Geochemical Journal, Vol. 33, pp. 1 to 12, 1999

The origin of natural gas emissions from island,

A. MINISSALE,1 G. MAGRO,2 F. TASSI,3 F. FRAU4 and O. VASELLI3

1CNR-Centro di Studio per la Minerogenesi e la Geochimica Applicata , Via G. La Pira 4, 50121 Firenze, Italy 2CNR -Istituto di Geocronologia e Geochimica Isotopica , Via C. Maffi 36, 56100 Pisa, Italy 3Dipartimento di Scienze della Terra , University di Firenze, Via G. La Pira 4, 50121 Firenze, Italy 4Dipartimento di Scienze della Terra , University di , Via Trentino 51, 09127 Cagliari, Italy

(Received December 16, 1997; Accepted July 18 , 1998)

The geochemical study carried out on seven natural gas emissions from Sardinia island (western central Italy) has allowed to distinguish two different groups: i) C02-high and He-low gases in the Logudoro area (northern Sardinia) associated with low temperature Na-HCO3 type waters and ii) N2 and He-rich gases bordering the western limit of Paleozoic basement crystalline rocks associated with long-term Na-Cl and Na(Ca)-Cl thermal water circulation therein. The emission of CO2 is prevalently related to outcrop areas of recent Quaternary extensional basaltic volcanism. The different origin of the two types of gas is even more evident when considering the helium (as R/ Ra) and argon isotopic ratios for the Logudoro and the remaining areas, being 3.0-3.5 Ra and >320 and <0.4 Ra and 295, respectively. Both CO2 and such isotopic ratios suggest: i) a deep source for the Logudoro samples where a contribution of 40-50% mantle gas can be assessed; ii) a prevalent atmospheric origin for the N2-rich gases emerging in association with meteoric-derived thermal waters circulating in aquifers reaching 2,000 m in depth and 80°-110°C maximum temperature inside the Paleozoic formations. Although both the N2/Ar and 40Ar/36Ar ratios in the N2-rich gases point, roughly, to an atmospheric origin, their relative Ne concentration, being this element almost exclusively of atmospheric origin, suggests that contributions of N2 from other sources than air can be envisaged.

INTRODUCTION pressive volcanic activity (andesites and ignim brites; 28-15 Ma) and promoting the uplift of Sardinia island (western-central Italy) is located magmas with formation of magma reservoirs in at the centre of the western Mediterranean Sea the upper crust (Lecca et al., 1997). The subse and, although very close to one of the most active quent Plio-Quaternary mainly extensional tectonic seismic areas in the world (central-southern Italy), phases were accompained with effusive activity has a deeply rooted basement which makes it a (alkaline basic lavas containing mantle-derived stable block. The present tectonic stability of xenoliths; Beccaluva et al., 1989). Sardinia derives from a rather complex The presence of several thermal manifestations geodynamic evolution. In fact, the Sardinia base (warm gas-water emissions) focused, since the '70s ment is considered as a segment of the south-Eu , the attention of studies on the geothermal ropean Hercynian Chain, faulted and rifted during potentiality of Sardinia (Bertorino et al., 1979; the Oligo-Miocene when the Sardinian-Corsican CNR-PFE-RF10, 1982; Caboi et al., 1983, 1988; microplate broke off from the European basement Fanfani, 1987). More recent studies were ad (Provence region, southern France) and began its dressed towards a better understanding of water southeastward drifting and counterclockwise rota rock interaction processes which take place inside tion in the western Mediterranean area the main geothermal reservoirs (Caboi et al., 1993; (Carmignani et al., 1992, 1994). These move Frau, 1993, 1994). ments played an important role in driving an im It is not clear if the geothermal systems of

1 2 A. Minissale et al.

