Use of compositional variations and in situ Sr-isotopes in plagioclase feldspars in arc rocks and mid-ocean ridge basalts to trace the effects of magma mixing and crustal contamination; use of U-series disequilibria (U, Th, Pa, Ra) in whole rocks and minerals to decipher the sources of slab-derived components in arc magmas, and the time scales and character of melting processes within the mantle wedge beneath ocean islands and continental arcs; use of major- and trace-element diffusional profiles in mineral phases to determine the time scales of magma mixing processes occurring in sub-volcanic reservoirs.