Fe K XANES, Fe KI3 HERFD XANES and EPMA flank method determinations of the oxidation state of Fe in garnet

The ferric to total iron ratios (Fe3+/EFe) 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+/EFe ratios at the percent level and to accurately measure garnets that are zoned or contain inclusions. We acquired conventional Fe K alpha and high-resolution energy fluorescence detection (HERFD) Fe KI3 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+/EFe ratios previously determined by Mo center dot ssbauer 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+/EFe ratios: 1) the intensity ratio of two-post edge features (I-ratio; Fe K alpha only); 2) the energy of the Fe edge at 90% normalized intensity (E0.9; Fe K alpha only) and 3) the pre-edge centroid energy (Fe K alpha and HERFD Fe KI3). In accordance with previous work, we find the energies of garnet pre-edge centroids are relatively insensitive to Fe3+/EFe ratios. The I-ratios of peridotitic and eclogitic garnets are offset from each other at low Fe3+/EFe ratios (<= 0.13); I-ratio garnet XANES calibrations are composition-specific. The E0.9 feature is independent of garnet major element composition in spectra that have been corrected for the effects of self-absorption. We produce two Fe K alpha garnet XANES calibrations based on variations in the E0.9 feature; one calibration with all garnet reference materials included (Fe3+/EFe up to 1.0; "all garnet calibration") and another calibration specific to garnets with low Fe3+/EFe ratios ("low ferric calibration"). Fe3+/EFe 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+/ EFe ratios measured by Mo center dot ssbauer. The standard error of the mean Fe3+/EFe ratio calculated from flank method approaches the Mo center dot ssbauer-determined Fe3+/EFe 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.