Sardinia are linked to recent volcanic activity, but with the Sardinian-Corsican paleomicroplate in the it is a fact that they are generally located along western Mediterranean (Dewey et al., 1989). regional faults where the Hercynian basement is The Tertiary complex fills the several branches in tectonic contact with the Alpine complex. Be in which the Oligo-Miocene rift system may be sides, the most important geothermal area of divided. The infillings mainly consist of Miocene Sardinia, i.e. the Pliocene graben of Campidano, marine sediments and Oligo-Miocene andesitic and shows a significant heat flow anomaly (up to 4.5 ignimbritic suites (Lecca et al., 1997). The Plio H.F.U.), with a geothermal gradient of about 1°C Quaternary tectonic phases caused dismemberment every 12-15 m, referable to a local thinning of of the Oligo-Miocene Sardinian rift and originated the lithosphere (Loddo et al., 1982). the Campidano graben in the southern branch of To have a complete view of natural fluid dis the rift (Marini and Murru. 1983; Carmignani et charges at surface and processes and temperatures al., 1994). governing the composition and circulation of flu The stratigraphic sequence of the Campidano ids inside the main tectonic units in the island, graben consists of a succession of Oligo-Miocene this work deals with the natural gas emissions in calc-alkaline volcanics (400-800 m thick), Mio the island, generally emerging in association with cene marine sediments (at least 1000 m thick) and the thermal springs. We carried out major, minor, Plio-Quaternary continental and marine sediments and trace gas analyses and measurements of 3He/ (over 800 m thick) (Pala et al., 1982 and refer 4He, 40Ar/36Ar,and 13C/12Cin CO2 isotopic ratios ences therein). Geological and geophysical data of seven gas emissions from different localities suggest the presence of the Paleozoic basement at (Anglona, Tirso Valley, Campidano and Logudoro) variable depths between 2,000 and 5,000 m (Balia representing the main gas-water manifestations of et al., 1991). Sardinia (Fig. 1). The Logudoro basin includes the northernmost branch of the Oligo-Miocene rift. Geophysical studies (Pecorini et al., 1988) pointed out the GEOLOGICAL SETTINGS AND GEOTHERMAL BACKGROUND presence of a buried Mesozoic carbonate platform, which outcrops in the Nurra area (NW Sardinia), The structure of Sardinia is mostly at a depth of about 2,000 m. Plio-Pleistocene characterised by the presence of two large geo basalts (up to 100 m thick) extensively outcrop in logical domains: the Paleozoic crystalline base the Logudoro and surrounding areas (Maciotta and ment and the Tertiary complex (Fig. 1). The Pa Savelli, 1984). This volcanism took place in an leozoic unit is dismembered in several sub-regional extensional regime and upper mantle-derived ul blocks, mainly by NE-SW and NW-SE fault sys tramafic xenoliths are included in lavas (Rutter, tems of late-Hercynian tectonic phases, reactivated 1987; Dupuy et al., 1987; Beccaluva et al., 1989). during the Alpine orogenesis (Carmignani et al., The main geothermal areas of Sardinia 1992). The basement is composed of (Anglona, Tirso Valley and Campidano) are related metasediments with varying metamorphic grade to regional faults where the Paleozoic basement and of a Carboniferous granitic pluton, whose is in tectonic contact with the Alpine complex. largest outcrops occur in the eastern and south They are water-dominated systems at low-medium western parts of Sardinia. They represent the temperatures, in which the groundwater evolution structural heights of the Oligo-Miocene rift system from recharge to discharge areas prevalently takes which roughly cuts the island in two, stretching place inside the Paleozoic basement (Dettori et al., from the Cagliari Gulf to the south to the Asinara 1982; Frau, 1993). In it, in fact, meteoric waters Gulf to the north. The rift system is generally re may infiltrate and reach depths of 1,000-2,000 m, lated to the eastward drifting of the Sardinia block warm up at 80°-120°C and rise to the surface induced by indentation of the Maghrebian orogen through regional faults (Nuti et al., 1977; Bertorino 3 Natural gas from Sardinia, Italy

7yrrhenion sea .*S. Martin,

Abbarghente Tyrrhenian 0 Sea

basement

Post Paleozoic formations / fault Cagliari

spring site

Cagliari Gulf 30km

Fig. 1. Simplified geological map of Sardinia with sampling locations (1: Abbarghente; 2: Terme Aurora; 3: S. Martino; 4: Monastir; 5: ; 6: Casteldoria; 7: Fordongianus).

et al., 1982; Caboi and Noto, 1982; D'Amore et scribed by Giggenbach (1975), by means of two al., 1987). The Logudoro basin is another area of 100 cc pre-evacuated glass tubes, one containing seemingly geothermal interest because of the 50 cc of 4N NaOH to concentrate the non reactive presence of both several slightly warm springs components. The in-situ Rn activity measurement (20°-24°C) with high PC02 values and dry CO2 has been done using an EDA RDA-200 portable emissions. Chemical geothermometers in this area detector according to the Washington and Rose indicate deep temperatures of 40°-80°C (D'Amore (1992) counting method. et al., 1987; Caboi et al., 1993) Gas components have been determined in the laboratory by gas-chromatography, after molecular sieve separation, using a thermal conductivity de SAMPLING AND ANALYTICAL METHODS tector for C02, N2, 02, H2S, He, Ar and H2 and a The location of gas-water manifestations from flame ionization detector for CH4 and CO. Either Sardinia is reported in Fig. 1. The gas sampling He or Ar were used as gas carrier whereas air has been carried out following the procedure de was used as standard. Analytical precision was

0 4 A. Minissale et al.

