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Determining the origin of carbon dioxide and methane in the gaseous emissions of the San Vittorino plain (Central Italy) by means of stable isotopes and noble gas analysis

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The chemistry and isotope ratios of He, C (δ13C) and H (δD) of free gases collected in the San Vittorino plain, an intramontane depression of tectonic origin, were determined to shed light on mantle degassing in central Italy. The C isotopic
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  Determining the srcin of carbon dioxide and methane in the gaseousemissions of the San Vittorino plain (Central Italy) by means of stableisotopes and noble gas analysis Francesca Giustini a , Michaela Blessing b , Mauro Brilli a, ⇑ , Salvatore Lombardi c , Nunzia Voltattorni d ,David Widory e a Istituto di Geologia Ambientale e Geoingegneria, CNR, Area della Ricerca di Roma 1, Via Salaria Km 29.300, 00015 Monterotondo Staz., Rome, Italy b BRGM, 3 avenue Claude Guillemin, BP 36009, 45060 Orleans Cedex 02, France c Dipartimento di Scienze della Terra, Università di Roma La Sapienza, P.le A. Moro, 5, 00185 Rome, Italy d Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Rome, Italy e UQAM/GEOTOP, 201 ave du Président Kennedy, Montréal, H2X 3Y7, Canada a r t i c l e i n f o  Article history: Available online 6 March 2013 a b s t r a c t The chemistry and isotope ratios of He, C ( d 13 C) and H ( d D) of free gases collected in the San Vittorinoplain, an intramontane depression of tectonic srcin, were determined to shed light on mantle degassingincentralItaly.TheCisotopiccompositionofCO 2 ( d 13 C–CO 2 À 2.0 ‰ to À 3.8 ‰ )andHeisotoperatios( R / R  A 0.12–0.27) were used to calculate the fraction of CO 2 srcinating from mantle degassing vs. sedimentarysources. The results show that CO 2 predominantly (average of 75%) derives from the thermo-metamor-phic reaction of limestone. Between 6% and 22% of the CO 2 in the samples derives fromorganic-rich sed-imentarysources.Themantlesourceaccountsfor0–6%ofthetotalCO 2 ;however,intwosamples,locatedinproximitytothemostimportantfaultsoftheplain,themantleaccountsfor24%and42%. Thepresenceof faults and fractures allows upward gas migration froma deep source to the Earth’s surface, not only inthe peri-Tyrrhenian sector, as generally reported by studies on natural gas emissions in central Italy, butalso inthe pre-Apennine and Apennine belts. Isotope ratios of CH 4 ( d 13 C–CH 4 À 6.1 ‰ to À 22.7 ‰ ; d D–CH 4 À 9 ‰ to À 129 ‰ ) showthat CH 4 does not appear toberelatedtomantleor magmadegassing, butit istheproduct of thermal degradation of organic matter (i.e. thermogenic srcin) and/or the reduction of CO 2 (i.e. geothermal srcin). Most of the samples appear to be affected by secondary microbial oxidationprocesses. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Central Italy is characterized by widespread CO 2 gas emissions.Numerous hypotheses have been proposed for the srcin of suchemissions, including carbonate hydrolysis (Panichi and Tongiorgi,1976), metamorphic decarbonation (Gianelli, 1985; Duchi et al.,1992) and magma degassing (Minissale, 1991). The CO 2 in thegas emissions of the peri-Tyrrheniansector is generally consideredto be a mixture, of varying proportions, of mantle degassingsources and the byproduct of decarbonation of crustal carbonates(Chiodini et al., 1995, 2000; Minissale et al., 1997; Minissale,2004). In the eastern sector of central Italy, particularly in thepre-Apennine and Apennine belts, the srcin of CO 