Decompression and degassing, repressurization, and regassing during cyclic eruptions at Guagua Pichincha volcano, Ecuador, 1999–2001

Abstract In 1999–2001, Guagua Pichincha volcano, Ecuador, produced a series of cyclic explosive and effusive eruptions. Rock samples, including dense blocks pumiceous clasts collected during the eruption sequence, ballistic bombs later from crater floor, provide information about magma storage, ascent, decompression, degassing, repressurization, regassing prior to eruption. Pairs Fe-Ti oxides indicate equilibrium within 1.2–1.5 log units above NNO oxidation buffer temperatures 805 905 °C. Melt inclusions record H 2 O contents 2.7–4.6 wt% CO (uncorrected for segregation into bubbles) 19 310 ppm. Minimum melt inclusion saturation pressures fall between 69 168 MPa, or equilibration depths 2.8 6.8 km, lower end which is coincident with maximum inferred most vesicular breadcrust sampled. Amphibole phenocrysts lack breakdown rims (except one sample) plagioclase have abundant oscillatory compositional zones. Plagioclase areal microlite number densities ( N ) range over less than order magnitude (8.9×10 3 –8.7×10 4 mm -2 among all exception dense, low crystallinity sample = 3.0×10 −2 erupted on 17 December 1999 1.7×10 ). shapes include tabular, hopper, swallowtail forms. Taken together, relatively high densities, zones in phenocrysts, presence groundmass glass, seismicity, time-varying tilt cycles picture sudden evacuation residing at different levels shallow conduit. Explosive eruptions punctuate inter-eruptive repose periods marked by rates degassing (volatile fluxing) re-pressurization. Shallow residence time conduit was sufficient allow precipitation silica-phase groundmass, but insufficient hornblende final dome 2000, has longest preceding time. These results support model pressure cycling, volatile exsolution regassing, decompression decoupled ascent.