Abstract
Niobium K-edge and tantalum LIII-edge XANES spectra were recorded for silicate glasses quenched from melts equilibrated at oxygen fugacities (in log units relative to the iron–wüstite buffer) ranging from IW+6.7 to IW−4.3 at 1400–1650°C and atmospheric pressure to 1.5GPa. Niobium and tantalum were found to occur exclusively as Nb5+ and Ta5+. This suggests that changes in oxidation state do not contribute to the fractionation of Nb from Ta in arc settings or the generation of the hidden Nb-rich reservoir that is necessary to explain the deviation of the Nb/Ta ratio of the bulk silicate Earth from the chondritic value.
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