Noble gases in six ordinary chondrites: comparison of exposure ages from noble gases with 26Al ages

Noble gases were redetermined in six ordinary chondrites in order to clarify some discrepancies between cosmic-ray exposure ages from noble gas isotopes and ²⁶Al activity. Generally, samples used for noble-gas analyses were from the same specimen which earlier had been used for ²⁶Al determinations in this laboratory. The high ²¹Ne ages (> 3 × 10⁶ y) for Dimmitt, Menow and Pierceville are consistent with ²⁶A1 activity at saturation level. Variations in both ²¹Ne_c age and ²⁶Al activity, in earlier measurements on Dimmitt, were due to “shielding”; very low noble-gas ages in earlier analyses on Menow and Pierceville were due to mis-labelling of samples. Earlier observation of very low noble-gas ages for Seres, which is inconsistent with its saturation activity of ²⁶Al, is confirmed. Ladder Creek yields consistent ages both from noble gases and ²⁶Al. Malotas contains a substantial excess of ²¹Ne, as revealed by a higher ²¹Ne age, compared to its ²⁶Al age; the excess ²¹Ne is probably due to “pre-irradiation”. Isotopic compositions for trapped Kr and Xe in these chondrites, excepting Seres, are very similar to compositions determined earlier for ordinary chondrites. The Xe in Seres is enriched in light isotopes and is similar to solar Xe.