The contribution of fluid geochemistry to the volcano monitoring of Stromboli.

Abstract

The persistent open-vent mild explosive activity of Stromboli volcano is episodically interrupted by more violent and dangerous explosive events (major explosions and paroxysms). According to the nature of erupted products, paroxysms can be related either to the explosion of overpressured gas pockets located in the proximity of the magma column or to the sudden uprise of hot, 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 produce gas leakage anomalies at the surface in sites of high vertical permeability, such as deep-reaching faults. In order to identify such gas leaking sites, a systematic CO2 soil flux survey has been carried out on the island using an accumulation chamber. Four hundred sixty-one points have been measured with a high density in the summit crater area, where high flux values 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 decreases from the crater zone to the base of the volcanic cone, where, however, an interesting gas leakage anomaly occurs (Pizzillo mofette). In the Pizzillo area thermal water wells also occur, whose chemistry indicates an origin by sea water heated by hot gas. Soil gases have been sampled in the sites 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 component especially in the crater zone samples (high CO2, appreciable contents of He and H2, 3He/4He values up to 3.55, δ13C of CO2=−2 ‰). The SC5 low-flux fumarole on the crater rim has shown, during six years, a remarkable persistence of temperature (93–95°C), with only minor fluctuations of chemistry. Appearance of anomalous peaks of H2 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 that strombolian blasts produce rapid fluctuations in the fumarolic gas composition. In the light of this study, SC5 fumarole and Pizzillo mofette look promising sites for the testing of a continuous geochemical monitoring system of Stromboli volcano.