Active Degassing of Deeply Sourced Fluids in Central Europe: New Evidences From a Geochemical Study in Serbia

We report on the results of an extensive geochemical survey of fluids released in the Vardar zone (central‐western Serbia), a mega‐suture zone at the boundary between Eurasia and Africa plates. Thirty‐one bubbling gas samples are investigated for their chemical and isotopic compositions (He, C, Ar) and cluster into three distinct groups (CO2‐dominated, N2‐dominated, and CH4‐dominated) based on the dominant gas species. The measured He isotope ratios range from 0.08 to 1.19 Ra (where Ra is the atmospheric ratio), and reveal for the first time the presence of a minor (<20%) but detectable regional mantle‐derived component in Serbia. δ13C values range from −20.2‰ to −0.1‰ (versus PDB), with the more negative compositions observed in N2‐dominated samples. The carbon‐helium relationship indicates that these negative δ13C compositions could be due to isotopic fractionation processes during CO2 dissolution into groundwater. In contrast, CO2‐rich samples reflect mixing between crustal and mantle‐derived CO2. Our estimated mantle‐derived He flux (9.0 × 109 atoms m−2 s−1) is up to 2 orders of magnitude higher than the typical fluxes in stable continental areas, suggesting a structural/tectonic setting favoring the migration of deep‐mantle fluids through the crust.


Geological Setting
The sector of the Vardar zone in Serbia (south eastern Europe, SEE) is part of the mega-suture stretching along the entire Balkan Peninsula (e.g., Cvetković et al., 2016). Its present-day geological setting is the result of a complex geodynamic and tectonic evolution over the last ∼200 Ma (from the middle Mesozoic to the present) that progressively involved subduction, continental collision, and finally lithospheric extension (e.g., Belinic et al., 2021;Cvetković et al., 2004). The engine of the regional geodynamic evolution is the interaction between Eurasian (Europe) and Gondwana (Africa) continental plates (Cvetković et al., 2016). More in detail, Serbia is part of the orogenic system composed by the Alpine, Carpathian, and Dinaride belts (e.g., Marović et al., 2007;Schmidt et al., 2008Schmidt et al., , 2019 and its territory can be divided into distinct tectonic units: (a) the Pannonian basin (northern part), (b) the Dinaric Alps (central-western part), (c) the Vardar zone, divided in East and West zones (the study area, Figure 1), (d) the Serbian-Macedonian Massif, a belt stretching in north-south direction into north-western Macedonia and northern Greece, (e) the Carpatho-Balkan Region (eastern part), and (f) the Dacia basin (Bazylev et al., 2009;Cvetković et al., 2004;Jelenković et al., 2008;Moores & Fairbridge, 1997).
The final stage of the regional geodynamic evolution involved an extensional phase of lithospheric thinning (Cvetković et al., 2016) that culminates in the Pannonian Basin and the Serbian-Macedonian Massif (40-50 km of lithospheric crust) and is associated to an asthenosphere up-rise (Cvetković et al., 2016;Milivojević, 1993). The heat flow distribution in the Pannonian basin is consequently high (from 50 to 130 mW/m 2 ), and the highest values are observed in the Great Hungarian Plain, the Pannonian part of Serbia (Vojvodina) including its continuation into the Vardar zone (Horvath et al., 2015;Lenkey et al., 2002).
The region is characterized by active seismicity with earthquake magnitudes up to 6.5 and hypocenters down to 20-30-km depth (http://www.seismo.gov.rs/Seizmicnost/Katalog-zemljotresa.pdf) and this depth coincides with the regional crust-mantle boundary (Marović et al., 2007;Metois et al., 2015). The studied area is highlighted by the dashed ellipse on the map. Small inset at the bottom left indicates the European areas in which geochemical studies on natural degassing have been conducted (shaded yellow area); (b) geological map of Serbia with sample locations, main regional faults (red), and neotectonic faults (black). The small inset is a zoom on the area in which a sample with lowest R/Ra has been collected in correspondence to a granitoid intrusion.

