Boron in chondrules

Abstract

Isotopic compositions and abundances of boron were measured in sixteen chondrules from seven chondrites by ion microprobe mass spectrometry. The chondrules are of the porphyritic, barred, and radial type and host meteorites include carbonaceous, ordinary, and enstatite chondrites. Boron abundances are generally low with average boron concentrations of between 80 and 500 ppb. These abundances are lower than those of bulk chondrites (0.35 to 1.2 ppm; Zhai et al., 1996), confirming earlier suggestions that boron is mostly contained in the matrix. No significant variation in the ¹¹B/¹⁰B ratio is observed among these chondrules, outside our experimental error limits of several permil, and B-isotopic compositions agree with those reported for bulk chondrites. The lack of a significant isotope fractionation between chondrules and matrix implies that the low boron abundances are not the result of a Rayleigh fractionation during chondrule formation. Isotopic heterogeneities within individual chondrules are constrained to be ± 20%0 at 95% confidence level at a spatial scale of 20-30 μ m, significantly lower than the value of about ± 40%0 previously reported for chondrules from carbonaceous and ordinary chondrites (Chaussidon and Robert, 1995, 1998). The observed B-isotopic homogeneity does not conflict with the presence of decay products from extinct ¹⁰Be, with (¹⁰Be/⁹Be)₀≅ 10^-3, as was inferred for calcium-aluminum-rich inclusions. Extinct 10Be in chondrules would shift the abundance ratio ¹¹B/¹⁰B at best by several permil because of their commonly observed low Be/B ratios (2). The results show that potential B-isotopic heterogeneities in the solar nebula due to the presence of components with different B-isotopic signatures, such as boron produced by high-energy galactic cosmic rays (¹¹B/¹⁰B ≅ 2.5), or by the hypothetical low-energy particle irradiation (¹¹B/¹⁰B ≅ 3.5-11) or boron from type II supernovae (¹¹B/¹⁰B » 1), did not survive the chondrule formation processes to a measurable extent.