High-Precision Fe and Mg Isotope Ratios of Silicate Reference Glasses Determined In Situ by Femtosecond LA-MC-ICP-MS and by Solution Nebulisation MC-ICP-MS

In this study, a technique for high precision in situ Fe and Mg isotope determinations by femtosecond-laser ablation-multi collector-ICP-MS (fs-LA-MC-ICP-MS) was developed. This technique was employed to determine reference values for a series of common reference glasses that may be used for external standardisation of in situ Fe and Mg isotope determinations in silicates. The analysed glasses are part of the MPI-DING and United States Geological Survey (USGS) reference glass series, consisting of basaltic (BIR-1G, BCR-2G, BHVO-2G, KL2-G, ML3B-G) and komatiitic (GOR128-G and GOR132-G) compositions. Their Fe and Mg isotope compositions were determined by in situ fs-LA-MC-ICP-MS and by conventional solution nebulisation multi-collector ICP-MS. We determined Fe-56 values for these glasses ranging between -0.04 and 0.10 parts per thousand (relative to IRMM-014) and Mg-26 values ranging between -0.40 parts per thousand and -0.15 parts per thousand (relative to DSM-3). Our fs-LA-MC-ICP-MS results for both Fe and Mg isotope compositions agreed with solution nebulisation analyses within analytical uncertainties. Furthermore, the results of three USGS reference glasses (BIR-1G, BHVO-2G and BCR-2G) agreed with previous results for powdered and dissolved aliquots of the same reference materials. Measurement reproducibilities of the in situ determinations of Fe-56 and Mg-26 values were usually better than 0.12 parts per thousand and 0.13 parts per thousand (2s), respectively. We further demonstrate that our technique is a suitable tool to resolve isotopic zoning in chemically-zoned olivine crystals. It may be used for a variety of different applications on isotopically-zoned minerals, e.g., in magmatic or metamorphic rocks or meteorites, to unravel their formation or cooling rates.