Details

Autor(en) / Beteiligte

• Cinti, D.
• Procesi, M.
• Tassi, F.
• Montegrossi, G.
• Sciarra, A.
• Vaselli, O.
• Quattrocchi, F.

Titel

Fluid geochemistry and geothermometry in the western sector of the Sabatini Volcanic District and the Tolfa Mountains (Central Italy)

Ist Teil von

• Chemical geology, 2011-05, Vol.284 (1), p.160-181

Ort / Verlag

Elsevier B.V

Erscheinungsjahr

2011

Link zum Volltext

Quelle

Alma/SFX Local Collection

Beschreibungen/Notizen

• 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)–HCO 3 and Ca(Mg)–HCO 3 compositions when trapped in volcanic and sedimentary formations, respectively; and 2) a deep reservoir, which is hosted in the Mesozoic carbonate sequence, rich in CO 2 and having a Ca–SO 4(HCO 3) composition. Dissolution of a CO 2-rich gas phase into the shallow aquifers produces high-TDS and high-pCO 2 cold waters, while oxidation of deep-derived H 2S to SO 4 2− generates low-pH (< 4) sulfate waters. The δ 13C–CO 2 values for gas emissions (from − 2.8 to + 2.7‰ vs. VPDB) suggest that the origin of CO 2 associated with the deep fluids is mainly related to thermo-metamorphic reactions within the carbonate reservoir, although significant mantle contribution may also occur. However, R/R a 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 δ 13C–CH 4 and δD–CH 4 values (from − 25.7 to − 19.5‰ vs. VPDB and from − 152 to − 93.4‰ vs. VSMOW, respectively) CH 4 production is associated with thermogenic processes, possibly related to abiogenic CO 2 reduction within the carbonate reservoir. The δ 34S–H 2S 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 CO 2–CH 4 and, separately, H 2S suggest that the reservoir equilibrium temperature is up to ~ 300 °C. The δD and δ 18O data indicate that water 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. ► We studied the geochemical features of fluids emerging from northern Latium (Italy); ► Waters emerge from both shallow (cold) and deep (thermal and CO 2-rich) reservoirs; ► CO 2 is mainly originated by thermo-metamorphic reactions within carbonate formations; ► CH 4 and H 2S are mainly produced by thermogenic processes within the carbonates; ► Reservoir temperatures calculated by using CH 4–CO 2 and H 2S are of 260–310 °C