Materials and Methods
Thirty-one bubbling gas samples were collected from north to south in the central and western sectors of Serbia ( Figure 1b). The bubbling gases were sampled by using an inverted funnel that was positioned above the bubbles, so the gases fluxed through a two valves glass or steel bottle to avoid air contamination. Once the bottles had been flushed with an amount of gas at least tens of times the volume of the bottles (20-30 cc) the valves were closed to trap the gases into the bottles. All chemical and isotopic analyses were carried out at the laboratories of the INGV-Palermo within 1 month from the sampling in order to prevent isotopic fractionation due to storage of gases. Water temperature and pH were measured in the field by using a portable multiparameter instrument (WTW Multi 350i), which was previously calibrated using standard solutions ( Table 1). The chemical composition of the gases was analyzed by an Agilent 7890B gas chromatograph using Ar as carrier gas, and equipped with 4-m Carbosieve S II and PoraPlot-U columns. A thermal conductivity detector (TCD) was used to measure the concentrations of O 2 , N 2 , and CO 2 , while a flame ionization detector (FID) was used for CH 4 . Analytical errors of the measured concentrations are always within 5%.
The 13 C/ 12 C ratios of CO 2 (expressed as δ 13 C-CO 2 in ‰ versus the V-PDB standard) were measured with a Finnigan Delta S mass spectrometer after purification of the gas mixture by standard procedures using cryogenic traps with precision of ±0.1‰. He isotopes were analyzed using a static vacuum mass spectrometer (GVI Helix SFT), using a double collector in order to detect 3 He and 4 He ion beams simultaneously (precision for isotopic ratio within ±0.5%). The 3 He/ 4 He ratio was determined by measuring 3 He in an electron multiplier detector and 4 He in an axial Faraday detector. 20 Ne was measured with a multicollector Thermo-Helix MC Plus mass spectrometer. Helium isotope compositions are expressed as R/Ra, normalizing the 3 He/ 4 He ratio of the sample against the atmospheric 3 He/ 4 He ratio (Ra = 1.386 × 10 −6 ; Ozima & Podosek, 2002). The Ar concentrations and its isotope compositions ( 40 Ar, 38 Ar, and 36 Ar) were analyzed by multicollector Helix MC-GVI mass spectrometer with analytical uncertainty (1σ) for single 40 Ar/ 36 Ar measurements of <0.1%.

Results
The chemical composition of the sampled gases, together with the isotopic composition of He, Ar, and C (CO 2 ), are presented in Table 1.
On the base of their chemical compositions, the studied gases are subdivided into three different groups: CO 2 -dominated (CO 2 > 50%), N 2 -dominated (N 2 > 50%), and CH 4 -dominated (this includes only sample SRB31, being methane-rich: 87%) ( Table 1). CO 2 -dominated and N 2 -dominated samples have CH 4 concentrations ranging from 0.02% to 19. Ar and O 2 concentrations are typically ≤1% (Table 1), and He and Ne are present in trace amounts (ppmv). The CH 4 -dominated sample has a CO 2 concentration of 0.51% and N 2 of 11%. Also, for this sample, the O 2 and Ar concentration are very low (<1%) with He and Ne present in trace (37.3 and 0.24 ppmv). CO 2 and N 2 exhibit a negative correlation, as implied by their being the two dominant gas species (Figure 2). Only the CH 4 -rich sample, and two other N 2 -dominated samples (10.1-51.6% of N 2 ) that are bubbling gases from hyperalkaline waters (pH from 11.6 to 12.2) depart from a pure CO 2 -pure N 2 mixing line. Gases in hyperalkaline waters have high amount of H 2 (85% and 34%) and very low CO 2 amounts (<0.15%).
The δ 13 C CO2 values vary from −20.2‰ to −0.1‰ (Table 1). The N 2 -dominated gases exhibit the lowest δ 13 C-CO2 values, especially those with CO 2 <3% that plot in the field of biogenic CO 2 (Figure 3). The CO 2 -C isotopic compositions of bubbling gases in hyperalkaline waters also plot in the same biogenic field (Figure 3).
