title: A furnace design for XANES spectroscopy of silicate melts under controlled oxygen fugacities and temperatures to 1773 K.
abstract: 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 Cr(2+)/Cr(3+) ratio has been identified directly in an Fe-bearing melt. At typical terrestrial oxygen fugacities around half the Cr is present as Cr(2+), even though this oxidation state has never been identified in a terrestrial material and only Cr(3+) is observed after quenching to a glass. Cr(2+) oxidizes to Cr(3+) on cooling in the presence of Fe(3+) according to the electron exchange reaction Cr(2+) + Fe(3+) --> Cr(3+) + Fe(2+). This illustrates the importance of the in situ determination of metal oxidation states in melts.
authors.affiliation: Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia. andrew.berry@anu.edu.au
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Berry
authors.forename: J Michael G
authors.initials: JM
authors.lastname: Shelley
authors.forename: Garry J
authors.initials: GJ
authors.lastname: Foran
authors.forename: Hugh St C
authors.initials: HS
authors.lastname: O'Neill
authors.forename: Dean R
authors.initials: DR
authors.lastname: Scott