The origin of the fumaroles of La Solfatara (Campi Flegrei, South Italy)

• Published in: Geochimica et cosmochimica acta 2007-06, Vol.71 (12), p.3040-3055
• Main Authors: Caliro, S., Chiodini, G., Moretti, R., Avino, R., Granieri, D., Russo, M., Fiebig, J.
• Format: Article
• Language: English
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2007.04.007

The analysis of gaseous compositions from Solfatara (Campi Flegrei, South Italy) fumaroles since the early 1980s, clearly reveals a double thermobarometric signature. A first signature at temperatures of about 360 °C was inferred by methane-based chemical–isotopic geoindicators and by the H 2/Ar geothermometer. These high temperatures, close to the critical point of water, are representative of a deep zone where magmatic gases flash the hydrothermal liquid, forming a gas plume. A second signature was found to be at around 200–240 °C. At these temperatures, the kinetically fast reactive species (H 2 and CO) re-equilibrate in a pure vapor phase during the rise of the plume. A combination of these observations with an original interpretation of the oxygen isotopic composition of the two dominant species, i.e. H 2O and CO 2, shed light on the origin of fumarolic fluids by showing that effluents are mixture between fluids degassed from a magma body and the vapor generated at about 360 °C by the vaporization of hydrothermal liquids. A typical ‘andesitic’ water type (δD ∼ −20‰, δ 18O ∼10‰) and a CO 2-rich composition ( X CO 2 ∼ 0.4 ) has been inferred for the magmatic fluids, while for the hydrothermal component a meteoric origin and a CO 2 fugacity fixed by fluid-rock reaction at high temperatures have been estimated. In the time the fraction of magmatic fluids in the fumaroles increased (up to ∼0.5) at each seismic and ground uplift crisis (bradyseism) which occurred at Campi Flegrei, suggesting that bradyseismic crises are triggered by periodic injections of CO 2-rich magmatic fluids at the bottom of the hydrothermal system.