Oxybarometry of reduced silicate glasses: Using multivariate methods to constrain Cr oxidation states with application to lunar glasses

Abstract Iron redox works well for constraining oxygen fugacity (fO2) in terrestrial igneous materials due to the relatively high fO2 of the Earth’s atmosphere, crust, and upper mantle [fO2≥ quartz-fayalite-magnetite(QFM)], where there are large changes in Fe2+/Fe3+ with relatively small changes in fO2. At fO2 values <QFM, Fe redox becomes less sensitive, and analytical uncertainties may make it difficult to determine fO2 differences between samples. The valence change between Cr2+ and Cr3+ occurs at lower fO2 values than for iron, potentially making it a more sensitive oxybarometer for materials equilibrated under reducing conditions. The current approach to measuring fO2 from X-ray absorption (XAS) measurements derives Cr valence first from the 1s → 4s transition and then uses that redox couple to estimate fO2 as a function of temperature and composition. Here, that method is compared to an alternate approach of predicting fO2 directly from the spectra of experimentally homogenized glasses of geological relevance without an intermediate step of attempting to discern Cr2+/Cr3+. In this study, partial least-squares (PLS) multivariate (MVA) regression models were trained on the whole XAS energy spectral range, and accuracy was quantified using root mean square error (RMSE). MVA results showed significantly higher accuracy (RMSE-C of ±0.75 log units) for predicting fO2 ΔIW relative to known experimental conditions relative to the two-step method, which yielded RMSE-C of ±2.75 to ±7.65 log units for our data set vs. those of Berry and O’Neill (2004) and Berry et al. (2006), respectively. The MVA results calibrate a new Cr oxybarometer for use in low-fO2 glasses with a cross-validated (RMSE-CV) accuracy of ±0.84 log units fO2 relative to a standard oxygen buffer. Finally, the new Cr oxybarometer was applied to lunar glasses, both volcanic and impact metamorphosed, to assess the range in oxidation conditions the materials experienced during formation. Lunar volcanic glasses cluster ∼IW±1, close to that of previous studies while agglutinates and lunar impact melts record a wide range of fO2 values using Cr oxybarometry.