The He isotopic ratios, expressed as R/Ra, vary from 0.08 to 1.2 Ra, and the N 2 -dominated gases have the lowest He isotopic ratios ( Figure 4). The 4 He/ 20 Ne ratios mostly range from 13 to 1,300, and are much higher than the atmospheric ratio (0.318; Ozima & Podosek, 2002) Note.
Major elements in %, H 2 and noble gases in ppm. δ 13 C of CO 2 in ‰ versus V-PBD. Percentage of atmosphere (Atm) and mantle calculated following Sano et al. (1985).

Table 1
Location, Geochemical, and Isotopic Composition of the Sample Purple open circles that along the y axis are two samples of bubbling gases in hyperalkaline water. The light blue diamonds and the dark blue crosses depict data from some central and eastern Europe areas (Eger rift; Weinlich et al., 1999) and from the Austria/Slovenia border, Pannonian basin (Bräuer et al., 2016).

Figure 3.
A scatter plot of CO 2 concentrations versus CO 2 carbon isotopic composition (δ 13 C). N 2 -dominated gases (especially those with CO 2 <3%) exhibit the lowest δ 13 C CO2 values, falling in the field of the biogenic CO 2 (green bar). Gases from hyperalkaline waters also plot in the same field of biogenic CO 2 . CO 2 -rich samples have more positive carbon isotopic compositions, falling within the magmatic (orange) and metamorphic (blue) fields. The three colored boxes indicate the typical δ 13 C ranges for the three different sources: green = biogenic, orange = magmatic, blue = metamorphic. Note the overlap between the two field (magmatic-metamorphic) at ∼4‰ (from Holland and Gilfillan [2013]).

Discussion
He in natural fluids from tectonically active regions is typically interpreted as originating from three distinct sources: the mantle, the crust, and air (e.g., Burnard et al., 2013;O'Nions & Oxburgh, 1988;Sano et al., 1997). These three sources are characterized by distinct He isotopic signatures: (a) 6.1 ± 0.9 Ra, for the European Subcontinental Lithospheric Mantle, ESCLM (Gautheron & Moreira, 2002); (b) 0.01-0.02 Ra, for pure crustal fluids dominated by radiogenic 4 He produced by U and Th decay (Ballentine & Burnard, 2002); (c) 1 Ra, for air (Ozima & Podosek, 2002). 4 He/ 20 Ne ratios are >1,000 for crust and mantle and 0.318 for air respectively (Sano et al., 1985). Because of these different end-member compositions, He isotopes in natural fluids, coupled with their 4 He/ 20 Ne ratios, can be used to resolve the relative He contributions from the three sources (e.g., Caracausi & Sulli, 2019;Sano & Wakita, 1985;Sano et al., 1997, and references therein). Using the approach proposed in Sano et al. (1997), and assuming that all 20 Ne is atmospheric, we estimate low atmospheric contributions (<3%, Table 1) for all samples, except those collected from the hyperalkaline waters, and mantle helium fractions of 1% to ∼20%, with the highest fractions calculated for the CO 2 -dominated samples ( Figure 4).
It is interesting to note that the two N 2 -rich samples (SRB10 and SRB11) with the lowest He isotopic signatures (R/Ra < 0.1; mantle component ∼1%) have been collected nearby two large granite intrusions (see inset in Figure 1) that are characterized by high U and Th concentrations (of, respectively, 563 and 270 ppm) (Schefer et al., 2011). Hence, it is reasonable that these low He isotopic ratios reflect the high radiogenic 4 He production in the U-Th-rich lithologies.

