Monitoring of active but quiescent volcanoes using light hydrocarbon distribution in volcanic gases: the results of 4 years of discontinuous monitoring in the Campi Flegrei (Italy)

During the period from January 1991 to January 1994, the 10-yr geochemical monitoring in the Campi Flegrei (Southern Italy), usually performed using the inorganic gas fraction, was implemented by discontinuous monitoring of the hydrocarbon gas fraction. The measured compositions have revealed the presence of significant amounts of at least 11 hydrocarbon species, related to alkane, alkene, and aromatic classes. Thermodynamic calculations showed the possible existence of partially independent equilibria within the C1/C2/C3 and C1/C2/C3/C4 alkane systems, as well as among alkene/alkane pairs at redox conditions controlled by the geothermal (FeO/Fe1,5O) redox buffer system, at temperatures between 200 and 400°C and PH2O controlled by the presence of a saline brine. Unlike the constancy of the measured and calculated `inorganicʹ temperature, equilibrium temperature based on organic species has displayed fluctuations and a significant increase starting from the second half of 1993. According to their apparent correlation with other independent observations, such as the measured increases of vapor fraction, H2S/CO2 and H2/N2 ratios on the monitored fumarole in addition to a general increase of dissolved CO2 in the Campi Flegrei groundwaters, the observed variations of the organic parameters were interpreted as the result of a general increase in acid-bearing gas flux throughout the re-equilibration zone.