ABSTRACT

The Kobe CK4 carbonaceous chondrite is depleted in spallogenic and radiogenic He as well as trapped noble gases. The loss of spallogenic and radiogenic He is likely due to heating by solar radiation. Concordant cosmic-ray exposure ages are obtained from spallogenic ²¹Ne, ³⁸Ar and ⁸³Kr. The ²¹Ne-basis exposure age is 42.3 Myr, using the production rate for CV chondrites which are compositionally similar to CK chondrites. We have 3.3 Ga for K-Ar age. Kobe contains neutron-induced ⁸⁰Kr, ⁸²Kr and ¹²⁸Xe. The neutron-induced ⁸⁰Kr/⁸²Kr ratio agrees with that for Kr produced by epithermal neutron captures on ⁷⁹Br and ⁸¹Br. From the slowing-down density and the Fermi age of epithermal neutrons, we have the minimum radius (or mass) of 24 cm (or 200 kg with the density of 3.5 g/cm³) for the precursor meteoroid during neutron-capture reactions. Kobe was also analyzed for He, Ne and Ar by laser-microprobe techniques. The preliminary result shows heterogeneous distribution of noble gas components. A barred olivine chondrule is highly enriched in spallogenic gases and radiogenic He and Ar. It is suggested that a potential source for these components in the chondrule is mesostasis, because it is usually enriched in target elements O, Na and Ca for spallogenic gases and parent nuclides K and U for radiogenic gases, though the chondrule has never been analyzed for concentrations of these elements. A search for carriers of the trapped noble gas component, including matrix minerals, magnetite and also sulfides, olivine and mesostasis of chondrules did not yield positive results.