Trace element partitioning between pyrochlore, microlite, fersmite and silicate melts.

We present experimentally determined trace element partition coefficients (D) between pyrochlore-group minerals (Ca(Nb,Ta)O(O,F)), Ca fersmite (CaNbO), and silicate melts. Our data indicate that pyrochlores and fersmite are able to strongly fractionate trace elements during the evolution of SiO-undersaturated magmas. Pyrochlore efficiently fractionates Zr and Hf from Nb and Ta, with D and D below or equal to unity, and D and D significantly above unity. We find that D pyrochlore-group mineral/silicate melt is always higher than D, which agrees with the HFSE partitioning of all other Ti-rich minerals such as perovskite, rutile, ilmenite or Fe-Ti spinel. Our experimental partition coefficients also show that, under oxidizing conditions, D is higher than corresponding D and this implies that pyrochlore-group minerals may fractionate U and Th in silicate magmas. The rare earth element (REE) partition coefficients are around unity, only the light REE are compatible in pyrochlore-group minerals, which explains the high rare earth element concentrations in naturally occurring magmatic pyrochlores.