Temperature and pressure gas geoindicators at the Solfatara fumaroles(Campi flegrei)

Abstract

Long time series of fluid pressure and temperature within a hydrothermalsystem feeding the Solfatara fumaroles are investigated here, on the basisof the chemical equilibria within the CO2-H2O-H2-CO gas system. ThePisciarelli fumarole external to Solfatara crater shows an annual cycle ofCO contents that indicates the occurrence of shallow secondary processesthat mask the deep signals. In contrast, the Bocca Grande and Bocca Novafumaroles located inside Solfatara crater do not show evidence ofsecondary processes, and their compositional variations are linked to thetemperature-pressure changes within the hydrothermal system. Theagreement between geochemical signals and the ground movements of thearea (bradyseismic phenomena) suggests a direct relationship between thepressurization process and the ground uplift. Since 2007, the gasgeoindicators have indicated pressurization of the system, which is mostprobably caused by the arrival of deep gases with high CO2 contents inthe shallow parts of the hydrothermal system. This pressurization processcauses critical conditions in the hydrothermal system, as highlighted bythe increase in the fumarole temperature, the opening of new vents, andthe localized seismic activity. If the pressurization process continues withtime, it is not possible to rule out the occurrence of phreatic explosions. © 2011 by the Istituto Nazionale di Geofisica e Vulcanologia. All rights reserved.