Europium is a redox variable element that occurs as both Eu2+ and Eu3+ in the Earth's crust. The oxidation state will affect its partitioning between coexisting minerals, melts and fluids and hence its behaviour in magmatic processes. The oxidation state of Eu in a natrocarbonatite melt was determined in situ over a range of temperatures (730, 780 and 830 ˚C) and oxygen fugacities (quartz-fayalite-magnetite, QFM, -5 to ~8) using Eu L-edge X-ray absorption near edge structure (XANES) spectra. The intensities of peaks comprising the absorption edge were found to scale with the proportion of Eu3+ present. Eu3+/EuTOT was quantified by fitting the intensity of each feature as a function of oxygen fugacity to the thermodynamically expected relationship. Increasing temperature stabilises Eu3+ at a greater rate than the change in QFM. This means that temperature changes at a constant oxygen fugacity relative to QFM will produce changes in Eu3+/EuTOT in a carbonatite melt, whereas little or no change occurs for Eu3+/EuTOT in a silicate melt and most other redox pairs. For example, at 1200 ˚C and QFM+1.3, Eu3+/EuTOT = 0.97 in a nephelinite and 0.76 in a carbonatite, while at 750 ˚C Eu3+/EuTOT in the silicate melt would be effectively unchanged but increase to 0.98 in the carbonatite melt. Eu3+/EuTOT can be predicted in carbonatite melts at atmospheric pressure by the empirical equation:

log(Eu3+/Eu2+) = 9861/T + 0.25(DQFM +8.58 – 25050/T) – 4.42

where T is temperature in K. At the average oxygen fugacity of carbonatite melts (QFM+1.3) and temperature range (1100-700 ˚C) Eu3+ will be the dominant oxidation state