Utilizing Raman spectroscopy to constrain Fe3+/Fe2+ ratios in silicate glasses: Implications for terrestrial and extraterrestrial investigations.

Abstract:

Silicate melt structure is affected by Fe speciation. Knowledge of the redox Fe³⁺/Fe²⁺ ratio of a silicate system can provide constraints on physical properties including viscosity and oxygen fugacity (f_O2). Raman spectroscopy has the potential to quantify these ratios in glasses similar to the amorphous phases likely to be encountered by the Mars 2020 and ExoMars rovers, provided a robust calibration exists. Recently, 79 silicate glass samples (basaltic to rhyolitic in composition) were equilibrated at f_O2 values ranging from highly oxidizing (CO₂, Air) to more reducing (IW/Mo-MoO_2,QFM). Raman spectra were excited at 532 nm and subjected to multivariate analysis using the Data Exploration, Visualization, and Analysis for Spectroscopy (DEVAS) program. The acquired Raman spectra of these samples are shown in the accompanying figure. When plotted as a function of Fe³⁺/Fe_tot, the acquired spectra exhibit distinct variability in peak position and band intensity as a function of both sample composition and f_O2. Overall, band intensity increases as iron oxidation state (i.e., Fe³⁺ concentration) decreases. A peak shift towards higher wavenumbers as %Fe³⁺ decreases is also observed. The most reduced samples along the QFM and IW/Mo-MoO₂ buffers trend toward higher intensities, while the oxidized glasses along the CO₂ and Air buffers exhibit lower intensity features. As silica content increases between the samples, band intensity generally drops off, and peak position shifts towards lower wavenumbers. Specifically, in basaltic samples, the 770, 870, 980, and 1550 cm^-1 peaks shift to ~ 775, 875, 990, and 1555 cm^-1 respectively as %Fe³⁺decreases. In rhyolitic glasses, the 700 – 900 cm^-1 region results in higher intensities than lower concentration silica samples, while the 980 cm^-1 peak generally shows lower intensities, and similar peak shifts are observed. An optimized partial lease squares regression (PLS) model was applied to the spectra to determine Fe³⁺/SFe, using a described component (q) value of both 6 and 10. Individual compositional ranges yield significantly better RMSE values and R²predictions than does the full spectrum. When plotted against Mössbauer %Fe3+, model R² values with q = 10, range from 0.981 – 1.00 for all singular rock types along all f_O2’s, and 0.9766 – 1.00 along individual buffers.