Publication Date:
2024-05-09
Description:
A geochemical survey of 197 fluid discharges (cold and thermal waters and bubbling pools) and 15 gas
emissions from the western sector of the Sabatini Volcanic District and the Tolfa Mountains (Latium, Central
Italy) was carried out in 2007–2008. The chemical and isotopic compositions of the fluid discharges indicate
the occurrence of two main sources: 1) relatively shallow aquifers with Ca(Na,K)–HCO3 and Ca(Mg)–HCO3
compositions when trapped in volcanic and sedimentary formations, respectively; and 2) a deep reservoir,
which is hosted in the Mesozoic carbonate sequence, rich in CO2 and having a Ca–SO4(HCO3) composition.
Dissolution of a CO2-rich gas phase into the shallow aquifers produces high-TDS and high-pCO2 cold waters,
while oxidation of deep-derived H2S to SO4 2− generates low-pH (b4) sulfate waters.
The δ13C–CO2 values for gas emissions (from−2.8 to+2.7‰vs. VPDB) suggest that the origin of CO2 associated
with the deep fluids ismainly related to thermo-metamorphic reactions within the carbonate reservoir, although
significant mantle contribution may also occur. However, R/Ra values (0.37–0.62) indicate that He is mainly
produced by a crustal source, with a minor component from a crust-contaminated mantle. On the basis of the
δ13C–CH4 and δD–CH4 values (from −25.7 to −19.5‰ vs. VPDB and from −152 to −93.4‰ vs. VSMOW,
respectively) CH4 production is associated with thermogenic processes, possibly related to abiogenic CO2
reduction within the carbonate reservoir. The δ34S–H2S values (from+9.3 to +10.4‰ vs. VCDT) are consistent
with the hypothesis of a sedimentary source of sulfur from thermogenic reduction of Triassic sulfates.
Geothermometric evaluations based on chemical equilibria CO2–CH4 and, separately, H2S suggest that the
reservoir equilibriumtemperature is up to ~300 °C. The δDand δ18O data indicate thatwater recharging both the
shallow and deep aquifers has a meteoric origin. Fluid geochemistry, coupled with gravimetric data and tectonic
lineaments, supports the idea that significant contributions from a deep-seated geothermal brine are present in
the Stigliano thermal fluid discharges. Exploration surveys investigated this area during 70's–90's for geothermal
purposes. Nevertheless, presently the area is still under-exploited. The presence of thermal waters and
anomalous heat flow together with the demographic growth of the last years,makes this site a suitable location
for direct applications of the geothermal resource.
Description:
Published
Description:
160-181
Description:
2.4. TTC - Laboratori di geochimica dei fluidi
Description:
JCR Journal
Description:
reserved
Keywords:
Geochemistry Water Gas Stable isotope Geothermometry Central Italy
;
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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