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

TLDR: In this paper, the authors reported 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.

About: This article is published in Applied Geochemistry.The article was published on 2017-09-01 and is currently open access. It has received 13 citations till now. The article focuses on the topics: Aquifer & Meteoric water.

Figures

Fig. 8. Ar-N2-He ternary diagram (Giggenbach, 1992) for free gases. The proportions among the three gases were calculated by using the concentrations in mmol/mol. ASW ¼ air saturated water at 20 C. Data for the western SVD gases are from Cinti et al. (2011). Symbols as in Fig. 6a.

Fig. 1. Geological sketch map of the Sabatini Volcanic District (SVD) showing the location of Cinti et al. (2011). Location of test-holes and geothermal wells are from Buonasorte et a Interpretative section corresponding to trace A-B is reported in Fig. 14.

Fig. 3. a) dD vs. d18O plot for the collected waters. The solid and the dashed lines delineate the isotopic domain for Global Meteoric Waters (dD ¼ 8d18O þ 10; Craig, 1961) and that for Mediterranean Meteoric Waters (dD ¼ 8d18O þ 22; Gat and Carmi, 1970), respectively. LSW ¼ local seawater; b) d18O vs. emergence elevation for the collected springs. The dashed line delineates the d18O gradient with elevation of 0.24‰/100 m. The d18O values of rainwater samples (black stars) from the SVD and surrounding areas are from Longinelli and Selmo (2003). Symbols as in Fig. 2.

Fig. 7. (a) d13C-CH4 vs. dD-CH4 diagram for free gases. Fields and boxes are from Etiope (2015). MRC ¼ microbial carbonate reduction, MAF ¼ microbial acetate fermentation, ME ¼microbial in evaporitic environment, TO ¼ thermogenic with oil, TC ¼ thermogenic with condensate, TD ¼ dry thermogenic, TH ¼ thermogenic with high-temperature CO2-CH4 equilibration, TLM ¼ thermogenic low maturity; (b) CH4/(C2H6þC3H8) vs. d13C-CH4 diagrams for free gases. Fields and boxes are from McCollom and Seewald (2007). Data for the western SVD gases are from Cinti et al. (2011). Symbols as in Fig. 6a.

Fig. 4. HCO3 vs. pH binary diagram for the collected waters. CO2 saturation curves at pCO2 ¼ 0.10, 0.05 and 0.025 bar are also reported. Symbols as in Fig. 2.

Table 1 Chemical and isotopic composition of the gas discharges. Gas contents are in mmol/mol except for Ne, C2H6 and C3H8 which are expressed in mmol/mol; d13C in CO2 and CH4 as ‰ V-PDB; dD in CH4 as ‰ V-SMOW; d15N as ‰ NBS air; d34S as ‰ V-CDT. Data from samples 190e216 are taken from Cinti et al. (2014).

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Geochemistry of hydrothermal fluids from the eastern sector of the sabatini volcanic district (central italy)" ?

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. 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. 3‰ V-CDT ) suggests that H2S is mainly related to thermogenic reduction of Triassic anhydrites. 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. 

Q2. What is the important fact about the SVD gases?

Since the eastern SVD gases are associated with bubbling pools, the concentrations of water vapor and CO mostly depend on interaction with liquid water at the surface, and therefore only low soluble gas compounds, such as CH4, CO2 and H2, can be used for geothermometric applications. 

Q3. What is the reliable explanation for the SVD gas equilibrium?

The most reliable explanation is that re-equilibrium processes at lower temperatures affect H2 and CH4 during the uprising of fluids from the hydrothermal reservoir. 

Q4. What is the effect of the shallow volcanic and sedimentary formations on the hydrothermal reservoir?

in the absence of tectonic disturbance, the shallow volcanic and sedimentary formations efficiently isolate the hydrothermal reservoir from the surface. 

Q5. What is the hydrogeological setting of the Bracciano Lake?

The hydrogeological setting is dominated by a regional hydrothermal reservoir hosted in the carbonate-evaporite units and a shallow, mainly unconfined, regional aquifer within the volcanic rocks (Capelli et al., 2005). 

Q6. What is the role of the low-permeability sedimentary deposits in the Bracciano Lake?

permeable layers within the low-permeability sedimentary deposits host perched aquifers that feed numerous springs of limited and discontinuous extent (Dall’Aglio et al., 1994). 

