| Popis: |
Chromium K -edge X-ray absorption near-edge structure (XANES) spectra were recorded at room temperature for 27 CaO-MgO-Al 2 O 3 -SiO 2 (CMAS) glass compositions quenched from melts equilibrated at various oxygen fugacities ( f O 2 ) 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 Cr 2+ in these glasses. For each composition, Cr 2+ /∑Cr could be quantified as a function of f O 2 , using a theoretical expression, from as few as three samples (Cr 2+ /∑Cr 0, 0.5, and 1). This allowed log K ′, or the reduction potential of the Cr 3+/2+ half-reaction, and hence the relative change in the ratio γ melt Cr 3+ O 1.5 /γ melt Cr 2+ O , to be determined for each composition. At constant f O 2 , log[Cr 2+ /Cr 3+ ] was found to decrease linearly with increasing optical basicity. The variation in log K ′ with composition is controlled by γ melt Cr 3+ O 1.5 , corresponding to the capacity of the melt to stabilize both the charge and the preferred solvation site of Cr 3+ . The method was then applied to spectra recorded in situ at 1400 °C for a synthetic mid-ocean ridge basalt (MORB) composition, allowing Cr 2+ /∑Cr to be quantified in a Fe-bearing melt for the first time. Cr 2+ /∑Cr was found to vary from ~0.45 at the nickel-nickel oxide (NNO) f O 2 buffer to ~0.90 at iron-wustite (IW). This indicates that Cr 2+ is likely to be the dominant oxidation state in terrestrial basaltic melts. |
