First observational evidence for the CO2-driven origin of Stromboli's major explosions

We report on the first detection of CO2 flux precursors of the till now unforecastable "major" explosions that intermittently occur at Strombolivolcano (Italy). An automated survey of the crater plume emissions in the period 2006-2010, during which 12 such explosions happened, demonstrated that these events are systematically preceded by a brief phase of increasing CO2/SO2 weight ratio (up to > 40) and CO2 flux (> 1300 t d(-1)) with respect to the time-averaged values of 3.7 and similar to 500 t d(-1) typical for standard Stromboli's activity. These signals are best explained by the accumulation of CO2-rich gas at a discontinuity of the plumbing system (decreasing CO2 emission at the surface), followed by increasing gas leakage prior to the explosion. Our observations thus supports the recent model of Allard (2010) for a CO2-rich gas trigger of recurrent major explosions at Stromboli, and demonstrates the possibility to forecast these events in advance from geochemical precursors. These observations and conclusions have clear implications for monitoring strategies at other open-vent basaltic volcanoes worldwide.