Fluid geochemistry of the Sardinian Rift-Campidano Graben (Sardinia, Italy): fault segmentation, seismic quiescence of geochemically “active” faults, and new constraints for selection of CO2 storage sites

Applied Geochemistry Applied Geochemistry 20 (2005) 317–340 www.elsevier.com/locate/apgeochem

Fluid geochemistry of the Sardinian Rift-Campidano Graben (Sardinia, Italy): fault segmentation, seismic quiescence of geochemically ‘‘active’’ faults, and new constraints for selection of CO2 storage sites M. Angelone b, C. Gasparini c, M. Guerra c, S. Lombardi a, L. Pizzino c, F. Quattrocchi c,*, E. Sacchi d,f, G.M. Zuppi e,f a

DST, Universita` ‘‘La Sapienza’’ di Roma, Piazzale Aldo Moro 5, 00100 Roma, Italy b ENEA PROT-CHIM, C.R. Casaccia, P.O. Box 2400, 00100 Rome, Italy c Istituto Nazionale di Geofisica e Vulcanologia (INGV), Via di VignaMurata 605, 00143 Roma, Italy d Dipartimento di Scienze della Terra (DST), Universita` di Torino, Via Valperga Caluso 35, 10125 Torino, Italy e Dipartimento di Scienze Ambientali, Universita` Ca
Foscari di Venezia, Dorsoduro 2137, 30123 Venezia, Italy f DST, Universita` degli Studi di Pavia, Via Ferrata 1, 27100 Pavia, Italy Received 7 November 2003; accepted 20 August 2004 Editorial handling by R. Fuge

Abstract Sardinia is typically seismically quiescent, displaying an almost complete lack of historical earthquakes and instrumentally recorded seismicity. This evidence may be in agreement with the presence of a ductile layer in the northern sector of the island, as suggested by the He isotopic signature in fluids rising to the surface through quiescent fault systems. The fault systems have been found to be ‘‘segmented’’ and therefore isolated in fluid circulation. The study of fluid behaviour along fault systems becomes strategically important when applied to solve some geological risk assessments such as Rn-indoor, or to define geological structures like potential CO2 storage sites. Both of these have been recently requested by the exploitation in Italy of the Euratom Directive and the evolution of the KyotoProtocol policy. Four water-dominated hydrothermal areas of Sardinia, located along regional fault systems, were considered: Campidano Graben, Tirso Valley, Logudoro and Casteldoria. A fluid geochemical survey was carried out taking into account physical–chemical and environmental parameters, major elements within gaseous and liquid phases, a few minor and trace elements, selected isotope ratios (2H, 18O, 13C, 3He/4He), 222Rn concentration, and some dissolved gases. Two different fluids have been recognised as regards both water chemistry and dissolved gases: (i) CO2-rich gases, poor in He and Rn, with a relatively high 3He/4He ratio (up to R/Ra = 2.32), associated with Na–HCO3–(Cl) thermal and cold groundwater; (ii) gases rich in He and N2, poor in CO2 and Rn, with a low 3He/4He ratio, associated with alkaline thermal and cold waters. The distribution of these two groups of fluids characterises the Sardinian tectonic systems. In fact, gas fluxes are not homogeneous, being mainly related to the different fault segments and to the areas where Quaternary basalts crop out. The underground geochemical evolution of the Sardinian fluids, as a function of the

*

Corresponding author. Fax: +39 6 51860302/507. E-mail address: [email protected] (F. Quattrocchi).

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2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2004.08.008

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geological and tectonic systems, provides some suggestions for solving one of the most important problems: CO2 geological sequestration. In order to reduce the CO2 excess produced by human activity, the best geological disposal sites are reservoirs with low hydraulic conductivity, sealed to fluid movement, or aquifers characterised by maximum pH buffering capacity of their mineralogical matrix. The knowledge of the role of faults, as permeability barriers or as deep fluid uprising pathways, is prerequisite.
2004 Elsevier Ltd. All rights reserved.

1. Introduction In the framework of the Geochemical Seismic Zonation EC funded program (EEC GSZ Project, 1996–98; Lombardi et al., 1999), Sardinia was selected as an aseismic site, in order to discriminate the differences in the relationships between fluid geochemistry and seismo-tectonics with respect to the other selected seismic areas (Quattrocchi, 1999; Quattrocchi et al., 1999, 2000, 2003; Salvi et al., 2000; Pizzino et al., 2004a). As a consequence of the obtained results and the experience gained in this project, further EC funds have been devoted to the same topic in the Corinth Rift, Greece (Pizzino et al., 2004b, Giurgea et al., 2004). Sardinia represents a typical seismically quiescent site, displaying both an almost complete lack of historical earthquakes (Boschi et al., 1997) and of instrumentally recorded seismicity (with few exceptions, e.g., Peronaci, 1953). Concerning the relations between fluid movement and seismic activity, the subject has been of increased interest in recent years (Hickman et al., 1995; Ingebritsen and Sanford, 1998; Teisseyre and Majewski, 2001), following the work of Sibson et al. (1975). In fact, the importance of fluid circulation along fault systems is now of strategic importance in order both to rework the earthquake prediction approach (Kingsley et al., 2001) and to solve some environmental risk assessments, for instance, Rn-indoor (Mancini et al., 2000). Moreover, the study of fluid behaviour through geological structures was recently enhanced in the framework of the Kyoto Protocol directivities to define storage sites for CO2 geological sequestration (Riding et al., 2002; Quattrocchi et al., 2004a,b). In contrast to other European aseismic countries (UK, Germany, etc.), the Italian Inventory of the best CO2 Storage Sites and their Storage Capacity (www.cslf.org), started recently. It needs accurate information about seismogenic sources: the GETSCO inventory structure, developed in the framework of an FP5 EC Project managed by TNO (The Netherlands Geological Survey), must be improved by new GIS strata encompassing the seismicity and the seismogenetic fault segments as geo-referenced objects (Quattrocchi et al., 2004b). Taking into account this statement and the future strategy to be exploited in Italy involving the GETSCO partnership, the authors completely re-

worked the information about seismicity and seismogenic structures of Sardinia. Fluid geochemistry study may give hints about the reological state of the crust: the presence of fluids may affect the seismicity style, enhancing the pore pressure at depth (Quattrocchi, 1999). On the other hand, the presence of ductile layers near surface may change the reological properties of the crust, allowing aseismic movements. The last characteristic may reduce seismic activity also in the presence of a regional active stressfield (Dragoni et al., 1996): despite the convergent point of view of the Authors stating the ‘‘non-active’’ geodynamical settings of the Sardinian ‘‘craton’’, it is necessary to review the previous statement by using new geochemical data and reworking the pre-existing seismological information. Four water-dominated hydrothermal areas, located along regional faults, were considered in this study: the Campidano Graben (CB as a geochemical family), the Tirso Valley (TV as a geochemical family), the Logudoro (LOG as a geochemical family) area and the Casteldoria area (CA as a geochemical family). Sixty-five sites, including thermal and mineralised waters, have been sampled.

