This study is focused on fluids characterization and circulations through the crust of the Irpinia region, an active seismic zone in Southern Italy, that has experienced several high-magnitude earthquakes, including a catastrophic one in 1980 (M = 6.9 Ms). Using isotopic geochemistry and the carbon‑helium system in free and dissolved volatiles in water, this study aims to explore the processes at depth that can alter pristine chemistry of these natural fluids. Gas-rock-water interactions and their impact on CO2 emissions and isotopic composition are evaluated using a multidisciplinary model that integrates geochemistry and regional geological data. By analyzing the He isotopic signature in the natural fluids, the release of mantle-derived He on a regional scale in Southern Italy is verified, along with significant emissions of deep-sourced CO2. The proposed model, supported by geological and geophysical constraints, is based on the interactions between gas, rock, and water within the crust and the degassing of deep-sourced CO2. Furthermore, this study reveals that the Total Dissolved Inorganic Carbon (TDIC) in cold waters results from mixing between a shallow and a deeper carbon endmember that is equilibrated with carbonate lithology. In addition, the geochemical signature of TDIC in thermal carbon-rich water is explained by supplementary secondary processes, including equilibrium fractionation between solid, gas, and aqueous phases, as well as sinks such as mineral precipitation and CO2 degassing. These findings have important implications for developing effective monitoring strategies for crustal fluids in different geological contexts and highlight the critical need to understand gas-water-rock interaction processes that control fluid chemistry at depths that can affect the assessment of the CO2 flux in atmosphere. Finally, this study highlights that the emissions of natural CO2 from the seismically active Irpinia area are up to 4.08·10+9 mol·y-1, which amounts is in the range of worldwide volcanic systems.
Aiming at understanding the source of the fluids that mineralizing within seismically active fault zones, we study the noble gas isotopes (i.e. helium (He), neon (Ne) and argon (Ar)) in the fluid inclusions (FIs) within the calcite veins sampled along high-angle fault zones of the Contursi hydrothermal basin, southern Italy. The latter basin lies in close vicinity of the MW = 6.9, 1980 Irpinia earthquake and exposes numerous fault scarps dissecting Mesozoic shallow-water carbonates. The isotopic analyses are conducted to identify the origin of the volatiles circulating along the faults at the time of calcite precipitation. Then, outcomes of this discussions are compared with currently outgassing of deep-sourced CO2 coupled to mantle-derived He is in that area, whose output is larger than those from some volcanic areas worldwide. The results indicate that He in FIs is dominated by a crustal radiogenic component (4He), and by an up to 20% of a mantle-derived component (3He), with a highest isotopic signature of 1.38 Ra. This value is consistent with the highest percentage of mantle He associated to CO2 high-flux emission in the investigated area (1.41 Ra). The variability of the He isotopic signature in FIs can result from either pre-or-post trapping processes and seismic activity that modify the pristine He isotopic signature (i.e. derived from the crust and/or mantle) in groundwater along the faults during periods of background seismicity. Such investigations are fundamental to understand fluid migration in fault systems and the role of fluids in processes of earthquake nucleation.
Abstract Aiming at understanding the source of the fluids that mineralizing within seismically active fault zones, we investigate the noble gas isotopes (i.e., helium (He), neon (Ne), and argon (Ar)) in the fluid inclusions (FIs) trapped in the calcite veins sampled along high‐angle fault zones of the Contursi hydrothermal basin, southern Italy. The latter basin lies in close vicinity of the M W = 6.9, 1980 Irpinia earthquake and exposes numerous fault scarps dissecting Mesozoic shallow‐water carbonates. The analyses of noble gases (He, Ne, Ar) are conducted to identify the origin of the volatiles circulating along the faults at the time of calcite precipitation. Then, outcomes of this discussions are compared with currently outgassing of deep‐sourced CO 2 coupled to mantle‐derived He in that area, whose output is larger than those from some volcanic areas worldwide. The results indicate that He in FIs is dominated by a crustal radiogenic component ( 4 He), and by an up to 20% of a mantle‐derived component ( 3 He), with a highest isotopic signature of 1.38 Ra. This value is consistent with the highest percentage of mantle‐derived He associated to high‐flux CO 2 gas emission in the investigated area (1.41 Ra). We propose that the variability of the He isotopic signature measured in primary FIs can result from early trapping of fluid inclusions or post trapping processes and seismic activity that modify the pristine He isotopic signature (i.e., derived from the crust and/or mantle) in groundwater along the faults during periods of background seismicity. Such investigations are fundamental to understand fluid migration in fault systems and the role of fluids in processes of earthquake nucleation.