<1% for CO2, N2, 02, H2S and CH4, and <5% for position, are fed mainly by aquifers hosted in the minor and trace compounds. Oligo-Miocene volcanics. The C02, N2-rich Sampled gas for isotopic measurements was springs of Campidano (Sardara: 55°C; Aquacotta: purified in a stainless steel vacuum line equipped 45°C) with a salinity of 2.0-3.5 g/L and Na with cold and hot traps to separate noble gases. HCO3(Cl) composition are fed by aquifers hosted Helium-4 and, after Ar entrapping, total Ne were inside the Paleozoic crystalline basement. By measured in a static way using a Spectrolab 200 contrast, the alkaline, N2-rich springs of the Tirso VG-Micromass quadrupole mass spectrometer Valley (Terme Aurora: 42°C; Fordongianus: (QMS). After desorbing Ar from activated charcoal 55°C), fed by aquifers in granite rocks, have about trap cooled at liquid nitrogen temperature, 40Ar and 0.6 g/L low salinity and a Na-Cl composition, 36Ar were also measured on the QMS (Magro and whereas the N2-rich groundwaters of Monastir Pennisi, 1991). 3He/4He ratios were determined (43°C) and Casteldoria (75°C) (Campidano graben with a Map 215-50 magnetic mass spectrometer and Anglona area, respectively) have the highest (MMS) equipped with an ion counting device. salinity of 4.5-5.0 g/L and a Na(Ca)-Cl composi Resolution was close to 600 AMU for HD-3He at tion. The latter are fed mostly by aquifers in the 5% of the peak. Pipetted atmospheric noble gases Paleozoic basement, but a likely circulation in the were used to estimate the sensitivity of the two Tertiary volcano-sedimentary formations is also to mass-spectrometers. No blank corrections were be taken into account. The anomalous Na-Cl applied to the He measurements due to its con chemical feature of Monastir water, compared to centration, generally several orders of magnitude the other known Na-HCO3 thermal waters emerg higher than that of the extraction line and the in ing at the borders of the Campidano graben, is strumental static background. Overall analytical justified with a groundwater evolution in a errors were estimated to be less than 10% for Campidano sub-system, with particular deep abundances and less than 5% for ratios. Using air structural conditions which would prevent rising as standard, reproducibility was about 7% for the of CO2 from depth (Frau, 1994 and references 3He/4He ratio , 5% for He/Ne ratio and 4% for therein). 40Ar/36Ar. The outlined evolutionary trends are corrobo After separating CO2 in the gas phase in a rated by a study concerning trace elements in the vacuum line using standard procedures, 13C/12C thermal solutions (Li, Rb, Cs, TI, B, Ge, Mo, W, isotopic ratios of CO2 were analyzed by using a Ga, Be, Zr, Co), which led to the recognition of Finningan MAT 251 mass spectrometer at the some hydrogeochemical markers directly related USGS laboratories in Menlo Park (U.S.A.). Sev to the geochemical characteristics of the reservoir eral internationally accepted standards (such as rocks (Frau, 1993). Iceland spar) have been used with a typical error of ±0.05%o (PBD). GAS GEOCHEMISTRY

Similarly to what described for thermal wa GEOCHEMISTRY OF THERMAL WATERS ters, the main composition of the Sardinia gases The evolution of groundwaters in the main allowed to distinguish two different groups (Table Sardinian geothermal areas and in the Logudoro 1): C02-rich and N2-rich gases. The former basin can better be outlined according to the (CO2 > 96% and N2 < 1.9% by vol.: Abbarghente presence or the absence of deep originated CO2 and S. Martino) have been found related to the (Dettori et al., 1982; Caboi et al., 1993; Frau, Na-HCO3 waters. Delta 13C isotopic values of CO2 1993). The highly C02-rich springs of Logudoro in these samples are -6.1 and -4.2 (%o PDB), re (S. Martino: 21'C; Abbarghente: 24°C) with about spectively, and they are in the same range of those 5.0 g/L of salinity and Na-HCO3 chemical com reported for the same waters by Caboi et al. Natural gas from Sardinia, Italy 5

w Q 0 O~ NO\ MN O0' N N N N z \ d N \O 0 O v

G1 V) O O\ V) 1'Y 00 K 0) Q O 1 N 0 ON 0 M 4 N O .- et d'- -400 MN 00 \D\O O O x ~--~ ~--~M 1-+

M 0 V N Q .O N N N 00 0 0 N 000 W)i r M N w M N C C N N N O O O U o o o O N 00 N C N V C~ 00 N •-N N N O O N M M x.~ M 0 fV O •-O 0 of h F z C C C ~ C C C zo h

n O N V)

M C W W W W W L b Z NM 00 cn ^+ M N .--i I V) O M 0 OVi d V1 V) In ~ 0 M O dam"V~1 O\ 00 N' .0 '0 0 0 0 0 0 0 - d •- N N M 00 O\ 00 00 00 00 .~ O\ w

O z z z z z z -d 11 00 U O 00 \0 M ~ o 0> h I.. NN M W) O C O .--O --~ O

to 0 0 0 M d0 a O ~ ' M h N y O V O U 00 ,- M N +-~ o O Obi O 00 O O II \O ~O \O \O \D \O \D 0) c0 O~ (71 O. O ON O\ a\ w C.) V A a. a a a a Q. a Q Q Q Q Q Q Q

N O~ID N M llN M zo .-, 00 0 00 O\ N C V o Cti y c, O\ 't C\ ON

N CO 0 C~. cd R n V 0 w x~ V N 0 O N M O~ M~ U °0 00 'd +~ b 0 O -t3 '1~3 Q E-Hv~ 00U w° O O O 0 O o~iU O O z O z h N Q

•--~ N M 'q V) \O N 0 .~ N M d' 'n \O N O V

UMvu 6 A. Minissale et al.