2 , which oftenmanifests itself in solution in highly mineralized waters, remainsa matter of debate. Some authors (Heinicke et al., 2006; Italianoet al., 2008) have suggested, onthe basis of  d 13 CinCO 2 and He iso-tope ratios, that the srcin of CO 2 -dominated gases is purely crus-tal, the gases being released by mechanical energy during seismicevents or microseismicity. In recent work,Chiodini et al. (2011)studied the presence of a deep active CO 2 source in the epicentralarea of the Abruzzo region during the 2009 earthquake; theseauthors conducted a hydrogeochemical study that included theanalysis of major solutes, dissolved gases, water/DIC/He isotopiccompositions of the aquifer in the area. The authors of that studysuggested that the CO 2 was not generated by the seismic eventsbut, on the contrary, contributed significantly to the generationoftheearthquake.Accordingtothishypothesis,thedeeplysourcedCO 2 is confined to stratigraphic and/or structural traps and is, fol-lowing a long residence time, contaminated by crustal fluids.Natural CH 4 gas emissions in central Italy are prevalently lo-catedintheApennineforedeepbasin.MostofthisCH 4 isgeneratedby microbial activity (Mattavelli and Novelli, 1987), as the rela-tively shallow depth of the organic source and the d 13 C–CH 4 (< À 50 ‰ ) vs. the C 1 /(C 2 +C 3 ) concentration ratios (>1000) indicate(Tassi et al., 2012). This areaalso containsnumerousCH 4 -richmud 0883-2927/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.apgeochem.2013.02.015 ⇑ Corresponding author. Tel.: +39 0690672645. E-mail address: mauro.brilli@cnr.it(M. Brilli).Applied Geochemistry 34 (2013) 90–101 Contents lists available atSciVerse ScienceDirect Applied Geochemistry journal homepage:www.elsevier.com/locate/apgeochem  volcanoes that are, instead, of thermogenic srcin (Etiope et al.,2007). In the main geothermal and volcanic–hydrothermal sys-temsoftheperi-Tyrrhenianbelt, theCO 2 -dominatedgasemissionscontain above all abiogenic CH 4 deriving from CO 2 reduction athigh temperature (Tassi et al., 2012). By contrast, the CO 2 -richgas vents located in the sedimentary Tyrrhenian domain appearto produce CH 4 , with a significant contribution being made bythermogenic sources (Tassi et al., 2012).The San Vittorino plain is a non-volcanic area, located in theApennine chain in central Italy (Fig. 1A). The area is characterizedby marked gas emissions, with a maximum CO 2 flux (5.7  10 À 5 -kgm À 2 d À 1 ) that is comparable to that of volcanic areas locatedfurther west in central Italy (i.e. Latera and Ciampino;Lombardiet al., 2006).Annunziatellis et al. (2004), on the basis of the chem- ical composition of fluids associated with the sinkholes of theplain, tentatively explained the srcin of this CO 2 as the combina-tion of both a mantle source and the product of thermo-metamor-phic reactions within Mesozoic limestone.TheaimofthisstudywastodeterminetheoriginoftheCO 2 andCH 4 present in the gas emissions of the San Vittorino plain usingthe gas chemistry and isotope composition of C in CO 2 , of C andH in CH 4 and of He isotopes from free gases. Biological processesthat may modify gas composition during its migration towardsthe surface, as well as their relationship with the tectonic settingofthestudyarea,werealsoinvestigated.Thistypeofdatahasbeenwidely and successfully used to determine the relative contribu-tion of different sources of CO 2 and CH 4 . In particular, the well-knownapproachdescribedbySanoand Marty(1995)will be used,whichhasbeenexploitedextensivelyintheliterature(WernerandBrantley, 2003; Crossey et al., 2009; Ray et al., 2009; Cinti et al.,2011; among others); it disentangles the various CO 2 sourceschiefly by using the d 13 C–CO 2 and the CO 2 / 3 He ratio, and allowsthe percentages of the three main CO 2 sources to be calculated:(i) magmatic or mantle-derived, (ii) carbonate and (iii) sedimen-tary or organic. The C and H isotope composition of CH 4 allowsits sources to be distinguished and secondary processes to be in-ferred (Schoell, 1988; Welhan, 1988). 