2. The Sardinian structural setting Sardinia represents, together with Corsica (France), a fragment of continental crust. Its structural setting, developed during the Variscan deformation, points to the presence of strong compressive Alpine tectonics, with S-verging (North) and W-verging (South) folding and thrusting processes (Egger et al., 1988; Carmignani et al., 1992; Carmignani and Rossi, 2000; Faccenna et al., 2002). During Oligocene–Miocene times the island detached from the Iberico-Provenzale margin and separated from the European continent (Cherchi and Montadert, 1982) and reached its present position after a south-eastward drift and a simultaneous almost 30 anti-clockwise rotation. During the Miocene, Sardinia underwent a further extensional episode, inducing the formation of the Sardinian Rift. The latter is the result of superimposed extensional and trans-tensional episodes (Faccenna et al., 2002), and is coeval with the western Mediterranean Sea rift. The Sardinian Rift is

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approximately N–S oriented, from the Gulf of Cagliari to the Gulf of Asinara, superimposed on its south-western portion by the Campidano Basin, also defined as the Campidano Graben (Carmignani et al., 1994; Cherchi and Montadert, 1982, Fig 1(a)). This phase ended with andesitic volcanism, probably linked to the subduction zone, related to the opening of the Alghero Provenzale Basin. As a consequence, a magma reservoir, with compositional patterns very close to the underlying mantle, developed inside the crust (Rutter, 1987; Dupuy et al., 1987; Beccaluva et al., 1989). The Campidano Graben was subjected to a new subsiding cycle during the Middle Pliocene. Thus, while the southernmost portion of the graben (Graben of Cagliari) was filled by sands, muds, conglomerates, lacustrine clays and, more recently, by fluvial deposits, the northernmost part (Oristano Graben) was affected by volcanic activity, which composition changed from acidic (rhyolites of Mt. Arci) to alkaline. According to Pala et al. (1982a,b), Egger et al. (1988) and Casula et al. (2001), the Campidano Graben basement mostly consists of granitic–metamorphic rocks of Palaeozoic age, with a variable metamorphic grade (Ciminale et al., 1985). The Tertiary sedimentary complex crops out along different branches of the rift system, as a consequence of several marine transgressions. Detailed geophysical investigations were carried out in the early 1980s both by EC and Italian National Research Centres (CNR; Balia et al., 1983a,b, 1984, 1991; Bertorino et al., 1982a,b; Caboi et al., 1986, 1993; Pala et al., 1982a,b). The geophysical information collected up to now, indicates that the petro-physical data (andesites, Palaeozoic rocks) cannot explain a laterally extended, shallow, unique geothermal reservoir. Consequently, all the thermal springs recognised along the rift correspond to local manifestations, related to the rise up, from the granitic–metamorphic complex, of deep fluids along fault segments or fractured zones. 2.1. Southern Sardinia From a structural point of view, the Campidano Graben (Fig. 1(a)) is a narrow deep graben, extending approximately in a NW–SE direction, limited by two Palaeozoic granitic–metamorphic horsts, bounded by regional faults. The above-mentioned geological and geophysical data suggest that the central depression is about 3000–5000 m deep, 100 km long and 30 km wide. None of the oil exploratory wells located in the centre of the graben reached the Palaeozoic Crystalline Basement. The Campidano Graben is characterised by a remarkable heat flow anomaly, probably linked to a local thinning of the lithosphere (Loddo et al., 1982; D
Amore et al., 1987; Cataldi et al., 1995). The heat flow of the area also shows an anomalous peak, which extends over the whole graben with a mean value of 188.4 mW/m2 (4.5 lcal/cm2 s). This heat anomaly is located close to Sard-

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ara, and is probably related to the presence of a local thinning of the lithosphere. The anomalous thermal gradient is around 1 C every 15–20 m. According to Panichi (1982), predicted temperatures at a depth of 2000 m are over 150 C in the centre of the graben. The Hercynian crust of Sardinia is characterised by a value of about 60 mW/m2; this value is probably due to a thermal event related to recent tectonic phases of the Ligurian, Provenc¸al and Sardo-Balearic rifting. The remarkably good correspondence between the heat flow dispersion (HFD) and the Bouguer gravity anomaly strongly supports the existence of transient thermal regimes in the recent deformational areas of the European Geo-Traverse South (EGT-S), as opposed to the steady regime mostly affecting the central and northern segments of the EGT (Della Vedova et al., 1995). In the regional gravity map, the western part of the graben shows a regular trend of positive anomalies and fault systems on both sides of the graben throughout its length: the contour lines are elongated parallel to the axis of the graben and form a positive gravity trend from the centre of the graben towards the SW (Guerra, 1981; Egger et al., 1988). The Bouguer map is dominated by a large elongated low extending approximately NW–SE from Oristano to Decimomannu, along the axis of the Campidano Graben, in which 3 pronounced gravity minima (Arborea, S. Gavino, Decimomannu) are present. Due to its regional nature, this negative anomaly, correlated with magnetometric exploration (Balia et al., 1984, 1991; Ciminale et al., 1985), may be related to the Tertiary filling of the graben, made up of Miocene–Pliocene sediments. Gravity highs correspond predominantly to the exposure of the Paleozoic Crystalline Basement and to the Tertiary andesites. There is good agreement between gravity and geo-electrical structures, especially along the Guspini–S. Gavino–Sardara–Lunamatrona line, where the interpretation indicates the presence of a significant tectonic dislocation, which affects the Paleozoic Crystalline Basement of the Central Campidano Graben up to a depth of about 3 km. A remarkable correspondence between high gravimetric–magnetometric gradients and thermal springs was reported by Balia et al. (1984): the prevailing orientation of the iso-anomaly lines along a NNW–SSE direction, a very high horizontal gradient in correspondence with the thermo-mineral springs, and a clear asymmetry of the Campidano Graben were indicated. In particular, in correspondence with the Sa Guardia and Su Concali sites (e.g., SAR 11 and SAR 15 springs in this work), a fault was clearly recognised (western border of the graben); this area corresponds with a maximum in the heat flux (Loddo et al., 1982). Also the Capoterra sector, in the South-Western part of the graben, is affected by high geophysical horizontal gradients. The weak gravimetric anomaly extending from S
Acquacotta (Fig. 1(b)) to Villacidro (namely the

