Excess argon in Mount St. Helens plagioclase as a recorder of magmatic processes

Excess argon in plagioclase crystals from young (< 4000 ybp) tephra layers from Mount St. Helens, Washington, illustrates the importance of argon isot opes in understanding magmatic processes. Ar-40/Ar-39 step-heating identifi es two distinct argon reservoirs in the plagioclase;, a high Cl/K, low Ca/K reservoir with atmospheric Ar-40/Ar-36 and a low Cl/K, higher Ca/K reservo ir with variable Ar-40/Ar-36. The first is probably glass, whereas the seco nd is "true" plagioclase. Felsic dacite plagioclases have little or no exce ss argon, but those from more mafic layers have significantly higher 40Ar/3 6Ar, indicating a non-atmospheric source. These variations are seen within a layer that contains both mafic and felsic dacite, and are inconsistent wi th 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 Ar-40/Ar-63 basalt. The relative timing b etween mixing and eruption can affect the amount of excess argon recorded i n plagioclase.