Geochemistry Of Water And Gas Discharges From The Mt Amiata Silicic Complex And Surrounding Areas (Central Italy)

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Journal Article: Geochemistry Of Water And Gas Discharges From The Mt Amiata Silicic Complex And Surrounding Areas (Central Italy)

The Mt. Amiata volcano in central Italy is intimately related to the post-orogenic magmatic activity which started in Pliocene times. Major, trace elements, and isotopic composition of thermal and cold spring waters and gas manifestations indicate the occurrence of three main reservoir of the thermal and cold waters in the Mt. Amiata region. The deepest one is located in an extensive carbonate reservoir buried by thick sequences of low-permeability allochthonous and neo-autochthonous formations. Thermal spring waters discharging from this aquifer have a neutral Ca-SO4 composition due to the presence of anhydrite layers at the base of the carbonate series and, possibly, to absorption of deep-derived H2S with subsequent oxidation to SO42- in a system where pH is buffered by the calcite-anhydrite pair (Marini and Chiodini, 1994). Isotopic signature of these springs and N2-rich composition of associated gas phases suggest a clear local meteoric origin of the feeding waters, and atmospheric O2 may be responsible for the oxidation of H2S. The two shallower aquifers have different chemical features. One is Ca-HCO3 in composition and located in several sedimentary formations above the Mesozoic carbonates. The other one has a Na-Cl composition and is hosted in marine sediments filling many post-orogenic NW-SE-trending basins. Strontium, Ba, F, and Br contents have been used to group waters associated with each aquifer. Although circulating to some extent in the same carbonate reservoir, the deep geothermal fluids at Latera and Mt. Amiata and thermal springs discharging from their outcropping areas have different composition: Na-Cl and Ca-SO4 type, respectively. Considering the high permeability of the reservoir rock, the meteoric origin of thermal springs and the two different composition of the thermal waters, self-sealed barriers must be present at the boundaries of the geothermal systems. The complex hydrology of the reservoir rocks greatly affects the reliability of geothermometers in liquid phase, which understimate the real temperatures of the discovered geothermal fields. More reliable temperatures are envisaged by using gas composition-based geothermometers. Bulk composition of the 67 gas samples studied seems to be the result of a continuous mixing between a N2-rich component of meteoric origin related to the Ca-SO4 aquifer and a deep CO2-rich component rising largely along the boundaries of the geothermal systems. Nitrogen-rich gas samples have nearly atmospheric N2/Ar (=83) and 15N/14N (Δ=0‰) ratios whereas CO2-rich samples show anomalously high Δ15N values (up to +6.13 ‰), likely related to N2 from metamorphic schists lying below the carbonate formations. On the basis of average 13C/12C isotopic ratio (Δ13C around 0‰), CO2 seems to originate mainly from thermometamorphic reactions in the carbonate reservoir and/or in carbonate layers embedded in the underlying metamorphic basement. Distribution of 3He/4He isotopic ratios indicates a radiogenic origin of helium in a tectonic environment that, in spite of the presence of many post-orogenic basins and mantle-derived magmatics, can presently be considered in a compressive phase.

A. Minissale, G. Magro, O. Vaselli, C. Verrucchi and I. Perticone

Published Journal 
Journal of Volcanology and Geothermal Research, 1997

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A. Minissale,G. Magro,O. Vaselli,C. Verrucchi,I. Perticone. 1997. Geochemistry Of Water And Gas Discharges From The Mt Amiata Silicic Complex And Surrounding Areas (Central Italy). Journal of Volcanology and Geothermal Research. (!) .