Boron Isotope Variations in Chondrules: Consequences for Chondrule Formation and Boron Cosmochemistry (original) (raw)

Abstract

A boron isotope study of chondrules was undertaken in several chondrites (Semarkona LL3, Allende CV3, Hedjaz L3, and Estacado H6) to (1) evaluate the potential of B isotopes for constraining the mechanism of chondrule formation and to (2) document the possible B isotope heterogeneity of nebular precursors and of chondrites. Measurements were made with a IMS 3f ion microprobe at CRPG (Nancy) on ~25 micrometer spots, following the procedures developed for mantle rocks [1,2] and with careful attention paid to problems of surface contamination. delta ^11B values are given vs. NBS 951 (^11B/^10B = 4.04558) with an accuracy varying between +/-5 permil to +/-10 permil 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 discrepencies exist between the only two existing B isotope studies of bulk chondrites which report three delta ^11B measurements between -35+/- 4 and -57+/-4 permil for the oldest one[3] and six delta^11B measurements between +1.8 and -8.5 permil for the most recent[4], while they both report B concentrations in the same range, between 0.4 and 0.9 ppm. These differences are un-explained, but large isotopic heterogeneities may have existed in the solar nebula since calculations of the ^11B/^10B 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 [5,6]. The delta ^11B values found in this study are extremely variable between ~+40 +/- 10 and -50 +/- 10 permil in chondrules from Semarkona, Hedjaz, and Estacado, and more homogeneous at -25 +/- 15 permil in chondrules from Allende. Strong linear correlations are observed in several cases (see Fig. 1) between the delta ^11B values and some major element ratios (e.g., Na/Si, Mg/Si, Ca/Al). In addition, B/Be ratios are rather homogeneous at 18 +/- 4, significantly higher than B/Be ratios predicted by spallation models. 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 of the presence in the interstellar matter of ^10B and ^11B produced by spallation and of ^11B produced by other mechanisms (e.g., thermonuclear reactions). References: [1] Chaussidon M. and Libourel G. (1993) GCA, 57, 5053-5062. [2] Chaussidon M. and Jambon A. (1994) EPSL, 121, 277-291. [3] Shima M. (1963) GCA, 27, 911-913. [4] Agyei E. K. and McMullen C. C. (1978) Geol. Surv. Op. File Rep. 78-701, 3-6. [5] Reeves H. (1974) Ann. Rev. Astr. Astrophys., 12, 437-469.[6] Curtis D. et al. (1980) GCA, 44, 1945-1953.