2. Geological and hydrogeological setting  The San Vittorino plain is an intramontane depression locatedin the central Apennines (elevation of 400–412m a.s.l.). The plainis the result of extensional and/or transtensive tectonics, displace-ment along major fault planes and regional uplift. This uplift wasparticularly intense in the Lower–Middle Pleistocene, which gaverisetoablockpatternwithdifferential verticalmovementsandre-sulted in the plain subsequently being filled by Upper Pleistocene Fig. 1. (A) Geological map of the San Vittorino plain and sampling locations of gas vents. (B) Geological cross-section of San Vittorino plain (modified fromCentamore et al.(2009)). Location of the cross-section is shown in (A). F. Giustini et al./Applied Geochemistry 34 (2013) 90–101 91  and Holocene alluvial and fluvio-lacustrine deposits (Centamoreand Nisio, 2003; Centamore et al., 2009). The northern edge of the plain is characterized by travertine deposit outcrops that lieon two orders of the Middle Pleistocene alluvial terraces (Centa-more et al., 2009). These are likely to have been formed by theemergence of mineralized water (Manfra et al., 1976; Minissaleet al., 2002) following intense tectonic activity at the end of theMiddle Pleistocene.The tectonic extensional network in the plain resulted in fourmain fault systems (Centamore et al., 2009), the most importantbeing the Fiamignano-Micciani and the Velino River transtensivefaults, trending NNW–SSE and NE–SW, respectively (Fig. 1A). Allthese systems result primarily from the reactivation of formerstructural elements, often of different natures (e.g. the Fiamig-nano-Micciani fault). The srcin of the Fiamignano-Micciani faultis still debated. Some authors believe that it is merely a recent(Holocene)andactivenormalfault,linkedtothecurrentseismicityin the area (Bosi, 1975; Morewood and Roberts, 2000). Accordingto other authors this fault is characterized by intense polyphaseactivity: extensional in the post-rift stage; then compressive dur-ing the Apennine chain building phase (Late Miocene–Early Plio-cene); finally extensional, with transtensive kinematics duringthe Plio-Pleistocene (Centamore and Nisio, 2002; Centamoreet al., 2009). Whatever the srcinof the Fiamignano-Micciani fault,it is highly likely that its transtensive activity and associated sys-temsplayedafundamentalroleintheformationoftheSanVittori-no plain by dislocating the Middle Pleistocene deposits.The presence of faults (which are not easily recognizable in thefield owing to the alluvial cover and landscape changes due toanthropogenicreworking),aswellastherecentandhistoricalseis-micity, point to very intense Quaternary tectonic activity (Michettiet al., 1994). The fault network, dissecting the bedrock buried un-der a thick alluvial cover, is highlighted by the alignments of sink-holes, small springs (0.1–2L/s) and gas emissions (Ciotoli et al.,2001; Annunziatellis et al., 2004; Petitta, 2009). The plain is alsocharacterized by the emergence of important water sources fromregional aquifers fed by both the Mount Paterno and Giano-Nuri-a-Velino Mountain hydrological