This study explores the characteristics of fluids flowing through the earth's crust and the underground mechanisms that can alter their chemistry. We are investigating an active seismic region in southern Italy, Irpinia, which has experienced several high-magnitude earthquakes, last catastrophic in 1980 with a magnitude of 6.9 Ms.Our research uses isotopic geochemistry and the carbon-helium system to determine the sources of CO2 in free gases and dissolved in water. We highlight the importance of gas-rock-water interactions in controlling the amounts and isotopic composition of CO2 discharged in atmosphere as dry natural gas emissions and dissolved in cold and thermal groundwater.Through analysis of the He isotopic signature, we verify the release of mantle-derived He on a regional scale in southern Italy, along with significant emissions of deep-sourced CO2. Our proposed model, supported by geological and geophysical constraints, is grounded on the interactions between gas, rock, and water (either groundwater or brine) within the crust, and the degassing of deep-sourced CO2.Our study shows that the Total Dissolved Inorganic Carbon (TDIC) in cold waters results from mixing between a shallow and a deeper carbon endmember. In contrast, the geochemical signature of TDIC in thermal carbon-rich water is explained by complex secondary processes, involving equilibrium fractionation between the solid, gas, and aqueous phases, and sinks such as mineral precipitation and CO2 degassing.Our findings have important implications for the development of effective monitoring strategies for crustal fluids in different geological contexts. The study highlights the critical need to understand the gas-water-rock interaction processes that control the chemistry of fluids at depths. This understanding is crucial for resolving geochemical changes in natural fluids observed during the monitoring of dynamic processes such as seismogenesis. In addition, comprehension of these processes is necessary to constrain natural CO2 emissions into the atmosphere in continental areas.
<p>Degassing of volatiles across the Earth crust towards the atmosphere is mainly controlled by long last diffusion. However, it can also be episodic. In fact, tectonic control of solid phase release of radiogenic helium (<sup>4</sup>He) due to, e.g. fracturing, may contribute to explain variance of the continental <sup>4</sup>He degassing flux over multiple time and space scales.&#160;Rock rheology have a controlling influence on a wide range of crustal-scale processes including fluid flow, tectonic deformations and seismicity. Though faults comprise a small volume of the crust, they influence the mechanical and fluid flow properties of the crust, and are mechanisms for accommodating most of the elastic strain in the crust through a variety of slip-behaviours. Helium isotopes (<sup>3</sup>He,<sup>4</sup>He) are useful tracers for investigating many important geological processes because helium is a stable and conservative nuclide that does not take part in any chemical or biological process. Indeed, <sup>4</sup>He released from rocks in the porefluid can be used to trace the deformation of rocks in a field of stress [Bauer 2017; Torgersen and O&#8217;Donnell 1991]. In fact, a volume of rock starts to be affected by micro-fractures from since &#160;it is subjected to stress conditions exceeding about half its yield strength [Bauer, 2017]. Hence, the network of fractures evolves in a volume of rock progressively increase as a function of the evolution of deformation, improving the release of <sup>4</sup>He that is trapped since its production. Consequently, <sup>4</sup>He in natural fluids that outgas in a region of active tectonic can record the evolution of the field of