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Fig. 1. (a) Sardinia geological features as reported by Casula et al., 2001; (1) Paleozoic basement (PCB in the text); (2) Permian– Mesozoic; (3) Paleocene–Eocene; (4) Oligo-Miocene volcanics; (5) Oligocene to Neogene deposits; (6) Pliocene continental deposits; (7) Pliocene-Quaternary volcanics; (8) Quaternary; (9) generic faults; (10) normal faults; (11) sampling points; (12) geological crosssections. (b) Geological–stratigraphical cross-sections of Campidano along the S. Gavino basin (A), S
Acquacotta and SerramannaSerrenti basins (B) (after Balia et al., 1991): (1) schists (Paleozoic); (2) granitic rocks (Paleozoic); (3) sandstones (Eocene); (4) andesitic lavas and breccias (Oligocene–Lower Miocene); (4 0 ) andesitic dikes (Miocene); (5) prevalently marine arenaceous-marly or calcareous sediments (Miocene); (6) alluvial and lacustrine deposits (Pliocene). WCF = Western Campidano Fault; ECF = Eastern Campidano Fault. The centre of the S. Gavino sedimentary basin is the possible location for CO2 geological sequestration.

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Fig. 1 (continued)

‘‘Villacidro anomaly’’) is now displaced along the new NNW–SSE direction and it is probably connected with an intrusive body, at greater depth, where the western master fault may have undergone a dislocation. The S
Acquacotta spring (SAR 33 in this work) is associated with the main fault along the western side of the S
Acquacotta Basin (Fig. 1(b)), which is the extreme extension of the S. Gavino Basin (e.g., SAR 9 and 10 wells in this work).

2.2. Northern Sardinia The Fordongianus thermal area along the Tirso Valley (e.g., SAR 5 spring in this work) is located along a tectonic depression bordered by NW–SE and NE–SW trending fault systems. In this area, the thermal pools pertain to a circuit inside the granitic–metamorphic basement (Caboi et al., 1989). The Logudoro Basin, (e.g., SAR 6–8 springs in this work) a syncline with a N–S axis, is part of the northernmost Oligo-Miocene Sardinian Rift (Fig 1(a)). Andesites and east-dipping ignimbrites, covered by Middle Miocene sedimentary formations, fill it. During the Quaternary, the basin was affected by intense volcanic activity.

It is characterised by the intersection of NE–SW and NNW–SSE trending fault systems, which dislocated the basement in correspondence with the present location of CO2-rich outflows. Owing to its geothermal interest (presence of slightly thermal springs with high PCO2 values, Nuti et al., 1977; Caboi et al., 1993), geophysical studies were performed, suggesting the presence at depth of a Mesozoic Carbonate Plate despite its east to west continuity not being experimentally proved (Pecorini et al., 1988). According to Rutter (1987), the Plio-Pleistocene basalts overlie the Miocene sediments or the Oligo-Miocene volcanic rocks or the Paleozoic basement. The Casteldoria thermal area (e.g., SAR 46 spring in this work) is located near the sea at the intercept of two orthogonal fault systems, separating granites of the Gallura Hercinian basement from the Cenozoic and Quaternary sediments of the Anglona area. Cenozoic sediments, locally crossed by lava bodies and lamprophiric dikes, cover outcropping granites and Palaeozoic schist. Not far westward, the Mesozoic Carbonate Plate is also exposed: a limited outcrop is visible at Erula, about 15 km SSE of Casteldoria town (Carmignani, 1996).

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3. Reworking of seismicity studies There is no significant seismic activity in the Sardinian plate and the fault systems located along the Sardinian Rift seem to be almost totally quiescent from the seismic point of view (Fig. 2, Table 1). Seismic catalogues only report indications of events above a certain damage threshold. Historical information about Sardinian seismicity delineate a zone of low seismicity in the southern part of the region (Campidano Graben) as suggested by the 1610, 1771, 1835 and 1855 events (Boschi et al., 1997). In fact, some seismic events occurred during the last centuries, delineating some active tectonic structures. In particular, the historical seismicity was merely confined to the discussion on two possible seismically active areas: the Sassari province and the Cagliari county, both characterized by off-shore events, with a probable epicentral area far from the coast. The instrumental seismicity of the last four decades brought to light other seismic sources. 3.1. Southern Sardinia seismic sources The most ancient event mentioned in the historical catalogue (Boschi et al., 1997) occurred in 1610 in the Cagliari area. Its epicenter may correspond with the extensive structures bordering the Cagliari gulf (e.g., Capoterra Fault, as mentioned in this study, where the highest 222Rn values were found in this study, see later). Seismicity located in the northwestern sector of the Cagliari province may be connected with the Campidano Graben fault system. These structures were re-activated, e.g., during 1835 and 1855, both events being felt in Cagliari town. The map of the iso-seismic lines does not give a final indication about the effective seismic source. In the southern part of Sardinia, another seismic source may be delineated near both the Sant
Antioco and San Pietro islands (1771). Another low-energy event was felt in the Sant
Antioco island in 1923 (Ingrao, 1928). Southward of these areas, another seismic event was localized in August, 1977 with Ml = 5.2, which is an historically extraordinary event. This event was located in the vicinity of the Quirino Mount, south of Cagliari, 50 km from the coast. The sub-marine relief is made of calc-alkaline rocks (Finetti and Morelli, 1973). It is a seat of a positive magnetic anomaly. Gasparini et al. (1986) calculated the focal mechanism resulting in a right-lateral strike-slip fault with Apenninic direction (NW–SE).

