Compositions of andesitic to dacitic melts in equilibrium with plagioclase, orthopyroxene, and clinopyroxene have been experimentally calibrated as a geohygrometer. Application to Melson's data on melt inclusions and matrix glasses from the 1980–1982 cycle of eruptions of Mount St. Helens reveals systematic dewatering trends—ca. 4% (1980) to 1% (1982) H2O in subliquidus (intratelluric) melt in pyroclastics and 2%–3% (1980) to 1% (1982) in subliquidus melt in dome-building dacites. Before each pyroclastic event, intratelluric crystallization achieved melt H2O contents of ca. 6% (at a depth of at least 8 km), a necessary condition for crystallization of amphibole. Dewatering trends indicate that individual magma blobs or ribbons then ascended to successively higher levels over the 3-yr cycle, from a minimum depth of 4 km in 1980 to 0.35 km in 1982, and pyroclastically erupted.
Comparison of Mount St. Helens compositions to worldwide andesites and dacites suggests that near-liquidus arc magmas that crystallize plagioclase and pyroxenes contain 1%–2% H2O, on average, whereas magmas in continental margins contain appreciably more H2O.