Other
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29s.455c&link_type=abstract
Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 455-456
Other
Boron Isotopes, Chemical Composition, Chondrites, Chondrule, Meteoritic Composition, Abundance, Cosmic Rays, Cosmochemistry, Interstellar Matter, Solar Corona
Scientific paper
A B isotope study of chondrules was undertaken in several chondrites to (1) evaluate the potential of B isotopes for constraining the mechanism of chondrule formation and (2) document the possible B isotope heterogeneity of nebular precursors and of chondrites. Measurements were made with an IMS 3f ion microprobe at CRPG on approximately = 25 micron spots, following the procedures developed mantle rocks and with careful attention paid to problems of surface contamination. Values of delta B-11 vs. NBS 951 (B-11/B-10 = 4.04558) are given with an accuracy varying between +/- 10% according to the B contents. Elemental ratios of B, Na, Mg, Al, K, and Ca vs. Si were determined simultaneously on the same spots by ion probe. Large discrepancies exist between the only two existing B isotope studies of bulk chondrites, which report three delta B-11 measurements between -35 +/- 4 and -57 +/- 4% for the oldest one and six delta B-11 measurements between +1.8 and -8.5% for the most recent, while they both report B concentrations in the same range, between 0.4 and 0.9 ppm. These differences are unexplained, but large isotopic heterogeneities may have existed in the solar nebula since calculations of the B-11/B-10 ratios, produced during the synthesis of B via the interaction between high-energy cosmic rays and the interstellar matter, can theoretically vary between 2.5 and 4.1. The delta B-11 values found in this study are extremely variable. These variations can be explained by (1) isotopic and chemical fractionations of B occurring in the solar nebula before and/or during the formation of chondrules and/or (2) by an isotopic heterogeneity of the nebular precursors resulting from the presence in the interstellar matter of B-10 and B-11 produced by spallation and of B-11 produced by other mechanisms (e.g., thermonuclear reactions).
Chaussidon Marc
Robert Frédéric
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