Insights From CO 2 / 3 He Ratios
Additional insights into volatile sources and sinks, and into the processes occurring during (a) the migration of fluids through the crust and (b) their storage in shallow crustal layers can be derived from a joint analysis and interpretation of He and carbon isotopic signatures (e.g., Barry et al., 2020;Holland & Gilfillan, 2013). Our study highlights that natural gases in the Vardar zone of Serbia are dominated by either CO 2 or N 2 ( Figure 2) and are characterized by a significant spread of δ 13 C compositions ( Figure 3) and R/ Ra ratios (Figure 4) that could reflect a multiplicity of gas sources involved. 3 He in natural fluids is mainly primordial and sourced from the mantle. Thus, combining CO 2 and 3 He (into the CO 2 / 3 He ratio) allows evaluating enrichments or depletions relative to a mantle-like signature ( Figure 5).
Our CO 2 -rich and N 2 -rich fluids are characterized by distinct CO 2 / 3 He ratios that are, respectively, higher (up to 5.26 × 10 11 ) and lower (as low as 5.89 × 10 6 ) than the above defined mantle range ( Figure 5). In tandem with gas samples from nearby regions (Eger rift, Weinlich et al., 1999; Austria/Slovenia border region, Pannonian basin; Bräuer et al., 2016), our samples identify a continuous trend from (a) a CO 2 -rich, high CO 2 / 3 He ratio end-member, and (b) a 4 He-20 Ne-rich, low CO 2 / 3 He ratio end-member (Figures 5a-5c). The high CO 2 / 3 He ratios (10 12 -10 14 ) of most CO 2 -rich crustal continental gases are commonly interpreted (Sano & Marty, 1995;Sherwood Lollar et al., 1997) to result from decarbonation reaction and biological processes in the crust that produce a CO 2 -rich, 3 He-free gas. We thus propose that the CO 2 -dominated gases are mix- tures of CO 2 -rich crustal gas with a 5-20% mantle-derived component (Figure 4). This is additionally supported by Figure 6, in which the CO 2rich samples fall along hypothetical mixing curves between a SCLM pole and a set of hypothetical crustal end-members with same radiogenic R/ Ra ratio but different CO 2 / 3 He ratios.
Moreover, a crustal (limestone + organic-biogenic) carbon addition to a SCLM-like gas is suggested by the δ 13 C versus CO 2 / 3 He ratio plot of Figure 7.
Interpreting the N 2 -dominated samples is less straightforward. However, except for sample SRB12, He in all the investigated N 2 -dominated gases is minimally contributed by the mantle (<5%; Figure 4) and by atmosphere ( Figure 4). Furthermore, these samples exhibit the highest 4 He and 20 Ne contents (Figures 5b and 5c), and the lowest CO 2 / 3 He ratios and He isotopic signatures ( Figure 6). Although there is no a priori reason to expect a correlation between 4 He and 20 Ne with the CO 2 / 3 He ratio, such a correlation has been found regionally in natural gases Gilfillan et al., 2009). 4 He is constantly produced in the subsurface by the radiogenic decay of U, Th, while 20 Ne enters subsurface groundwater systems as a component of air-saturated meteoric water (Ballentine & Sherwood Lollar, 2002). This atmospheric component can then be transferred to natural fluids in crustal layers, interacting with the groundwater that are able to trap the air component together with large amount of radiogenic volatiles (e.g., 4 He) produced over time into the crust and degassing through it (e.g., . Previous studies indicated that such correlations are the result of 4 He accumulating in the groundwater which also contains atmospheric-derived 20 Ne, and subsequent quantitative partitioning of both 4 He and 20 Ne into the gas phase due to fractionation events, probably in the groundwater (e.g., Gilfillan et al., 2008). It is worth noting that gases from gas-fields from central and eastern Europe (e.g., Eger rift, Austria/Slovenia border region, Pannonian basin) fit with similar CO 2 -N 2 -He concentration arrays (e.g., Bräuer et al., 2016;Weinlich et al., 1999), supporting the recurrence of solubility-dependent volatile fractionation.
