Publication Date:
2024-05-09
Description:
The Vicano–Cimino Volcanic District (VCVD) is related to the post-orogenic magmatic activity of the peri-
Tyrrhenian sector of Central Italy. The chemical and isotopic compositions of 333 water discharges and 25 gas
emissions indicate the occurrence of two main sources: 1) cold Ca-HCO3 to Ca(Na, K)-HCO3 type waters from
relatively shallow aquifers hosted in volcanic and sedimentary formations; and 2) thermal Ca-SO4(HCO3) type
waters located in a deep CO2-pressurized reservoir, hosted in carbonate–evaporite rocks and separated from
the shallow aquifers by thick sequences of low-permeability formations. Carbon dioxide is mainly produced by
thermal metamorphic decarbonation within the deepest and hottest parts of the carbonate–evaporite reservoir
(δ13C–CO2 from−3.1 to+2.2‰vs. VPDB), likely affected by a mantle-rooted CO2. ReleaseofCO2-rich gases from
the deep aquifer into the overlying shallow aquifers produces high-CO2 springs and bubbling pools. The spatial
distribution of thermal waters and CO2-rich cold discharges is strongly controlled by fractures and faults located
in correspondencewith buried structural highs. Stable isotopes (δD and δ18O) suggest thatmeteoric water feeds
both the shallowand deep reservoirs. The relatively lowR/Ra values (0.27–1.19) indicate that He ismainly deriving
from a crustal source, with minor component from the mantle affected by crustal contamination related to
the subduction of the Adriatic plate. Consistently, relatively high N2/Ar and N2/3He ratios and positive δ15N–N2
values (from0.91 to 5.7‰vs. air) characterize the VCVD gas discharges, suggesting the occurrence of a significant
“excess” nitrogen. Isotopic compositions of CH4 (δ13C–CH4 and δD–CH4 values from−28.9 to−22.1‰vs. VPDB
and from −176 to −138‰ vs. VSMOW, respectively), and composition of light alkanes are indicative of prevalent
thermogenic CH4, although the occurrence of abiogenic CH4 production cannot be excluded. The δ34S–H2S
values (from+9.3 to+11.4‰vs. VCDT) are consistentwith the hypothesis of H2S production fromthermogenic
reduction of Triassic anhydrites. Gas geothermometry in the H2O–H2–Ar–H2S system suggests that the VCVD
gases equilibrated in a liquid phase at redox conditions controlled by interactions of fluids with the local mineral
assemblage at temperatures lower (b200 °C) than that andmeasured in deep (N2000 m) geothermalwells. This
confirms that secondary processes, i.e. steam condensation, gas dissolution in shallow aquifers, re-equilibration
at lower temperature, and microbial activity, significantly affect the chemistry of the uprising fluids. Thermal
water chemistry supports the occurrence in this area of an anomalous heat flowthat, coupledwith the recent demographic
growth, makes this site suitable for direct and indirect exploitation of the geothermal resource, in
agreement with the preliminary surveys carried out in the 1970's–1990's for geothermal exploration purposes.
Description:
Published
Description:
96-114
Description:
5A. Energia e georisorse
Description:
JCR Journal
Description:
restricted
Keywords:
geothermal resources
;
03. Hydrosphere::03.04. Chemical and biological::03.04.03. Chemistry of waters
;
03. Hydrosphere::03.04. Chemical and biological::03.04.05. Gases
;
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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