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

Abstract

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/melt D Nb and rt/melt D Ta increase by almost one order of magnitude with SiO 2 content and polymerization, but decrease with TiO 2 content in the melt. The ratio rt/melt D Nb/ D Ta 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/melt D Nb/ D Ta is smaller than unity can be explained by a slightly smaller ionic radius of Ta 5+ than Nb 5+ and thus a preferred incorporation of Ta into rutile. The variation of rt/melt D Nb, rt/melt D Ta, and rt/melt D Nb/ D Ta strongly depends on melt composition without any significant correlation with rutile composition. The strong positive correlation of rt/melt D Nb and rt/melt D Ta with rt/melt D Ti and SiO 2 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/melt D Nb/ D Ta with rt/melt D Ti and SiO 2 contents is more difficult to understand and might be related to the higher polarizibility of Nb 5+ compared to Ta 5+. 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.