Calibration for iron redox state and oxygen fugacity in silicate glasses from x-ray absorption spectroscopy

Abstract

Prior work to quantify glass Fe-redox ratios in situ using x-ray absorption spectroscopy (XAS) has relied heavily on the pre-edge region. The pre-edge-based methods do not take advantage of other information in the spectra above the pre-edge, up to and past the main edge. This study updates the previous calibration (Dyar et al., 2016a) with additional experimental glasses spanning a wide range of geologically relevant compositions (SiO2 weight % = 38.5–78.0), equilibrated over a range of oxygen fugacity (fO2) values from reducing (IW-2) to oxidizing (air), for which Fe XAS analysis has been collected for the whole spectral region (pre-edge, main edge, EXAFS). The resulting dataset was used to evaluate multiple multivariate statistical models for their ability to accurately predict log fO2 or %Fe3+. Accuracies are estimated using cross-validated root mean square error, for which purpose unseen test data are held out from the regressions. Datasets covering small compositional ranges were tested and compared with datasets covering broad compositional ranges. These ‘stratified’ sets improved predictive accuracy in some cases; however, the extra complications associated with data stratification did not justify the small improvement in accuracy. Overall, a simple partial least squares (PLS) model based on all data collected over the entire XAS region was found to result in a universal calibration for determination of log fO2 and %Fe3+ in glasses.