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 (fo(2)) at 1400 degrees C. Values of Cr2+/Sigma 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+/Sigma Cr could be quantified as a function of fo(2), using a theoretical expression, from as few as three samples (Cr2+/Sigma Cr approximate to 0, 0.5, and 1). This allowed logK ', or the reduction potential of the Cr3+/2+ half-reaction, and hence the relative change in the ratio gamma(Cr3)+(melt)(O1.5)/gamma(Cr2)+(melt)(O), to be determined for each composition. At constant fo(2) log[Cr2+/Cr3+] was found to decrease linearly with increasing optical basicity. The variation in logK ' with composition is controlled by gamma(Cr3)+(melt)(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 degrees C for a synthetic mid-ocean ridge basalt (MORB) composition, allowing Cr2+/Sigma Cr to be quantified in a Fe-bearing melt for the first time. Cr2+/Sigma Cr was found to vary from similar to 0.45 at the nickel-nickel oxide (NNO) fo(2) buffer to similar to 0.90 at iron-wustite (IW). This indicates that Cr2+ is likely to be the dominant oxidation state in terrestrial basaltic melts.