Partitioning of Nb and Ta between rutile and carbonatitic melts: Implications for genesis of arc magmas and carbon cycle at subduction zones

Abstract

Carbonatitic melts are crucial media in element transfer at subduction zones, however, their role in Nb/Ta fractionation is unclear. Here we experimentally determined the rutile-carbonatitic melt partition coefficients (D values) of Nb and Ta at 2.5–8.0 GPa and 1000–1300 ℃. The results show that D_Nb ranges from 17 to 79, D_Ta from 39 to 393, and D_Nb/D_Ta from 0.20 to 0.27. D_Nb and D_Ta strongly depend on pressure and temperature, decreasing initially and then increasing as pressure/temperature rises. Conversely, the narrow range of D_Nb/D_Ta suggests that this ratio is insensitive to either pressure or temperature. Using these new partition coefficients, we modeled behaviors of Nb and Ta in carbonatitic melts and residual rutile-bearing eclogites during slab melting. Our results demonstrate that slab melting imparts lower Nb/Ta ratios to rutile-bearing eclogites and higher Nb/Ta ratios to carbonatitic melts than their protoliths. Consequently, deeply subducted rutile-bearing eclogites cannot serve as a superchondritic Nb/Ta reservoir that could explain Earth’s Nb paradox. We quantify the contribution of slab-derived carbonatitic melts on the Nb/Ta variations of natural carbonatite assuming that their mantle sources have been impregnated by such melts. The results indicate slab-derived carbonatitic melts explain less than two orders of magnitude of Nb/Ta variation in carbonatites. After fully clarifying the Nb/Ta fractionation in various types of slab-derived liquids, we identify that the mantle source of silica-undersaturated, high Nb/Ta arc magmas is metasomatized by slab-derived carbonatitic melts, while those of silica-saturated magmas are by silicic melts originating from the basaltic part of the slab.