Quantitative Analysis of Fe³⁺/∑Fe in Silicate Glasses Based on XPS Fe 3p Core-Level Spectroscopy

Abstract

Natural silicate glasses are widespread in diverse geological and planetary settings. The oxidation state of iron (Fe) preserved within these glasses is highly sensitive to variations in oxygen fugacity (fO₂) during their formation, providing critical constraints on magmatic evolution, volcanic processes, and planetary surface oxidation histories. X-ray photoelectron spectroscopy (XPS) is a crucial technique for determining the valence of Fe. Conventional approaches using Fe 2p XPS are often limited in complex silicate systems due to overlapping peaks and interference from satellite structures, which could reduce the accuracy of spectral fitting. Fe 3p XPS spectra, on the other hand, are more suitable for quantitative analysis due to their straightforward spectral characteristics. Despite the benefit, a systematic quantitative framework and standardized calibration method for Fe 3p analysis have not been fully developed yet. In this study, a quantitative analytical method has been developed for quantification of Fe³⁺/ΣFe ratio from Fe 3p XPS spectra, using a suite of basaltic synthetic glasses as reference materials. We have systematically investigated the effects of binding energy position, peak fitting strategy, beam spot size, and scan repetition on spectral stability and quantification of Fe³⁺/ΣFe ratio. Based on these evaluations, a robust correlation has been established between the Fe³⁺/ΣFe ratio and the Fe 3p peak area ratio. Two universal calibration curves have been constructed under different beam spot sizes. The reliability of the two curves has been validated using a leave-one-out cross-validation method, yielding root mean square errors (RMSE) of 0.03 and 0.04, respectively. These results showcase a practical Fe valence quantification method of Fe 3p-based XPS for complex silicate glasses, highlighting the potential for integrating advanced spectroscopy methods into broader geological research frameworks.