Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.362h&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 362
Other
Accelerator Mass Spectrometry, Aluminum-26, Beryllium-10, Cosmic-Ray Ages, Lodranites
Scientific paper
Noble gas measurements by [1] indicate that four lodranites LEW 88280, Lodran (a fall), MAC 88177, and Yamato 791491 have the same cosmic ray exposure age of a few million years. The elevated ^22Ne/^21Ne ratios of these lodranites, from 1.22 to 1.28 [1], suggest that shielding was light and production rates appreciably lower than in average chondrites. Cosmic-ray irradiation in space for, say, 4 My would bring ^26Al and ^10Be to within 2% and 16% of their respective saturation values. Thus measurement of ^26Al may provide information about production rates and shielding and ^10Be about exposure age. We separated magnetically metal- and silicate-rich material from the four lodranites mentioned above and from Winona. The ^26Al and/or ^10Be activities (Table 1) were measured by accelerator mass spectrometry [2] with the statistical 1-sigma precision shown; the activities are thought to have an overall accuracy of 6-8%. Although the metal phases were etched with HF, they retained some silicate. To get a quantitative indication of the amounts of silicate present, the Mg concentrations in aliquots of the dissolved metal samples (Table 1) were measured by ICP/MS. The Mg, Al, Ca, Ti, Mn, and Fe contents of the silicate phases were determined by DCP emission spectrometry [3]. The measured activities in silicates from LEW 88280, Lodran, and Y 791491 resemble one another closely: The average ^26Al and ^10Be activities are 50.9 and 16.7 dpm/kg compared to estimated production rates of about 55 and 23 dpm/kg. These results lead to an exposure age of ~3.3 My, but do not indicate substantial lowering of production rates. The ^26Al and ^10Be contents of MAC 88177 are about half the values expected at saturation under normal shielding and are lower than those in the other three lodranites. These results are consistent with the very light shielding inferred from the exceptionally high ^22Ne/^21Ne ratio of 1.28, and perhaps with some lowering due to terrestrial age. Kirsten et al. [4] found a ^21Ne content of 25.2 x 10^-8 cm^3 STP/g and a low ^22Ne/^21Ne ratio of 1.071 for Winona, a find of uncertain age with heavily weathered metal. The measured ^10Be activities are also low, about half the estimated production rates. A ^21Ne production rate of about 0.314 x 10^-8 cm^3 STP/g- My would be expected under normal shielding in a body with the bulk composition of Winona [5,6]. If we assume a short terrestrial age and a constant ratio of ^10Be to ^21Ne production [7], then an exposure age on the order of 150 My is implied. Use of the measured ^26Al activity in the same way gives a shorter but more uncertain exposure age of ~110 My. The high ^26Al activity in Winona "metal" may indicate the presence of sulfide [5]. Table 1, which appears here in the hard copy, shows ^10Be and ^26Al (dpm/kg) in silicate- and metal-rich samples from lodranites and Winona. References: [1] Eugster O. and Weigel A. (1993) LPS XXIV, 453- 454. [2] Middleton R. and Klein J. (1986) Proc. Workshop Tech. Accel. Mass Spectrom., England, 76-81; Middleton R. and Klein J. (1987) Phil. Trans. R. Soc. London, A323, 121-143. [3] Feigenson and Carr (1985) Chem. Geol., 51, 19-27. [4] Schultz L. and Kruse H. (1989) Meteoritics, 24, 155-172. [5] Mason B. and Jarosewich E. (1967) GCA, 31, 1097-1099. [6] Eugster O. (1988) GCA, 52, 1649-1659. [7] Graf Th. et al. (1992) GCA, 54, 2521-2534.
Herzog Gregory F.
Juenemann D.
Klein Jeff
Middleton Richard
Xue She-Sheng
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