Models for the estimation of Fe³⁺/Fe_tot ratio in terrestrial and extraterrestrial alkali- and iron-rich silicate glasses using Raman spectroscopy

To develop Raman spectroscopy as a quantitative tool in both geosciences and planetary sciences the effect of iron oxidation state (Fe³⁺/Fe_tot) on the Raman spectra of basaltic and pantelleritic glasses has been investigated. We have used remelted pantellerite from Pantelleria Island and synthetic iron-rich basaltic glasses [from Chevrel et al. (2014)]. The Raman spectra of pantelleritic glasses reveal dramatic changes in the high wavelength region of the spectrum (800-1200 cm^-1) as iron oxidation state changes. In particular the 970 cm^-1 band intensity increases with increasing oxidation state of the glass (Fe³⁺/Fe_tot ratio from 0.24 to 0.83). In contrast, Raman spectra of the basaltic glasses do not show the same oxidation state sensitivity (Fe³⁺/Fe_tot ratio from 0.15 to 0.79). A shift, however, of the 950 cm^-1 band to high wavenumber with decreasing iron oxidation state can be observed. We present here two empirical parameterizations (for silica- and alkali-rich pantelleritic glasses and for iron-rich basaltic glasses) to enable estimation of the iron oxidation state of both anhydrous and hydrous silicate glasses (up to 2.4 wt% H₂O). The validation of the models derived from these parameterizations have been obtained using the independent characterization of these melt samples plus a series of external samples via wet chemistry. The "pantelleritic" model can be applied within SiO₂, FeO, and alkali content ranges of ∼69-75, ∼7-9, and ∼8-11 wt%, respectively. The "basaltic" model is valid within SiO₂, FeO, and alkali content ranges of ∼42-54, ∼10-22, and ∼3-6 wt%, respectively. The results of this study contribute to the expansion of the compositionally dependent database previously presented by Di Genova et al. (2015) for Raman spectra of complex silicate glasses. The applications of these models range from microanalysis of silicate glasses (e.g., melt inclusions) to handheld in situ terrestrial field investigations and studies under extreme conditions such as extraterrestrial (i.e., Mars), volcanic, and submarine environments.