Thermobarometry, vesicularity and microlite populations in pyroclastic density current deposits from the last 4500 years of activity at Galeras, Colombia

Abstract

Vulcanian eruptions during the 19th and 20th Centuries at the Galeras volcano have produced small eruption columns and bomb fields around the active vent. Four pyroclastic density current (PDC) deposits from the early history of the currently active Galeras cone which have ages between 4500 and 2300 years before present have a much wider distribution. The first two (ML 1 and ML 2) were deposited from hot, dilute, pumice-rich pyroclastic density currents. Amphibole thermobarometry indicates that the magma for these eruptions tapped reservoirs between 5 and 28 km (ML 1) and 33–41 km (ML 2) below the crater. Debse, degassed magma formed a conduit plug which persisted for up to several weeks before the eruptions based on the thickness of breakdown rims around the amphibole. Amphibole-bearing pumice came from a reservoir 5–26 km below the crater and since it lacks breakdown rims it must have reached surface in <8 days. The vesicularity of the pumice indicates two phases of bubble nucleation and growth which coincides with growth of plagioclase and orthopyroxene microlites. The upper two layers (ML 3 and ML 4) are block and ash flows. ML3 shows significant hydrothermal alteration of the dense lithic clasts and has only minor vesicular scoria. The magma for the ML 3 eruption came from a reservoir 17–29 km below the crater and the presence of 20 μm thick breakdown rims on the amphibole implies that the degassing plug was stable for ∼20 days, though the degree of hydrothermal alteration of the clasts appears to require a significantly longer time. The uppermost layer (ML4) is relatively unaltered with a large proportion of pumice. Dense lithics indicate plug formation from magma that had its source at ∼17 km below the crater and remained stable during degassing for ∼20 days. The absence of amphibole from the pumice in ML 4 implies a distinct magma composition was emplaced after the plug.