Geochemical Journal, Vol. 33, pp. 1 to 12, 1999 The origin of natural gas emissions from Sardinia island, Italy 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 Cagliari, 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: Sardara; 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.