BUILDING CONTINENTS IN MAGMATIC OROGENS: NB/TA SYSTEMATICS IN ARC MAGMA DIFFERENTIATION AND THE ROLE OF ARCLOGITES (Invited Presentation)

Most of the crust on Earth and other rocky planets is basaltic, but Earth differs from other planets in that it also has extensive regions of felsic crust, manifested in the form of continents. Exactly how felsic crust forms when basaltic magmas are the dominant products of melting the mantles of rocky planets is unclear. Insights into crust formation processes may come from element pairs whose ratios remain constant during mantle melting but fractionate during crustal differentiation. The similar Nb/Ta ratios (~15) of the mantle, mid-ocean ridge basalts and their derivative liquids indicate that Nb and Ta behave identically during mantle melting and basaltic differentiation. However, the continental crust has a significantly lower Nb/Ta ratio of 11-13. The origin of this low Nb/Ta ratio and the identity of the necessary high Nb/Ta complementary reservoir have remained elusive. Here, we show that Nb/Ta fractionation can happen during intracrustal differentiation in thick continental arcs, where subducting slabs descend beneath thick crust. In island arcs, where the crust is thin, Nb/Ta does not fractionate from ~15 until extreme differentiation is attained (SiO₂ > 70 wt.%), whereas in mature continental arcs, the decline in Nb/Ta occurs earlier, in magmas of intermediate (andesitic) compositions (SiO₂ > 60 wt.%). We further show that pyroxenite cumulates in deep continental arcs (arclogites) have high Nb/Ta (average ~19) due to the presence of high Nb/Ta magmatic rutile. This crustal thickness controlled Nb/Ta fractionation results from the pressure dependence of rutile saturation in silicate melts. Together with their high iron and low silica contents, these high Nb/Ta deep arc cumulates are likely candidates for the “missing” components that drive the formation of felsic continental crust on Earth. Deep-seated fractionation in magmatic orogens, such as continental arcs, is thus necessary for making continents, which in turn suggests a strong link between continent formation and plate tectonics.