Chemical controls on niobium and zirconium mobility inferred from dissolution experiments on wöhlerite in alkaline silica-undersaturated melts

Abstract

The solubility of wöhlerite, ideally Na2Ca4ZrNb(Si2O7)2O3F, in peralkaline SiO2-undersaturated melts (Na2O–CaO–Al2O3–SiO2 ± F ± H2O) was studied at the following temperature (T) and pressure (P) conditions: 750 °C, 200 MPa, 850 °C, 100 MPa, and 1000 °C, 200 MPa. Wöhlerite appeared to be stable up to high temperature, with increasing solubility, which is consistent with field observations of early wöhlerite crystallization in pegmatites in the Larvik Plutonic Complex. A lovozerite-group mineral (combeite–zirsinalite–lovozerite) was frequently observed to crystallize in the experiments at 750 °C, and, at very high Na/Al ratios, also at 850 °C. Fluorcalciopyrochlore formed at 1000 °C in experiments where water was added. The concentrations of Zr and Nb in the quenched glasses were generally very high and found to increase strongly with temperature. Moreover, they also increase with addition of water and with increasing peralkalinity. The effect of CaF2 addition was insignificant. Based on comparison with the limited information in the available literature, the solubilities of Ca-bearing Zr and Nb minerals (wöhlerite, lovozerite-group minerals, fluorcalciopyrochlore) in peralkaline silica-undersaturated melts may be lower than those of nominally Ca-free Zr and Nb minerals (zircon, baddeleyite, wadeite, columbite-(Mn)). The experimental data do not support the existence of significant ZrF or NbF complexation. The high concentrations of Nb and Zr in the reacted peralkaline melts can be explained by strongly enhanced formation of alkali-Nb/Zr-silicate complexes compared to that at peraluminous conditions. These complexes are the underlying chemical reason for characteristic high-field-strength-element (HFSE) enrichment in agpaitic and hyperagpaitic rocks.