Antarctic Meteorites with Unusual Exposure and Terrestrial Histories

Details Antarctic Meteorites with Unusual Exposure and Terrestrial Histories Antarctic Meteorites with Unusual Exposure and Terrestrial Histories

Sep 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.598w&link_type=abstract

Meteoritics, vol. 30, no. 5, page 598

Computer Science

2

Ages, Terrestrial, Aluminum-26, Beryllium-10, Cosmic Rays, Galactic, Solar, Exposure Ages, Isotopes, Cosmogenic, Meteorites, Elephant Moraine 83246, Lewis Cliff 85324, Lewis Cliff 86011, Lewis Cliff 86012, Lewis Cliff 86013, Lewis Cliff 86016, Lewis Cliff 86019, Lewis Cliff 86360, Lewis Cliff 87169, Lewis Cliff 88008, Noble Gases

Scientific paper

A gamma-ray survey of 26Al (t1/2 = 0.7 Ma) in 90 samples from the Lewis Cliff area revealed 26Al activities between 30 and 70 dpm/kg, with a few unusual values ranging from 7 dpm/kg to 96 dpm/kg [1]. Shielding effects can explain deviations of +/- 30% from the average saturation value of 60 dpm/kg, but values below 40 dpm/kg must be due to short exposure ages and/or long terrestrial ages. High values may be ascribed to an increased galactic cosmic ray (GCR) flux or to solar cosmic ray (SCR) produced 26Al in a meteorite which suffered low atmospheric ablation. In order to obtain a better understanding of the exposure and terrestrial history of Lewis Cliff meteorites subsequent 10Be and 26Al AMS analyses were carried out. The 26Al/10Be ratio is relatively insensitive to shielding effects [2], but due to the 1.51 Ma half-life of 10Be, it discriminates between short exposure ages and long terrestrial ages. It also distinguishes between SCR- and GCR-produced 26Al, since SCR particles are too low-energetic to produce 10Be. In addition noble gas isotopes were measured in six 'suspect' samples in order to determine the 3He, 21Ne and 38Ar exposure ages and estimate shielding conditions on the basis of the 22Ne/21Ne ratio. In addition, cosmic-ray tracks were counted in two high-26Al samples in order to estimate the pre-atmospheric size. SCR-produced 26Al. The high 26Al activities of LEW 87169 and EET 83246 (table 1) hint at SCR-produced 26Al. This is confirmed by the 10Be activities which exclude an increased GCR flux. In addition, a third meteorite (LEW 86016) with possible SCR-26Al was identified on the basis of its high 26Al/10Be ratio. For LEW 86016 and LEW 87169 track densities were measured in unshocked olivine grains. The calculated track production rates of 2.8 x 10^5 and 3.7 x 10^5 cm^-2 Ma^-1, respectively, indicate higher shielding depths (> 2.0 - 2.5 cm) than expected, but are comparable with track densities found in the Salem meteorite [3]. Short exposure ages. On the basis of high 26Al/10Be ratios and low 26Al and 10Be activities, exposure ages of 1.0 - 1.3 Ma were estimated for LEW 85324 and for four members of the LEW 86011-shower. These estimates were confirmed by noble gas measurements (table 1). Anomalous exposure histories. The exposure history of the small diogenite LEW 88008 (18 g) is not well understood. Its 10Be content and 22Ne/21Ne ratio indicate irradiation in a large object, whereas the 26Al activity is more characteristic for a small object. A two-stage exposure history with an initial 2pi-exposure can be excluded on the basis of its low 26Al/10Be ratio (1.9). High terrestrial ages. The exceptionally low 26Al value of 7 dpm/kg for LEW 86360 was tentatively ascribed to a complex exposure history [1], but the 26Al/10Be ratio of 1.1 rather hints a terrestrial age of about 2.2 Ma. The noble gas records show that LEW 86360 was exposed for 4.5 Ma under average shielding conditions. Presently, 36Cl measurements are in progress to provide conclusive evidence on its terrestrial history. If the terrestrial age of 2.2 Ma is confirmed, LEW 86360 would be the oldest stony meteorite found on Earth. References: [1] Welten K. C. et al. (1994) Workshop on Meteorites from Hot and Cold Deserts, LPI, Houston, in press. [2] Welten K. C. et al., this volume. [3] Nishiizumi K. et al. (1990) Meteoritics, 25, 392-393.

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