U–Th dating of single zircons from young granitoid xenoliths: new tools for understanding volcanic processes

Multiple U–Th isotopic analyses of individual zircon crystals by ion microprobe define isochrons that discriminate between different crystallization ages of granitoid xenoliths in lavas erupted 1065 and 2000 years ago from Medicine Lake volcano, CA, USA. Zircon ages indicate at least two intrusive episodes, ∼25 and ∼90 ka, at times when silicic volcanism was rare, but basaltic volcanism was prevalent. Ar–Ar spectra require that the granitoids were completely crystalline thousands of years prior to their mobilization and eruption. These techniques demonstrate that individual zircon crystals can form rapidly enough to provide unique U–Th ages, and allow dating of <300 ka xenoliths from volcanic eruptions.