Nb and Ta incorporation and fractionation in titanian pargasite and kaersutite: crystal-chemical constraints and implications for natural systems

New partition coefficients between liquid and pargasitic/kaersutitic amphiboles ((Amph/L)D(Nb,Ta)) experimentally determined for Nb and Ta at upper-mantle conditions, combined with single-crystal structure refinement of the synthesised amphiboles, show that (Amph/L)D(Nb,Ta) are strongly dependent on the structure and composition of both amphibole and melt. The correlation of the (Amph/L)D(Nb,Ta) with the amphibole oxy-component is explained by the ordering of Nb and Ta at the M1 site, contributing with the fraction of Ti at M1 to loclaly balance the (O3)O2- ⇆ (O3)(OH)- substitution. In our set of dehydrogenated amphiboles, variations in the SiO2 content of the melt from 41.5 to 54.6 correspond to a six-fold increase of the (Amph/L)D(Nb,Ta), in which (Amph/L)D(Nb) varies from 0.14 to 0.71 and (Amph/L)D(Ta) from 0.11 to 0.54. Partition coefficients for Nb and Ta abruptly increase in Ti-depleted compositions ((Amph/L)D(Nb) up to 1.63 and (Amph/L)D(Ta) to 1.00). The ratio of D(Nb) to D(Ta) ((Amph/L)D(Nb/Ta)) varies from 0.71 to 1.63, and is a function of the M1 site dimension, which in turn depends on its Fe, Mg and Ti contents. The observed variations can be explained by assuming that the ionic radius of Nb is (~0.01-0.02 Å) larger than that of Ta, contrary to the common assumption that they are both equal to 0.64 Å. We calibrated a simplified model for the prediction of (Amph/L)D(Nb/Ta) values shown to be negatively related mainly to mg [Mg/(Mg+Fe)] and to Ti content. High-mg amphiboles have (Amph/L)D(Nb/Ta) close to unity, so the low Nb/Ta found in convergent margin volcanics and in the continental crust cannot be explained by the involvement of amphibole in the mantle wedge. Amphibole in the subducting slab may have lower mg and consequently high Nb/Ta values, and thus may give rise to subchondritic Nb/Ta values in coexisting melts. Nb/La is also negatively correlated with mg, and strongly increases in Ti-depleted compositions.