(1993). These authors supposed the CO2 originat Fordongianus (6.5 Bq/L). The highest Rn value ing from a mantle source as the high-Pco 2 springs (935 Bq/L) is the one measured at Casteldoria, of Logudoro are aligned along important faults where a likely very high rising speed of the fluid from which basaltic lavas effused up to 0.2 Ma. from its source prevents the radioactive decay of Similar b'3C values were also measured for HCO3 radon formed at depth. Still low CH4 contents ions of Campidano thermal waters (Caboi and characterize these gases (<1.7%), but much higher Noto, 1982). Methane, He and Ar contents in these than the C02-rich samples, probably indicating samples are quite low (<0.0017%, <0.0027%, and greater contribution of altered organic matter from <0.39% respectively), and the Rn activity is as Miocene marine sediments (especially for Monastir low as <20 Bq/L. and Casteldoria). As for the C02-rich samples, CO, The N2-rich gases (N2 > 97% and CO2 < 0.3% H2 and H2S contents are always below their de by vol.: Casteldoria. Monastir su Campu, Terme tection limits of 0.0001 % (CO and H2) and 0.005% Aurora, Fordongianus) are associated with the Na (H2S), generally suggesting low temperatures of (Ca)-Cl thermal waters. The N2/Ar ratio in these equilibration at depth (Giggenbach, 1991). samples is similar to either that of the air (83) or The Sardara gas sample, located on the eastern the air-saturated water (ASW: 38), indicating an border of the Campidano graben, presents CO2 and atmosphere-derived N2, where 02 is consumed in N2 contents (48.6 and 49.7%, respectively) inter sub-surface oxidation processes. With respect to mediate between the two above-described groups, the C02-rich emissions they have much higher He whereas He and Ar contents (0.167% and 1.0%, (0.12% to 0.36%) and Ar (1.0% to 1.5%), and respectively) are within the range of the N2-rich higher Rn contents (>28 Bq/L; Fig. 2) except waters (Table 1).

SOURCE OF GASES AND GEOTHERMOMETRY 1000 0 N2-rich gas decreasing 96 , To relate the composition of gases with po 4C02-rich gas 0 Sardara spea y: spew' tential source components, the N2/100-Ar-He* 10 triangular diagram, proposed by Giggenbach et al.

5' (1983), is shown in Fig. 3. All the samples are 100 clearly aligned along the atmospheric (air or ASW)-crust array suggesting a general increase _M 0' 2:0 in crustal helium produced by radioactive decay. C 40 '4 cc To better differentiate the two types of gases 3 10 recognised, the 'He* 106-4He*2-N2/1000 plot is 1 0' shown in Fig. 4 (Giggenbach et al., 1993). In this 7: diagram all the N2 and He-rich gases fall in the

increasing residence tim e in th a crus t ~ crustal domain whereas the C02-rich , He-poor gases trend towards the upper-mantle domain. The 0.00001 0.0001 0.001 0.01 0.1 1.o Sardara gas locates in an intermediate position He (% by vol.) between the two. The different position of the two Fig. 2. Rn-He binary diagram. The figure shows that C02-rich samples in Fig. 4 is directly related to N2-rich gases have similar He concentrations but very the extremely low He content of the San Martino different Rn values. Since the circulation of the associ sample (0.00003%) with respect to that of ated waters takes place in similar formations (gran Abbarghente (0.0027%) and to their He/N2 ratios ites) with likely similar parent elements (U, Th) con , 0.0001 and 0.001, respectively. Nevertheless, their centration (Fanfani, 1987), Rn-rich gases may reflect higher flowing speed of fluids from their source. For similar 3He/4He ratio points to a common source . sample numbers refer to Table 1. The 3He/4He and 40Ar/36Ar isotopic ratios are Natural gas from Sardinia, Italy 7

N2/100 corrected for air contamination using the He/Ne ratio (Craig et al., 1978) according to the follow ing equation: Andesite 0N2-rich gas •C02-rich gas O Sardara R/Ra = ((Rm/Ra)X 1)/(X 1),

where Rm = 3He/4Hemeasured for the sample and air X = (He/Ne)sample/(He/Ne)air.The N2-rich gases are 00 11ment-- . characterized by low R/Ra ratios (<0.4) between 3, ASW the typical crustal value (0.02) and that of the air G ~at en 2 G~at0% en 2 -Og-~Q•1 (1.0), while the Abbarghente and San Martino 104 .5 crust. ©rustal enrichment C02-rich gases have R/Ra values as high as 3.48 /0* and 3.03, respectively. Similarly, the 40Ar/36Ar ratios are identical to that of the atmospheric value He*10 Ar (295) in the N2-rich gases whereas the Fig. 3. N2/100-Ar-He*10 ternary diagram (after Abbarghente sample has a higher ratio (322). Giggenbach et al., 1983) with location of the atmo If R/Ra values are plotted against the concen spheric, crustal and andesitic endmembers.A general He enrichment after long circulation in the crust is tration of CO2 (in vol.%), a significant positive evident in all samples. The sample numbers are the correlation is evident (Fig. 5), where the two same as in Fig. 1. groups described so far are clearly separated and the Sardara gas sample lies between them. Since the isotopic composition of He is a very impor N2/100 tant parameter in discriminating the origin of gas source (Ozima and Posodek, 1983 and references /air\ therein), it is very likely that mantle-originated 0N2-rich gas 0C02-rich gas O Sardara

