Laser ablation multicollector ICPMS determination of δ11B in geological samples

A method for the in situ single spot delta B-11 characterisation of geological materials with laser ablation multicollector ICP mass spectrometry (LA-MC-ICPMS) has been developed. The mass spectrometer was equipped with both Faradays and multiple ion counters. Four samples with different B contents (12-31,400 ppm) and isotopic compositions (sigma B-11 are between -8.71 and +13.6 parts per thousand) were analysed. Samples include the B4 tourmaline and 3 MPI-DING glasses (StHs6/80-G, GOR132-G and GOR128-G). All sources of B isotopic fractionation during the analysis (mass bias, laser-induced isotopic fractionation and detector efficiency drift) have been evaluated and quantified. Instrumental mass bias is the major source of fractionation, altering the original isotopic ratio up to 13%. Fractionation related to laser sampling and transport to the ICP was found to be very low (less than 0.0015% s(-1)). Fractionation effects due to drift in ion counter efficiencies were found to be significant. Nevertheless, the "standard-sample-standard" bracketing approach could be used to correct for the above fractionation effects using NIST SRM 610 as external standard. With spot sizes of 60-80 mu m in diameter, geologically meaningful results can be achieved on samples containing at least 10 ppm B, i.e., results with precisions that can discriminate between the different reservoirs on Earth. Data obtained with Faraday detectors on NIST SRM 610 and B4 tourmaline show high precision (down to 0.04 parts per thousand, 1 sigma) and accuracy. Boron isotope ratios measured in the glass samples using multiple ion counting show significantly higher standard deviations (up to 2.5 parts per thousand, 1 sigma), but they are very close to the values that can be expected from counting statistics. No significant variations with spot size or B contents were observed. Most of the values are within 1 sigma level of the reference values. The developed method was applied to a series of ashes from Mt. Etna erupted in 1995 having B contents between 14 and 20 ppm. The B isotope compositions of the ashes are between -4.8 and -10.7 parts per thousand, with a weighted average value of -8.0 +/- 1.9 parts per thousand (1 sigma). (c) 2006 Elsevier Ltd. All rights reserved.