Cosmic ray exposure history of two basaltic lunar meteorites: Asuka 881757 and Yamato 793169

Details Cosmic ray exposure history of two basaltic lunar meteorites: Asuka 881757 and Yamato 793169 Cosmic ray exposure history of two basaltic lunar meteorites: Asuka 881757 and Yamato 793169

Jul 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29q.540t&link_type=abstract

Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 540

Computer Science

4

Abundance, Basalt, Cosmic Rays, Lunar Rocks, Meteoritic Composition, Radiation Effects, Radioactive Age Determination, Rare Gases, Solar Wind, Wind Effects

Scientific paper

We obtained lunar meteorite samples Asuka 881757,106 (0.503 g) and Yamato 793169,46 (0.024 g) for the determination of their noble gas isotopic abundances. The data relevant for the discussion of the cosmic-ray exposure history are given. Preliminary results have been published earlier; recently Nagao and Miura (1993) reported noble gas data on Asuka 881757 that generally agree with our results. Based on Be-10 Nishiizumi et al. determined the duration of the Moon-Earth transfer, T(4 pi), and obtained 0.9 +/- 0.1 m.y. and 1.1 +/- 0.2 m.y. for Asuka 881757 and Yamato 793169 respectively. We adopt these times for the exposure in 4 pi space and calculate the noble gas concentrations produced in 4 pi geometry using the procedure given by Eugster and Michel (1994). For both meteorites we adopt an average eucritic shielding depth in 4 pi geometry, d(4 pi), of about 40 g/sq cm. After subtraction of the amounts produced in 4 pi geometry from the observed noble gas concentrations, fractions of the Ne-21, Ar-38, Xe-126, and Xe-131 concentrations are left over. These amounts must have been produced during residence in the lunar regolith. The average duration necessary to produce the above concentrations on the Moon at this depth is 0.8 +/- 0.2 m.y. This brief lunar surface residence time for Asuka 881757 is consistent with the fact that the abundance of solar wind particles is low or absent. Using production rates for 500 g/sq cm the resulting cosmic ray exposure time for Yamato 793169 on the Moon, T(2 pi), is greater than or equal to 70 m.y. A terrestrial age of 30,000 +/- 8,000 yr was calculated for Asuka 881757, in agreement with the value of less than 50,000 yr from Cl-36 reported by Nishiizumi et al (1992).

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