Noble gases of the Yamato 000027 and Yamato 000097 lherzolitic shergottites from Mars

Abstract We used total melting and stepwise heating methods to measure noble gases within bulk samples from Yamato 000027 and Yamato 000097 lherzolites as well as a melt-vein sample from Yamato 000027. He and Ne are dominated by cosmogenic noble gases. The obtained cosmic-ray exposure age of 4.6 ± 1.5 Ma, an average age based on cosmogenic 3 He, 21 Ne, and 38 Ar for these samples, is consistent with the ages of other lherzolitic shergottites, indicating a common impact event for the ejection of lherzolites from Mars. Heavy noble gases released from the bulk samples at low temperatures were elementally fractionated terrestrial atmosphere. Martian noble-gas isotopic signatures, 40 Ar/ 36 Ar = 1900 and 129 Xe/ 132 Xe = 1.3, were observed at high temperatures (>1000 °C). The melt-vein sample released greater amounts of atmospheric Ar, Kr, and Xe at low temperatures than the bulk samples. Large amounts of Ar and Kr, as well as excess 40 Ar and 129 Xe, were evolved from the melt-vein sample at 1400 °C, and the gas shows very high 36 Ar/ 132 Xe (=3100) and 84 Kr/ 132 Xe (=76) values. Maximum 40 Ar/ 36 Ar and 129 Xe/ 132 Xe values of the melt sample were 1100 and 1.6, respectively, at 1600 °C. Cosmogenic Kr shows an absence of 80 Kr and 82 Kr produced by neutron capture on Br, which suggests a small pre-atmospheric body. Overall noble gas compositions for Y000027 and Y000097 support pairing for the Yamato 00 shergottites.