ORIGIN OF FUMAROLIC FLUIDS AT VULCANO (ITALY) - INSIGHTS FROM ISOTOPEDATA AND NUMERICAL MODELING OF HYDROTHERMAL CIRCULATION

Hydrothermal activity is often associated with active volcanic systems . During quiescent times, geochemical monitoring of discharged fluids commonly is carried out to gain insights on the state of evolution of the whole volcanic system. The interest in geochemical monitoring deri ves from the observation that compositional variations of discharged f luids are commonly observed as a new eruptive phase is approaching. In particular, an increase in the gas components of direct magmatic orig in may indicate a higher magma degassing rate at depth, potentially re lated to a renewal of explosive activity. Surveillance programs devote d to hazard mitigation in active volcanic areas generally include peri odic analyses of discharged fluids at various locations on the volcani c edirice. Unfortunately, when significant changes are observed in gas composition, their correct interpretation in terms of system evolutio n is not always clear and straightforward. Several mechanisms may in f act be responsible for differences in the proportion of magmatic gases and shallower fluid components, and it is not always possible to reco gnize the magmatic gas fraction. Discrimination among fluids of differ ent origin ideally is accomplished based on the isotope composition th at each fluid acquires at the time of its generation. However, this is otope signature can be altered before the fluids reach the surface, be cause of mixing between fluids of different origin, or due to reaction s that modify the original isotope composition. Thus, the interpretati on of isotope data sometimes is quite complex and it can be highly mis leading. In this work, the origin of the steam discharged at the hydro thermal system of Vulcano (Italy) was investigated by the means of a d ual approach: first, the available isotope data on the discharged stea m were analyzed. On the basis of on these and other data, a conceptual model of the hydrothermal system was developed and numerical simulati ons of a multiphase, multicomponent flow through a heterogeneous porou s medium were carried out to evaluate the effects of various parameter s on the system evolution. Results suggest that the hydrothermal syste m of Vulcano is controlled by the entrance of large quantities of seaw ater whose original isotope composition has been altered by water-rock interaction. Rock permeability and its distribution at depth were sho wn to be a primary parameter, controlling the overall system evolution , the fluid composition, and fluid discharge rate at the surface. (C) 1997 Elsevier Science B.V.