Geochemical modeling of mixing between magmatic and hydrothermal gases: the case of Vulcano Island, Italy

In this study we present a geochemical approach to model the mixing of magmatic and hydrothermal gases. Our model is based on a rigorous formulation of a perfectly dimensioned system of mass and energy balance equations. Inputs of the equation set are the H2O and CO2 content and gas emission temperature at the surface, together with some constraints gained from the chemical–physical features of the hydrothermal system. The output data give the composition of magmatic gas, mixing fractions and temperatures of gas mixtures in the mixing zone. The comparison of the emission temperature of fumarolic gases with their calculated mixing temperatures provides valuable indications on the expansion process that gases undergo during their ascent towards the surface. Our mixing model is exemplified by the fumarolic system of Vulcano Island, Italy. We calculate the thermodynamic parameters of the hydrothermal system by means of an H2O–CO2–NaCl equation of state, while the thermo-baric conditions of the magma have been assessed by previous geophysical and petrologic investigations. The obtained results show that, during volcanic crises at Vulcano Island and in agreement with magma degassing processes, both the CO2 and the He contents of magmatic gas display peaks with a progressively declining amplitude, suggesting a progressive magmatic impoverishment of volatiles strongly partitioned in the gas phase. Mixing fractions of various fumaroles are normally variable, while they display almost homogeneous values during periods of increased volcanic activity. On the basis of results gained, some implications regarding the hazard of phreato-magmatic explosions are also discussed. The high temperature reached by fumarolic gases strongly constrains their probable expansion process during the ascent towards the surface. A comparison between the temperature of the fumarolic gas and the calculated mixing temperature at depth indicates that the expansion process can be reasonably described as almost iso-enthalpic, although field evidences suggest that the fumarolic emission temperature could be affected by the sharp permeability changes in the last few hundred meters. Therefore, our results and field evidences clearly indicate that variations in gas emission temperature cannot be considered a good indicator of volcanic activity, as it is strongly dependent on expansion process and, ultimately, on rock permeability changes. The model greatly improves the knowledge of the volcanic system of Vulcano Island and provides an important tool for the interpretation and evaluation of volcanic activity. Moreover, a similar approach can be considered for other volcanic systems having analogous characteristics.