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 carries of the trapped noble gas component, including matrix minerals, magnetite and also sulfides, olivine and mesostasis of chondrules did not yield positive results.
ASJC Scopus subject areas
- Geochemistry and Petrology