Quantitative models of hydrothermal fluid–mineral reaction: The Ischia case

Abstract

The intricate pathways of fluid-mineral reactions occurring underneath active hydrothermal systems are explored in this study by applying reaction path modelling to the Ischia case study. Ischia Island, in Southern Italy, hosts a well-developed and structurally complex hydrothermal system which, because of its heterogeneity in chemical and physical properties, is an ideal test sites for evaluating potentialities/limitations of quantitative geochemical models of hydrothermal reactions. We used the EQ3/6 software package, version 7.2b, to model reaction of infiltrating waters (mixtures of meteoric water and seawater in variable proportions) with Ischia's reservoir rocks (the Mount Epomeo Green Tuff units; MEGT). The mineral assemblage and composition of such MEGT units were initially characterised by ad hoc designed optical microscopy and electron microprobe analysis, showing that phenocrysts (dominantly alkali-feldspars and plagioclase) are set in a pervasively altered (with abundant clay minerals and zeolites) groundmass. Reaction of infiltrating waters with MEGT minerals was simulated over a range of realistic (for Ischia) temperatures (95-260°C) and CO 2 fugacities (10 À0.2 to 10 0.5 ) bar. During the model runs, a set of secondary minerals (selected based on independent information from alteration minerals' studies) was allowed to precipitate from model solutions, when saturation was achieved. The compositional evolution of model solutions obtained in the 95-260°C runs were finally compared with compositions of Ischia's thermal groundwaters, demonstrating an overall agreement. Our simulations, in particular, well reproduce the Mg-depleting maturation path of hydrothermal solutions, and have end-ofrun model solutions whose Na-K-Mg compositions well reflect attainment of full-equilibrium conditions at run temperature. High-temperature (180-260°C) model runs are those best matching the Na-K-Mg compositions of Ischia's most chemically mature water samples, supporting quenching of deep-reservoir conditions for these surface manifestations; whilst Fe, SiO 2 and, to a lesser extent, SO 4 contents of natural samples are better reproduced in low-temperature (95°C) runs, suggesting that these species reflect conditions of water-rock interaction in the shallow hydrothermal environment. The ability of model runs to reproduce the compositional features of Ischia's thermal manifestations, demonstrated here, adds supplementary confidence on reaction path modelling as a realistic and insightful representation of mineral-fluid hydrothermal reactions. Our results, in particular, demonstrate the significant impact of host rock minerals' assemblage in governing the paths and trends of hydrothermal fluids' maturation.

A tool for evaluating geothermal power exploitability and its application to Ischia, Southern Italy

“…The still active nature of the volcano creating vigorous hydrothermal circulation beneath Ischia, is testified by the intense seismicity [30]; and by numerous numerous fumarolic gas emissions and hot water discharges widespread over the entire island (e.g., [33][34][35][36]). …”

“…(Società Anonima Forze Endogene Napoletane) [36]. About 100 wells were drilled down to a depth of 1156 m in the western and southern sectors of the island, demonstrating high subsurface temperatures from T > 70°C at the free-water surface up to 225°C at $1 km depth [37,38].…”

“…Since the early 80's, numerous geochemical investigations on surface thermal manifestations (shallow thermal waters and fumarolic gas emissions) were conducted, with the aim of extracting information about the structures of the deep hydrothermal system and fluid circulation dynamics. According to these studies [33,39,[34][35][36], both cold ($15°C) and hot (up to 100°C) geofluids coexist on Ischia. The northern and southwestern sectors were identified as the most thermalized areas (T P 80°C); while geofluids with temperatures of <50°C were encountered in the eastern part.…”

Geochemical and isotopic insights into the assembly, evolution and disruption of a magmatic plumbing system before and after a cataclysmic caldera-collapse eruption at Ischia volcano (Italy)

“…Precision is variable: 1-5%, for major elements, with the exception of P (<10%); <5%, for trace element contents higher than 50 ppm; 5-10%, for trace element contents in the range 10-50 ppm, and 5-15%, for trace element contents in the range 1-10 ppm. Napoli et al (2013).…”

“…The investigated pre-MEGT and MEGT rocks are variably vesicular and porphyritic, with a total phenocryst content of pumice clasts in the range 5-25 vol%, whereas scoria, obsidian and lava clasts may reach 40 vol% ( Napoli et al (2013).…”