MAGMA ASCENT RATES FROM AMPHIBOLE BREAKDOWN - AN EXPERIMENTAL-STUDY APPLIED TO THE 1980-1986 MOUNT ST-HELENS ERUPTIONS

Recent 1980-1986 Mount St. Helens dacites contain the phenocryst assem blage, plagioclase, amphibole, low-Ca pyroxene, magnetite, ilmenite, a nd rare high-Ca pyroxene, which indicates that they all originated fro m an 8 km deep reservoir at 900-degrees +/- 20-degrees-C with X(H2O) = 0.67 in fluid according to experimental data. Iron-titanium oxide phe nocryst compositions indicate that all post May 18 dacitic magmas erup ted at 900-degrees +/- 20-degrees-C except for the final lava extrusio n in October 1986; the magma reservoir may have cooled to 866-degrees- C by October 1986. Amphiboles in the post May 18, 1980, magma contain one or more amphibole populations characterized by reaction rims of di fferent thicknesses. The development of the amphibole reaction rims in these rocks is a response to water loss from the coexisting melt duri ng an approximately adiabatic ascent from a deep reservoir. Constant P and T and isothermal decompression experiments show that during a 900 -degrees-C constant rate decompression from 8 km to the surface, no re action film develops on amphibole in 4 days, a 10-mum nm develops in 1 0 days, and a 35-mum rim develops in 20 days. These experimental data and histograms of film widths in 1980-1986 Mount St. Helens dacites sh ow that post May 18 eruptions are composed in large part of magma repr esented by a population of thin-rimmed amphiboles, magma which ascende d from the deep (8 km) reservoir in 6 to 10 days. The remainder of eac h sample consists of magma containing amphiboles with reaction rims ra nging from 14 to 60 mum, magma which apparently spent from 8 to 25 day s along the conduit margins before being mixed thoroughly (millimeter scale) into the erupting magma. The mixing in a viscous, slowly ascend ing dacite may be enhanced by its flow through partially crystallized magma emplaced earlier and by the evolution and loss of a large vesicl e population. The experimental calibration of amphibole reaction rim w idth versus decompression time yields average ascent velocities for po st May 18 dacites of about 15-30 m/h for magma represented by the thic k-rimmed amphiboles and from 35 to 50 m/hr for magma represented by th e thin-rimmed crystals. An ascent rate of >66 m/h is indicated for the May 18, 1980, eruption, which contains amphiboles with no reaction ri ms. The volume of endogenous dome growth which preceded extrusion of m agma newly derived from the deep source region suggests that the effec tive conduit volume beneath Mount St. Helens in 1981-1982 was equivale nt to a cylinder 8 km long and 8-9 m in radius.