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

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• 作者：D. Cinti^a ; ^{daniele.cinti@ingv.it} ; M. Procesi^b ; F. Tassi^c ; ^d ; G. Montegrossi^d ; A. Sciarra^a ; O. Vaselli^c ; ^d ; F. Quattrocchi^a
• 关键词：Geochemistry ; Water ; Gas ; Stable isotope ; Geothermometry ; Central Italy
• 刊名：Chemical Geology
• 出版年：2011
• 出版时间：9 May 2011
• 年：2011
• 卷：284
• 期：1-2
• 页码：160-181
• 全文大小：2355 K

文摘

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 SO₄²⁻ generates low-pH (< 4) 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 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 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 equilibrium temperature is up to ~ 300 °C. The δD and δ¹⁸O 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.