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 microp robe define isochrons that discriminate between different crystallization a ges of granitoid xenoliths in lavas erupted 1065 and 2000 years ago from Me dicine Lake volcano, CA, USA. Zircon ages indicate at least two intrusive e pisodes, similar to 25 and similar to 90 ka, at times when silicic volcanis m was rare, but basaltic volcanism was prevalent. Ar-Ar spectra require tha t the granitoids were completely crystalline thousands of years prior to th eir mobilization and eruption. These techniques demonstrate that individual zircon crystals can form rapidly enough to provide unique U-Th ages, and a llow dating of <300 ka xenoliths from volcanic eruptions. (C) 2000 Elsevier Science B.V. All rights reserved.