ABSTRACT

We have investigated elemental and isotopic compositions of noble gases in the Kobe (CK4) chondrite by a stepwise heating method. The previously observed inconsistency in cosmic-ray exposure ages derived from cosmogenic ²¹Ne and ³⁸Ar was eliminated in this study, and the averaged value of 38.2 Ma was obtained. Based on neutron-produced ⁸⁰Kr, a pre-atmospheric size of Kobe is estimated to be larger than ∼23 cm radius, whereas the results from cosmogenic radionuclides indicate a pre-atmospheric radius of ∼10 cm. These results indicate a two-stage exposure history for the Kobe meteorite. The release pattern of the primordial noble gas from Kobe indicates that Kobe has lost its primordial gas by thermal metamorphism. However, the observed Q-Xe concentration is lower than that expected from the HL-Xe as a carbonaceous chondrite, indicating preferential loss of Q-Xe. This cannot be explained solely by thermal metamorphism, suggesting that shock events at high temperature have caused the preferential loss of Q-Xe over HL-Xe in the Kobe meteorite.