The low CO 2 concentrations and low CO 2 / 3 He ratios in the N 2 -dominated gases (Figures 4-6), combined with their more negative 13 C-compositions (Figure 7), imply some mechanism of CO 2 removal during gas-water-rock interactions. During their migration through the crust, volatiles can interact with groundwater and, due to its high solubility, CO 2 dissolves preferentially in water relative to He (in the range of temperature up to 90 °C: CO 2 solubility > He solubility; Clever et al., 1979;Gilfillan et al., 2009;Scharlin et al., 1996). Furthermore, groundwater can also precipitate carbonate minerals, additionally modifying the dissolved carbonate equilibria Gillfillan et al., 2009). In both cases, CO 2 is retained either in form of carbonate minerals (mineral trapping) or dissolved in solution (solubility trapping) (e.g., Baines et al., 2004;Bradshaw et al., 2005) leading to decreased CO 2 / 3 He ratios and more negative δ 13 C in the residual gases.  20 Ne concentrations. The panel shows a trend from CO 2 -rich, high CO 2 / 3 He (low in He and Ne) samples to He-Ne-rich, low CO 2 / 3 He ratio samples. The SCLM range is given by the shaded area (CO 2 / 3 He = 2-7 × 10 9 ; Bräuer et al., 2016;Marty et al., 2020). Data for other central and eastern Europe areas follow the same trend (dark blue crosses, Bräuer et al., 2016;light blue diamonds, Weinlich et al., 1999).
In order to interpret the variability of CO 2 / 3 He ratios coupled to that of δ 13 C that we recognized in the Vardar zone samples, we investigate the processes of CO 2 partial dissolution in water, and calcite precipitation, by modeling (see Gillfillan et al., 2009) their potential control on CO 2 / 3 He ratios and CO 2 carbon isotopic compositions (δ 13 C) (Figure 7). According to Gillfilan et al. (2009), the process can be modeled as (a) an open-system degassing (Rayleigh type) at isotopic equilibrium (between phases) and (b) calcite precipitation (Figure 7). We model the progressive variation of the CO 2 / 3 He ratio in the residual gas assuming that the CO 2 / 3 He ratio and the δ 13 C CO2 of the pristine gas are of mantle-type (CO 2 / 3 He range = 2-7 × 10 −9 , δ 13 C = −3.5‰; Bräuer et al., 2016;Marty et al., 2020;Rizzo et al., 2018). We stress that here we consider the case of a pristine gas as the mantle end-member, but the choice of a different end-member, resulting from the mixing between crustal (limestone + organic-biogenic) and mantle-derived fluids, would lead to similar (but shifted) model curves.
Our model curves, obtained over a range of pH values for increasing extents of gas dissolution, are plotted in Figure 7. Overall, we find the model CO 2 dissolution lines at pH between 5.6 and 7 fit the data set nicely. This comparison demonstrates the N 2 -dominated samples can be interpreted as due to different degrees of CO 2 loss by dissolution, from about 50% (for samples SRB 14, SRB 25, SRB 26) to about 99% for more fractionated samples. These gas/water fractionations ultimately result in 13 C-depleted compositions and CO 2 / 3 He spanning over 3 orders of magnitude.
We caution that, for a thick crustal sector with a potentially high number of stratified aquifer such as in Serbia, a simple open-system degassing (Rayleigh type) model approach is evidently a simplified approach. In fact, it is possible that more complex gas-aquifer interactions, such as complete gas dissolution in deep aquifer, followed by multistep degassing upon groundwater upward migration (Chiodini et al., 2011), could have taken place instead. Also, we cannot exclude the lowest δ 13 C CO2 values are not at least partially reflecting a biogenic origin, and carbonate precipitation (together with CO 2 dissolution at a lower pH than 5.6-7; Gillfillan et al., 2009) has not taken a role. This notwithstanding, although simplified, our model clearly highlights the role played by gas-water interaction in determining the composition of Serbian gas manifestations.

