The oxidation state of chromium in basaltic silicate melts

Cr³⁺/Cr²⁺ in melts in the systems CaO-MgO-Al₂O₃-SiO₂ ± Na₂O ± K₂O doped with Cr added as 0.5 wt% Cr₂O₃ were determined as a function of oxygen fugacity (fO₂) at 1400 °C by XANES spectroscopy of their quenched glasses, using the intensity of the shoulder on the Cr K-edge due to the 1 s → 4 s transition. The addition of Na and K to the system CMAS increases Cr³⁺/Cr²⁺ at constant temperature and fO₂, in good agreement with the predictions from the “ideal optical basicity”. The new results have been combined with previous results to calibrate a model for Cr³⁺/Cr²⁺ in silicate melts as a function of temperature, pressure and melt composition: log₁₀(Cr³⁺/Cr²⁺) = 1/4 ΔQFM + 3031/T − 2.26 + (843P – 158 P²)/T + ∑c_ZX_Z. where X_Z are the mole fractions of the oxide components Z defined on the single-cation basis and ∑c_ZX_Z = 2.00 X_NaO0.5 + 1.00 (X_MgO + X_Fe2+O) + 0.37 (X_AlO1.5 + X_Fe3+O1.5) + 2.12 X_KO0.5 + 2.44 X_CaO + 3.69 X_TiO2, T is temperature in K, P is pressure in GPa, and ΔQFM is the difference between the fO₂ of the silicate melt and the Quartz-Fayalite-Magnetite buffer at 10⁵ Pa, given by log₁₀fO₂(QFM) = 8.58–25050/T, relative to the conventional standard state of pure O₂ at 10⁵ Pa. The effect of pressure is markedly non-linear, so the model should not be extrapolated above 4 GPa. Combining this model with the similar one for Fe³⁺/Fe²⁺ in silicate melts gives: log₁₀(Cr³⁺/Cr²⁺) = log₁₀(Fe³⁺/Fe²⁺) + 3031/T − 0.9 + (1317P – 219 P²)/T + ∑j_ZX_Z. where ∑j_ZX_Z = 1.00 (X_MgO + X_Fe2+O) + 0.37 (X_AlO1.5 + X_Fe3+O1.5) −1.59 X_KO0.5 + 0.04 X_CaO + 3.69 X_TiO2. High Cr²⁺/∑Cr in silicate melts is promoted by high temperature and low pressure, as well as low Fe³⁺/∑Fe. For a parental MORB melt composition at 1250 °C (1523 K), 10⁵ Pa, with Fe³⁺/∑Fe = 0.10, the model predicts Cr²⁺/ΣCr = 0.27. The effect of pressure is very large: the Cr²⁺/ΣCr in the above example would drop to 0.03 at 2 GPa and 1250 °C. The Cr²⁺ present in Fe³⁺-containing melts at magmatic temperatures decreases on cooling because of the electron exchange reaction: Cr²⁺ + Fe³⁺ = Cr³⁺ + Fe²⁺.