systems (Boni et al., 1986, 1995).The discharge of the main springs, which are located along thenorth- and south-eastern boundaries of the plain, ranges from2000L/s to 18,000L/s. The plain of San Vittorino does not hostany aquifer units owing to the low permeability of the alluvialsediments.A soil gas survey (NASCENT, 2005) revealed very high concen-trations of CO 2 (>70%), CH 4 (2%) and He (80ppm) closely linkedto the sinkhole phenomena. They are believed to be associatedwiththemigrationofdeepCO 2 alongmajorfaultsinthearea(Lew-icki et al., 2006). 3. Methods Nine gas samples (Fig. 1A) were collected in 25mL stainlesssteel canisters sealed with two vacuumstop-cocks. Sampling loca-tions were associated with small sinkholes, filled with water char-acterized by vigorous bubbling of free gases. HeliumconcentrationsweremeasuredusingaVarianMass4spectrometer(analytical error±20ppb). Nitrogen, O 2 and CO 2 concentrationswere determined using a Fisons GC-8000 Series gas-chromato-graph coupled witha Thermal ConductivityDetector (TCD; analyt-ical error±1%). For CH 4 , the instrument was coupled with a FlameIonization Detector (FID) (analytical error±100ppb). Chemicalconcentrations were corrected for air contamination on the basisof the N 2 /O 2 ratio in the samples compared with the same ratioin the atmosphere.After off-line purification of CO 2 in a vacuum line, the C isotopecompositionof CO 2 ( d 13 C–CO 2 ) was determinedonaFinniganMAT252 (analytical error±0.1 ‰ ). Carbon ( d 13 C–CH 4 ) and H ( d D–CH 4 )isotope compositions of CH 4 were measured by GC-C/TC-IRMS(analytical error±0.5 ‰ and ±5 ‰ , respectively). The system con-sists of a gas chromatograph (Trace GC ultra) coupled via an inter-face (GC-C/TC III, operated at 940 ° C and 1420 ° C, respectively)with a Thermo Finnigan Delta plus XP. Separation of the gaseswas performed on a CP PoraPLOT Q column (Varian,25m  0.32mm, 10 l mfilmthickness). d 13 Cand d Dare expressedin delta notation relative to VPDB and VSMOW, respectively,following: d 13 C ‰ ¼ 13 C 12 C   sample 13 C 12 C À Á VPDB À 1 264375  10 3 and d D ‰ ¼ D 1 H À Á sampleD 1 H À Á VSMOW À 1 " #  10 3 Helium isotope ratios were measured using a static vacuummass spectrometer VG-5400TFT, according to the method de-scribed byCaracausi et al. (2005). The He isotope analysis wasnot performed for samples where it was under 10ppm. Typicaluncertainties for low 3 He samples are below±5%. The 3 He/ 4 He iso-tope ratio is expressed as R / R  A , where R is the 3 He/ 4 He ratio in thesample and R  A is the same ratio in the air ( R  A =1.4  10 À 6 ;Mamy-rin and Tolstikhin, 1984). The R / R  A values were corrected for aircontamination according to the He/Ne ratios (Craig et al., 1978).Temperature, pH, redox potential (Eh) and electrical conductiv-ity (EC) values were measured in the field from the waters of thegas vents using a multi-parameter probe (VWR Symphony). Alka-linity was measured by titration with 0.05N HCl. Major anions(Cl and SO 4 ) and cations (Ca, Mg, Na and K) were determined by  Table 1 Chemical and isotope compositions of the San Vittorino plain gas samples. Latitude and longitude are expressed in decimal degrees in relation to the WGS84 coordinate system.Gas concentrations are expressed as l mol/mol, d 13 C as ‰ vs. VPDB, d D as ‰ vs. VSMOW, He isotope composition as R / R  A = ( 3 He/ 4 He) sample /( 3 He/ 4 He) air . Chemical concentrationsare corrected for air contamination. % M  , % L , % S  are the percentages (rounded to an integer) of C of the CO 2 derived from the magmatic, limestone and organic