Publication Date:
2024-05-09
Description:
This study reports a complete geochemical dataset of 215 water and 9 gas samples collected in 2015 from
thermal and cold discharges located in the eastern sector of the Sabatini Volcanic District (SVD), Italy.
Based on these data, two main aquifers were recognized, as follows: 1) a cold Ca-HCO3 to Ca(Na)-HCO3
aquifer related to a shallow circuit within Pliocene-Pleistocene volcanic and sedimentary formations and
2) a deep CO2-pressurized aquifer hosted in Mesozoic carbonate-evaporitic rocks characterized by a Ca-
HCO3(SO4) to Na(Ca)-HCO3(Cl) composition. A thick sequence of low-permeability formations represents
a physical barrier between the two reservoirs. Interaction of the CO2-rich gas phase with the shallow
aquifer, locally producing high-TDS and low-pH cold waters, is controlled by fractures and faults related
to buried horst-graben structures. The d18O-H2O and dD-H2O values indicate meteoric water as the main
source for both the shallow and deep reservoirs. Carbon dioxide, which is characterized by d13C-CO2
values ranging from 4.7 to þ1.0‰ V-PDB, is mostly produced by thermo-metamorphic decarbonation
involving Mesozoic rock formations, masking possible CO2 contribution from mantle degassing. The
relatively low R/Ra values (0.07e1.04) indicate dominant crustal He, with a minor mantle He contribution.
The CO2/3He ratios, up to 6 1012, support a dominant crustal source for these two gases. The d34SH2S
values (from þ9.3 to þ11.3‰ V-CDT) suggests that H2S is mainly related to thermogenic reduction of
Triassic anhydrites. The d13C-CH4 and dD-CH4 values (from 33.4 to 24.9‰ V-PDB and from 168
to 140‰ V-SMOW, respectively) and the relatively low C1/C2þ ratios (〈100) are indicative of a prevailing
CH4 production through thermogenic degradation of organic matter. The low N2/Ar and high N2/
He ratios, as well as the 40Ar/36Ar ratios (〈305) close to atmospheric ratio, suggest that both N2 and Ar
mostly derive from air. Notwithstanding, the positive d15N-N2 values (from þ0.91 to þ3.7‰ NBS air)
point to a significant extra-atmospheric N2 contribution. Gas geothermometry in the CH4-CO2-H2 and
H2S-CO2-H2 systems indicate equilibrium temperatures 〈200 C, i.e. lower than those measured in deep
geothermal wells (~300 C), due to either an incomplete attainment of the chemical equilibria or secondary
processes (dilution and/or scrubbing) affecting the chemistry of the uprising fluids. Although the
highly saline Na-Cl fluids discharged from the explorative geothermal wells in the study area support the
occurrence of a well-developed hydrothermal reservoir suitable for direct exploitation, the chemistry of
the fluid discharges highlights that the uprising hydrothermal fluids are efficiently cooled and diluted by
the meteoric water recharge from the nearby Apennine sedimentary belt. This explains the different
chemical and isotopic features shown by the fluids from the eastern and western sectors of SVD,
respectively, the latter being influenced by this process at a lesser extent. Direct uses may be considered a
valid alternative for the exploitation of this resource.
Description:
Published
Description:
187-201
Description:
6A. Geochimica per l'ambiente
Description:
2IT. Laboratori sperimentali e analitici
Description:
1VV. Altro
Description:
JCR Journal
Keywords:
Fluid geochemistry
;
Central Italy
;
Water-gas-rock interaction
;
Geothermometry
;
Sabatini Volcanic District
;
03.02. Hydrology
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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