Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..467z&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 467
Other
B Abundance, Falls, Meteorites
Scientific paper
We requested from participating museum curators interior fragments of chosen falls, never touched by water or other possible sources of B contamination. Thirty six were obtained, crushed, and analyzed for B by PGNAA (prompt gamma- ray neutron activation analyses) at McMaster University and at The National Institute of Standards and Technology. Boron concentrations are close to the sensitivity limit in both laboratories. Results agree well, but with slight systematic differences attributable to blank and background correction factors. Our results (Table 1) are similar to previous measurements on falls [1], but lower than in Antarctic meteorites [2,3], some of which are altered. To calculate the solar system abundance of B, the four carbonaceous chondrite analyses (Table 1) were used as follows. Since CM and CV meteorites contain 48% and 42% matrix [4], if the B ratio of inclusions/matrix is 0.17 [3], then the matrix of the four carbonaceous chondrites averages 0.97 ppm B. Taking the average Si abundance in CI to be 10.64% [5], the calculated solar system abundances from the four carbonaceous chondrites are 23.51, 22.54, 16.95, and 34.20, with a geometric mean of 23.5 B atom/10^6 Si atoms. For comparison, of 18 analyses of interior samples of falls and Antarctic carbonaceous chondrites [1,2,3], 12 have normalized matrix B between 17.6 and 31.4. A composite chondrite atomic composition was calculated for Mg, Na, Li, B, Ga, S, and Zn using their average abundances in H, L, LL, E, and CC meteorites, weighted by their fall frequencies [7] and normalized to Si and the CI abundance [5]. The values show a systematic decrease (Table 2). If this trend is related to volatility [8], then the condensation temperature of B should be between the condensation temperature of Li and Ga, at about 1125 degrees K. If the relative abundances of these elements are similar in the Earth's mantle except for Na [8], then Na appears to show a lower condensation temperature, similar to B. This difference may be due to different evolution paths of meteorites and the Earth. References: [1] Curtis D. B. and Gladney E. S. (1985) EPSL, 75, 311-320. [2] Shaw D. M. et al. (1992) Meteoritics, 27, 289. [3] Shaw D. M. et al. (1988) GCA, 52, 2311-2319. [4] Dodd R. T. (1981) Meteorites, 53, Cambridge Univ. [5] Anders E. and Grevesse N. (1989) GCA, 53, 197-214. [6] Mason B., ed. (1971) Handbook of Elemental Abundances in Meteorites, Gordon and Breach, New York. [7] Sears D. W. G. and Dodd R. T. (1988) in Meteorites and the Early Solar System (J. F. Kerridge and M. S. Matthews, eds.), 4, Univ. of Arizona, Tucson. [8] Jagoutz E. et al. (1979) Proc. LPSC 10th, 2031-2050. Table 1, which appears here in the hard copy, shows boron in meteorite falls. Table 2, which appears here in the hard copy, shows silicon and solar-system- normalized abundances of some elements in chondrites.
Shaw Matthew D.
Zhai Meng
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