| Summary: | Young igneous rocks from a range of tectonic settings exhibit various Uranium-series (Useries) disequilibria. The U-series systematics of Mid-ocean-ridge basalts (MORBs) have been attributed to mantle melting, while those of arc basalts are widely thought to be slab fluid signatures. Mid-ocean-ridge and arc U-series models generally assume that U4+ is the only oxidation state relevant to mantle melting processes, however the potential for the stabilisation of U5+ and/or U6+ has recently been proposed for some arc lavas. To determine the oxidation state of U in geological melts, and to assess the relative stabilities of U4+, U5+, and U6+ under mantle conditions, X-ray absorption near edge structure (XANES) spectra were recorded from synthetic analogues, including an Fe-bearing MORB composition. Spectra were collected from quenched glasses equilibrated at 1400 °C and oxygen fugacities (fO2s) between logfO2 = -18 and +4.7 (QFM-11.7 to QFM+11), and from equivalent high temperature melts using a custom designed XAS furnace. Spectra were collected at both the U L3- and M4-edges, as the variation of the M4-edge spectral line-shape could be unambiguously linked to oxidation state changes in the glasses, while the higher energy of the L3-edge was better suited to in situ studies. The variation of the XANES spectra as a function of fO2 allowed U5+ to be identified as a major component in both the MORB glasses and their original melts, and a methodology was developed to accurately quantify their U oxidation state proportions. The proportion of U4+ was found to be highly sensitive to fO2 at conditions relevant to mantle melting, with U5+/ΣU varying between ~0.1 and 0.5 between QFM-1 and QFM+2 and pressures equivalent to mantle depths of ≥ 15 km. U-series models assuming melting of a variably oxidised mantle wedge can produce a wide range of U-series signatures that are consistent with many arc basalts. In contrast, the stability of even small proportions of U5+ in the melt may present a problem for those models currently attributing the 230Th excesses of MORBs to mantle melting in the spinel lherzolite field.
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