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Published July 1978 | public
Journal Article

Boron concentrations in carbonaceous chondrites

Abstract

We have analyzed B in carbonaceous chondrites in order to clarify a factor of 100 difference between the solar system B abundance derived from the solar photosphere and that inferred from previous meteorite data. Consistent results were obtained from two instrumental methods for B analysis: (a) counting of the high energy betas from ^(12)B produced by the ^(11)B(d,p) reaction, and (b) measurement of particle track densities from ^(10)B(n,α)^7Li in a plastic track detector affixed to a homogenized meteorite sample. Contamination is a major problem in B analyses, but extensive testing showed that our results were not seriously affected. Our B concentrations are typically 1–2 ppm and are a factor of 2–6 lower than previous carbonaceous chondrite measurements. Our data for the Cl chondrites Ivuna and Orgueil would indicate a solar system B/Si atomic abundance ratio of 58 × 10^(−6), but this is still a factor of 2–10 higher than the photospheric estimates. It may be that B is depleted in the sun by thermonuclear processes; however, the similarity of photospheric and meteoritic Be abundances is a problem for this point of view. Alternatively, B may be enhanced in carbonaceous chondrites, but this would make B a cosmochemically unique element. A mm-sized (Fe,Mn,Mg)CO_3 crystal from Orgueil shows no B enrichment. We find ^(10)B ≤ 10^(16) atoms/g in two Allende fine-grained inclusions suggesting that B is not a refractory element under solar nebula conditions. This ^(10)B limit, when taken as a limit on ^(10)Be when the inclusion formed, puts constraints on the possibility of a solar system synthesis of ^(26)Al. For a proton spectrum of E^(−a), a must be ≥ 3 if a solar gas is irradiated or a ≥1.5 if dust of solar composition is irradiated.

Additional Information

© 1978 Published by Elsevier Ltd. Received 26 October 1977; accepted in revised form 24 February 1978. Meteorite samples for this study were generously supplied by R. CLARKE (U.S. National Museum), E. OLSEN (Field Museum, Chicago), C. MOORE (Arizona State), G. J. WASSERBURG (Caltech) and J. WASSON (U.C.L.A.). We profited greatly from a discussion with D. HEYMANN on ^(10)Be production. The co-operation of the U.C.L.A. Nuclear Energy Laboratory in arranging reactor irradiations is acknowledged. This work was supported by NSF Grants EAR 76-84402 (D. S. Burnett) and PHY 76-83685 (T. Tombrello).

Additional details

Created:
August 22, 2023
Modified:
October 20, 2023