Isotopic records of deep and shallow magmatic evolution of Mount Mazama, Crater Lake, Oregon

Crater Lake caldera (southern Oregon) was formed by the climactic eruption of Mt. Mazama at 7.7 ka. This dissertation presents three related studies of U-Th, O, and Re-Os variations recorded in lavas, pyroclastic rocks, and plutonic wall rocks erupted during the buildup to the climactic eruption of Mt. Mazama, with an emphasis on exploring the development of the climactic magma system at deep and shallow crustal levels. The high proportion of mafic lavas erupted in the Crater Lake region and the extended upper crustal history of the caldera-forming magma chamber allow us to better assess the influence of both lower and upper crustal processes on petrogenesis, as well as the age and composition of the continental crust. Chapter 1 is focused on the role of the lower crust, particularly young, mafic crust, in moderating and attenuating ascent of magmas from the mantle. Modeling of U-Th isotope and trace element compositions of volcanic rocks erupted from ∼36 to 7.8 ka suggests that strong 230Th-excesses were generated by garnet formation in the lower crust via dehydration melting, a process which has been under-appreciated in arc systems, but is likely widespread. Chapter 2 provides an "outside in" view of the buildup to the climactic eruption via in situ oxygen and whole-rock U-Th isotope ratios measurements in granitoid wall rocks of the magma chamber. Oxygen self-diffusion models of oxygen isotope zoning present in quartz crystals indicate partial melting of the granitoids occurred within decades to centuries of the climactic eruption, possibly reflecting injection of the final batches of hot, mafic to andesitic recharge into the magma chamber. Finally, Chapter 3 is aimed at evaluating the age and composition of the crust beneath Crater Lake using Os and U-Th isotope measurements in lavas and pyroclastic rocks erupted from 73 to 7.7 ka. We document age heterogeneity between the lower and mid- to upper-crust beneath Mt. Mazama, where lower crustal rocks that contributed to Mazama magmas were ∼4 Ma in age, followed by young (<300 ka) garnet-formation during dehydration melting, whereas upper crustal rocks that interacted with the shallow magma chamber were ∼15 Ma in age.