Monitoring active volcanoes: The geochemical approach

Abstract

Shallow magmas located beneath active volcanoes release volatiles both during eruptive activity and during inter-eruptive periods (passive degassing). The fluids in active volcanic areas rise directly from magma, and their composition is characterized mainly by H2O, CO2, SO2, H2S, HF and HCl (condensable gases), and by some non-condensable gases (e.g. He, H2, N2, CO, CH4). The chemical composition of fumarolic gases can reflect the pressure, temperature and oxygen fugacity conditions of the deep magmatic source, provided that during their rise towards the surface, the gases do not undergo re-equilibration processes [Giggenbach 1980, Giggenbach 1996, Nuccio and Paonita 2001, Paonita et al. 2002]. As the equilibrium kinetics of several chemical reactions is much slower than the rising velocities of the gases, the gas molecular compositions often undergo quenching phenomena, so that the gases show temperature and pressure equilibria higher than their outlet values. The concentration of magmatic species or their molecular ratios can be determined by means of direct sampling of fumarole gases or by using telemetric methods of observation. The extensive parameters (mass output) of volcanic fluids, coupled with the intensive parameters described before, provide basic and useful information for the formulation of volcanic fluid degassing models [Italiano et al. 1997, Brusca et al. 2004, Inguaggiato et al. 2011]. The first step in the framework of the geochemical investigation of a volcanic system aimed at surveillance is the chemical and isotopic characterization of the fluids, and the putting forward of a geochemical model [Inguaggiato et al. 2011]; within this geochemical model, it is possible to interpret the observed changes in any single investigated parameter. The geochemical approach is to identify the following topics: • The main end-members involved in the studied system; • The possible type and degree of interaction processes: e.g. water-rock and gas-water interactions; • The mixing among the individual end-members; • The chemical and isotopic characterization of a possible hydrothermal system; • The formulation of a geochemical model.