Fractionation of Nb and Ta by biotite and phengite: implications for the “missing Nb paradox”

The subchondritic Nb/Ta in both the continental crust and the depleted mantle remains enigmatic and is called the “missing Nb paradox.” We present partitioning data between biotite and granitic melt for experimental and natural samples that provide evidence that Nb is compatible in biotite and phengite. Nb can thus be enriched in the residue during partial melting of crustal rocks. Additionally, biotite and phengite in equilibrium with granitic melts preferentially incorporate Nb over Ta. Therefore incipient partial melting of biotite-rich crustal rocks produces restites with high Nb/Ta. Progressive melting of such rocks leads to the consumption of biotite and the formation of peritectic rutile or ilmenite, which retain the high-Nb/Ta signature. We suggest that such mid to lower crustal granulites could represent an important Nb-rich reservoir with high Nb/Ta. Similarly, high-Ti phengite that is present in deeply subducted sediments preferentially incorporates Nb over Ta. High-pressure incipient partial melting in the presence of residual phengite thus also produces restites with high Nb/Ta that could be subducted to the deeper mantle.