Mantle Helium Source and Tectonic Implications
The chemistry of both CO 2 -dominated and N 2 -dominated gas samples unravels the active outgassing of mantle-derived volatiles (He and, to a lesser extent, CO 2 ) in Serbia. In continental areas far from any evidence of active volcanism, the possible main sources of mantle-derived volatiles are (a) reservoirs of fossil mantle-derived volatiles (e.g., Ballentine et al., 2001), (b) the presence of magmatic intrusions into the crust, and (c) the transfer of mantle He through lithospheric faults (e.g., Burnard et al., 2013;Caracausi & Sulli, 2019;Kennedy et al., 2007;Lee et al., 2019). A reservoir of fossil mantle-derived volatiles as a source of the mantle He should not be associated to a heat-excess, as presently observed at regional scale in Serbia (up to 130 mW/m 2 ). Magmatic intrusions in the crust could in principle supply both mantle-derived heat and fluids toward the surface. However, at a regional scale, a magmatic intrusion can be considered as a localized source of both volatiles and heat. In spite of some possible long-range transport through groundwaters, the He isotopic ratio and heat flux anomaly should thus decrease upon increasing distance from the position of the source at depth. In the study area, in contrast, we recognize a fairly homogeneous and generalized outgassing of mantle-derived He (Figure 1) and high regional heat flow. Therefore, it is unlikely that isolated magmatic intrusions in the crust are involved. Figure 6. 3 He/ 4 He (expressed as R/Ra) versus CO 2 / 3 He ratio plot of the sampled gases. Binary mixing curves are shown between the SCLM (6.1 ± 0.9 Ra and CO 2 / 3 He of 7 × 10 9 ; Bräuer et al., 2016;Gautheron & Moreira, 2002) and different hypothetical crustal end-members with same helium isotopic composition (0.02 Ra) but variable CO 2 / 3 He ratios. N 2 -dominated, CH 4 -dominated, and alkaline springs require CO 2 loss via gas-water-rock interactions. Volatiles (i.e., CO 2 , He) can reach the surface directly from the mantle through lithospheric faults (e.g., Burnard et al., 2012;Caracausi & Sulli, 2019;Lee et al., 2019), acting as a network of pathways of high permeability enhancing the transfer of deep fluids and heat through the crust. The study area is strongly affected by active tectonics as indicated by seismicity (http://www.seismo.gov.rs/Seizmicnost/Katalog-zemljotresa. pdf.). All the investigated emissions are located along tectonic discontinuities, even if all of them are not in correspondence of the main regional faults (Figure 1). Hence, a system of well-connected faults with roots down to the mantle, through which the fluids and heat from the mantle can cross the crust and reach the surface, seems the most plausible mechanism to explain the combined high heat flux and regional-scale outgassing of mantle He in the study area.
In Serbia, crustal thickness progressively increases in ∼260 km, from about 25 km in the north up to 35 km in the south (Horváth et al., 2015;Marovic et al., 2007). The greater thickness in the south of Serbia could lead to a higher production of 4 He by the U and Th decay if we assume a homogeneous and constant distribution of U and Th concentrations in the crust below the study area. However, we find no geographical control on He isotopic signature, and a large He isotope variability occurs sometimes over short distances (e.g., 1 order of magnitude change in only 27 km) (see Figure S1 in Supporting Information S1). Therefore, the variability of the He isotopic signature does not appear to correlate with crustal thickness. Moreover, we highlight that the lowest He isotopic signatures (SRB10 site, 0.08 Ra; SRB11 site, 0.10 Ra) have been measured in fluids that circulate in U-rich and Th-rich granitic rocks. Thus, it is reasonable that the lowest He isotopic signatures could be due to local high production of 4 He ( Figure S1 in Supporting Information S1; Section b) from granitoid lithologies.
