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ABSTRACT Quantification of Fe redox state and hydrogen content of amphibole provides information regarding the relationship between oxygen and water concentrations in terrestrial and planetary materials. Raman spectroscopy is a powerful technique due to its ability to characterize both %Fe³⁺and H₂O from single crystal measurements, in addition to other chemical, mineralogical, and structural properties. Raman spectral measurements of amphibole minerals are used here to estimate %Fe³⁺(relative to total Fe) and H₂O (wt%) contents using partial least squares (PLS) multivariate modeling. The accuracy of our model for prediction of %Fe³⁺is ± 8.11% (absolute) expressed as root‐mean‐square error (RMSE) of the entire data set, covering the range from 0 to 100% with anR²value of 0.85. The model for prediction of H₂O has an internal RMSE of ± 0.09 wt% over the range from 0.1 to 1.9 wt% with anR²value of 0.95. Additional compositional model variables for predicting FeO, Fe₂O₃, MgO, CaO, Cr₂O₃, Al₂O₃, and TiO₂have highR²values above 0.82; theR²value for SiO₂is 0.63. Reliable models could not be achieved for MnO, Na₂O, and K₂O. The successful creation of our compositional models along with detailed analysis of the PLS model coefficients indicates that Raman spectroscopy can be used as a quantitative compositional tool in characterizing the amphibole mineral group. Quantifying amphibole compositions is useful for evaluating repositories of hydrogen, constraining the water budget of the terrestrial crust and interior, developing geothermobarometers and geohygrometers, and quantifying magma ascent rates. 

Abstract Oxygen fugacity is an important but difficult parameter to constrain for primitive arc magmas. In this study, the partitioning behavior of Fe3+/Fe2+ between amphibole and glass synthesized in piston-cylinder and cold-seal apparatus experiments is developed as an oxybarometer, applicable to magmas ranging from basaltic to dacitic composition. The partitioning of Fe2+ is strongly dependent on melt polymerization; the relative compatibility of Fe2+ in amphibole decreases with increasing polymerization. The Fe2+/Mg distribution coefficient between amphibole and melt is a relatively constant value across all compositions and is, on average, 0.27. The amphibole oxybarometer is applied to amphibole in mafic enclaves, cumulates, and basaltic tephra erupted from Shiveluch volcano in Kamchatka with measured Fe3+/FeTotal. An average Fe3+/Fe2+ amphibole-glass distribution coefficient for basalt is used to convert the Fe3+/FeTotal of amphibole in samples from Shiveluch to magmatic oxygen fugacity relative to NNO. The fO2 of primitive melts at the volcano is approximately NNO+2 and is faithfully recorded in amphibole from an amphibole-rich cumulate and the basaltic tephra. Apparently, higher fO2 recorded by amphibole in mafic enclaves likely results from partial dehydrogenation of amphibole during residence in a shallow andesite storage region. We identify three pulses of mafic magma recharge within two weeks of, a month before, and two to three months before the eruption and find that, at each of these times, the host andesite was recharged by at least two magmas at varying stages of differentiation. Application of the amphibole oxybarometer not only gives insight into magmatic fO2 but also potentially details of shallow magmatic processes.