Hydrothermal systems hosted within active volcanic systems represent an excellent opportunity to investigate the
interactions between aquifer rocks, infiltrating waters and deep-rising magmatic fluids, and thus allow deriving
information on the activity state of dormant volcanoes. From a thermodynamic perspective, gas-water-rock
interaction processes are normally far from equilibrium, but can be represented by an array of chemical reactions,
in which irreversible mass transfer occurs from host rock minerals to leaching solutions, and then to secondary
hydrothermal minerals. While initially developed to investigate interactions in near-surface groundwater environments,
the reaction path modeling approach of Helgeson and co-workers can also be applied to quantitative
investigation of reactions in high T-P environments.
Ischia volcano, being the site of diffuse hydrothermal circulation, is an ideal place where to test the application
of reaction-path modeling. Since its last eruption in 1302 AD, Ischia has shown a variety of hydrothermal
features, including fumarolic emissions, diffuse soil degassing and hot waters discharges. These are the superficial
manifestation of an intense hydrothermal circulation at depth. A recent work has shown the existence of several
superposed aquifers; the shallowest (near to boiling) feeds the numerous surface thermal discharges, and is
recharged by both superficial waters and deeper and hotter (150-260° C) hydrothermal reservoir fluids.
Here, we use reaction path modelling (performed by using the code EQ3/6) to quantitatively constrain the
compositional evolution of Ischia thermal fluids during their hydrothermal flow. Simulations suggest that
compositions of Ischia groundwaters are buffered by interactions between reservoir rocks and recharge waters
(meteoric fluids variably mixed - from 2 to 80% - with seawater) at shallow aquifer conditions. A CO2 rich
gaseous phase is also involved in the interaction processes (fCO2 = 0.4-0.6 bar). Overall, our model calculations
satisfactorily reproduce the main chemical features of Ischia groundwaters. In the model runs, attainment of
partial to complete equilibrium with albite and K-feldspar fixes the Na/K ratios of the model solutions at values
closely matching those of natural samples. Precipitation of secondary phases, mainly clay minerals (smectite and
saponite) and zeolites (clinoptilolite), during the reaction path is able to well explain the large Mg-depletions
which characterise Ischia thermal groundwaters; while pyrite and troilite are shown to control sulphur abundance
in aqueous solutions. SiO2(aq) contents in model simulations fit those measured in groundwaters and are being
buffered by the formation of quartz polymorphs and Si-bearing minerals. Finally, our simulations are able to
reproduce redox conditions and Fe-depletion trends of natural samples. We conclude that reaction path modelling is an useful tool for quantitative exploration of chemical process within volcano-hosted hydrothermal systems