We report on the first detection of CO<sub>2</sub> flux precursors of the till now unforecastable larger than normal ("major") explosions that intermittently occur at Stromboli volcano (Italy). Automated survey of the crater plume emissions in the period 2006–2010, during which 12 such explosions happened, demonstrate that these events are systematically preceded by a brief phase of increasing CO<sub>2</sub>/SO<sub>2</sub> weight ratio (up to >40) and CO<sub>2</sub> flux (>1300 t/d) with respect to the time-averaged values of 3.7 and ~500 t/d typical for standard Stromboli's activity. These signals are best explained by the accumulation of CO<sub>2</sub>-rich gas at a discontinuity of the plumbing system (decreasing CO<sub>2</sub> emission at the surface), followed by increasing gas leakage prior to the explosion. Our observations thus support the recent model of Allard (2010) for a CO<sub>2</sub>-rich gas trigger of recurrent major explosions at Stromboli, and demonstrate 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
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