sedimentarycomponents, respectively, calculated by considering the CO 2 / 3 He ratio of mantle equal to 2  10 9 ; values in parentheses indicate the upper and lower limits of each fraction,which is estimated from the uncertainty of the CO 2 / 3 He ratio of the mantle (7.5  10 8 and 3  10 9 ;Sano and Marty, 1995). R / R  A in parentheses are values used to calculate theCO 2 / 3 He ratios of those samples in which the helium isotopes are not determined directly; values are selected on the basis of the location of the sample sites, considering 0.12 asrepresentative of the fault bordering the northern side of the plain and 0.27 as representative of the Fiamignano-Micciani fault. Site Lat. Long. CO 2 N 2 CH 4 He d 13 C CO2 d 13 C CH4 d D CH4 R / R  A He/Ne CO 2 / 3 He % M  % L % S  SV1 42.374579 12.996482 877,951 111,991 616 71.8 À 2.26 À 16.3 À 79 0.13 40.6 6.72E+10 3 (4–1) 80 (79–82) 17 (16–17)SV2 42.375006 12.997394 939,880 49,617 455 48.4 À 2.65 À 18.9 À 69 (0.12) 1.16E+11 2 (2–1) 80 (79–80) 19 (19–19)SV3 42.376769 13.002721 983,016 7630 280 9.8 À 2.02 À 18.2 À 103 (0.12) 6.00E+11 0 (0–0) 83 (83–83) 17 (17–17)SV4 42.376769 13.002721 954,215 36,604 117 3.0 À 3.29 À 17.2 À 112 (0.12) 1.92E+12 0 (0–0) 78 (78–78) 22 (22–22)SV5 42.370227 12.986428 948,686 42,337 239 13.6 À 2.71 À 10.6 À 9 0.27 1.6 1.84E+11 1 (1–0) 80 (79–80) 19 (19–19)SV6 42.376846 12.997968 942,167 48,055 833 41.2 À 2.12 À 18.8 À 118 (0.12) 1.36E+11 1 (2–1) 82 (81–83) 17 (17–17)SV7 42.375646 12.989392 844,639 145,962 3 164.2 À 2.04 0.12 32.1 3.06E+10 6 (10–2) 79 (77–82) 15 (14–16)SV8 42.382605 13.030152 579,994 409,278 618 422.1 À 3.80 À 22.7 À 129 0.12 29.5 8.18E+09 24 (37–9) 59 (51–70) 16 (13–21)SV9 42.368549 12.981363 964,258 25,675 661 539.1 À 2.45 À 6.1 (0.27) 4.73E+09 42 (63–16) 52 (35–71) 6 (2–14)92 F. Giustini et al./Applied Geochemistry 34 (2013) 90–101  ion-chromatography (Metrhom) on filtered and filtered/acidifiedsamples, respectively. Transition metals (Fe and Mn) were deter-mined on the filtered/acidified samples by ICP-OES. The analyticalprecisionsformajorandminorconstituents,as%RSDbasedonrep-licate analysis of standard solutions, were <5% for solutes in ionchromatography, 2% for Fe and 1% for Mn.The C isotope compositions of DIC were determined by acidifi-cation of water samples for extraction of CO 2 , which was subse-quently introduced into the Mass Spectrometer Finnigan MAT252for isotopediscrimination. Datawerereportedvs. V-PDB(ana-lytical error±0.1 ‰ ).The spatial distribution of mantle CO 2 was mapped using ESRIArcGIS 9.2™, applying a multivariate interpolation function. Thenumerical technique used is the Inverse Distance Weighted(IDW) interpolation method. It is based on the principle wherebythe(geographically)spatiallycloserobjectstendtobemoresimilarinvaluethanthoselyingfurtheraway.Adescriptionofthemethodused here is detailed inShepard (1968). 4. Results Table 1shows the chemical and isotope composition of the gassamples. The sampling locations are shown inFig. 1A. All the sam-ples are CO 2 dominated, with concentrations in the range of 579,994 and 983,016 l mol/mol. Methane concentrations rangewidely from 3 to 833 l mol/mol. The He concentration is higherthan the atmospheric concentration (i.e. 5.2 l mol/mol;Hollandand Emerson, 1990) in all the samples but SV4, whose He concen-tration is lower (3.0 l mol/mol) thanthat found in the atmosphere.Fig. 2shows the CO 2 concentrations vs. d 13 C, highlighting quitea uniformCO 2 isotope compositionof the samples around anaver-age value of  À 2.6 ‰ vs. VPDB, with only one sample displaying aslightly more negative isotope composition corresponding to alower concentration. The d 13 C and d D of CH 4 vary markedly, asshown inFig. 