An experimental study of Nb and Ta partitioning between Ti-rich minerals and silicate liquids at high pressure and temperature

Experimental determination of partition coefficients (D) for Nb and Ta between Ti-rich minerals (sphene, rutile, ilmenite and Ti-magnetite) and coexisting silicate liquids of basaltic andesite, andesite and trachyte composition, indicate that these elements partition strongly into sphene and rutile, and moderately into ilmenite and Ti-magnetite. The D values for sphene increase with decreasing temperature and with increasing SiO₂ content of the coexisting liquid, but appear unchanged by pressure between 7.5 and 16 kb, and oxygen fugacity between magnetite-wustite and haematite-magnetite buffers. Also, D_Nb D_Ta. varies from 0.3-0.4 for sphene, increasing to 0.6-0.7 for rutile, 0.7-0.8 for ilmenite and 0.8 for Ti-magnetite. Thus fractionation involving these minerals may cause an increase in the Nb Ta ratio of any derivative liquid; this will be most noticeable where sphene is involved.