Analytical procedures for the measurement of boron isotope compositions by ion microprobe in meteorites and mantle rocks

An analytical procedure has been developed to measure in situ the B-11/B-10 ratio in terrestrial basaltic rocks and meteoritic chondrules having B concentration of less than 1 mu g 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 B-10 and B-11. Contamination experiments conducted with isotopically labelled B enriched in B-10 showed that ultrasonic cleaning in bi-distilled water (< 1 ng g(-1) B) and presputtering of the analysed area decrease B contamination to the level of 0.01 mu g g(-1). Analyses of isotope standards spanning a range of B-11/B-10 between 3.93 and 4.20 showed that instrumental mass fractionation was constant within approximate to 1 parts per thousand 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 parts per thousand. Taking into account all sources of error, boron isotope measurements are accurate to within +/-5 parts per thousand for meteoritic samples having B contents in the range 0.1 to 1 mu g g(-1). A slightly better accuracy of +/-1.5 parts per thousand can be achieved for basaltic glasses which can be sputtered with very intense primary beams.