The effect of composition on Cr²⁺/Cr³⁺ in silicate melts

Abstract

Chromium K-edge X-ray absorption near-edge structure (XANES) spectra were recorded at room temperature for 27 CaO-MgO-Al₂O₃-SiO₂ (CMAS) glass compositions quenched from melts equilibrated at various oxygen fugacities (f_O₂) at 1400 °C. Values of Cr²⁺/ΣCr were determined from the intensity of a shoulder on the main absorption edge, attributed to the 1s → 4s transition, which is characteristic of Cr²⁺ in these glasses. For each composition, Cr²⁺/ΣCr could be quantified as a function of f_O₂, using a theoretical expression, from as few as three samples (Cr²⁺/ΣCr ≈ 0, 0.5, and 1). This allowed logK’, or the reduction potential of the Cr^3+/2+ half-reaction, and hence the relative change in the ratio γ^melt_Cr³⁺O1.5/γ^melt_Cr²⁺O, to be determined for each composition. At constant f_O₂, log[Cr^2+/Cr3+] was found to decrease linearly with increasing optical basicity. The variation in logK’ with composition is controlled by γ^melt_Cr³⁺O1.5, corresponding to the capacity of the melt to stabilize both the charge and the preferred solvation site of Cr³⁺. The method was then applied to spectra recorded in situ at 1400 °C for a synthetic mid-ocean ridge basalt (MORB) composition, allowing Cr²⁺/ΣCr to be quantified in a Fe-bearing melt for the first time. Cr²⁺/ΣCr was found to vary from ~0.45 at the nickel-nickel oxide (NNO) f_O₂ buffer to ~0.90 at iron-wüstite (IW). This indicates that Cr²⁺ is likely to be the dominant oxidation state in terrestrial basaltic melts.