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

Abstract

Abstract 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°C with H 2 O‐added (4−10 wt.%) mixtures of granitic and biotitic glasses as starting materials. Two series of experiments (graphite‐buffered and unbuffered f O 2 conditions) were conducted with calculated f O 2 values ranging from ∼FMQ–1.5 to ∼FMQ+4. Under these experimental conditions, biotite–melt D Nb , D Ta , and D Nb /D Ta 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 H 2 O content, and biotite Mg# T value [molar 100 × MgO/(MgO + FeO T )]. 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 ∼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 ∼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.

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