The oxidation state of chromium in basaltic silicate melts

Abstract

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 + ∑cZXZ. where XZ are the mole fractions of the oxide components Z defined on the single-cation basis and ∑cZXZ = 2.00 XNaO0.5 + 1.00 (XMgO + XFe2+O) + 0.37 (XAlO1.5 + XFe3+O1.5) + 2.12 XKO0.5 + 2.44 XCaO + 3.69 XTiO2, 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 + ∑jZXZ. where ∑jZXZ = 1.00 (XMgO + XFe2+O) + 0.37 (XAlO1.5 + XFe3+O1.5) −1.59 XKO0.5 + 0.04 XCaO + 3.69 XTiO2. 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+.