Q7. What is the SVD's thermal and cold fluid discharges?

In thisregion, mantle degassing and thermo-metamorphic processes feed a regional pressurized gas system (e.g., Chiodini et al., 1999; Minissale, 2004) that is connected to CO2-rich thermal and cold fluid discharges through fractures and faults related to an extensional tectonic regime (Barberi et al., 1994), producing a number of CO2-rich thermal and cold fluid discharges. 

Q8. What was the chemical and isotopic data used to determine the concentrations of the water samples?

pH, Eh, electrical conductivity and alkalinity concentrations (acidimetric titration with 0.05 N HCl) concentrations were determined in situ. 

Q9. What is the recent unconventional technology?

the most recent unconventional technologies can make possible the exploitation of the geothermal resources through binary systems where a working fluid, generally constituted by organic compounds (propane), is used in place of the geothermal fluid to produce electricity (Di Pippo, 2008). 

Q10. What is the main reason why the deep hydrothermal reservoir of eastern SVD is considered?

Although masked by mixing processes with the shallow aquifer, the deep hydrothermal reservoir of eastern SVD is to be considered an important potential resource, especially considering the recent renewed interest for geothermal energy in Italy. 

Q11. What is the reason for the unrealisticestimations of fluid temperatures?

The unrealisticestimations of fluid temperatures provided by solute geothermometers suggest that, even along the main tectonic lineaments where the fast rising of deep fluids is favored, the chemistry of the thermal fluids is affected by mixing with shallow waters. 

Q12. What are the main reasons for the lack of a valid alternative for the exploitation of these?

direct uses (e.g. building heating and cooling, fish farming, greenhouse heating), which imply relatively low costs, may represent, at a local scale, more than a valid alternative for the exploitation of these thermal fluids. 

Q13. What was the aliquot of water collected?

One filtered (0.45 mm) and two filtered and acidified (with ultra-pure HCl and HNO3, respectively) aliquots were collected and stored in high-density polyethylene bottles for laboratory analysis. 

Q14. What is the main chemical feature of the acque acetose?

The main chemical features of the acque acetose, i.e. high HCO3 concentrations and slightly acidic pH (Fig. 4), likely derive from the interaction of the shallow aquifers with the CO2-rich gas phase released from the deep pressurized regional fluid reservoir (Minissale, 2004). 

Q15. How much of the SVD isotopic values are consistent with the prevailing thermogenic?

The eastern SVD isotopic values reflect a prevailing crustal-radiogenic source with a minor amount of mantle He. Assuming R/Ra ~6.5 as themantle end-member in central-southern Italy (Tedesco, 1997), i.e. similar to the European sub-continental mantle (Dunai and Baur, 1995), the mantle He fraction in the study area is estimated at 0.8e16%. 

Q16. What is the d34S value of sulfide deposits in the eastern ?

It is worth mentioning that the d34S values of sulfide deposits in the SVD are mainly negative (averaging 2.6‰ V-CDT; Cavarretta and Lombardi, 1992), whereas evaporitic sulfates from Triassic anhydrites (Burano Fm.) have d34S values ranging from 13.6 to 17.4‰ V-CDT (Cortecci et al., 1981), and the d34S-SO42- values in Ca-SO4 thermal waters are typically ~þ14‰ V-CDT (Zuppi et al., 1974; Cavarretta and Lombardi, 1992; Cinti et al., 2014). 

Q17. What is the origin of CO2 in central Italy?

It is widely accepted (e.g., Minissale et al., 1997b; Chiodini et al.,1999; Minissale, 2004) that the origin of CO2 in central Italy is related to a twofold source, i.e. mantle degassing (d13C-CO2 from 7.0 to 3.0‰ V-PDB; Javoy et al., 1982; Rollinson, 1993) and thermo-metamorphic reactions on carbonates and/or the underlying metamorphic Palaeozoic basement (d13C-CO2 from 2.0 toþ2.0‰ V-PDB; Craig, 1963). 

Q18. What is the chemistry of the water-rock reactions?

When water-rock reactions attain a chemical equilibrium, reliable estimations of the fluid reservoir temperatures can be carried out, as described by the following equations (Giggenbach, 1988): Tð CÞ ¼ 1;390 .