Understanding crustal modification of magmas in volcanic arcs is a fundamental problem of igneous petrology and geochemistry. This dissertation examines the role of crustal processes including crustal assimilation, magma mixing, magma differentiation, and volatile transport in generating erupted magmas from four volcanic systems in the Kamchatka Volcanic arc: Bezymianny, Klyuchevskoy, Shiveluch, and Karymsky. High-precision Pb isotope data from the Central Kamchatka Depression (CKD) record lower crustal assimilation and magma mixing of less radiogenic crust and melt into the magmas of Bezymianny volcano. Unradiogenic Pb isotope compositions in CKD magmas suggest that lower crustal assimilation occurs to a larger extent beneath the CKD than in other segments of the Kamchatka arc. Klyuchevskoy compositional variations support this argument and also show that the degree of assimilation beneath an individual volcanic center may vary on a decadal timescale. Documented lower crustal assimilation by magmas beneath the CKD modifies uranium-series isotope disequilibrium compositions of CKD magmas. U-series isotopes are the only radioactive isotope system with half-lives useful for understanding the timing of volcanic processes. Constraining the affect of crustal processes that alter U-series compositions is essential to interpret isotope signatures. Chapter 2 of this dissertation shows that lower crustal assimilation and fractional crystallization of high-pressure clinopyroxene modifies U-excess to form Thexcess compositions. Global documentation of Th-excess reveals a correlation of Th-excess disequilibria with hot lower crustal geologic settings. Th-excess may therefore be an important chemical tracer of lower crustal assimilation. Shallow magma storage and degassing is investigated using (210Pb)/(226Ra) compositions at periodically erupting volcanoes with well-constrained degassing and eruptive histories. Current models that predict (210Pb)/(226Ra) as a function of gas loss do not accurately predict the observed compositions of erupted products in Kamchatka. This paradox has important implications for the timescales of magma storage and degassing in the crust. Chapter 4 of this work is a detailed isotopic study of environmental uranium contamination from an abandoned open-pit mine. Uranium isotope data fingerprint alteration of the mine source and transport of uranium from the mine to sediment in Red Rock Creek on National Forest Service recreational land
