Study of the primordial noble gas components in achondrites, with special interest in U-Xe

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We measured many achondrites, i.e., eucrites, diogenites, howardites, martian meteorites, and both silicates and metal phases of pallasites. Several different methods for noble gas extraction from meteorites were tried to separate different components trapped in meteorites. However, because the concentrations of trapped noble gases are 2-3 orders of magnitude lower than those in chondrites, precise determination of isotopic ratios was difficult when we measure small amounts of meteorite samples. Hence, only the noble gas data obtained by total melt and stepwise heating experiments are presented in this report. Techniques and apparatus applicable to smaller sample size are now under developments in our laboratory.Remarkable results obtained from the project are followings :1) Trapped Xe isotopic compositions are dificient in heavier isotopes compared with those of chondrites. Chondrites have H-Xe component enriched in heavier Xe isotopes trapped in diamonds. On the other hand, Xe in achondrites has isotopic composition similar to the solar type Xe or suggestive U-Xe.2) Xe isotopic ratios indicate a presence of extinct ^<244>Pu(T_<1/2>=8.2x10^7y) in most eucrites at the sollidification of eucrite parent body. Estimated ^<244>Pu-Xe formation age are in the range of 1.5 Ga after the formation of solar system at about 45 Ga ago.3) Relative abundances of heavy noble gases, Ar, Kr and Xe, in chondrites show a trend clearly different from that of chondrites. Noble gases of the Earth and the Mars are also in the trend for achondrites.4) Achondrites might not have included pre-solar grains such as diamonds at the formation of achondrite parent body, or the gases trapped in pre-solar grains might have lost at high temperature during the formation of parent body.5) Solar type noble gases may have acquired from the solar nebula.