Revisiting the EPMA Flank Method for Determination of Fe Oxidation State of Garnet, Amphibole and Biotite

The oxidation state of Fe serves as a direct indicator of oxygen fugacity in geological systems. We present new results of electron probe microanalysis (EPMA) flank method for determining Fe 2+ /ΣFe ratios in garnet, amphibole, and biotite, using a field‐emission microprobe equipped with TAPL diffraction crystal. For each mineral group, the Fe 2+ mass fractions exhibit a linear correlation with the intensity ratios measured at the flank positions of Fe L α and Fe L β , albeit with distinct slopes and intercepts. The flank positions of Fe L α and Fe L β , which theoretically correspond to minimal Fe 3+ L 3 absorption and maximal Fe 2+ L 2 absorption, are constrained using two synthetic ferric and ferrous garnet end‐members. Eight newly collected natural garnets, for which the Fe 2+ /ΣFe ratios have been measured by an independent method of wet‐chemistry or Mössbauer spectroscopy, constitute a useful set of reference materials with FeO T mass fractions ranging from 3 to 39% m/m for calibrating the EPMA flank method. Additionally, we re‐analysed the amphibole and biotite reference materials proposed by Li et al . (2019, Chem. Geol., 509, 152–162). The Fe 2+ /ΣFe ratios obtained using our EPMA flank method exhibited small bias relative to reference values, with deviations below 0.05 for garnet, amphibole and biotite in most cases.