Meteorites with short cosmic-ray exposure ages, as determined from their Al26 content

摘要: The Al26 content was determined by γ-γ coincidence spectrometry in thirty-one stony meteorites: ten carbonaceous, four enstatite, three bronzite, five hypersthene and amphoteric chondrites; one angrite, eucrite, ureilite diogenite. In the following eight meteorites, 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 Bath have gas contents suggesting ages <1 106 yr, but their correspond ≥1.5 ≥ 1.9 yr. These two discordant cases appear be due diffusion losses sample mixup, respectively. Goalpara has only about 60 % its expected Al26content. If this result is attributed shielding, must had preatmospheric mass ≥700 kg. The mean Al26content found for sixteen samples ordinary enstatite chondrites 63 dpm/kg, appreciably higher than value 47 dpm/kg means γ-ray (Rowe et al., 1963). discrepancy may calibration error latter method, sampling statistics. present data scatter little from mean, thus indicating constant production rate most chondrites. Production rates meteorites been estimated 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.