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

¹ Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
² Australian Synchrotron Research Program, Australian Nuclear Science and Technology Organisation, PMB 1, Menai, NSW 2234, Australia
³ Australian National Beamline Facility, KEK, Photon Factory, Oho 1-1, Tsukuba-shi, Ibaraki-ken 305-0801, Japan

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 ^melt_Cr³⁺O1.5/^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 ^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.

Key Words: XANES spectroscopy • chromium oxidation states • silicate melts • MORB

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