A furnace design for XANES spectroscopy of silicate melts under controlled oxygen fugacities and temperatures to 1773 K

A controlled-atmosphere furnace has been constructed for X-ray absorption spectroscopy experiments under imposed oxygen fugacities at temperatures up to 1773 K. The use of the furnace is demonstrated in a study of the oxidation state of Cr in a basaltic silicate melt (mid-ocean ridge basalt) by K-edge XANES spectroscopy. This is the first time the Cr²⁺/Cr³⁺ ratio has been identified directly in an Fe-bearing melt. At typical terrestrial oxygen fugacities around half the Cr is present as Cr²⁺, even though this oxidation state has never been identified in a terrestrial material and only Cr³⁺ is observed after quenching to a glass. Cr²⁺ oxidizes to Cr³⁺ on cooling in the presence of Fe³⁺ according to the electron exchange reaction Cr²⁺ + Fe³⁺ → Cr³⁺ + Fe²⁺. This illustrates the importance of the in situ determination of metal oxidation states in melts.