Noble gases in two shergottites and one nakhlite from Antarctica: Y000027, Y000097, and Y000593

Schwenzer, S. P.; Herrmann, S. and Ott, U. (2009). Noble gases in two shergottites and one nakhlite from Antarctica: Y000027, Y000097, and Y000593. Polar Science, 3(2) pp. 83–99.

DOI: https://doi.org/10.1016/j.polar.2009.06.001

Abstract

We have investigated secondary influences on the noble gas budget in rim and interior pairs of three Martian meteorites from Antarctica: the lherzolitic shergottites Y000027 and Y000097, and the nakhlite Y000593. Three factors have been found to influence the original Martian noble gas budget: shock metamorphic overprint, cosmic irradiation, and terrestrial weathering. The 3He/4He ratio of the shergottites is between 0.189 and 0.217, which indicates almost complete loss of radiogenic 4He. This is expected from the high shock pressure observed in the shergottite samples. The concentration of 4He in these shergottite samples ranges from 33.8 to 39.4 × 10−8 ccSTP/g. 22Ne in the shergottites is on the order of 14 × 10−9 ccSTP/g. The nakhlite has 800 × 10−8 ccSTP/g 4He and 26 × 10−9 ccSTP/g 22Ne. An indication for solar cosmic ray contribution to the neon budget can be found in the shergottites. As Y000027 and Y000097 are reported to be paired we conclude the cosmic ray exposure (CRE) age T(3+21) of this shergottite to be 4.41 ± 0.54 Ma. For the nakhlite Y000593 T(3+21) is 11.8 ± 0.3 Ma. Heavy noble gas concentrations show large differences between rim and interior samples with the rim samples having 1.3–2.9, 1.7–38, and 1.4–20 times as much 36Ar, 84Kr, and 132Xe, respectively. The enrichment of heavy noble gases in the rim samples indicates severe terrestrial contamination. The relation between 129Xe/132Xe and 84Kr/132Xe in the rim samples shows that the incorporation mechanism caused elemental fractionation of Kr and Xe to the extent that in the Y000027 shergottite samples any Martian signature is completely masked by terrestrial contamination, if the total is taken. Only the 1400 °C steps show clear evidence for Martian atmosphere. The Y000593 nakhlite interior sample, on the other hand, shows low 84Kr/132Xe in relation to 129Xe/132Xe, which is characteristic for fractionated Martian atmosphere observed in nakhlites.

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