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 ha ving B concentration of less than 1 mu g g(-1) using a small radius im s3f ion microprobe. The central difficulties for these measurements ar e (i) the removal of the trace amount of B contamination introduced in the sample during polishing, (ii) the precise calibration of instrume ntal 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-dist illed water (< 1 ng g(-1) B) and presputtering of the analysed area de crease B contamination to the level of 0.01 mu g g(-1). Analyses of is otope standards spanning a range of B-11/B-10 between 3.93 and 4.20 sh owed that instrumental mass fractionation was constant within approxim ate to 1 parts per thousand during one session of analyses. Repeated a nalyses of a standard glass showed a reproducibility of instrumental m ass fractionation between February 1991 and October 1996 of +/-1.3 par ts per thousand. Taking into account all sources of error, boron isoto pe measurements are accurate to within +/-5 parts per thousand for met eoritic 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.