little from the mean, thus indicating a constant Al 26 production rate for most chondrites. Production rates of Al 26 in stony meteorites have been estimated for the following target elements: Mg 85±110, Al 410±410, Si 210±110, S 100±90, Ca 280±130, Fe + Ni 1.8 ±0.6 dpm/kg.