Bismuth and 208Pb microdistributions in enstatite chondrites |
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| Authors: | Dorothy S Woolum DS Burnett LS August |
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| Institution: | Physics Department, California State University, Fullerton, CA 92634, U.S.A.;Division of Geology & Planetary Science, California Institute of Technology, Pasadena, CA 91125, U.S.A.;Radiobiology Branch, Naval Research Laboratory, Washington, DC 20375, U.S.A. |
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| Abstract: | Polished sections of 5 enstatite chondrites have been irradiated with 30 MeV 4He ions to produce the alpha-radioactive nuclei 211At and 210Po from 209Bi and 208Pb, respectively. The distribution of alpha activity can be mapped, using cellulose nitrate as an alpha track detector, to give the corresponding Bi or Pb distributions in the meteorite. No strong localization of Bi or 208Pb was found; relatively uniform track distributions were observed. In particular, metal or sulfide grains are not enriched in Bi or Pb (relative to bulk), which is in agreement with the predictions of nebular condensation calculations. While the track distributions appear uniform, the results of detailed, track-by-track mappings of the Bi detectors indicate that the Bi is not totally randomly distributed; the statistical fluctuations in the observed track density are different for the cases where the Bi is totally randomly distributed and where the Bi is localized in point sources. Assuming that the Bi in a given sample is localized in identical point sources which are uniformly distributed throughout the sample, the observed relative population densities of clusters (‘stars’) of small numbers of tracks (2–5) corresponds to Bi being localized, with ~90% in grains with about 10?16g-Bi (~3 × 105Biatoms), and with ~10% in 4 × 10?14 g-Bi sources. If these are elemental Bi, as predicted theoretically, they are ~ 102 Å and 103 Å in size, respectively. |
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