Recent 1980-1986 Mount St. Helens dacites contain the phenocryst assemblage, plagioclase, amphibole, low-Ca pyroxene, magnetite, ilmenite, and rare high-Ca pyroxene, which indicates that they all originated from an 8 km deep reservoir at 900¿¿20 ¿C with XH2O=0.67 in fluid according to experimental data. Iron-titanium oxide phenocryst compositions indicate that all post May 18 dacitic magmas erupted at 900¿¿20 ¿C except for the final lava extrusion in October 1986; the magma reservoir may have cooled to 866 ¿C by October 1986. Amphiboles in the post May 18, 1980, magma contain one or more amphibole populations characterized by reaction rims of different thicknesses. The development of the amphibole reaction rims in these rocks is a response to water loss from the coexisting melt during an approximately adiabatic ascent from a deep reservoir. Constant P and T and isothermal decompression experiments show that during a 900 ¿C constant rate decompression from 8 km to the surface, no reaction rim develops on amphibole in 4 days, a 10-μm rim develops in 10 days, and a 35-μm rim develops in 20 days. These experimental data and histograms of rim widths in 1980-1986 Mount St. Helens dacites show that post May 18 eruptions are composed in large part of magma represented by a population of thin-rimmed amphiboles, magma which ascended from the deep (8 km) reservoir in 6 to 10 days.

The remainder of each sample consists of magma containing amphiboles with reaction rims ranging from 14 to 60 μm, magma which apparently spent from 8 to 25 days along the conduit margins before being mixed thoroughly (millimeter scale) into the erupting magma. The mixing in a viscous, slowly ascending dacite may be enhanced by its flow through partially crystallized magma emplaced earlier and by the evolution and loss of a large vesicle population. The experimental calibration of amphibole reaction rim width versus decompression time yields average ascent velocities for post May 18 dacites of about 15--30 m/h for magma represented by the thick-rimmed amphiboles and from 35 to 50 m/hr for magma represented by the thin-rimmed crystals. An ascent rate of >66 m/h is indicated for the May 18, 1980, eruption, which contains amphiboles with no reaction rims. The volume of endogenous dome growth which preceded extrusion of magma newly derived from the deep source region suggests that the effective conduit volume beneath Mount St. Helens in 1981-1982 was equivalent to a cylinder 8 km long and 8--9 m in radius. ¿ American Geophysical Union 1993