The contribution of fluid geochemistry to the volcano monitoring of Stromboli

The persistent open-vent mild explosive activity of Stromboli volcano is ep isodically interrupted by more violent and dangerous explosive events (majo r explosions and paroxysms). According to the nature of erupted products, p aroxysms can be related either to the explosion of overpressured gas pocket s located in the proximity of the magma column or to the sudden uprise of h ot, gas-rich magma from a deep part of the plumbing system. In both cases, these more energetic explosive events should be preceded by an escape or a preferential uprise of the highly mobile volatiles which, in turn, should p roduce gas leakage anomalies at the surface in sites of high vertical perme ability, such as deep-reaching faults. In order to identify such gas leakin g sires, a systematic CO2 soil flux survey has been carried out on the isla nd using an accumulation chamber. Four hundred sixty-one points have been m easured with a high density in the summit crater area, where high flux valu es have been found (10(-3)-10(-2) cm/s). Anomalous points are concentrated along the main NE-SW axial feeding system of the volcano. CO2 soil flux dec reases from the crater zone to the base of the volcanic cone, where, howeve r, an interesting gas leakage anomaly occurs (Pirzillo mofette). In the Piz zillo area thermal water wells also occur, whose chemistry indicates an ori gin by sea water heated by hot gas. Soil gases have been sampled in the sit es with the highest CO2 flux in the crater area and at the base of the cone . Chemical and isotopic analyses indicate the presence of a deep gas compon ent especially in the crater zone samples (high CO2. appreciable contents o f He and H-2, He-3/He-4 values up to 3.55, delta(13)C of CO2 = - 2 parts pe r thousand). The SC5 low-flux fumarole on the crater rim has shown, during six years, a remarkable persistence of temperature (93-95 degrees C), with only minor fluctuations of chemistry. Appearance of anomalous peals of H-2 in correspondence with strombolian explosions was observed during a 2-h 30- min experiment of continuous recording of hydrogen content of the fumarole. Together with previous data on H2O, CO2 and He, this experiment confirms t hat strombolian blasts produce rapid fluctuations in the fumarolic gas comp osition. In the light of this study, SC5 fumarole and Pizzillo mofette look promising sites for the testing of a continuous geochemical monitoring sys tem of Stromboli volcano. (C) 2000 Elsevier Science B.V. All rights reserve d.