Abstract
Basaltic lavas from the Three Sisters and Dalles Lakes were erupted from two isolated vents in the central Washington Cascades at 370-400 k.y. and 2.2 m.y. respectively, and have distinct trace element compositions. The Three Sisters calc-alkaline basalts have trace element contents typical of most arc magma: high LIL/HFSE ratios and strong negative Nb and Ta anomalies. In contrast, the Dalles Lakes lavas have relatively low LIL/HFSE and no Nb or Ta anomalies, similar to OIB. Nearly all Washington Cascade basalts with high to moderate incompatible trace element contents show this compositional distinction, there is a pronounced divergence between the two magma types. This distinction is explained by a model of flux-melting of the mantle wedge by a fluid-rich subduction component (SC) in which the degree of melting of the peridotite source is correlated with the amount of SC added to it.