Fe Kα XANES, Fe Kβ HERFD XANES and EPMA flank method determinations of the oxidation state of Fe in garnet

The ferric to total iron ratios (Fe³⁺/∑Fe) of garnets can be paired with thermodynamic mineral activity models to quantify the oxygen fugacity of garnet-bearing rocks. However, techniques with a high analytical and spatial resolution are necessary to distinguish differences in garnet Fe³⁺/∑Fe ratios at the percent level and to accurately measure garnets that are zoned or contain inclusions. We acquired conventional Fe Kα and high-resolution energy fluorescence detection (HERFD) Fe Kβ X-ray absorption near edge structure (XANES) spectra and electron microprobe flank method analyses on a suite of 27 peridotitic and eclogitic garnets with Fe³⁺/∑Fe ratios previously determined by Mössbauer spectroscopy to evaluate the precision of each technique. We examined variations in the energy and intensity of three XANES spectral features as a function of Fe³⁺/∑Fe ratios: 1) the intensity ratio of two-post edge features (I-ratio; Fe Kα only); 2) the energy of the Fe edge at 90% normalized intensity (E_0.9; Fe Kα only) and 3) the pre-edge centroid energy (Fe Kα and HERFD Fe Kβ). In accordance with previous work, we find the energies of garnet pre-edge centroids are relatively insensitive to Fe³⁺/∑Fe ratios. The I-ratios of peridotitic and eclogitic garnets are offset from each other at low Fe³⁺/∑Fe ratios (≤0.13); I-ratio garnet XANES calibrations are composition-specific. The E_0.9 feature is independent of garnet major element composition in spectra that have been corrected for the effects of self-absorption. We produce two Fe Kα garnet XANES calibrations based on variations in the E_0.9 feature; one calibration with all garnet reference materials included (Fe³⁺/∑Fe up to 1.0; "all garnet calibration") and another calibration specific to garnets with low Fe³⁺/∑Fe ratios ("low ferric calibration"). Fe³⁺/∑Fe ratios calculated from the mean of up to 25 flank method measurements on eight garnet reference materials fall within 4% absolute of a one-to-one correlation with Fe³⁺/∑Fe ratios measured by Mössbauer. The standard error of the mean Fe³⁺/∑Fe ratio calculated from flank method approaches the Mössbauer-determined Fe³⁺/∑Fe ratio within estimated error (3%) after three analyses. Flank method precision is enhanced at higher beam current; however, the precision of the flank method does not approach the precision of XANES under any microprobe analytical condition tested here. Garnet reference materials detailed here are available by request to the Smithsonian Institution.