Abstract
The surfaces of rocky planets are mostly covered by basaltic crust, but Earth is unique 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. A fundamental part of the debate is centered on the low Nb/Ta of Earth’s continental crust (11–13) compared to basalts (15–16). Here, we show that during arc magma differentiation, the extent of Nb/Ta fractionation varies with crustal thickness with the lowest Nb/Ta seen in continental arc magmas. Deep arc cumulates (arclogites) are found to have high Nb/Ta (average ~19) due to the presence of high Nb/Ta magmatic rutiles. We show that the crustal thickness control of Nb/Ta can be explained by rutile saturation being favored at higher pressures. Deep-seated magmatic differentiation, such as in continental arcs and other magmatic orogens, is thus necessary for making continents.
Highlights
The surfaces of rocky planets are mostly covered by basaltic crust, but Earth is unique in that it has extensive regions of felsic crust, manifested in the form of continents
We show that Nb and Ta can fractionate during arc magma differentiation, but the fractionation primarily happens in arcs built on thickened crust—magmatic orogens
We find that those rutile-bearing cumulates possess high Nb/Ta ratios that are complementary to the low Nb/ Ta continental crust
Summary
The surfaces of rocky planets are mostly covered by basaltic crust, but Earth is unique in that it has extensive regions of felsic crust, manifested in the form of continents. 1234567890():,; On average, continental crust and subduction zone magmas have remarkably similar major and trace element compositions, suggesting an intrinsic relationship between continent growth and arc magmatism[1,2]. Both evolved arc magmas and continental crust exhibit low Nb/ Ta ratios[1,3,4]. We turn to the Nb/Ta systematics of deep arc cumulates, represented by garnet pyroxenite xenoliths from Arizona, USA, which crystallized at 45–80 km[6] We find that those rutile-bearing cumulates possess high Nb/Ta ratios that are complementary to the low Nb/ Ta continental crust
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