Non-matrix-matched analysis of Fe isotopes in silicates by laser ablation MC-ICP-MS and potential silicate in-house standards for microbeam Fe isotopic analysis†

Precise and accurate Fe isotope data of thirteen silicate minerals, including olivine, garnet, amphibole and biotite are reported here using the non-matrix-matched BCR-2G reference glass for isotope ratio calibration combined with a Ni doping method for instrumental mass bias correction by LA-MC-ICP-MS under wet-plasma conditions. All the in situ Fe isotope data are consistent with the recommended Fe isotopic values determined by SN-MC-ICP-MS. These results indicate that in situ Fe isotopic determinations by LA-MC-ICP-MS under wet-plasma conditions are matrix-independent for a variety of silicate minerals, and BCR-2G glass is suitable as the reference material for microbeam Fe isotopic analysis for silicate minerals. However, for other microbeam Fe isotopic analysis, e.g., SIMS, finding matrix-matched standards for data quality assurance is also required. Multiple analyses on two international olivine standards (San Carlos and ALM) and four natural silicate minerals (YY09-04 olivine, YS-13 garnet, YS-31 amphibole and Moosilauke biotite) reveal homogeneous Fe isotopic compositions with the external reproducibility better than 0.20‰ for δ⁵⁶Fe values (2SD). Thus, these silicate minerals are proposed as matrix-matched in-house standards for isotope ratio calibrations and data quality control during microbeam Fe isotopic analysis. YY11-12 olivine, YS-42 garnet, YS-37 amphibole and YS-20 biotite with the external reproducibility of 0.23‰, 0.28‰, 0.23‰ and 0.25‰ for δ⁵⁶Fe values (2SD), respectively, are only suitable as in-house standards for data quality monitoring during microbeam Fe isotopic analysis.