Excess argon in plagioclase crystals from young (<4000 ybp) tephra layers from Mount St. Helens, Washington, illustrates the importance of argon isotopes in understanding magmatic processes. 40Ar/39Ar step-heating identifies two distinct argon reservoirs in the plagioclase; a high Cl/K, low Ca/K reservoir with atmospheric 40Ar/36Ar and a low Cl/K, higher Ca/K reservoir with variable 40Ar/36Ar. The first is probably glass, whereas the second is true plagioclase. Felsic dacite plagioclases have little or no excess argon, but those from more mafic layers have significantly higher 40Ar/36Ar, indicating a non-atmospheric source. These variations are seen within a layer that contains both mafic and felsic dacite, and are inconsistent with either xenocrystic or restitic origin for plagioclase. The magma chamber exhibits long-term open-system degassing behavior punctuated by short-term fluctuations from influx of high 40Ar/36Ar basalt. The relative timing between mixing and eruption can affect the amount of excess argon recorded in plagioclase. ¿ 2001 American Geophysical Union