Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30..585t&link_type=abstract
Meteoritics, vol. 30, no. 5, page 585
Other
5
Cosmogenic-Isotopes, Exposure Ages, Meteorites, Allan Hills 81005, Lunar, Macalpine 88105, Queen Alexander Range 93069, Noble Gases, Ar, Ne, Trapped
Scientific paper
We obtained lunar meteorite QUE 93069,7 (0.304 g) from the NASA/MWG for the determination of its noble gas isotopic abundances and exposure history. The data relevant for the discussion of the exposure history and trapped noble gases are given in Tables 1 and 2. Exposure history: The duration of Moon-Earth transfer was determined by Nishiizumi et al. [1]. Based on 10Be these authors obtained 1.9 +/- 0.4 Ma for a 4 pi model (all radionuclides produced in 4 pi space) and <0.1 Ma for a 2 pi model (most radionuclides produced on the Moon). Adopting these times we find that less than one percent of the cosmogenic noble gases were produced during Moon- Earth transfer. The overwhelming amounts of 21Nec and 38Arc must have been produced during residence in the lunar regolith. Using lunar regolith production rates [2] at 5-10 g/cm2 shielding [1], we calculated the exposure times, T (2 pi), on the Moon. Table 2 gives the results and compares them with the exposure times for other anorthositic lunar meteorites (MAC 88105 and ALHA 81005). QUE 93069 shows the longest exposure to cosmic rays (1100 +/- 400 Ma) of all lunar meteorites if we compare the T38 values. Based on 21Nec we obtain 420 +/- 60 Ma. Typically for lunar surface material the T21 are lower than those based on 38Arc, 83Krc, and 126Xec due to 21Ne loss. This effect is also observed for MAC 88105 and ALHA 81005. Characteristics of the trapped noble gases: The long lunar surface residence time and the shallow shielding depth are consistent with the very large amounts of trapped solar wind particles (20Ne and 36Ar, Table 1) for QUE 93069. The concentration of trapped 36Ar is quite similar to that of Y-791197: Takaoka [3] and Ostertag et al. [4] obtained 33900 and 36600 x 10-8 cm3 STP/g, respectively. The trapped ratio 40Ar/36Ar, an antiquity indicator for lunar soil, yields information on the time when the breccia was compacted from regolith material [5]. For QUE 93069 we obtain (40Ar/36Ar)trapped = 1.9 +/- 0.1 indicating exposure of the breccia material on the lunar surface about 600 Ma ago. Conclusions: Based on 38Arc the lunar surface exposure to cosmic rays for QUE 93069 lasted about 1100 +/- 400 Ma, similar to Y-791197, about twice as long as for ALHA 81005, and about seven times longer than for MAC 88104/5. The trapped 40Ar/36Ar ratio of 1.9 +/- 0.1 suggests that exposure to solar particles occured around 600 Ma ago. Since relatively large amounts of solar wind particles were accumulated, it is reasonable to assume that most cosmogenic noble gases were also produced during this exposure period. Kr and Xe analysis are in progress with the main purposes to determine 81Kr, T83, T126, and the shielding in the lunar regolith based on (131Xe/126Xe)c. Acknowledgments: We thank the NASA/MWG for the meteorite sample. This work was supported by the Swiss N.S.F. References: [1] Nishiizumi K. et al. (1995) LPS XXVI, 1051. [2] Hohenberg C. M. et al. (1978) Proc. LPSC 9th, 2311. [3] Takaoka N. (1986) Mem. NIPR, Spec. Iss. 41, 124. [4] Ostertag R. et al. (1986) Mem. NIPR, Spec. Iss., 41, 17. [5] Eugster O. et al. (1983) LPS XIV, 177. [6] Eugster O. et al. (1991) GCA, 5, 3139. [7] Nishiizumi K. et al. (1988) Meteoritics, 23, 294. Table 1, showing cosmogenic and trapped solar wind noble gases, appears here in the hard copy. Table 2, showing exposure ages, lunar surface exposure ages T(2pi), and Moon-Earth transfer time, appears here in the hard copy.
Eugster Otto
Thalmann Ch.
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