3; 100 3 0 4 0 1 0 80

'e .0' tea, 2 8 7 ~ 60 0 crust • mixing mantle10' 05 660 4 0 40 5' • 4H e 3He*106*2 Fig. 4. 3He*106-N2/1000-4He*2ternary diagram (af 20 ter Giggenbach et al., 1993). A partial mantle origin 46 ON2-rich gas for the Logudoro samples (#1 and #3) is evident. The 0 •C02-rich gas sample numbers are the same as in Fig. 1. 7 20*6 Sardara 0 1 2 3 4 R/Ra also reported in Table 1, where the helium iso Fig. 5. C02-R/Ra binary diagram for the samples in tope composition is referred to that of the air as vestigated. Two families are clearly recognizable, where R/Ra (where Ra is the 3He/4He ratio of the air = Sardara sample #5 might represent a mixed term. The 1.39* 10-6; Mamyrin and Tolstikhin, 1984) and sample numbers are the same as in Fig. 1.

0r 8 A. Minissale et al.

gases are present in the Logudoro area. guish among: 1) atmospheric nitrogen dissolved As far as the equilibrium temperatures of pos in ASW, 2) free nitrogen deriving from air enter sible deep source reservoirs are concerned, due to ing directly into the system in the shallower part the very low CO, H2, and H2S concentration in all of the flowing path of associated water, 3) con samples, any gas geothermometric technique tamination with air bubbles during sampling, and (D'Amore and Panichi, 1980; Arnorsson and 4) enrichments of N2 from other sources than air. Gunnlaugsson. 1985; Bertrami et al., 1985; However, speculations can be done by plotting the Giggenbach, 1991) produces always temperatures N2, Ar, and Ne triangular diagram (Fig. 6), Ne <100°C, which agree well with deep equilibration being re-calculated from total helium and the Ne/ temperatures reported in the literature for their He ratio. This diagram shows the position of the associated liquid phases (max temperature of two endmembers represented by air and ASW, as 110°C at Casteldoria: D'Amore et al., 1987). well as lines delimiting areas where excess N2, 40Ar and Ne are expectable with respect to air and ASW. If we consider that Ne mainly derives DISCUSSION from the atmosphere, it is very unlikely, far from As mentioned before, the N2/Ar ratio for the active volcanic areas, to have gas vents at surface Sardinia gases varies between 37 and 99, well in with excess Ne with respect to the air (Kaneoka, the range between ,the ASW ratio and that of pure 1980). With this in mind, if nitrogen and argon air. With the exception of Abbarghente, showing would derive only from air driven underground a slight excess of deep-originating radiogenic 40Ar by rainfalls without any crustal contamination, (Table 1), all the other samples have 40Ar/36Ar samples should lie in the area delimited by the ratios close to that of the air. Although both the position of air, ASW and the Ne-Ar axis. The N2/Ar and the 40Ar/36Ar ratios suggest an atmo diagram confirms that, as expected, no samples spheric origin for N2, it is not possible to distin have "excess" Ne with respect to air and that a

N2/100 A Q N2-rich gas •C02-rich gas

, p Sardara ,

,

,

t , , , ~~ it Qs ~' 4' 5 air, ~2 7

ASW ~'

Ne* 1000 Ar Fig. 6. Ne* 1000-N2/100-Ar ternary diagram . Areas of natural gas composition with excess Ar, N2 and Ne with respect to air and air saturated water (ASW) are reported (see text). The sample numbers are the same as in Fig . 1.

