Cosmic-ray exposure age and heliocentric distance of the parent body of H chondrites Yamato-75029 and Tsukuba

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Many small pieces of the H chondrites Yamato (Y-) 75029 and Tsukuba were investigated to characterize signatures of light noble gases. These meteorites contain large amounts of solar gases as well as cosmogenic ones. A simple regolith exposure model was developed in order to explain the correlations among solar ^<20>Ne and ^<36>Ar and cosmogenic ^<21>Ne concentrations. Based on the regolith model, the parent body exposure ages, heliocentric distances, and space exposure ages of the two meteorites were calculated. The parent body exposure ages were more than 5.5Ma and 11.8Ma for Y-75029 and Tsukuba, respectively. The heliocentric distances were 2.2±^<0.3>_<0.6> AU and 4.2±^<0.4>_<1.2> AU for Y-75029 and Tsukuba, respectively. The space exposure age of Y-75029 was 5.2-5.8Ma, whereas that of Tsukuba was 8.1±0.6Ma. It has been suggested that the parent bodies of H chondrites are S-type asteroids with orbits that range from 2 to 3.5AU in the present solar system. On the other hand, the obtained heliocentric distances of Y-75029 and Tsukuba indicate locations of the parent bodies in the past when some parts of the meteorites were exposed to the sun. The heliocentric distances for Y-75029 is in a good agreement with current S-type asteroid distribution, while that for Tsukuba is at the upper tail of the distribution.
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