the northeastern lobe of the big positive anomaly of the Campidano Basin, while within the second area (Borore village) is located on the fault segment responsible for both the 1922 and 2001 events, in the vicinity of the zero value of the Bouguer anomaly of the Abbasanta Plateau. The epicenters fall in the proximity of the tectonic structures possibly defined as strike-slip (Cocozza et al., 1974). 3.3. Northern Sardinia seismic sources This is the seismically most active sector of the island. On February 2, 1838 a VI MCS scale event was located in the eastern Gallura (a few kilometers away from the Casteldoria CO2–Rn-rich thermo-mineral spring studied in this paper). Well-defined events occurred in 1970, 1977 and 1997, slightly eastward from Alghero, up to the Nurra Escarpment. These seismic sources seem to be related to the regional NW–SE normal fault cutting the Sassari province limestone formations. Together with the N–S trending Ittireddu Fault, this regional fault is linked to the 1870 event (with a V MCS scale Intensity). The 1838, 1906 and 1960 (V MCS intensity) events may be associated with to the Libbra Mount structure throughout the Tempio Pausania area. The iso-seismic lines of the 1948 event, studied by Peronaci (1953) are in agreement with the position of the epicenter inside the Castelsardo Canyon, located about 40 km from the coast, being characterized by extensive faults with a NW–SE direction. Recently, two earthquakes of Md = 4.2 and Md = 4.8 occurred on April, 26, 2000 at 15:29 and 15:38 UTC, respectively. They were located offshore from the eastern coast of northern Sardinia (Fig. 2), causing noteworthy seismic concern throughout the whole region. The most important seismic sources of this sector are off-shore and they were activated also recently, mostly along the Olbia Canyon, made up of E–W extensive faults and along the Sardinia Basin, bounded by the same extensive faults, but elongated N–S. Compared with the described tectonics, the recent earthquakes are characterized by compressive mechanisms (buried thrusts) with a N–S main direction. This evidence is in total disagreement with the actual tectonic models (Faccenna et al., 2002). It seems that the ‘‘meso-alpine thrust’’ of the early Authors (e.g., Finetti and Morelli, 1973) is still active. The question remains intriguing for future detailed seismological studies.

3.2. Central Sardinia seismic sources This sector of the island is characterized by the presence of macro-seismic and instrumentally recorded events. The southernmost epicentral zone is close to a zero value of the Bouguer anomaly (see references above). The first area, near Gergei, is located inside

4. Geochemical methods From February 1997 to June 1997, a fluid geochemical survey was carried out, taking into account physicochemical and environmental parameters, major elements

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Fig. 2. Seismicity, focal mechanisms (together with the seismic moment values) and main historical seismic events of the Sardinia island (extracted from INGV Historical Catalogue, Boschi et al., 1997). Seismic events that occurred inside the Italian territory are listed in Table 1. See the text for the description of most of the quakes.

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Table 1 Historical and main instrumental seismicity Year

Month

Day

Time hh:mm:ss

Lat. North

Long. East

Depth km

Int.

Mag.

Ref.

1610 1771 1771 1835 1838 1855 1870 1901 1906 1922 1922 1923 1948 1948 1948 1948 1948 1948 1948 1948 1949 1960 1970 1972 1977 1977 1977 1985 1993 1997 1997 2000 2000 2000 2000 2000 2001 2001 2001 2001 2001 2001 2002

06 08 08 03 02 06 06 03 04 07 07 11 11 11 11 12 12 12 12 12 01 05 06 06 05 06 08 08 10 04 06 04 04 06 06 09 03 04 04 04 11 12 02

04 17 17 06 02 11 20 22 03 18 18 24 13 16 21 08 08 08 08 29 06 25 18 14 29 27 28 12 23 21 23 26 26 10 27 20 03 21 24 24 07 20 10

nd 13:30 18:30 nd 22:30 nd 08:22 13 16:20 20:30 22:30 18:48 09:52 21:57:43 21:50 04:50 13:15 13:45 23:00 21:45 17:30 22 09:03:03 19:21:44 16:19:56 19:36:44 09:45:14.5 01:23:29.2 14:27:54.1 11:38:00.9 02:55:02.3 13:28:41.1 13:37:48.3 04:11:26 04:07:55.9 19:26:31.8 01:54:51.9 17:31:38.3 16:01:58.1 16:22:38 09:40:43.3 16:05:37.5 16:21:36.7

39.22 39.00 39.00 39.25 41.00 39.25 40.55 39.683 40.90 40.217 40.217 39.15 41.067 41.067 41.067 41.067 41.067 41.067 41.067 41.067 41.067 40.933 41.000 40.783 40.800 40.60 38.21 38.16 41.70 41.45 41.81 40.92 40.98 41.90 40.95 41.22 40.82 41.01 41.68 41.68 41.34 40.16 40.96

9.11 8.43 8.43 9.25 9.50 9.167 8.933 9.10 9.117 8.800 8.800 8.25 8.683 8.683 8.683 8.683 8.683 8.683 8.683 8.683 8.683 9.117 7.800 10.383 8.200 8.40 8.21 8.98 7.69 7.74 7.75 10.07 10.10 7.88 10.03 7.66 10.04 9.90 9.55 9.55 10.15 8.65 10.28

nd nd nd nd nd nd nd nd nd nd nd nd 13 nd nd nd nd nd nd nd nd nd nd 183 nd nd 10 10 10 10 10 10 10 19 10 10 10 10 10 10 10 10 10

IV–V IV V III VI nd V IV–V III–IV III III II VI nd VI IV–V IV–V VI IV–V V VI V nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd

3.3 3.0 3.5 2.5 4.1 nd 3.5 3.3 2.8 nd nd 2.2 4.4 4.5 4.4 3.3 3.3 4.4 3.3 3.5 4.2 3.5 nd 3.8 2.6 3.0 5.2 nd 3.3 3.8 3.2 4.5 5.1 2.9 4.3 2.9 4.8 3.9 2.7 2.7 4.6 3.5 3.6

nd nd nd nd nd nd nd nd nd nd nd nd ML RM ML RM ML RM nd nd ML RM nd nd ML RM nd nd ML RM ML RM ML RG ML RMP nd ML LDG ML STR ML LDG ML GEN mb GS MD LDG mb GS ML MDD mb GS mb GS ML LDG ML LDG mb GS ML ROM ML LDG

Depth, calculated depth; Int., evaluated macroseismic intensity in MCS degrees; Mag., evaluated or calculated magnitude; Ref., reference magnitude (see INGV catalogue reference, Boschi et al., 1997). Mb is the body wave magnitude, Ml is the local magnitude as calculated by both the listed seismic stations (e.g., RMP: Roma Monte Porzio) and European seismic networks (e.g., LDG: Laboratoire de Detection Geophysique). Md is the magnitude based on the seismic signal duration. nd: not determined.

within gaseous and liquid phases, a few minor and trace elements, as well as selected stable isotope ratios (2H, 18 O, 13C, He). Cation/anion balance is ±10%. Besides temperature, electrical conductivity, pH, Eh, 222 H2S and HCO
Rn concentration was also mea3 , the sured directly in the field (Mancini et al., 2000). Samples selected for dissolved gas analyses (N2, O2, Ar, CO2, CH4 and He) were collected by admitting groundwater

into special glass vessels, and operating to avoid any atmospheric contamination. Dissolved gases were extracted in the laboratory using a customised vacuum extraction line connected to a Quadrupole Mass Spectrometer (Quattrocchi et al., 2000). Water samples were also collected to analyse the 3He/4He isotopic ratio by the CNRS-CRPG all-metal rare gas analytical facility (Marty et al., 1992).

