Syneruptive mixing, degassing, and crystallization at Redoubt Volcano, eruption of December, 1989 to May 1990

The 1989–1990 eruption of Redoubt Volcano included initial energetic vertical venting of pumiceous tephra followed by shedding of dense pyroclastic flows from successive unstable domes. The suite of quenched magmatic materials thus produced, coupled with detailed seismic observations, provides a record of establishment and subsequent evolution of an active magma column in the upper crust. The initial eruptive event involved a dacite magma with rhyolite melt and an andesite magma with dacite melt. The dacite magma contains reaction rim-free hornblende, high Cl concentrations (0.12±0.02 wt.% of melt), and is microlite-free: all evidence for rapid ascent from depth. In contrast, the andesite component is microlite-rich, though this characteristic may have arisen by mixing between the two magmas rather than by slow ascent. Abundant dense juvenile lava-like clasts, with low H2O (0.1–0.3 wt.%), were ejected in the initial eruption event, implying that the magma column had reached the near surface and its upper portion had undergone degassing and bubble collapse prior to explosive disruption. Later eruptive events contain evidence of progressive blending of the two magmatic components, as well as mixing between fresh reservoir-derived and old conduit-resident magma. Magma apparently ascended to the surface from the 6–10-km-deep source chamber in a series of pulses, rather than by steady flow or gradual emptying of a shallowly intruded body. Increased shallow long-period seismicity, which may indicate subsurface degassing from the flowing magma column, occurred ≤5 days before each major eruptive event. Petrologic evidence indicates that each eruption contained some magma that ascended quickly from the chamber. With time, the proportion of new magma in eruptive batches may have decreased. However, even the final dome contains magma that left the chamber no more than a few days before extrusion. Together, the pulses apparently record passage of an andesite plume through Redoubtʹs dacite magma reservoir.