Nb/Ta systematics in arc magma differentiation and the role of arclogites in continent formation

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.

Fate of the lower lithosphere during shallow-angle subduction: The Laramide example

“…As with the sub-SNB suite, garnet clinopyroxenite xenoliths from Chino Valley and Camp Creek are arclogitic in composition, and equilibrated between 600 and 900 °C and 12-28 kbar (45-100 km depth assuming a 2800 kg/m 3 overburden density; Smith et al, 1994;Esperança et al, 1988;Erdman et al, 2016). Furthermore, high field strength element compositions and major element systematics of Arizona xenoliths balance those expected from calc-alkaline magmatic differentiation (Tang et al, 2018(Tang et al, , 2019. Finally, Mesozoic plutons of the Mojave Desert and arclogite recovered from Chino Valley and Camp Creek all share similar isotope systematics, with 87 Sr/ 86 Sr and eNd values ranging from 0.706 to 0.711 and −2 to −10, respectively (Esperança et al, 1988;Smith et al, 1994;Miller et al, 1996).…”

A Late Cretaceous felsic magmatic suite from the Tengchong Block, western Yunnan: integrated geochemical and isotopic investigation and implications for Sn mineralization

“…7b) from the GPM to the XMS and XFS, and this outcome indicates the fractional crystallization of Ti-bearing oxides, such as ilmenite and titanite (Liu et al 2009;Stepanov et al 2014). Ti-bearing oxides or silicates (micas and amphiboles) are the major hosts for Ta and Nb in granites (Stepanov et al 2014;Tang et al 2019). According to Stepanov et al (2014), fractionation of Ti-bearing oxides decreases the Ta/Nb ratios and Ta content (ilmenite and titanite), whereas biotite increases the Ta/Nb ratios and Ta content.…”

Effects of aseismic ridge subduction on the geochemistry of frontal arc magmas

“…The very low Nb and Ta contents of near equatorial volcanoes and the progressive increase going away from the equator require variable fractionation of rutile in the AFC modelling ( Fig. 6f: see also Tang et al, 2019) or, in the mixing model, very low Nb (2.2 ppm) and Ta (0.18 ppm) contents in the SiO2--rich melts for the volcanoes near the equator, increasing to 7--8 ppm Nb and 0.5--0.6 ppm Ta for the volcanoes farthest away from the equator (Fig. 6g--h).…”