Geochemistry of hydrothermal fluids from the eastern sector of the Sabatini Volcanic District (central Italy)

Abstract 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-HCO 3 to Ca(Na)-HCO 3 aquifer related to a shallow circuit within Pliocene-Pleistocene volcanic and sedimentary formations and 2) a deep CO 2 -pressurized aquifer hosted in Mesozoic carbonate-evaporitic rocks characterized by a Ca-HCO 3 (SO 4 ) to Na(Ca)-HCO 3 (Cl) composition. A thick sequence of low-permeability formations represents a physical barrier between the two reservoirs. Interaction of the CO 2 -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 δ 18 O-H 2 O and δD-H 2 O values indicate meteoric water as the main source for both the shallow and deep reservoirs. Carbon dioxide, which is characterized by δ 13 C-CO 2 values ranging from −4.7 to +1.0‰ V-PDB, is mostly produced by thermo-metamorphic decarbonation involving Mesozoic rock formations, masking possible CO 2 contribution from mantle degassing. The relatively low R/R a values (0.07–1.04) indicate dominant crustal He, with a minor mantle He contribution. The CO 2 / 3 He ratios, up to 6 × 10 12 , support a dominant crustal source for these two gases. The δ 34 S-H 2 S values (from +9.3 to +11.3‰ V-CDT) suggests that H 2 S is mainly related to thermogenic reduction of Triassic anhydrites. The δ 13 C-CH 4 and δD-CH 4 values (from −33.4 to −24.9‰ V-PDB and from −168 to −140‰ V-SMOW, respectively) and the relatively low C 1 /C 2+ ratios ( 4 production through thermogenic degradation of organic matter. The low N 2 /Ar and high N 2 /He ratios, as well as the 40 Ar/ 36 Ar ratios ( 2 and Ar mostly derive from air. Notwithstanding, the positive δ 15 N-N 2 values (from +0.91 to +3.7‰ NBS air) point to a significant extra-atmospheric N 2 contribution. Gas geothermometry in the CH 4 -CO 2 -H 2 and H 2 S-CO 2 -H 2 systems indicate equilibrium temperatures