A quantitative He flux estimate can provide insights into the transfer of volatiles through the crust. Estimates of the 4 He flux in continental regions are mainly based on calculations of in-situ production and steady-state degassing through the continental crust, and these calculations yield a crustal 4 He degassing flux of ∼3.3 ± 0.5 × 10 10 atoms m −2 s −1 (Buttitta et al., 2020, and references therein). However, experimental work highlights that the release of volatiles from rock increases in an active stress field, which implies that 4 He degassing through the crust can be episodic in active tectonic areas (e.g., Bräuer et al., 2016;Honda et al., 1982;Torgersen & O'Donnell, 1991). It is worthy of note that deformation and failure of rocks crack mineral grains, causing pervasive microfracturing. Consequently, the rocks can increase their porosities from 20% to as high as 400% prior to failure, opening new microfracture surfaces, and eventually causing macroscopic failure and fracture of rocks (Bräuer et al., 2016). These processes lead to a higher release of volatiles (e.g., He) previously trapped within mineral grains along fracture networks and the pore fluids transport these volatiles through the crust.
Considering that, during the transfer of mantle-derived fluids through the crust, the addition of crustal radiogenic 4 He produces a decrease of the pristine mantle He isotopic ratio, it is possible to assess the flux of mantle-derived He by using the approach proposed by O'Nions and Oxburgh (1988) and making a guess for the crustal He flux range. This method is based on the assumption that, if the degassing of He occurs at steady state, then it is possible to estimate the mantle He flux from the helium isotope composition of the system. This principle is illustrated in Figure 8 that shows the dependence of R/Ra in the surface gas on mantle He flux (for a crustal 4 He flux of 3.3 ± 0.5 × 10 10 atoms m −2 s −1 ). From this, we estimate a mantle-derived He flux in the study area of ∼2.1 × 10 8 to ∼9.0 × 10 9 atoms m −2 s −1 , up to 2 orders of magnitude higher than normally found in stable continental areas (<<10 8 ;e.g., O'Nions & Oxburgh, 1988). In the case of precipitation there is zero 3 He loss from the CO 2 phase and CO 2 / 3 He changes in proportion to the fraction of the remaining CO 2 phase while for CO 2 dissolution, the change in CO 2 / 3 He ratio is calculated following the Rayleigh equation.
However, in active tectonic regions an enhanced release of He from rocks occurs that is up to 10 4 times higher the steady-state values. Therefore, assuming a 4 He crustal flux of 10-10 4 times the average "steady-state" value, the mantle He fluxes increase to between 10 11 and 10 14 atoms m −2 s −1 (Figure 8). These are typical He fluxes encountered in active tectonic regions and/or in volcanic systems (Figure 9; Torgersen, 2010).
Active fault zones are regions of advanced permeability that permit a fast transfer of volatiles through the crust, and seismicity is a strong evidence of the capacity of faults to transfer fluids through the crust. However, the mechanisms that control the migration of fluids in the deep crust (e.g., ductile layers) are still not well recognized (e.g., Caracausi & Sulli, 2019;Kulongoski et al., 2005). In active tectonic regions, fluids can move via developing fault-fracture meshes with a mechanism analogous to the fault valve model that drives flow by fluid over-pressurization and stress switching (compression to extension) (Newell et al., 2015;Sibson, 2013Sibson, , 2020, or by creep cavitation that can establish a dynamic granular fluid pump in ductile shear zones (i.e., Fusseis et al., 2009). Therefore, considering: (a) that the study area is affected by extensional tectonics and active seismicity down to the crust-mantle boundary (Faccenna et al., 2014;Marović et al., 2007;Metois et al., 2015); (b) the high regional heat flow (up to 130 mW/m 2 ) due to the up-rise of the asthenosphere up to 50-60-km depth at regional scale (Horváth et al., 2015), (c) the presence of inherited lithospheric tectonic discontinues that allowed the up-rise of magmas since the Jurassic (Zelić et al., 2010), and that can still work today as pathways for the transfer of deep volatiles through the crust, (d) the computed high fluxes of mantled derived He, we conclude that the mantle below Serbia is the most obvious source of the surface-released heat and fluids.  (1988) that is based on the progressive addition (as a mixing) of a crustal He component that dilute the mantle He component producing a decrease of the He isotopic signature from the typical mantle-derived component (6.1 Ra; Gautheron & Moreira, 2002) to the radiogenic signature (0.02 Ra; Ballentine & Burnard, 2002). The solid curve refers to an average continental crust 4 He steady-state flux of 3.3 ± 0.5 × 10 10 atoms m −2 s −1 (Buttitta et al., 2020). The dotted curves refer to 10× and 100× the average continental crust steady-state He flux. The blue dotted line corresponds to minimum and dark blue dashed line to maximum R/Ra values in our samples, and are used to infer the mantle He flux range in Serbia region.

