Understanding the mechanisms of Nb mineralization in carbonatites is critical for addressing the growing demand for this strategic metal in high-tech industries. This study presents new experimental constraints on Nb solubility in carbonate melts, shedding light on the magmatic processes that control Nb ore formation. High-temperature crystallization and dissolution experiments were conducted at 0.1 GPa and 875 ~ 1025°C under oxidized conditions (log fO₂ ~ FMQ+3) using synthetic CaCO₃–Na₂CO₃–Ca(OH)₂–(F)–Nb₂O₅ mixtures. Experimental charges were encapsulated in Au/AuPd capsules and processed in internally heated pressure vessels (IHPV) with rapid quenching at ISTO, CNRS, Orléans, France. Starting compositions systematically varied Ca/Na ratios (0.5 ~ 2.0) and F contents (0 ~ 7.5 wt.%) to quantify their effects on Nb solubility. Equilibrium was verified through dissolution experiments using natural pyrochlore (Vishnevogorskoe deposit) as the Nb source.
Our results reveal that Nb solubility is strongly influenced by temperature and Ca/Na molar mass ratio, with F playing a secondary role. A robust multiple linear regression model (K_sp (Nb_2 O_5 )= EXP(-(11.86±1.01)×1000/T+(0.67±0.06)×ln⁡〖Ca/Na〗+(9.82±0.89)±0.34)+5.15×√(0.28&F^* )) was developed to predict Nb solubility as a function of melt composition. Phase equilibria modeling demonstrates that fractional crystallization of calcite and fluorite drives residual melts toward Na-rich compositions, triggering pyrochlore saturation within a narrow thermal window (700 ~ 735 °C). Textural evidence from SEM/EPMA analyses confirms late-magmatic pyrochlore growth under disequilibrium conditions induced by rapid Na enrichment.
These findings reconcile discrepancies in previous experimental data, showing that Nb solubility variations are primarily controlled by Ca/Na ratios and temperature. By establishing a quantitative framework linking carbonate melt evolution to Nb ore formation, this study provides critical insights into the magmatic processes that control Nb mineralization. Our findings may offer a robust tool for interpreting carbonatite-hosted Nb ore petrogenesis and guiding exploration strategies for high-grade Nb deposits.