Source versus differentiation controls on U-series disequilibria: Insights from Cotopaxi Volcano, Ecuador

Abstract

Although the majority of young volcanic rocks in island arcs typically have 238U excesses, continental arc rocks display both 238U and 230Th excesses. In fact, there is a global correlation between the sense of U-series disequilibria and crustal thickness that suggests that crustal thickness may somehow influence the processes that fractionate U from Th. At Cotopaxi Volcano, Ecuador, ( 238U) / ( 230Th) values of 1.03–1.14 in rhyolites are attributed to accessory phase fractionation, whereas ( 238U) / ( 230Th) values of 0.96–1.07 in andesites can be explained by several potential processes, including melting of garnet-bearing lower crust. The Cotopaxi rocks have non-fractionated HFSE ratios and La / Yb values that are consistent with melting of a garnet-bearing lithology, and we suggest a model of lower crustal melting and assimilation to account for the range of U-series data in the Cotopaxi andesites. Mantle like 87Sr / 86Sr and 143Nd / 144Nd values indicate that the assimilant was a relatively juvenile and/or mafic component. The rhyolites contain apatite and allanite, fractionation of which can generate 238U excesses during crystallization, and modeling shows that 70–90% crystallization of an assemblage containing these phases could generate the observed 238U excesses. These data suggest that multi-level AFC processes contribute to magma evolution at Cotopaxi Volcano as magma traverses the continental crust of the Northern Volcanic Zone. On the basis of the 238U– 230Th– 226Ra data, the time for assimilation and ascent of the andesites was < 8000 yr, whereas the rhyolites may have resided in the crust for 70–100 ky. The modification of U–Th isotopic signatures may be a common feature of differentiated magmas in continental arc settings and such potential effects should be considered along with interpretations involving variable mantle sources and melting regimes.