Geochemical prediction of the 2002–2003 Stromboli eruption from variations in CO2and Rn emissions and in helium and carbon isotopes

Significant changes in both the chemistry of coastal thermal waters and the soil CO2and Rn emissions in the crater area were recorded at Stromboli prior to the eruption that began on 28 December 2002. The dissolved CO2contents and the δ13C and3He/4He values were elevated in the thermal aquifer from July 2002. Synchronous variations in the same isotope ratios were recorded in the summit fumarolic gases, with both3He/4He and δ13C values of gases released from a fumarole in the summit area increasing between May and November 2002. These variations are indicative of early degassing of a new gas-rich magma batch with a13C- and3He-rich signature. This magma recharge probably fed the intense Strombolian activity recorded during that period. The eruption began with a major explosion that produced a glowing avalanche, immediately followed by a fluid lava overflow from the NE crater and subsequent lava effusion from vents opened in the Sciara del Fuoco depression. Sharp increases in CO2soil flux and Rn emissions—to values never observed previously—were recorded in the summit crater area 10 d before the eruption onset. These CO2and Rn anomalies are indicative of a high gas-driven magma supply rate and gas overpressure within the conduit. The sudden depressurization of the magma filling the upper conduit probably caused the major explosion that occurred on 28 December, which heralded the effusive phase. These data demonstrate the importance of collecting a wide spectrum of geochemical data from different geological sites when monitoring a volcano.