Determination of Fe oxidation state in tourmaline with the flank method by electron probe microanalysis

Abstract The Fe oxidation state in tourmaline is a potentially valuable indicator of oxygen fugacity in both magmatic and hydrothermal systems. Here we present the compositions of eight tourmaline crystals as reference materials to calibrate the electron probe microanalysis (EPMA) flank method for in-situ determination of Fe3⁺/ΣFe ratios in tourmaline. A field-emission microprobe equipped with two TAPL analyzing crystals provides stable and high-intensity FeL spectra. Two synthetic garnet end-members, ferric and ferrous, were used to constrain the measuring positions in the flanks of FeLα and FeLβ, which theoretically correspond to maximal Fe3+L3 absorption and minimal Fe2+L2 absorption, respectively. For the suite of tourmaline reference materials (mainly elbaite and schorl), spanning the ranges of FeOT content within 3–15 wt% and Fe3+/ΣFe within 0.0–0.3, the Fe2+ contents show a linear correlation with the Lβ/Lα intensity ratios measured at the flank positions of FeLα and FeLβ. Based on this EPMA flank-method calibration, the Fe3+/ΣFe ratios of the tourmaline reference materials agree well with wet chemistry results. The analytical accuracy in Fe3+/ΣFe is better than ±0.1 for tourmaline with FeOT ≥3 wt% and better than ±0.05 for those with FeOT ≥12 wt%.