Numerical modelling of gas-water-rock interactions in volcanic-hydrothermal environment: the Ischia Island (Southern Italy)case study.

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Abstract

Hydrothermal systems hosted within active volcanic systems represent an excellent opportunity to investigate theinteractions between aquifer rocks, infiltrating waters and deep-rising magmatic fluids, and thus allow derivinginformation on the activity state of dormant volcanoes. From a thermodynamic perspective, gas-water-rockinteraction 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 secondaryhydrothermal 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 quantitativeinvestigation of reactions in high T-P environments.Ischia volcano, being the site of diffuse hydrothermal circulation, is an ideal place where to test the applicationof reaction-path modeling. Since its last eruption in 1302 AD, Ischia has shown a variety of hydrothermalfeatures, including fumarolic emissions, diffuse soil degassing and hot waters discharges. These are the superficialmanifestation of an intense hydrothermal circulation at depth. A recent work has shown the existence of severalsuperposed aquifers; the shallowest (near to boiling) feeds the numerous surface thermal discharges, and isrecharged 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 thecompositional evolution of Ischia thermal fluids during their hydrothermal flow. Simulations suggest thatcompositions 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 richgaseous phase is also involved in the interaction processes (fCO2 = 0.4-0.6 bar). Overall, our model calculationssatisfactorily reproduce the main chemical features of Ischia groundwaters. In the model runs, attainment ofpartial to complete equilibrium with albite and K-feldspar fixes the Na/K ratios of the model solutions at valuesclosely matching those of natural samples. Precipitation of secondary phases, mainly clay minerals (smectite andsaponite) and zeolites (clinoptilolite), during the reaction path is able to well explain the large Mg-depletionswhich characterise Ischia thermal groundwaters; while pyrite and troilite are shown to control sulphur abundancein aqueous solutions. SiO2(aq) contents in model simulations fit those measured in groundwaters and are beingbuffered by the formation of quartz polymorphs and Si-bearing minerals. Finally, our simulations are able toreproduce 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.
Original languageEnglish
Number of pages0
Publication statusPublished - 2012

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