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 12B produced by the 11B(d,p) reaction, and (b) measurement of particle track densities from 10B(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 10B ≤ 10 16 atoms/g in two Allende fine-grained inclusions suggesting that B is not a refractory element under solar nebula conditions. This 10B limit, when taken as a limit on 10Be when the inclusion formed, puts constraints on the possibility of a solar system synthesis of 26Al. 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.