3; a positive linear correlation, likely attributableto partial oxidation processes, is evident.It was not possible to measure the He isotopic ratios in most of the samples because concentrations were too low or air contami-nation was too high. Therefore, for samples SV2, SV3, SV4 andSV6, i.e. those aligned along the fault bordering the northern sideof the plain, a R / R  A value of 0.12 was used, which is the result of the samples aligned along this fault (SV7 and SV8); for sampleSV9, located close to sample SV5 (whose R / R  A is 0.27) and overthe Fiamignano-Micciani fault, a R / R  A value of 0.27 was assigned.The hydrochemistry of groundwater from the San Vittorinoplain is shown inTable 2; the hydrochemical facies are shown inthe Piper diagram (Fig. 4). Groundwater can be classified as Ca–HCO 3 type; samples seemto be aligned along a mixing line, whoseend-members are Ca–HCO 3 and Ca–SO 4 –HCO 3 . Waters display ahigh electrical conductivity (up to 3382 l S/cm) and slightly acidicpH. Their temperature ranges between 13.9 and 16.5 ° C. The redoxpotential values (Eh) in the gas vent waters are generally negative(up to À 324mV in the SV7 sample), with the exception of the SV5and SV9 samples, which yielded positive values (248 and 26mV,respectively). The waters display equilibriumconditions for calcite( À 0.2<SI calcite <0.2) and are slightly under-saturated for dolomite(SI dolomite < À 0.2), with the exception of SV6, which is oversaturat-edforbothcalciteanddolomite.IronandMnconcentrationsrangebetween 0.83 and 135 l g/L and 0.08 and 64.5 l g/L, respectively.The lowest values correspond to the SV5 sample, where a remark-able precipitation of insoluble Fe oxides was evident upon sam-pling given the reddish color of the water pond; this sample alsoyielded the highest redox potential, thereby confirming thestrongly oxidative nature of this water compared with the otherwaters. The d 13 C of DIC, measured in a selection of samples, rangefrom À 0.13 ‰ to 6.23 ‰ vs. VPDB (Table 2). 5. Discussion 5.1. Source of CO  2 The proposed source of CO 2 in the San Vittorino plain is thecombined product of a thermo-metamorphic reaction of Mesozoiclimestone and primordial (magmatic)-derived CO 2 (Annunziatelliset al., 2004) that migrates from deep down towards the surfacealong major faults in the area (Lewicki et al., 2006). The d 13 C val-ues, which range from À 3.8 ‰ to À 2.0 ‰ , are likely too depletedin 13 C for the CO 2 to be the sole product of a thermo-metamorphic Fig. 2. CO 2 concentration vs. d 13 C–CO 2 (VPDB) of San Vittorino gas emissions. Fig. 3. Methane C and H isotope diagram, with genetic zonation updated byHosgormez et al. (2008). T  O =thermogenic with oil; T  C =thermogenic with condensate; T  D =drythermogenic; T  H =thermogenic with high-temperature CO 2 –CH 4 equilibration. F. Giustini et al./Applied Geochemistry 34 (2013) 90–101 93  process. As C isotope fractionation of the CO 2 –calcite system en-riches the CO 2 in the heavy isotope at high temperatures, theCO 2 produced by decarbonation reactions should result in a higher d 13 C than that present in the srcinal limestone. The San Vittorinoplain consists of Meso-Cenozoic carbonate bedrock, whose d 13 Cranges from À 3 ‰ to +2 ‰ (Iannace, 1991) that is the typical rangeof the carbonates of marine srcin. If it is assume that the carbon-aterocks inthecrustconstituteaninfinitereservoir of CcomparedwithCO 2 producedbydecarbonationandthat theCO 2 decarbonat-ed