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

We present experimentally determined trace element partition coefficients (D) between pyrochlore-group minerals (Ca(2)(Nb,Ta)(2)O(6)(O,F)), Ca fersmite (CaNb(2)O(6)), and silicate melts. Our data indicate that pyrochlores and fersmite are able to strongly fractionate trace elements during the evolution of SiO(2)-undersaturated magmas. Pyrochlore efficiently fractionates Zr and Hf from Nb and Ta, with D(Zr) and D(Hf) below or equal to unity, and D(Nb) and D(Ta) significantly above unity. We find that D(Ta) pyrochlore-group mineral/silicate melt is always higher than D(Nb), 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(Th) is higher than corresponding D(U) 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.