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5. Hydrogeochemical data and results Four water-dominated hydrothermal areas of Sardinia were considered, all located along regional fault systems: from S to N, the CB, the TV, the LOG area and the CA area: 65 sites were sampled, including thermalmineralised and cold springs and wells. Analytical data are reported in Tables 2a, 2b and 3. Groundwater evolves toward Na–Cl, Na–(Ca–Mg)–Cl, Na (Ca– Mg)–SO4 and Na–HCO
3 ðClÞ chemistry, as shown by the Ludwig–Langelier diagram (Fig. 3): (i) Forty-four sampling points, such as springs and wells were sampled in the CB, and their distribution follows the main structural patterns of the graben. Owing to their geographic and geological homogeneity, sampling points have been divided into 3 families: Western Campidano Fault (WCF), Eastern Campidano Fault (ECF), Central Campidano Graben (GR). This classification was performed to evidence possible differences in composition within the above-mentioned groups, pertaining to different fault segments. A fourth group of water points was named Capoterra Fault (CF, 16 samples) to indicate those discharging in the homonymous area (see Fig. 1(a)), characterised by outcrops of granitic rocks. The WCF family groups 14 sampling points: among these, some are Na–HCO3–Cl thermal waters (T > 25 C) (Sandalia Acquacotta spring, 46.1 C and Sa Guardia well, 31.2 C), one is Na–Cl thermal water (Santa Maria Neapolis well, 26 C) and some are cold or slightly thermal waters (highest temperature 23.1 C at Su in Sidu Concali well). With the exception of the Santa Maria Neapolis sample, all thermal waters show both high dissolved CO2 and Rn contents; a previous isotopic study on dissolved carbonates (d13C 
3&) suggests that CO2 may derive from the metamorphism of marine carbonates (Minissale et al., 1999). High Rn contents are probably due to the joint action of both the interaction with granitic rocks forming the western edge of Campidano Graben and the presence of fault systems, but this aspect will be discussed later. In the ECF family 21 sampling points were considered: 5 of these are thermal waters (Su Campu well, 43 C; Bagno Antico di Sardara spring, 57 C; Terme di Sardara well, 53.1 C, Soro Uras well, 30 C and Tiria Podere 60 well, 26.1 C), while the others are cold or slightly thermal (highest temperature 23.3 C at S. Margherita Tramatza well). The GR family groups 9 samples: only one is thermal and rich in CO2 (Fonderia discarica 1 well, 27.4 C with a Na–HCO3–Cl chemistry), while the others are cold and poor in CO2 (highest temperature 22.0 C at Fonderia Colombo 6 well). The hydrogeological circuits in the CB originate from the PCB and schist rocks at the Eastern and Western sides of the structure, representing the recharge areas; then they develop in the basement, below approximately 2500 m of a volcanic-sedimentary series, where deep ori-

325

gin CO2 is locally added. These suggestions are fully in agreement with previous studies (Bertorino et al., 1982a,b; Caboi and Noto, 1982). The fault system bordering the CB allows the geothermal fluids to reach the surface, locally accompanied by CO2 gas carrier; this is the case for all thermal waters, except for that of Su Campu well (SAR 13), where CO2 is completely absent and the prevailing gas is N2 (see dissolved gases data, Table 2b). This borehole, 210 m deep, was drilled in May 1989 (Frau, 1994). Groundwater is alkaline (pH 8.01) and indicates negative Eh (
10 mV), high electrical conductivity (9000 lS/cm) and very low Rn contents (2 Bq/L). These data suggest that groundwater circulates through PCB, in an environment completely closed to CO2, either atmospheric, geogenic or biogenic. In contrast, the Sardara geothermal area is characterised by fluids with pH close to neutral, high salinity, and high CO2 and HCO3 contents (Table 2a and 2b). Groundwater with similar physical and chemical characteristics has been found in different wells at about 30 km NW of Sardara, near Uras. This area could be considered as a new potential geothermal area. (ii) The Fordongianus spring (SAR 5, TV family), located along a NE–SW trending fault bordering the Tirso river graben, is the most significant water discharge of this hydrothermal system. Physical–chemical parameters of the Fordongianus spring (high temperature, low salinity, relatively low Eh value), together with the geological features of the area, seem to indicate a typical circulation in granite rocks, with mobilisation of Na and Cl, leached from a HCO3–CO2 poor crystalline basement. According to previous studies (Biddau et al., 2002; Cidu and Mulas, 2003), surface waters draining granite rocks in the Tirso Valley contain up to 85 mg/L Cl, and shallow-cold groundwater in nearby granite areas contain up to 130 mg/L Cl. In general, in granite environments, Cl-bearing minerals, such as micas, may be weathered at ambient temperature. At Fordongianus site, in particular, the breakdown of Cl-bearing minerals may be enhanced through a combination of tectonics and long rock-water interaction times along fractures at relatively high temperature (Kamineni, 1987). The authors
estimates of deep temperatures (<100 C), in agreement with previous studies (Caboi et al., 1989; Minissale et al., 1999), confirm this suggestion. The outlet water temperature at Fordongianus (54 C) strictly reflects zonal hydrodynamic behaviour. Thus the measured temperature might be due to the slow deepening process, to the long residence time and, finally, to the fast uprising of hydrological circuits along the faults interacting in the area (NW–SE and NE–SW structures). The discharge area is not characterised by an anomalous geothermal gradient. Discharging gases are essentially N2 with a very low CO2 content, possibly due to the low CO2 flux in this segment of the rift. In spite of water circulation in rocks likely to be U–Th-rich, the Rn content

Date

N. Lab.

Site

Family

Lat.

Long.

