Analytical Procedures for the Measurement of Boron Isotope Compositions by Ion Microprobe in Meteorites and Mantle Rocks |
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| Authors: | Marc Chaussidon François Robert Denis Mangin Pascal Hanon Estelle F Rose |
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| Institution: | CRPG-CNRS, BP 20, 54501 Vandoeuvre-lès-Nancy, France;Muséum National d'Histoire Naturelle - CNRS, URA 736, Laboratoire de Minéralogie, 61 rue Buffon, 75005 Paris, France |
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| Abstract: | 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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