does not isotopically fractionate on its way to the surface, it ispossible to envisage two cases of decarbonation isotope fraction-ation and resulting d 13 C–CO 2 : in the absence and presence of water. In the first case the temperature at which metamorphicreactions between carbonate and silicate occurs is at 600 ° C (Muf-fler and White, 1968), then the d 13 C of CO 2 would be enriched byapproximately +2.6 ‰ if compared with that of CaCO 3 (Ohmotoand Rye, 1979), thereby producing a gas with d 13 C between $À 0.5 ‰ and +4.5 ‰ . In the latter case (presence of water), the firstmetamorphic reactions between carbonate and silicate rocks startat T  >200 ° C(Muffler and White, 1968), and at temperatures of  around 250 ° C, isotope enrichment amounts to 1.3 ‰ (Ohmotoand Rye, 1979), producing a CO 2 with d 13 C> À 2 ‰ . Only at temper-atures lower than $ 193 ° C is the enrichment in the CO 2 –calcitesystem reversed, but CO 2 production by decarbonation should befar less abundant. The d 13 C of CO 2 in the San Vittorino plain maythus be ascribed to the mixing of (i) carbonate-derived deep crustCO 2 , and (ii) more 13 C-depleted CO 2 . The srcin of the latter end-member may vary: organic matter ( d 13 C= À 23 ‰ ,Cerling et al.,1991) or mantle( d 13 C= À 5 ‰ to À 8 ‰ ,Javoyet al., 1986). However,theuseofcoupled d 13 CandCO 2 concentrationsontheirowntodis-criminate between the mantle and organic srcins is a difficulttask.Helium isotopes provide a reliable means of distinguishing be-tween the crustal and mantle components in the gas emissionsources. Heliumis a noble gas withtwo isotopes: 3 He is of primor-dial srcin, whereas 4 He is the result of radioactive decay (Porcelliet al., 2002; Yang et al., 2005). Heliumis a good tracer owing to itschemical inertness, relatively low abundance in the atmosphereand large isotope variations in its various reservoirs (Porcellietal.,2002).InMORB(middle-oceanridgebasalts),the 3 He/ 4 Hera-tio is uniform at $ 8 R  A (Craig et al., 1978). The lower mantle is en-richedin 3 He,with 3 He/ 4 Heratios>30 R  A (FarleyandNeroda,1998).The cratonic areas of continents are characterized by low 3 He/ 4 Heratios (0.01–0.1 R  A ), due to the abundant production of  4 He by  Table 2 Chemical composition of water samples. Temperature ( T  ) is expressed in ° C; electrical conductivity (EC) is expressed in l S/cm; Eh is the redox potential expressed in mV; majorion concentrations are expressed in mg/L; Fe and Mn in l g/L. SI calcite and SI dolomite stand for calcite and dolomite saturation indexes, respectively, calculated by means of PHREEQC(Parkhurst and Appelo, 1999). Site T  pH EC Eh Ca Mg Na K HCO 3 SO 4 Cl SI calcite SI dolomite Fe Mn d 13 C DICSV1 15.8 6.2 2290 À 212 297 69.5 19.3 3.8 976 233 21.9 À 0.06 À 0.53 100 30.1 6.23SV2 15.9 6.1 2279 À 210 253 72.2 18.3 3.7 854 239 23.1 À 0.27 À 0.87 102 29.1SV3 15.1 6.3 1891 À 202 348 54.2 15.3 3.5 1159 146 16.5 0.18 À 0.23 135 14.6 3.14SV4 15.1 6.3 1888 À 205 352 56.6 14.9 3.2 1158 151 18.1 0.17 À 0.24 122 9.42SV5 13.9 6.3 2576 248 388 74.6 18.1 2.7 1281 227 24.7 0.21 À 0.10 0.83 0.08SV6 14.2 6.2 3382 À 198 571 86.9 27.3 2.8 1957 195 37.2 0.41 0.20 132 55SV7 14.4 5.9 2259 À 324 334 58.4 14.4 1068 199 18.2 À 0.34 À 1.24 45 15.1 À 0.13SV8 13.9 6.2 1637 À 262 353 34.8 6.4 1.8 1159 78 6.0 0.09 À 0.64 11.4 35.2 3.03SV9 16.5 6.0 2713 26 427 82.7 20.9 2.5 1360 271 39.1 0 À 0.48 96 64.5 1.69 Fig. 4. Piper diagram of the water samples associated with gas emissions.94 F. Giustini et al./Applied Geochemistry 34 (2013) 90–101
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