Geochemistry of rainfall at Stromboli volcano (Aeolian Islands): isotopic model and plume-rain interaction

Abstract

The chemical and isotopic compositions of the precipitation at Stromboli Island, Italy, were investigated between October 2003 and October 2005. We employed a rain gauge network designed to cover the range in exposures and elevations of the volcanic edifice. The hydrogen and oxygen isotopic ratios vary greatly on a seasonal basis and correlate with air temperature. Deuterium excess values show a positive correlation with altitude. No direct contribution of volcanogenic H or O is evident in the isotopic composition of the rainwater. The chemical composition of the rainwater is principally controlled by the sea aerosol contribution at the coastal sites, whereas it is significantly influenced by volcanic activity near the summit vents. Interaction with volcanic acid gases is indicated by the pH, which is usually 1–2 units lower near the craters than at the coastal sites. The S/Cl, Cl/F, and S/F molar ratios in rainwater 1.5 km from the craters are consistent with those measured in the volcanic plume using other methods (diffusive tubes and Fourier transform infrared spectroscopy). Rising of undegassed magmas changes these molar ratios because of the differential degassing of sulphur, chlorine, and fluorine from the magma. We therefore propose that the chemical composition of precipitation, within 1.5 km of the craters, provides additional information that is useful for monitoring volcanic activity at Stromboli Island. Moreover, this paper presents estimates of the fluxes of F, Cl, S, Na, K, Ca, and Mg to the soil that could be useful for geochemical studies on groundwater.