Temp. (C)

and total CO2 measured by ORION ion-selective WCF 4361.717 485.052 43.5 WCF 4361.436 487.037 20.0 WCF 4354.381 484.439 14.5 WCF 4360.841 477.539 14.9 TV 4427.543 483.519 54.1 LOG 4503.674 472.066 18.2 LOG 4477.553 465.489 22.5 LOG 4477.637 488.033 18.8 GR 4376.924 481.286 27.4 GR 4376.863 481.262 22.0 WCF 4344.689 493.504 31.2 GR 4355.325 492.747 17.5 ECF 4357.449 503.542 43.0 GR 4353.907 492.578 18.1 WCF 4346.199 493.434 23.1 ECF 4384.777 485.096 17.0 ECF 4384.754 481.543 16.0 ECF 4385.124 481.449 57.0 ECF 4385.186 481.377 53.1 ECF 4377.522 492.744 18.3 ECF 4378.478 492.339 18.3 ECF 4378.848 492.316 20.3 ECF 4378.602 491.647 18.5 CF 4337.812 496.234 20.5 CF 4336.549 496.089 21.0 CF 4330.999 499.784 12.8 CF 4323.724 498.894 18.5 CF 4321.936 498.702 14.5 CF 4320.949 498.966 20.6 ECF 4385.117 484.811 17.2 WCF 4370.146 479.573 15.0 WCF 4369.656 478.306 19.5 WCF 4361.717 485.052 46.1 GR 4367.201 487.118 16.5 ECF 4372.154 497.062 19.0 CF 4334.945 496.880 23.0 CF 4333.866 500.792 18.8 CF 4337.134 496.833 19.4 CF 4337.566 496.569 19.7 CF 4339.139 494.867 15.3

El. Cond. (ls/cm) electrode 3050 418 2020 481 1547 4570 5790 2180 2590 1428 1568 1230 9005 3100 1320 2470 3900 5300 5280 2020 2620 2610 2480 2090 1433 537 1615 481 1640 1844 635 734 3000 500 4500 840 1100 1400 1800 400

pH

Eh (mV)

Total CO2 (ppm)

Rn (Bq/l)

NH4 (ppm)

6.53 7.16 7.24 6.16 8.40 6.51 6.35 5.96 7.56 7.52 6.98 7.14 8.01 7.76 7.02 7.09 7.26 6.84 6.79 7.34 7.40 7.24 7.16 7.10 7.22 8.54 7.07 7.14 7.63 7.50 6.51 6.94 6.72 8.58 6.98 6.94 7.44 7.00 7.02 6.94

292 392 353 391 259 248 305 311 159 266 222 361
10 355 185 351 362 372 361 385 391 337 340 382 301 330 362 363 323 436 403 394 342 366 363 379 408 395 279 341

nd nd 2992 440 145 11440 9680 nd 7040 1760 3432 616 15 2640 3520 1760 1144 10120 9680 2113 nd 2596 2728 3784 2244 nd nd nd 836 1848 nd 128 1892 nd 462 167 110 185 194 46

31 63 14 27 3 4 45 12 12 16 45 14 2 28 51 43 16 13 33 18 18 35 16 95 95 30 13 27 14 16 35 76 41 4 6 251 107 114 142 98

nd nd nd 0.04 0.07 0.04 0.04 0.04 0.04 0.06 0.05 nd 0.43 0.04 0.04 0.04 nd 0.16 0.27 nd nd nd nd 0.04 0.04 nd nd nd nd nd nd nd 0.04 nd nd 0.04 nd nd 0.04 nd

M. Angelone et al. / Applied Geochemistry 20 (2005) 317–340

On all samples physical–chemical parameters, 222Rn, NH4 08/02/1997 SAR 1 W. Sandalia Acquacotta out. 08/02/1997 SAR 2 W. Giara 08/02/1997 SAR 3 W. Sa Domu 08/02/1997 SAR 4 S. Gonnu Forru 09/02/1997 SAR 5 S. Terme Fordangianus 09/02/1997 SAR 6 S. S. Martino 09/02/1997 SAR 7 S. Abbarghente 09/02/1997 SAR 8 W. Acqua S. Lucia 10/02/1997 SAR 9 W. Fonderia Discarica 1 10/02/1997 SAR 10 W. Fonderia Colombo 6 10/02/1997 SAR 11 W. Sa Guardia 11/02/1997 SAR 12 W. Sisinnio 11/02/1997 SAR 13 W. Su Campu San Sperate 11/02/1997 SAR 14 W. Giardinetti Decimopotzu 11/02/1997 SAR 15 W. Su in Sidu Concali 12/02/1997 SAR 16 W. Campo Sportivo Sardara 12/02/1997 SAR 17 W. Albergo Comune Sardara 12/02/1997 SAR 18 S. Bagno Antico Sardara 12/02/1997 SAR 19 W. Terme di Sardara 12/02/1997 SAR 20 S. Mitza de Gilladri 12/02/1997 SAR 21 W. Sanluri Pontis 12/02/1997 SAR 22 W. Sanluri Piras 12/02/1997 SAR 23 W. Sanluri Cadeddu 13/02/1997 SAR 24 W. Podere Montaldo 13/02/1997 SAR 25 W. Garau Capoterra 13/02/1997 SAR 26 W. Sgaravatti 13/02/1997 SAR 27 W. Su Argori 13/02/1997 SAR 28 W. Crabiliddu 13/02/1997 SAR 29 W. Bardini Villasanpietro 14/02/1997 SAR 30 W. S. Anastasia 14/02/1997 SAR 31 W. Riu Peis 14/02/1997 SAR 32 W. Saddanus Villacidro 14/02/1997 SAR 33 W. Sandalia Acquacotta 14/02/1997 SAR 34 W. Sinpisu S. Michele 14/02/1997 SAR 35 W. Serra Pucix 15/02/1997 SAR 36 W. Cardia - Capoterra 15/02/1997 SAR 37 W. S. Giuseppe Capoterra 15/02/1997 SAR 38 W. Bingiamanna Capoterra 15/02/1997 SAR 39 W. Sa Tumba S. Lucia 15/02/1997 SAR 40 W. S. Lucia

