• journal_title：American Mineralogist
• Contributor：Andrew J. Berry ; Hugh St.C. O’Neill ; Dean R. Scott ; Garry J. Foran ; J.M.G. Shelley
• Publisher：Mineralogical Society of America
• Date：2006-
• Format：text/html
• Language：en
• Identifier：10.2138/am.2006.2097
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：91
• issue：11-12
• firstpage：1901
• section：Articles

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 Cr ²⁺/∑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 f_O2, using a theoretical expression, from as few as three samples (Cr²⁺/∑Cr 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[Cr²⁺/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⁺/∑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) 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.