Os and U-Th isotope signatures of arc magmatism near Mount Mazama, Crater Lake, Oregon

Interaction of mantle melts with the continental crust can have significant effects on the composition of the resulting melts as well as on the crust itself, and tracing this interaction is key to our understanding of arc magmatism. Lava flows and pyroclastic deposits erupted from similar to 50 to 7.7 ka at Mt. Mazama (Crater Lake, Oregon) were analyzed for their Re/Os and U-Th isotopic compositions. Mafic lavas from monogenetic vents around Mt. Mazama that erupted during the buildup to its climactic eruption have lower Os-187/Os-188 ratios (0.1394 to 0.1956) and high Th-230 excess ((Th-230/U-238)(0) of 1.180 to 1.302), whereas dacites and rhyodacites tend to have higher Os-187/Os-188 ratios (0.2292 to 0.2788) and significant U-238 excess ((Th-230/U-238)(0) of 0.975 to 0.989). The less radiogenic Os isotope compositions of the mafic lavas can be modeled by assimilation of young (similar to 2.5 to 7 Ma), mafic lower crust that was modified during regional extension, whereas the more radiogenic Os isotope compositions of the dacites and rhyodacites can be attributed to assimilation of older (similar to 10 to 16 Ma), mid to upper crust that acquired its composition during an earlier period of Cascade magmatism. Production of Th excesses in the lower crust requires very young garnet formation accompanying dehydration melting in the lower crust at less than a few 100 ka by heat from recent basaltic magma injection. The results from this study suggest that the combination of Os and Th isotopes may be used to provide insights into the timescales of evolution of the continental crust in arc settings, as well as the influence of the crust on erupted magmas, and suggest a link between the age and composition of the lower and upper crust to regional tectonic extension and/or earlier Cascade magmatism. (C) 2015 Elsevier B.V. All rights reserved.