Key Factors Controlling Biotite-Silicate Melt Nb and Ta Partitioning: Implications for Nb-Ta Enrichment and Fractionation in Granites

Biotite-melt Nb and Ta partition coefficients (D-Nb and D-Ta) are crucial for understanding Nb-Ta enrichment and fractionation in rare metal granites (RMGs). However, the key factor(s) affecting biotite-melt D-Nb, D-Ta, and D-Nb/D-Ta values remain unclear. To elucidate the physicochemical factors that control the partition coefficients, we performed piston-cylinder experiments at 0.5-1.0 GPa and 850-1000 degrees C with H2O-added (4-10 wt.%) mixtures of granitic and biotitic glasses as starting materials. Two series of experiments (graphite-buffered and unbuffered fO(2) conditions) were conducted with calculated fO(2) values ranging from similar to FMQ-1.5 to similar to FMQ+4. Under these experimental conditions, biotite-melt DNb, DTa, and DNb/ DTa values are 0.30-2.63, 0.24-1.02, and 1.01-2.15, respectively. Biotite-melt D-Nb, D-Ta, and D-Nb/D-Ta values increase with decreasing melt NBO/T value (non-bridging oxygens per tetrahedron), melt H2O content, and biotite Mg# T value [molar 100 x MgO/(MgO + FeOT)]. In addition, D-Nb and D-Ta exhibit good correlations with D-Ti, suggesting that D-Nb and D-Ta are predictable via D-Ti values. With our and literature data, we used multiple linear regressions to obtain empirical expressions of D-Nb and D-Ta as functions of the three parameters. By applying the empirical models to granite differentiation process, we found that similar to 99% crystallization of biotite +/- muscovite-bearing assemblages results in an enrichment in magma Ta contents by >10 times with a decrease in Nb/Ta values from 10-13 to similar to 1, which reproduces the Ta-Nb/Ta features of most RMGs. However, additional processes, such as columbite-group mineral precipitation, may be required to account for the extremely low Nb/Ta values (<1) of some RMGs. Plain Language Summary Niobium (Nb) and tantalum (Ta) are twin elements with similar properties. However, in rare metal granites (RMGs) with economic concentrations of Nb and Ta, the Nb/Ta ratios are generally <5 or even <1, which are much lower than the Nb/Ta ratios of any other Earth reservoirs. Biotite is a major Nb- and Ta-bearing phase during granite differentiation. Therefore, understanding biotitemelt Nb and Ta partitioning is crucial for explaining the enrichment and fractionation of Nb and Ta in granites. Our experiments show that, compared with mafic-intermediate melts, Nb and Ta are much more compatible in biotite in felsic melts. The D-Nb and D-Ta values (i.e., ratios of the element concentrations in biotite vs. those in melt) also increase with decreasing biotite Mg#(T) value [molar 100 x MgO/(MgO + FeOT)] and melt H2O content. The D-Nb/D-Ta ratio is generally >1 and increases as D-Nb and D-Ta increase. By applying the compositionally dependent partition coefficients to granite differentiation, we found that fractionation of a biotite-bearing assemblage is the main cause for the low Nb/Ta ratios of most RMGs. However, this process cannot explain the extremely low Nb/Ta ratios of <1 in some RMGs.