„r~ Natural gas from Sardinia, Italy 9 certain enrichment in radiogenic Ar is evident only Apart from their low R/Ra ratio, the very high in the Abbarghente sample (as suggested also by total He concentration for the N2-rich samples is a its 40ArP6Ar isotopic composition), with smaller further proof of the "crustal" origin for these gases. enrichments in the samples from Casteldoria and It can be explained by either very long-term cir Sardara. On the other hand, apart from the culation of the associated waters before emerging, Logudoro samples (#1 and #3), a clear enrichment or circulation in rocks such as granites, since they of N2 of different origin with respect to air and/or have high contents of radioactive elements (U and ASW is evident in all the N2-rich samples. Such Th; Fanfani, 1987) from which helium derives. enrichments explain why in the N2-rich group there Considering that a large part of eastern Sardinia are several samples with N2/Ar ratio > 38, al is covered by granites, it is very likely that both though this should be the theoretically expectable processes may act together. value for nitrogen strictly deriving from long cir The decoupling between He and Rn contents culating ASW underground. Clearly, a contribu in the N2-rich samples, i.e. He-rich samples with tion of crustal sedimentary N2 and/or N2 deriving low Rn contents (Fig. 2), may be related to the from the alteration of ammonia ions in feldspars relatively low speed of gas flow for some of them, and micas (Jenden et al., 1988) is likely in all the which would deplete Rn, being its half-life time N2-rich samples. as low as 55 sec. for 222Rnand 3.8 days for 220Rn. A possible source for such "excess" N2 can be Moreover, mixing between Rn-high thermal wa envisaged in the metamorphic formations of ters with relatively Rn-low shallower aquifers (or basement which, as already reported for other ar vice versa) may produce a further dilution. Since eas in central Italy (Minissale et al., 1997), might they have common parent elements, such produce "metamorphic" nitrogen from both or decoupling and mixing mechanisms would support ganic and inorganic reactions. the lack of expected positive correlation between Further genetic evaluations on gas sources in He and Rn in Fig. 2. Nevertheless, the similar Sardinia can be drawn when considering the he high concentration of helium (0.1-0.4% by vol.) lium isotopic ratios. In fact, if a strong crustal and R/Ra ratio points to similar long circulation component or air is evident in all the N2-rich patterns in similar U-Th-rich rocks. gases of which R/Ra ranges from 0.02 to 0.37, Quite different is the situation related to the the Abbarghente and San Martino samples, being Abbarghente and San Martino gases in the their R/Ra values up to 3.48, seem to be clearly Logudoro area where their origin is likely to have affected by an upper-mantle gas contribution. a contribution from the mantle. This is also con When assuming a theoretical R/Ra value equal to firmed by considering their distribution of 3He and 8.0 for a MORB type mantle (Craig and Lupton, 4He and N2 concentrations in Fig. 4 (Giggenbach 1976) or R/Ra = 6.1-6.7 if the mantle beneath et al., 1993). Besides, CO2 uprising from deep Sardinia is similar to the European Subcontinen seated faults in Logudoro modifies the chemical tal Mantle (ESM; Dunai and Baur, 1995), a he composition of shallow aquifers. In fact, CO2 lium contribution from the mantle in the order of renders solutions more aggressive, makes water 40 or 50%, respectively, can be computed. rock interaction easier and causes, indirectly, the As already suggested by Caboi et al. (1993), a shifting of the water composition from Na-Cl to deep mantle component for the gases in the Na-HCO3. Logudoro area is also strongly suggested by the The Sardara sample is clearly an example of 13C/12C ratio of CO2. The 813C values measured in intermediate gas composition between the two end the present study (-4.2 and -6.1%o PDB) are well members described so far (Figs. 4 and 5). It gets in agreement with mantle C02, its typical values enriched in N2 by flowing through the Paleozoic being between -3.0 and -8.0%o PDB (Rollison, metamorphic basement, while, at the eastern mar 1993 and references therein). gin of the Campidano graben, it undergoes mix 10 A. Minissale et al. ing with C02-rich gases flowing along the Logudoro area, although characterized by low boundary fault systems. This is also proved by seismicity (as the entire Sardinia), hosts Quater the intermediate composition of the associated nary asthenospheric mantle-derived volcanic liquid phase, Na-HC03(C1), as reported by products which contain lithospheric mantle-related Bertorino et al. (1979). xenoliths. The high R/Ra values in the Logudoro gases may reflect the present extensional regime around the Alpine block as already observed in CONCLUDING REMARKS other aseismic European areas (e.g., the Pannonian The estimated temperatures of deep feeding Basin: Marty et al. (1992); and the Rhine Graben: aquifers for the Sardinia thermal spring discharges Griesshaber et al. (1992)). In addition, the marked indicate the presence of relatively low-enthalpy mantle-derived 3He signature in these gases may systems at depth, ranging in temperature from 80° imply the presence of mantle-related melts at depth to 110°C (Bertorino et al., 1979; D'Amore et al., and the fluid circulation along deep-seated fault 1987). They agree well with those measured dur systems would retrieve such imprinting at the ing drillings by ENSAE (Sardinian Electricity surface. Agency; Fanfani, 1987). The high concentration of N2 and the < 1 ppm Acknowledgments-We thank very much W. C. Evans content for H2 and CO in the gas phases also (U.S.G.S., Menlo Park, California) for the S13C mea surements of CO2 and R. Caboi (University of Cagliari, support the hypothesis that active hydrothermal Italy) for his useful suggestions. We are also grateful systems are not present at shallow depth. In to G. Nappi and B. Capaccioni (University of Urbino, agreement with geothermometric calculations for Italy) for kindly providing the radon detector. M. the associated liquid phase, several Kusakabe (Okayama University, Japan) and two geothermometers applied to all the Sardinia gas anonimous reviewers greatly improved an earlier ver samples suggest deep equilibrium temperatures sion of the manuscript. always below 100°C. More important speculations can be attained REFERENCES when considering the tectonic implications of the Arnorsson, S. and Gunnlaugsson, E. (1985) New gas chemical and isotopic compositions of the gases geothermometers for geothermal exploration-cali themselves. The crustal contribution of atmo bration and exploration. Geochim. Cosmochim. Acta spheric-derived N2 and radiogenic 4He, coupled 49,1307-1325. with N2 likely deriving from Paleozoic metamor Balia, R., Ciminale, M., Loddo, M., Patella, D., Pecorini, G. and Tramacenere, A. (1991) A new phic rocks, is particularly striking to assess that N2-rich gases participate in a hydrogeologic cycle geophysical contribution to the study of the Campidano geothermal area (Sardinia, Italy). that starts with rainfalls infiltrating at heights of Geothermics 20, 147-163. 300-600 m (Caboi and Noto, 1982). Considering Beccaluva, L., Macciotta, G., Siena, F. and Zeda, O. that the deep low-enthalpy systems from which (1989) Harzburgite-lherzolite xenoliths and gases should derive have a temperature of 80° clinopyroxene megacrysts of alkaline basic lavas 100°C, it is very likely that ASW in presence of from Sardinia (Italy). Chem. Geol. 77, 331-345. Bertorino, G., Caboi, R., Caredda, A. M., Cidu, R., normal thermal gradients should reach 2,000-3,000 Fanfani, L., Oala, A., Pecori, G. and Zuddas, P. m depth emerging as N2-rich thermal springs along (1979) Caratteri geochimici delle acque termali della the main fault systems in the island. Sardegna quale primo contributo alla prospezione If we consider the He isotopic composition and geotermica della regione. CNR-PFE SPEG Roma, the CO2 abundances and isotopes, it seems plau Italy 1, 570-586. sible to hypothesize a strong deep (upper-mantle Bertorino, G., Caboi, R., Caredda, A. M., Cidu, R., Fanfani, L., Sitzia, R. and Zuddas, P. (1982) related) component for the Abbarghente and San Idrogeochimica del graben del Campidano. CNR-PFE Martino and, partly, the Sardara gases. The Pisa, Italy 10, 104-123. Natural gas from Sardinia, Italy 11