Conclusions
We investigated the chemical and isotopic composition of natural gas manifestations along the Serbian Vardar zone, a mega-suture zone between the Eurasia and the African plate. Gas compositions are very heterogeneous and cluster into the groups of CO 2 -dominated, N 2 -dominated, and CH 4 -dominated gases. Based on their He isotope compositions (<1.19 Ra), the CO 2 -rich samples are interpreted as mixtures of crustal CO 2 -rich gas (from limestones and organic matter) and mantle-derived components. The latter accounts for up to 20% of He ( Figure 10). N 2 -dominated samples are more atmospheric/crustal in nature (mantle He, <5%), and are inferred to have experienced extensive chemical and isotopic fractionations during water-gasrock interactions in shallow crustal layers ( Figure 10).
We estimate a mantle-derived He flux of ∼2.1 × 10 8 to ∼9.0 × 10 9 atoms m −2 s −1 , or 2 orders of magnitude higher than normally found in stable continental areas. This elevated transport of mantle-derived volatiles in the Serbian crustal sector is interpreted to occur through lithospheric faults that work as regions of enhanced permeability and favor the migration of fluids thought the whole crust ( Figure 10). Our study thus confirms that elevated outgassing of mantle-derived fluids can occur in tectonically active continental regions, even far from active volcanism (e.g., Caracausi & Sulli, 2019;Chiodini et al., 2004;Lee et al., 2019;Tamburello et al., 2018). Finally, we recognize that at regional scale the mantle volatiles are sourced directly from the mantle together with heat and this scenario supports the asthenosphere up-rise and delamination processes at the mantle-crust boundary recognized by recent regional geophysical investigations (Belinić et al., 2021).  (1988). The assessed mantle-derived He flux for the Serbian gases (orange diamonds), using the highest R/Ra value, is ∼9.0 × 10 9 atoms m −2 s −1 while for the lowest R/Ra value the mantlederived He flux is ∼3 × 10 9 atoms m −2 s −1 . For crustal-derived 4 He fluxes being 10-10 4 times higher than the "steadystate" crust, the mantle helium fluxes would also be in the order of magnitude of values characteristic of "Volcanic field" and/or "Tectonic-strain field" (red dotted ellipse area; modified after Torgensen [2010]). The six orange diamonds (Serbian point) represent, respectively, the mantle 4 He flux values for maximum and minimum R/Ra calculated on the base of continental crust 4 He production (3.3 ± 0.5 × 10 10 atoms m −2 s −1 ; Buttitta et al., 2020) and for 10 times and 100 times this value.

Acknowledgments
This work was supported by MIUR project PRIN2017-2017LMNLAW "Connect4Carbon" and DCO Grant 10881-TDB "Improving the estimation of tectonic carbon flux", by the European Union and the State of Hungary, co-financed by the European Regional Development Fund in the project of GI-NOP-2.3.2-15-2016-00009 "ICER" and by UBB grant nr GTC 35290/18.11.2020. Field work was supported by Brem Group Belgrade. The authors thank INGV-Palermo for supporting the analysis carried out in its laboratories and Mariano Tantillo, Aldo Sollami, Ygor Oliveri, and Francesco Salerno (gases chemistry, noble-gas, and stable isotopes) for their analytical contribution. The authors also thank the two anonymous reviewers for their critical comment which contributed to improve the manuscript.