stress and this volatile component could be used to trace changes in stress and deformation field. For the purpose of quantifying the amount of <sup>4</sup>He present in the geological traps that feed the mud volcanoes of Regnano-Nirano mud volcanoes systems (Bonini et al., 2007), in the north Italy, in our study we have reconstructed the 3D geological model of the reservoirs, and proceeded to estimate the gas contained in them. Fluids emitted from these systems are thermogenic-CH<sub>4</sub> rich, which vertically migrates towards the surface. Helium is in traces and its isotopic signature (&#8776;0.01-0.02Ra, Ra is the <sup>3</sup>He/<sup>4</sup>He in air) shows that <sup>4</sup>He is mainly produced in the crust by U-Th decay. We have found that the present <sup>4</sup>He is greater than what should be available taking into account only the steady-state crustal production. Therefore, we compared the excess helium present in the reservoirs with the contribution coming from the seismic activity of the area, which is sufficient to explain this excess. Our study highlights that an intense fracturing of a volume of rock, due to the recent seismicity below the studied area, may explain the accumulation of helium in the reservoir higher than the steady-state condition. Therefore, the effective vertical rate of fluid transport in the Earth's continental crust can be characterized by episodic events controlled by fracturing.</p><p>Bonini (2007) - JGRes Solid Earth&#160;</p><p>Torgersen & O&#8217;Donnell (1991) - GRL, vol.18</p><p>Bauer (2017) - SAND2017-9438</p>
The geochemical characteristics of fluids that emerge at the Earth's surface are influenced by gas-rock-water interactions in the deep and shallow crustal layers, including mixing, outgassing of volatiles, and precipitation of minerals. The goal of the study was to understand the various interactions that influence the migration and behaviour of fluids within the Earth's crust and how they may change during the process of crustal fluid migration towards a hydrothermal system in the shallow crustal layers and within (Contursi basin, Italy). These processes can make it difficult to identify the source of deep gas by using the classical approach based on mixing processes of fluids and carbonate dissolution. Therefore, alternately the relationship between Total Dissolved Inorganic Carbon (TDIC) and the δ13CTDIC in groundwater from the Contursi hydrothermal system investigating the water-gas-rock interaction at the local scale through the detailed reconstructions of the geological framework at depth have been taken into consideration. We found that both the dissolved and free gas in the hydrothermal system probably originated from a deep CO2 endmember with a δ13CCO2 value ranging from +2.12‰ to +3.20‰ (PDB) depending on the presence of brine or freshwater in the local aquifers. However, we observed that this CO2 lost its pristine carbon isotopic signature during its storage in the deep dolomite-composed reservoirs (6-8 km), making it challenging to figure out its deep origin (decarbonation vs mantle/magmatic CO2). Our calculations also showed that the output of CO2, taking into account secondary processes (i.e. degassing CO2 and calcite precipitation) and interactions with water at different salt concentrations, could be at least 40% higher than estimates from the mixing-only approach, such that it is comparable with several active and quiescent worldwide volcanic systems. In order to interpret potential geochemical changes that may occur during future seismic events in sites like Contursi, which are earthquake-prone areas, it is necessary to implement models that can help us understand fluids origin and the processes that influence their chemical and isotopic signature.