Bertrami, R., Cioni, R., Corazza, E., D'Amore, F. and acque termali della Sardegna. Ric. Geoterm. Sard., Marini, L. (1985) Carbon monoxide in geothermal CNR-PFE Pisa, Italy, RF10, 56-86. gases. Reservoir temperature calculations at Dewey, J. F., Helman, M. L., Turco, E., Huttin, D. H. Larderello (Italy). Geothermal Resources Council W. and Knott, S. D. (1989) Kinematics of the west Trans. 9, 299-303. ern mediterranean. Alpine Tectonics (Coward, M. P., Caboi, R. and Noto, P. (1982) Dati isotopici sulle acque Dietrich, D. and Park, R. G., eds.), Geol. Soc. Sp. termali e fredde dell'area campidanese. CNR-PFE Pubbl., 45, 265-283. SPEG Rome, Italy 1, 556-584. Dunai T. J. and Baur, H. (1995) Helium, neon and Caboi, R., Cidu, R., Fanfani, L., Pecorini, G. and argon systematics of the European subcontinental Zuddas, P. (1983) Preliminary geologic and geo mantle: Implications for its geochemical evolution. chemical data for the evaluation of geothermal po Geochim. Cosmochim. Acta 59, 2767-2783. tential in Sardinia. Proc. 3rd Int. Semin. EC Dupuy, C., Dostal, J. and Bodinier, J. L. (1987) Geo Geotherm. Energy Res. (Strub, A. S. and Ungemach, chemistry of spinel peridotite inclusions in basalts P. eds.), Reidel Dordrecht, Munich, Germany, 206 from Sardinia. Miner. Mag. 51, 561-568. 213. Fanfani. L. (1987) Progetto Sardegna: studi geologico Caboi, R., Cidu, R., Cristini, A., Fanfani, L. and strutturali ed idrogeologici; prospezione Zuddas, P. (1988) Studio geochimico delle acque idrogeochimica per la valutazione del potenziale termali di Casteldoria. CNR-PFE SPEG Ferrara, geotermico. Final Report EEC Contract, EG-A2-052 Italy 5, 597-615. I. Caboi, R., Cidu, R., Fanfani, L., Zuddas, P. and Zanzari, Frau, F. (1993) Selected trace elements in groundwaters A. R. (1993) Geochemistry of the high PC0 from the main hydrothermal areas of Sardinia (Italy) 2 waters in Logudoro, Sardinia, Italy. Appl. Geochem. 8, 153 as a tool in reconstructing water-rock interaction. 160. Miner. Petrogr. Acta 36, 281-296. Carmignani, L., Barca, S., Cappelli, B., Di Pisa, A., Frau, F. (1994) A new hydrothermal manifestation in Gattiglio, M., Oggiano, G. and Pertusati, P. C. (1992) the Campidano graben, Italy: the Su Campu borehole A tentative geodynamic model for the Hercynian (Monastir). Miner. Petrogr. Acta 37, 155-162. basement of Sardinia. Contributions to the Geology Giggenbach, W. F. (1975) A simple method for the of Italy (Carmignani, L. and Sassi, F. P., eds.). IGCP collection and analysis of volcanic gas samples. Bull. 276, Newsletters, 5, 61-82. Volcanol. 39, 132-145. Carmignani, L., Barca, S., Disperati, L., Fantozzi, P., Giggenbach, W. F. (1991) Chemical techniques in Funedda, A., Oggiano, G. and Pasci, S. (1994) Ter geothermal exploration. Application of Geochemistry tiary compression and extension in Sardinian Base in Geothermal Reservoir Development (D'Amore, F., ment. Boll. Geof. Teor. Appl. 36, 45-62. ed.), UNITAR, Rome, Italy, 119-144. CNR (Italian Council for Research) (1982) Ricerche Giggenbach, W. F., Gonfiantini, R., Jangi, B. and geotermiche in Sardegna con particolare riferimento Truesdell, A. H. (1983) Isotopic and chemical com al graben del Campidano. CNR-PFE-FR10, Pisa, position of Parbaty valley geothermal discharges, Italy, pp. 222. NW-Himalaya. Geothermics 12, 199-222. Craig, H. and Lupton, J. E. (1976) Primordial neon, Giggenbach, W. F., Sano, Y. and Wakita, H. (1993) helium and hydrogen in oceanic basalts. Earth. Isotopic composition of helium, and CO2 and CH4 Planet. Sci. Lett. 31, 369-385. contents in gases produced along the New Zealand Craig, H., Lupton, J. E. and Horibe, Y. (1978) A mantle part of a convergent plate boundary. Geochim. helium component in circum-Pacific volcanic gases: Cosmochim. Acta 57, 3427-3455. Hakone, the Marianas and Mt. Lassen. Terrestrial Griesshaber, E., O'Nions, R. K. and Oxburgh, E. R. Rare Gases (Alexander, E. C. and Ozima, M., eds.), (1992) Helium and carbon isotope systematics in Central Academic Publishers, Tokyo, 1-16. crustal fluids from the Eifel, the Rhine Graben and D' Amore, F. and Panichi, C. (1980) Evaluation of deep Black Forest, F.R.G. Chem. Geol. 99, 213-235. temperatures of hydrothermal systems by a new gas Jenden, P. D., Kaplan, I. R., Poreda, R. J. and Craig, geothermometer. Geochim. Cosmochim. Acta 44, H. (1988) Origin of nitrogen-rich natural gases in 549-556. the California Great Valley: evidence from helium, D'Amore, F., Fancelli, R. and Caboi. R. (1987) Obser carbon and nitrogen isotope ratios. Geochim. vations on the application of chemical Cosmochim. Acta 52, 851-861. geothermometers to some hydrothermal systems in Kaneoka, I. (1980) Rare gas isotopes and mass frac Sardinia. Geothermics 16, 271-282. tionation: an indicator of gas transport into or from Dettori, B., Zanzari, A. R. and Zuddas, P. (1982) Le a magma. Earth Planet. Sci. Lett. 48, 284-292. 12 A. Minissale et al.

