Publication Date:
2017-04-04
Description:
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 CO2 fugacities (10 0.2 to 100.5) 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, SiO2
and, to a lesser extent, SO4 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.
Description:
Published
Description:
108-129
Description:
JCR Journal
Description:
restricted
Keywords:
Ischia
;
Hydrothermal systems
;
EQ3-6
;
Geochemical modelling
;
03. Hydrosphere::03.04. Chemical and biological::03.04.06. Hydrothermal systems
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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