326

Table 2a Chemical composition of groundwater

N. Lab.

SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR

Family

41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

W. Marmeria SISAL W. Su Caddu W. Soru Uras W. Pixeneris Uras W. Mattiaedas Terralba S. Casteldoria W. Is Bangius W. Sa Zeppara (coop) W. Sa Zeppara W. M.te Arcuentu W. Santa Maria Neapolis W. Gennaniedda S. Cava M.te Arci W. Tiria podere 60 W. Palmas di Arborea W. M.te Palla W. Marrubiu W. Senaseba S. Barigadu Miles S. Funtanaintru Miles S. Funtana Manna Miles W. S. Margherita Tramatza W. Fadda 1 W. Fadda 2 S. Sa Canna S. Lucia

Ca (meq/l)

Mg (meq/l)

On selected samples, major and minor elements SAR 1 WCF nd nd SAR 2 WCF 0.52 0.75 SAR 3 WCF 2.90 1.17 SAR 4 WCF 0.95 1.23 SAR 5 TV 0.67 0.01 SAR 6 LOG 3.01 6.55 SAR 7 LOG 3.55 3.78 SAR 8 LOG 3.85 1.45 SAR 9 GR 1.27 2.29 SAR 10 GR 0.70 0.83 SAR 11 WCF 1.76 0.84 SAR 12 GR 1.54 1.90 SAR 13 ECF 30.09 0.03 SAR 14 GR 8.18 5.95

CF CF ECF ECF WCF CA ECF GR GR WCF WCF WCF ECF ECF GR ECF GR ECF ECF WCF ECF ECF CF CF CF

4342.898 4346.845 4394.144 4393.133 4396.357 4528.825 4404.525 4380.000 4387.600 4385.450 4392.600 4390.600 4403.200 4412.100 4414.275 4416.750 4399.525 4430.500 4433.250 4433.450 4433.375 4427.350 4338.750 4338.800 4338.800

498.034 496.813 475.254 473.250 468.666 491.675 472.125 470.150 466.650 462.000 462.625 460.900 473.000 475.350 470.225 481.775 468.375 473.675 469.000 469.125 469.150 470.600 494.450 494.650 492.400

16.1 17.2 30.0 16.8 18.2 73.3 22.0 18.5 20.2 21.3 26.0 20.7 18.5 26.1 20.3 22.7 19.7 20.8 19.6 19.2 19.8 23.3 24.8 20.1 20.2

400 1450 1450 1600 1500 12740 970 584 2550 1050 2790 1243 488 1192 739 3130 10490 853 340 330 329 1845 956 789 370

7.00 7.51 7.25 7.16 7.34 6.85 7.62 6.71 7.40 7.27 7.35 6.88 6.23 7.82 7.82 7.15 8.06 7.65 7.14 7.13 7.07 6.95 6.75 6.81 6.63

361 369 396 384 390 66 310 376 294 291 362 313 310 296 307 187 276 321 301 287 286 281 289 308 276

57 207 471 290 268 22 308 158 1012 370 440 339 35 158 238 392 422 393 41 nd 42 411 396 nd 381

99 70 4 17 11 125 34 37 14 12 10 nd 11 20 15 37 nd 3 36 nd 46 21 40 65 48

nd nd 0.04 nd nd 0.05 0.04 0.04 2.47 0.04 0.04 0.04 0.04 0.04 0.04 0.17 0.04 0.04 0.04 nd nd 0.04 nd nd 0.04

Na (meq/l)

K (meq/l)

HCO3 (meq/l)

Cl (meq/l)

SO4 (meq/l)

B (mg/L)

SiO2 (mg/L)

Fe (mg/L)

Sr (mg/L)

Li (mg/L)

nd 1.96 14.74 2.26 9.70 33.52 50.39 19.48 22.00 10.83 10.78 7.80 49.92 16.39

nd 0.07 0.33 0.07 0.05 2.07 1.23 0.13 0.16 0.08 0.36 0.00 0.22 0.33

15.67 1.14 9.73 1.37 1.03 51.73 46.94 18.11 16.80 6.89 7.91 3.37 0.50 5.76

nd 2.05 9.63 2.93 7.70 5.97 11.38 7.41 5.92 5.52 4.66 5.64 69.49 20.61

nd 0.30 0.90 0.63 0.83 4.80 9.90 0.95 0.32 0.51 1.27 1.16 6.13 5.53

nd nd nd 0.05 0.29 0.65 1.30 0.15 2.70 0.85 0.10 nd nd 10.34

nd nd nd 14.56 37.02 15.36 43.33 40.83 29.96 27.20 37.99 nd nd 20.42

nd <0.1 <0.1 <0.1 <0.1 0.10 0.10 0.20 3.50 0.10 0.40 nd <0.1 0.70

nd 0.04 1.03 0.10 0.10 3.70 0.70 0.10 0.70 0.20 0.20 nd 1.89 1.20

nd 0.02 0.25 0.01 0.98 6.51 3.90 1.49 2.10 1.15 1.03 nd 0.30 0.20

M. Angelone et al. / Applied Geochemistry 20 (2005) 317–340

15/02/1997 15/02/1997 16/02/1997 16/02/1997 16/02/1997 03/06/1997 03/06/1997 04/06/1997 04/06/1997 04/06/1997 05/06/1997 05/06/1997 06/06/1997 06/06/1997 06/06/1997 06/06/1997 06/06/1997 06/06/1997 07/06/1997 07/06/1997 07/06/1997 07/06/1997 07/06/1997 07/06/1997 07/06/1997

327

328 (continued on next page) Table 2a (continued) Family

Ca (meq/l)

Mg (meq/l)

Na (meq/l)

K (meq/l)

HCO3 (meq/l)

Cl (meq/l)

SO4 (meq/l)

B (mg/L)

SiO2 (mg/L)

Fe (mg/L)

Sr (mg/L)

Li (mg/L)

SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR

WCF ECF ECF ECF ECF ECF ECF ECF ECF CF CF CF CF CF CF ECF WCF WCF WCF GR ECF CF CF CF CF CF CF CF ECF ECF WCF CA ECF GR GR WCF

2.74 4.39 8.33 1.13 1.17 6.43 nd 8.23 8.10 2.26 1.24 nd nd nd 3.72 3.45 nd 2.39 1.67 0.63 26.60 1.40 nd nd 3.88 nd nd nd 2.54 nd nd 30.74 0.43 nd 4.40 2.21

1.81 3.05 14.20 0.39 0.36 5.12 nd 8.31 8.09 0.73 0.42 nd nd nd 2.57 2.90 nd 0.82 0.74 0.37 20.64 0.94 nd nd 4.73 nd nd nd 3.07 nd nd 0.33 0.57 nd 4.41 2.33

