Boron in chondrules

Isotopic compositions and abundances of boron were measured in sixteen chon drules from seven chondrites by ion microprobe mass spectrometry. The chond rules are of the porphyritic, barred, and radial type and host meteorites i nclude carbonaceous, ordinary, and enstatite chondrites. Boron abundances a re generally low with average boron concentrations of between 80 and 500 pp b. These abundances are lower than those of bulk chondrites (0.35 to 1.2 pp m; Zhai et al., 1996), confirming earlier suggestions that boron is mostly contained in the matrix. No significant variation in the B-11/B-10 ratio is observed among these chondrules, outside our experimental error limits of several permil, and B-isotopic compositions agree with those reported for b ulk chondrites. The lack of a significant isotope fractionation between cho ndrules and matrix implies that the low boron abundances are not the result of a Rayleigh fractionation during chondrule formation. Isotopic heterogen eities within individual chondrules are constrained to be < +/- 20 parts pe r thousand at > 95% confidence level at a spatial scale of 20-30 mum, signi ficantly lower than the value of about +/- 40 parts per thousand previously reported for chondrules from carbonaceous and ordinary chondrites (Chaussi don and Robert, 1995, 1998). The observed B-isotopic homogeneity does not c onflict with the presence of decay products from extinct Be-10, with (Be-10 /Be-9)(0) approximate to 10(-3), as was inferred for calcium-aluminum-rich inclusions. Extinct Be-10 in chondrules would shift the abundance ratio B-1 1/B-10 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-isot opic signatures, such as boron produced by high-energy galactic cosmic rays (B-11/B-10 approximate to 2.5), or by the hypothetical low-energy particle irradiation (B-11/B-10 approximate to 3.5-11) or boron from type II supern ovae (B-11/B-10 > > 1), did not survive the chondrule formation processes t o a measurable extent.