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 parts per thousand for delta 56Fe 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 parts per thousand, 0.28 parts per thousand, 0.23 parts per thousand and 0.25 parts per thousand for delta 56Fe values (2SD), respectively, are only suitable as in-house standards for data quality monitoring during microbeam Fe isotopic analysis. Fe isotopic compositions of natural silicate minerals, including olivine, garnet, amphibole and biotite are directly calibrated against non-matrix-matched BCR-2G glass by LA-MC-ICP-MS under wet plasma conditions.