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

The analysis of gaseous compositions Prom Solfatara (Campi Flegrei, South Italy) fumaroles since the early 1980s, clearly reveals a double thermobarometric signature. A first signature at temperatures of about 360 degrees 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 degrees 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. H2O and CO2, 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 degrees C by the vaporization of hydrothermal liquids. A typical 'andesitic' water type (delta D similar to -20%, delta O-18 similar to 10%,) and a CO2-rich composition (X-CO2 similar to 0.4) has been inferred for the magmatic fluids, while for the hydrothermal component a meteoric origin and a CO2 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 similar 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 CO2-rich magmatic fluids at the bottom of the hydrothermal system. (C) 2007 Elsevier Ltd. All rights reserved.