The oxidation states of niobium and tantalum in mantle melts

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 1.5 GPa. 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.

► Nb K and Ta LIII-edge XANES spectra recorded for silicate melts as a function of fO2.
► Nb5 + and Ta5 + are the only oxidation states in melts at upper mantle P, T, and fO2.
► The fractionation of Nb from Ta cannot be attributed to reduced valence states.