Noble gases of Yamato 980459 shergottite
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概要
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Isotopic ratios and concentrations of noble gases were determined for the Yamato (Y) 980459 olivine-phyric shergottite with a stepped heating extraction method. Trapped noble gas concentrations are low, and especially He and Ne are dominated by cosmogenic nuclides. Heavy noble gases, Ar, Kr and Xe, in the high temperature fractions (1000-1750°C ) show the martian atmospheric signatures: 40Ar36Ar and 129Xe132Xe ratios corrected for cosmogenic gases are >1000 and >1.4, respectively, and the data points plot along the mixing line between the Mars atmosphere and Chassigny in the system of 129Xe132Xe vs. 84Kr132Xe. Contribution of elementally fractionated Earth's atmospheric noble gases is significant in the low temperature fractions (400-800°C ), which has been frequently reported for meteorites from hot deserts. Cosmic-ray exposure ages calculated based on cosmogenic 3He, 21Ne and 38Ar are 1.6, 2.5 and 2.1Ma, respectively. Considering partial loss of He from the meteorite, the exposure age of Y980459 would be around 2.1-2.5Ma. Though the terrestrial age of this meteorite has not been reported so far, the ages reported for Antarctic SNCs are 0.29Ma. Hence, the ejection age for Y980459 could be in the range of 2.1-2.8Ma, which is comparable to those of some basaltic shergottites, but different from other olivine-phyric shergottites ranging mostly 0.7-1.2Ma. Isotopic ratios of Kr show excesses in 80Kr and 82Kr, with 82Kr80Kr of 0.375, which results from epithermal neutron captures on 79Br and 81Br. The minimum pre-atmospheric size of Y980459 was calculated as 27cm in radius, based on the 21Ne exposure age and the n-capture 80Kr and Br concentrations of 3.0×1013cm3STPg and 0.205ppm, respectively. The calculated K-Ar age is 0.99Ga from the total 40Ar and reported K concentration of 157ppm. The age, however, gives an upper limit for the crystallization age of this meteorite, because of possible contribution of martian atmospheric Ar, as well as the terrestrial atmosphere.
著者
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Nagao Keisuke
Laboratory For Earthquake Chemistry Faculty Of Science The University Of Tokyo
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Okazaki Ryuji
Department Of Earth And Planetary Sciences Faculty Of Sciences Kyushu University
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