The High Agri Valley (southern Italy) is one of the largest intermontane basin of the southern Apennines affected by intensive agricultural and industrial activities. The study of groundwater chemical features provides much important information useful in water resource management. In this study, hydrogeochemical investigations coupled with multivariate statistics, saturation indices, and stable isotope composition (δD and δ18O) were conducted in the High Agri Valley to determine the chemical composition of groundwater and to define the geogenic and anthropogenic influences on groundwater quality. Twenty-four sampling point ( including well and spring waters) have been examined. The isotopic data revealed that groundwater has a meteoric origin. Well waters, located on recent alluvial-lacustrine deposits in shallow porous aquifers at the valley floor, are influenced by seasonal rainfall events and show shallow circuits; conversely, spring waters from fissured and/or karstified aquifers are probably associated to deeper and longer hydrogeological circuits. The -mode factor analysis shows that three factors explain 94% of the total variance, and F1 represents the combined effect of dolomite and silicate dissolution to explain most water chemistry. In addition, very low contents of trace elements were detected, and their distribution was principally related to natural input. Only two well waters, used for irrigation use, show critical issue for NO3- concentrations, whose values are linked to agricultural activities. Groundwater quality strongly affects the management of water resources, as well as their suitability for domestic, agricultural, and industrial uses. Overall, our results were considered fulfilling the requirements for the inorganic component of the Water Framework Directive and Italian legislation for drinking purposes. The water quality for irrigation is from “good to permissible” to “excellent to good” although salinity and relatively high content of Mg2+ can occasionally be critical.
Abstract In order to investigate the variability of helium degassing in continental regions, its release from rocks and emission into the atmosphere, here we studied the degassing of volatiles in a seismically active region of northern Italy (Mw MAX = 6) at the Nirano-Regnano mud volcanic system. The emitted gases in the study area are CH 4 –dominated and it is the carrier for helium (He) transfer through the crust. Carbon and He isotopes unequivocally indicate that crustal-derived fluids dominate these systems. An high-resolution 3-dimensional reconstruction of the gas reservoirs feeding the observed gas emissions at the surface permits to estimate the amount of He stored in the natural reservoirs. Our study demonstrated that the in-situ production of 4 He in the crust and a long-lasting diffusion through the crust are not the main processes that rule the He degassing in the region. Furthermore, we demonstrated that micro-fracturation due to the field of stress that generates the local seismicity increases the release of He from the rocks and can sustain the excess of He in the natural reservoirs respect to the steady-state diffusive degassing. These results prove that (1) the transport of volatiles through the crust can be episodic as function of rock deformation and seismicity and (2) He can be used to highlight changes in the stress field and related earthquakes.
With the aim of deepening our understanding of deep-seated fluids upwelling and mixing in large regional aquifers, we performed a hydrogeochemical study of twenty-two springs in the Contursi area (upper Sele river valley, southern Apennines) by means of the measurements of chemical-physical parameters, major ions, trace elements, and stable and radioactive isotopes. Besides, we realized two updated geo-structural cross-sections in order to reconstruct the groundwater flowpath in the study area. The hydrogeochemical composition, as well as the water temperature allow to identify-three main groups of groundwater: Cold and Low salinity Groundwater (CLGW), Intermediate Salinity Groundwater (ISGW), and Thermal Salinity Groundwater (TSGW). The CLGW group, mostly emerging at the boundary of carbonate aquifers, is characterized by alkaline earth-bicarbonate hydrofacies. Instead, ISGW and TSGW, situated in the inner zone of the valley, show gradually a hydrogeochemical evolution towards sodium-chloride type hydrofacies domain with the highest salinity value. Stable isotope (δ18O-δD) of CLGW reveal the local meteoric origin of groundwater, while isotopic signatures of ISGW and TSGW is associated with the deep fluids inflow. CLGW hydrogeochemistry is clearly related to dissolution of carbonate rocks. On the other hand, for ISGW and TSGW an additional contribution from evaporitic rocks is supported by saturation indices values (gypsum and anhydrite) and validated by isotopic signature of dissolved sulphate (δ34S-δ18O). The application of two models based on tritium data (i.e., the piston-flow and well-mixed reservoir) attributes longer and deeper groundwater flowpaths to TSGW. Through geothermometric calculations (e,g., K-Mg and SiO2-quartz), the equilibrium temperature of deep fluids reservoir is also extrapolated (i.e., 75–96 °C). The results of the adopted hydrogeochemical multi-component approach allowed us to propose an interpretative model of groundwater flowpath for the Contursi area, where deep-seated tectonic discontinuities play a significant role for the upwelling of saline deep thermal fluids in shallow aquifers.
