Abstract

New eruptive activity at volcanoes that have been long quiescent poses a significant challenge to hazard assessment, as it requires assessment of how the situation may develop. Such incipient activity is often poorly characterised as most quiescent volcanoes are poorly monitored, especially with respect to gas geochemistry. Here, we report gas composition and flux measurements from a new vent at the onset of eruptive activity at the Nevados de Chillán volcanic complex (Chile) in January-February 2016. The molar proportions of H2O, CO2, SO2, H2S and H2 gases are found to be 98.4, 0.97, 0.11, 0.01 and 0.5 mol % respectively. The mean SO2 flux recorded in early February 2016 during periods of eruptive discharge amounts to 0.4-0.6 kg s-1. We show that magmatic gases were involved in this activity, associated with a sequence of eruptions. Tephra ejected by the first blast of 8 January are dominated by lithic fragments of dacitic composition. By contrast the tephra ejected from a subsequent eruption contains both lithic fragments of dense dacite, and a fresher, sparsely vesicular material of basaltic andesite composition. By October 2017, the ejected tephra was again dominated by dense dacitic lithic material. Together with seismic and ground deformation evidence, these observations suggest that a small intrusion of basaltic to andesitic magma at shallow level led to the explosive activity. Our serendipitous survey, right at the onset of eruptive activity, provides a valuable window into the processes of reawakening of a dormant volcano.