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Title: Oxidation state of uranium in mantle melts.
Authors: Berry, AJ
O'Neill, HSC
Foran, GJ
Keywords: Oxidation
Nickel Oxides
Issue Date: Jul-2008
Publisher: Elsevier; Cambridge Publications
Citation: Berry, A. J., O'Neill, H. S. C., & Foran, G. J. (2008). Oxidation state of uranium in mantle melts. 18th Annual V.M. Goldschmidt Conference (Goldschmidt 2008) - "from Sea to Sky", 13th - 18th July 2008. Vancouver, Canada: University of British Columbia. In Geochimica et Cosmochimica Acta, 72(12), A79.
Abstract: The oxidation state of U exerts a first order control on its partitioning during partial mantle melting. An understanding of U partitioning, and its abundance relative to other members of the U decay series, is important for interpreting U series disequilibrium, with implications for the rate of melt transport at mid-ocean ridges. U LIII-edge X-ray absorption near edge structure (XANES) spectra were recorded for a synthetic mid-ocean ridge basalt (MORB), and a number of CMAS (CaO-MgOAl2O3- SiO2) compositions, containing 0.5 wt % U3O8, equilibrated at 1400 ˚C and controlled oxygen fugacities (fO2)at one-atmosphere. Spectra were obtained for both quenched glasses, and in situ at magmatic temperatures (Berry et al. 2003). In situ measurements allowed changes in the spectra in response to the imposed fO2 to be followed in real time. The spectra were recorded in fluorescence mode at the Australian National Beamline Facility (beamline 20B), Photon Factory, Japan. The spectra exhibit systematic variations in absorption edge energy and crest intensity with fO2. These changes occur between -4 and +6 log units of the nickel-nickel oxide (NNO) fO2 buffer. The fO2 range of the spectral variation indicates that U4+ must oxidise to U6+ through two stepwise oneelectron reactions, involving a U5+ intermediate, rather than a direct two-electron process. The results also suggest that U5+ may be the dominant oxidation state at the fO2 conditions of MORB generation.
ISSN: 0016-7037
Appears in Collections:Conference Publications

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