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

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 (fo₂ ) at 1400 °C. Values of Cr²⁺/∑Cr 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⁺/∑Cr could be quantified as a function of fo₂, using a theoretical expression, from as few as three samples (Cr²⁺/∑Cr ≈ 0, 0.5, and 1). This allowed logK¹, or the reduction potential of the Cr³⁺/²⁺ half-reaction, and hence the relative change in the ratio γ_Cr^melt3 +_O1.5/γ^melt_Cr²⁺_O, to be determined for each composition. At constant fo₂, log[Cr²⁺/Cr³⁺] was found to decrease linearly with increasing optical basicity. The variation in logK¹ with composition is controlled by γ_Cr^melt 3+O_1.5, 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⁺/∑Cr to be quantified in a Fe-bearing melt for the first time. Cr²⁺/∑Cr was found to vary from ∼0.45 at the nickel-nickel oxide (NNO) fo₂ buffer to ∼0.90 at iron-wüstite (IW). This indicates that Cr²⁺ is likely to be the dominant oxidation state in terrestrial basaltic melts.