Natural Amphibole and Biotite Crystals as Potential Reference Materials for In Situ Fe Isotope Measurement

The in situ measurement of stable isotope ratios (e.g., δ 56 Fe) in natural and experimental samples has become an indispensable tool for investigating processes such as diffusion, equilibrium isotope exchange, and crystal growth/dissolution across inter‐ and intra‐mineral domains as well as between minerals, melts and fluids. A key prerequisite for achieving accurate and precise data of such isotope ratios is the availability of well‐characterised solid reference materials (RMs) for use as bracketing standards (calibrators) during in situ measurement. In this study, we evaluate a suite of millimetre‐sized natural amphibole and biotite crystals using both solution nebulisation MC‐ICP‐MS and femtosecond LA‐MC‐ICP‐MS to assess their suitability as RMs for in situ δ 56 Fe measurement. These mineral separates, previously used to establish Fe 2+ /ΣFe measurement by electron probe microanalysis via the flank method, include amphiboles with FeO T mass fractions ranging from 4.2 to 12.7 g/100g and biotites with FeO T mass fractions ranging from 8.2 to 28.6 g/100g. Results from LA‐MC‐ICP‐MS in situ δ 56 Fe measurement indicate that the biotite and amphibole crystals are sufficiently homogeneous (except for Amp‐12 and Amp‐15), with intermediate precision better than 0.10‰ for δ 56 Fe values. Furthermore, the in situ data agree well with solution‐derived Fe isotope ratios within analytical uncertainty (except for Bt‐18 and Bt‐41). Specifically, sample Amp‐15 exhibits significant intra‐crystal heterogeneity, Amp‐12 shows considerable variability in δ 56 Fe values between different crystals, Bt‐18 shows an obvious discrepancy between solution and in situ data and Bt‐41 displays considerable inter‐laboratory inconsistency of solution data. Therefore, the homogeneous mineral separates of biotite (Bt‐25 and Bt‐26) and amphibole (Amp‐11 and Amp‐40) are recommended as matrix‐matched in‐house RMs for isotope ratio calibration and data quality control of spatially resolved Fe isotope measurements.