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Iron redox works well for constraining oxygen fugacity(    f    O  2)$\left( {{f}_{{{\text{O}}_{2}}}} \right)$    in terrestrial igneous materials due to the relatively highf    O  2${{f}_{{{\text{O}}_{2}}}}$    of the Earth’s atmosphere, crust, and upper mantle[f    O  2≥$\left[ {{f}_{{{\text{O}}_{2}}}}\ge  \right.$    quartz-fayalite-magnetite (QFM)], where there are large changes in Fe 2+ /Fe 3+  with relatively small changes inf    O  2.        At     f    O  2${{f}_{{{\text{O}}_{2}}}}.\,\,\text{At  }{{f}_{{{\text{O}}_{2}}}}$    values &lt;QFM, Fe redox becomes less sensitive, and analytical uncertainties may make it difficult to determinef    O  2${{f}_{{{\text{O}}_{2}}}}$    differences between samples. The valence change between Cr 2+  and Cr 3+  occurs at lowerf    O  2${{f}_{{{\text{O}}_{2}}}}$    values than for iron, potentially making it a more sensitive oxybarometer for materials equilibrated under reducing conditions. The current approach to measuringf    O  2${{f}_{{{\text{O}}_{2}}}}$    from X-ray absorption (XAS) measurements derives Cr valence first from the 1 s →4 s  transition and then uses that redox couple to estimatef    O  2${{f}_{{{\text{O}}_{2}}}}$    as a function of temperature and composition. Here, that method is compared to an alternate approach of predictingf    O  2${{f}_{{{\text{O}}_{2}}}}$    directly from the spectra of experimentally homogenized glasses of geological relevance without an intermediate step of attempting to discern Cr 2+ /Cr 3+ . 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 predictingf    O  2${{f}_{{{\text{O}}_{2}}}}$    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 inlow-   f    O  2$\text{low-}{{f}_{{{\text{O}}_{2}}}}$    glasses with a cross-validated (RMSE-CV) accuracy of ±0.84 log unitsf    O  2${{f}_{{{\text{O}}_{2}}}}$    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 off    O  2${{f}_{{{\text{O}}_{2}}}}$    values using Cr oxybarometry.

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