An analytical procedure has been developed to measure in situ the 11 B/10 B ratio in terrestrial basaltic rocks and meteoritic chondrules having B concentration of less than 1 μg g−1 using a small radius ims3f ion microprobe. The central difficulties for these measurements are (i) the removal of the trace amount of B contamination introduced in the sample during polishing, (ii) the precise calibration of instrumental mass fractionation of B isotopes and (iii) the low count rates of 10 B and 11 B. Contamination experiments conducted with isotopically labelled B enriched in 10 B showed that ultrasonic cleaning in bi‐distilled water (< 1 ng g−1 B) and pre‐sputtering of the analysed area decrease B contamination to the level of 0.01 μg g−1. Analyses of isotope standards spanning a range of 11 B/10 B between 3.93 and 4.20 showed that instrumental mass fractionation was constant within? during one session of analyses. Repeated analyses of a standard glass showed a reproducibility of instrumental mass fractionation between February 1991 and October 1996 of 1.3. Taking into account all sources of error, boron isotope measurements are accurate to within 5 for meteoritic samples having B contents in the range 0.1 to 1 μg g−1. A slightly better accuracy of 1.5 can be achieved for basaltic glasses which can be sputtered with very intense primary beams.

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Abstract-The distribution of Li, Be, and B was studied by ion microprobe mass spectrometry in 36 chondrules from the Semarkona, Bishunpur, Allende, Clovis #1, and Hedjaz meteorites. Within a single chondrule, Li-Be-B concentrations can vary up to one order of magnitude. For example, in a chondrule from Hedjaz, concentrations range from 0.3 to 2.4 ppm for Li, from <0.001 to 0.17 ppm for Be, and from 0.4 to 5.5 ppm for B. Among chondrules from Semarkona and Bishunpur, clear crystal-mesostasis partitioning was observed in nine chondrules for Li, in nine chondrules for Be, and in three chondrules for B. The heterogeneities in the distribution of Li, Be, and B in chondrules fiom Semarkona and Bishunpur appear to be primary features that were inherited from the chondrules' precursors and not totally obscured during the chondrules' formation. A redistribution of B was nevertheless observed at the whole-rock scale for Allende (B-Al2O3 correlation) and Hedjaz (B-SiO2 correlation).At the scale of bulk chondrules, a robust correlation is observed for all studied meteorites between the B/Be and the B/Li ratios, which indicates that Li and Be are much less hetero eneously distributed in chondrites ) is a factor of -4 depleted relative to Orgueil (Li/Be ratio of -78). Such a depletion is puzzling because no correlation between Li and Na or B has been found as would be expected to result from volatilization processes during chondrule melting and cooling. A s a consequence, the exact abundance of solar system Li remains an open question.