Origin of fumarolic fluids at Vulcano (Italy). Insights from isotope data and numerical modeling of hydrothermal circulation

Abstract

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 derives from the observation that compositional variations of discharged fluids are commonly observed as a new eruptive phase is approaching. In particular, an increase in the gas components of direct magmatic origin may indicate a higher magma degassing rate at depth, potentially related to a renewal of explosive activity. Surveillance programs devoted to hazard mitigation in active volcanic areas generally include periodic analyses of discharged fluids at various locations on the volcanic edifice. Unfortunately, when significant changes are observed in gas composition, their correct interpretation in terms of system evolution is not always clear and straightforward. Several mechanisms may in fact be responsible for differences in the proportion of magmatic gases and shallower fluid components, and it is not always possible to recognize the magmatic gas fraction. Discrimination among fluids of different origin ideally is accomplished based on the isotope composition that each fluid acquires at the time of its generation. However, this isotope signature can be altered before the fluids reach the surface, because of mixing between fluids of different origin, or due to reactions that modify the original isotope composition. Thus, the interpretation of isotope data sometimes is quite complex and it can be highly misleading. In this work, the origin of the steam discharged at the hydrothermal system of Vulcano (Italy) was investigated by the means of a dual approach: first, the available isotope data on the discharged steam were analyzed. On the basis of on these and other data, a conceptual model of the hydrothermal system was developed and numerical simulations of a multiphase, multicomponent flow through a heterogeneous porous medium were carried out to evaluate the effects of various parameters on the system evolution. Results suggest that the hydrothermal system of Vulcano is controlled by the entrance of large quantities of seawater whose original isotope composition has been altered by water-rock interaction. Rock permeability and its distribution at depth were shown to be a primary parameter, controlling the overall system evolution, the fluid composition, and fluid discharge rate at the surface.