The dependence of Nb and Ta rutile–melt partitioning on melt composition and Nb/Ta fractionation during subduction processes

Partition coefficients between rutile and silicate melts were determined experimentally for Nb and Ta with melt compositions varying from rhyolite to basalt. Experimental conditions were 1.7–2.5 GPa, 950–1300 °C, at oxygen fugacities between QFM−2 and QFM+3.5. Both rt/meltDNb and rt/meltDTa increase by almost one order of magnitude with SiO2 content and polymerization, but decrease with TiO2 content in the melt. The ratio rt/meltDNb/DTa is 0.45–0.55 for basaltic melt compositions, around 0.6 for andesitic melts and 0.8–1.0 for more silicic melts, remaining ≤1 for all examined silicate melts. The fact that rt/meltDNb/DTa is smaller than unity can be explained by a slightly smaller ionic radius of Ta5+ than Nb5+ and thus a preferred incorporation of Ta into rutile. The variation of rt/meltDNb, rt/meltDTa, and rt/meltDNb/DTa strongly depends on melt composition without any significant correlation with rutile composition. The strong positive correlation of rt/meltDNb and rt/meltDTa with rt/meltDTi and SiO2 contents is explained with the decreasing solubility of high charge cations in an increasingly polymerized melt where the concentration of non-bridging oxygens decreases. The positive correlation of rt/meltDNb/DTa with rt/meltDTi and SiO2 contents is more difficult to understand and might be related to the higher polarizibility of Nb5+ compared to Ta5+. Magmas resulting from slab melting with residual rutile are slightly Nb-enriched relative to Ta and do not explain the subchondritic Nb/Ta ratio of continental crust. Rutile in the residue during partial melting or dehydration of subducting crust is not capable of significantly enriching Nb over Ta in the residue. Excluding residual rutile as a reason for high Nb/Ta reservoirs outsources this problem to either partial melting of low-Mg amphibolite or metasomatic Nb-enrichment of rutile-bearing eclogite-lenses in the source region of kimberlites. However, both of these processes cannot produce Nb-enriched reservoirs sufficiently large to balance the silicate Earth's Nb/Ta ratio to chondritic, thus, this study supports previous suggestions that the “missing” Nb is stored in the core.