Characterization of recent pyroclastic density currents and determination of magma residence times in historic and pre-historic eruptions of Galeras volcano, Colombia: a contribution to the hazard assessment

Abstract

Textural and chemical patterns in plagioclase phenocrysts of three pre-historic and two 21st century eruptions of the andesitic Galeras volcano, Colombia, have been used to determine the residence time of magma within the subvolcanic system. Near-equilibrium, oscillatory zoned plagioclase and variably textured zones indicating dissolution / resorption, evidence changes in pressure, temperature, composition, and magma flux during crystal growth. Modelling of Sr and Ba diffusion in plagioclase, which crystallized at an average temperature of 975 °C, indicates that all five eruptions preserve evidence of at least two phases of magma chamber development. Oscillatory zoned grains give magma residence times of 10 to 81 years. Grains with disequilibrium textures always indicate a much shorter residence time in the subvolcanic chamber, generally less than 10 years. These results suggest that the initial magma chamber received additional fluxes of phenocryst bearing magma no more than a decade before eruption.