Estimation of Fe3+/Fetot. ratio in natural silicate glasses and analogues for extra-terrestrial basalt using Raman spectroscopy

Abstract:

The effect of iron oxidation state (Fe³⁺/Fe_tot.) on the Raman spectra of pantelleritic (from Pantelleria island) and basaltic glasses (from Etna) and synthetic analogues for extra-terrestrial basaltic glasses (iron-rich martian basalt analogues; Chevrel et al. 2014) has been investigated. The Raman spectra of pantellerite glasses show dramatic changes in the high wavelength region of the spectrum (800-1200cm^-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 (natural and synthetic) do not show the same oxidation state sensitivity as the pantelleritic samples (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.

To help develop Raman spectroscopy as a quantitative tool in both geosciences and planetary science we present here an empirical, compositionally-independent model, based on an ideal mixing equation applied to the acquired Raman spectra. This model yields estimates of the iron oxidation state of anhydrous and hydrous silicate glasses of basaltic and pantelleritic composition for Fe³⁺/Fe_tot. ranging between 0.15 and 0.83 and water contents up to 2.4 wt.%. The model has been validated using independently characterized natural and synthetic silicate glasses (both anhydrous and hydrous) with a FeO content varying from ~8 to ~22 wt%.

The results of this study contribute to increase the compositionally-dependent database previously presented by Di Genova et al. (2015) for Raman spectra of complex silicate glasses. The applications of this model range from microanalysis of silicate glasses (e.g. melt inclusions) to handheld in situ terrestrial field studies and under extreme conditions (e.g. extraterrestrial, volcanic and submarine environments).