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, KL 2‐G, ML 3B‐G) and komatiitic ( GOR 128‐G and GOR 132‐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 δ 56 Fe values for these glasses ranging between ‐0.04‰ and 0.10‰ (relative to IRMM ‐014) and δ 26 Mg values ranging between ‐0.40‰ and ‐0.15‰ (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 δ 56 Fe and δ 26 Mg values were usually better than 0.12‰ and 0.13‰ (2 s ), 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.