|Title:||Source versus differentiation controls on U‐series disequilibria: insights from Cotopaxi Volcano, Ecuador|
|Authors:||J.M. Garrison, J.P. Davidson, M.R. Reid, and S.P. Turner|
|Publication:||Earth Planet. Sci. Lett., v. 244, p. .|
|Publish Date:||Apr 2006|
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.