Genesis of Intermediate and Silicic Arc Magmas Constrained by Nb/Ta Fractionation.

Resolving the geochemical discrepancies in Nb/Ta between the bulk continental crust and its parental mantle melts can provide insights into the processes by which basaltic magmas differentiated into the felsic crust. Deep‐seated garnet pyroxenite cumulates (i.e., arclogite) have high Nb/Ta values due to the presence of high Nb/Ta‐rutile. For this reason, rutile‐bearing arclogites having Nb‐rich composition were proposed to complement the low Nb/Ta nature of the continental crust and evolved arc magmas, that is to say, continental arc magma evolution dominantly occurred in the stability field of garnet. However, this suggestion is challenged by the incomplete understanding of the role of rutile in the Nb/Ta fractionation during arc magma evolution due to insufficient partitioning data at low temperatures (T ≤ 1,000°C). Here, we determined Nb and Ta partitioning ratios in rutile‐felsic melt systems at 2.0 GPa, 850–1,000°C and ∼4–20 wt.% H2O. Our results confirm that equilibrium crystallization of rutile with respect to Nb and Ta will not impart lower Nb/Ta values to any derivative liquids, whereas diffusive fractionation of Nb/Ta in siliceous melts during the growth of rutile can reduce Nb/Ta ratios in melts. However, a positive correlation between Nb/Ta and Dy/Yb for mature continental arc magmas, incipient continental arc magmas, island arc magmas, and global arc magmas indicates that amphibole rather than rutile‐bearing arclogite was responsible for causing low Nb/Ta ratios of many evolved intermediate rocks. Accordingly, the chemical differentiation from primary basaltic to average andesitic compositions of the continental crust mainly occurred in the stability field of amphibole. Key Points: Equilibrium crystallization of rutile will not reduce melt Nb/Ta, whereas Nb/Ta diffusive fractionation in melt is a viable mechanismNb/Ta correlates positively with Dy/Yb for arc magmas, suggesting fractionation of amphiboles, rather than rutile‐bearing arclogitesThe chemical differentiation of continental arc magmas mainly occurred in the amphibole stability field [ABSTRACT FROM AUTHOR]