Fe K-edge X-ray absorption near-edge structure (XANES) and Mössbauer spectra were collected on natural basaltic glasses equilibrated over a range of oxygen fugacity (QFM - 3.5 to QFM 4.5). The basalt compositions and fO2 conditions were chosen to bracket the natural range of redox conditions expected for basalts from mid-ocean ridge, ocean island, back-arc basin, and arc settings, in order to develop a high-precision calibration for the determination of Fe3 /[summation operator]Fe in natural basalts. The pre-edge centroid energy, corresponding to the 1s --> 3d transition, was determined to be the most robust proxy for Fe oxidation state, affording significant advantages compared to the use of other spectral features. A second-order polynomial models the correlation between the centroid and Fe3 /[summation operator]Fe, yielding a precision of ± 0.0045 in Fe3 /[summation operator]Fe for glasses with Fe3 /[summation operator]Fe > 8%, which is comparable to the precision of wet chemistry. This high precision relies on a Si (311) monochromator to better define the Fe2 and Fe3 transitions, accurate and robust modeling of the pre-edge feature, dense fO2-coverage and compositional appropriateness of reference glasses, and application of a non-linear drift correction. Through re-analysis of the reference glasses across three synchrotron beam sessions, we show that the quoted precision can be achieved (i.e., analyses are reproducible) across multiple synchrotron beam sessions, even when spectral collection conditions (detector parameters or sample geometry) change. Rhyolitic glasses were also analyzed and yield a higher centroid energy at a given Fe3 /[summation operator]Fe than basalts, implying that major variations in melt structure affect the relationship between centroid position and Fe3 /[summation operator]Fe, and that separate calibrations are needed for the determination of oxidation state in basalts and rhyolites.
