Hydrothermal experiments have been conducted on the ∼3.5 ka cummingtonite-bearing Yn tephra from Mount St. Helens, Washington, to determine the stability field of cummingtonite in volcanic rocks and to gather additional data on the evolutionary history of the Mount St. Helens magmatic system. Cummingtonite is not stable in experiments conducted at or above 810 °C at pressures <4 kbar; it is stable between 2 and 3 kbar at 790 °C. Iron-oxide compositions from natural cummingtonite-bearing volcanic rocks also indicate an upper thermal stability limit of about 790 to 800 °C for cummingtonite. The experiments show that the Yn tephra last equilibrated at temperatures of 770 to 790 °C and water pressures of 2.5 to 3.0 kbar. This is confirmed by ion-probe H2O analyses of melt inclusions that yield preeruption melt H2O contents of 6.4 wt%.
The Yn tephra belongs to a series of cummingtonite-bearing dacites that erupted early in the history of Mount St. Helens. These early dacites have relatively homogeneous compositions and are characterized by low temperature (∼800 °C), high water pressure, and high oxygen fugacity (>NNO +1) phenocryst crystallization. Silicic magmas that erupted after eruption of basaltic andesite at Mount St. Helens (∼2.3 ka) are more heterogeneous in composition and last equilibrated under higher temperatures (∼900 °C), lower water pressures, and lower oxygen fugacities than the early dacites. These changes in the magma chamber conditions appear to have been related to the intrusion of mafic magma into an upper crustal dacitic magma chamber.