Lecca, L., Mongelli, F., Pecorini, G. and Tramacere, 175-192. A. (1997) Oligo-Miocene volcanic sequences and Nuti, S., Fancelli, R., Dettori, B., Passino, A. M. and rifting stages in Sardinia: a review. Per. Mineral. 66, D'Amore, F. (1977) Il termalismo della provincia di 7-61. . Possibile modello del circuito termale di Loddo, M., Mongelli, F., Pecorini, G. and Tramacere, Casteldoria. Boll. Soc. Geol. It. 96, 491-504. A. (1982) Prime misure di flusso di calore in Ozima, M. and Podosek, F. A..(1983) Noble Gas Sardegna. CNR-PFE-RF10, Pisa, Italy, 181-209. Geochemistry. Cambridge University Press, Cam Macciotta, G. and Savelli, C. (1984) Petrology and K/ bridge, U.K. Ar ages of Pliocene-Quaternary volcanics from north Pala, A., Pecorini, G., Porcu, A. and Serra, S. (1982) western Sardinia. Univ. Parma, 1st. Petrografia, Schema geologico-strutturale della Sardegna. CNR STEP Parma, Italy. PFE, RF 10, Pisa, Italy, 7-24. Magro, G. and Pennisi, M. (1991) Noble gases and Pecorini, G., Balia, R., Ciminale, M., Loddo, M., Pa nitrogen: mixing and temporal evolution in the fu tella, D. and Tramacere, A. (1988) Studio geofisico marolic fluids of Vulcano, Italy. J. Volcanol. del bacino del Logudoro (Sardegna) e delle aree Geotherm. Res. 47, 237-247. circostanti. Boll. Soc. Geol. It. 107, 547-560. Mamyrin, B. A. and Tolstikhin, I. N. (1984) Helium Rollison, H. (1993) Using geochemical data. Longman isotopes in nature. Development in Geochemistry 3, Group Lmt., London, U.K., pp. 352. Elsevier. Rutter, M. J. (1987) Evidence for crustal assimilation Marini, A. and Murru, M. (1983) Movimenti tettonici by turbulently convecting, mafic alkaline magmas: in Sardegna fra it Miocene Superiore ed it Pleisto geochemistry of mantle xenolith-bearing lavas from cene. Geogr. Fis. Din. Quater. 6, 39-42. northern Sardinia. J. Volcanol. Geotherm. Res. 32, Marty, B., O'Nions, R. K., Oxburg, E. R., Martel, D. 343-354. and Lombardi, S. (1992) Helium isotopes in Alpine Washington, J. W. and Rose A. W. (1992) Temporal regions. Earth Planet. Sci. Lett. 206, 71-78. , variability of radon concentration in the interstitial Minissale, A., Evans, W. C., Magro, G. and Vaselli, gas of soils in Pennsylvania. J. Geophys. Res. 97, O. (1997) Multiple source components in gas mani 9145-9159. festations from north-central Italy. Chem. Geol. 142,