8.44 18.22 19.52 39.61 42.96 10.00 nd 12.44 10.61 18.52 11.44 nd nd nd 8.13 9.74 nd 2.87 21.70 1.09 25.13 5.65 nd nd 12.74 nd nd nd 8.57 nd nd 52.35 7.44 nd 19.35 5.87

0.21 0.11 0.82 1.23 1.18 0.17 nd 0.18 0.36 0.28 0.31 nd nd nd 0.08 2.38 nd 0.07 0.33 0.04 0.42 0.16 nd nd 0.15 nd nd nd 0.33 nd nd 1.38 0.22 nd 1.05 0.18

7.87 8.12 5.00 27.90 28.32 3.55 8.10 8.37 8.63 12.50 8.40 nd nd nd 3.10 7.84 1.40 3.43 15.61 1.87 7.86 2.58 1.42 3.30 3.17 0.62 1.18 3.19 8.63 7.16 6.46 0.79 3.00 1.50 13.00 4.63

4.71 13.07 37.12 12.17 13.35 7.35 nd 8.71 8.37 6.00 3.67 nd nd nd 10.74 5.86 nd 2.63 5.33 nd 7.83 4.22 nd nd 15.15 nd nd nd 4.47 nd nd 78.39 5.54 nd 14.71 6.18

0.91 6.22 3.61 1.64 1.73 9.18 nd 12.98 11.59 2.05 0.65 nd nd nd 1.53 3.25 nd 0.43 1.75 nd 51.68 1.58 nd nd 2.38 nd nd nd 1.15 nd nd 2.16 0.47 nd 3.61 0.70

0.11 0.21 nd 2.43 nd nd nd nd nd 0.41 0.03 nd nd nd nd nd nd nd 0.76 nd nd 0.04 nd nd nd nd nd nd 0.02 nd nd nd nd nd nd nd

22.06 13.20 nd 42.85 nd nd nd nd nd 33.75 43.11 nd nd nd nd nd nd nd 38.36 nd nd 39.61 nd nd nd nd nd nd 43.08 nd nd nd nd nd nd nd

0.30 1.40 <0.1 0.30 0.50 <0.1 nd 2.04 <0.1 0.30 0.10 nd nd nd 1.74 <0.1 nd <0.1 0.30 <0.1 <0.1 0.10 nd nd <0.1 nd nd nd <0.1 nd nd <0.1 <0.1 nd 1.08 0.73

0.30 0.90 0.97 0.30 0.40 0.63 nd 1.16 0.70 0.30 0.20 nd nd nd 0.36 0.22 nd 0.12 0.40 0.06 3.04 0.10 nd nd 0.40 nd nd nd 0.20 nd nd 7.75 0.02 nd 0.25 0.05

0.61 0.45 0.23 7.48 7.68 0.03 nd 0.06 0.04 2.05 0.66 nd nd nd 0.02 0.02 nd 0.03 5.23 0.01 0.09 0.15 nd nd 0.22 nd nd nd 0.02 nd nd 1.42 0.01 nd 0.01 0.01

15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

Line missing

M. Angelone et al. / Applied Geochemistry 20 (2005) 317–340

N. Lab.

0.02 0.02 0.12 0.03 nd <0.01 nd nd 0.01 nd nd 0.01 0.04 0.01 1.82 0.05 nd 0.02 nd nd 0.09 nd nd 0.03 1.94 <0.1 0.79 1.18 nd <0.1 nd nd <0.1 nd nd <0.1 nd nd nd nd nd nd nd nd nd nd nd nd 7.00 6.57 21.61 9.13 nd 2.02 nd nd 12.65 nd nd 1.57

6. Discussion 6.1. Origin of the water and dissolved salts

nd: not determined.

ECF GR ECF GR ECF ECF WCF ECF ECF CF CF CF 54 55 56 57 58 59 60 61 62 63 64 65 SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR

SAR 51 SAR 52 SAR 53

WCF WCF ECF

3.74 nd nd

1.21 0.64 5.68 1.00 nd 0.52 nd nd 2.80 nd nd 5.09

5.95 nd nd

1.63 0.60 3.22 0.85 nd 0.84 nd nd 3.50 nd nd 30.46

18.00 nd nd

0.20 nd nd

0.31 0.14 0.15 0.20 nd 0.05 nd nd 0.18 nd nd 0.06

2.83 3.68 8.00 4.00 4.66 1.57 1.58 1.54 5.55 3.72 3.59 1.25

7.13 3.83 18.67 nd nd 1.53 nd nd 10.46 nd nd nd

0.50 0.56 6.24 nd nd 0.22 nd nd 1.93 nd nd nd

nd nd nd nd nd nd nd nd nd nd nd nd

nd nd nd 2.62 1.90 nd 17.36 7.83 nd 10.70 3.80 0.70

329

is very low (3 Bq/L), testifying to the ‘‘non secular equilibrium’’ conditions (see later in Section 6). (iii) The LOG springs differ from all other mineralised water points, essentially for the low temperatures (20–24 C), and for the high CO2 content. The 3 most mineralised springs (San Martino, Abbarghente and Santa Lucia, samples SAR 6, SAR 7, SAR 8, respectively), are characterised by electrical conductivity up to 5800 lS/cm and high CO2 flux. The origin of the CO2 could be due either to thermal decomposition of marine carbonates (Logudoro MCP) or to magmatic input connected with the recent basalt volcanism (as discussed later). In this area, indeed, groundwater flows out of the Oligocene–Miocene calc-alkaline volcanic rocks (andesites and ignimbritites). The high CO2 content causes relatively low pH values (range 6–6.5) and an increase in salinity as a consequence of enhanced water– rock interaction processes. These waters are thus characterised by very high HCO3 content (up to 3170 mg/L). The content of 222Rn is low both in groundwater and in the free gas phase sampled at S. Martino spring; the maximum content of Rn was measured at the Abbarghente spring: 45 Bq/L. Low Rn content indicates either an almost complete lack of water–rock interaction with U– Th-rich rocks in the Logudoro MCP and/or an enhanced gas stripping-effect (Pizzino et al., 2002) due to high CO2 flux through groundwater, as discussed later. (iv) The high temperature and the high Rn content, as well as the high Cl content of CA waters (SAR 46), indicate the presence of a deep fluid, poor in dissolved gases, mixed with a sea-water component of almost 11–15% (Nuti et al., 1977; Caboi et al., 1986, 1993).

nd nd nd

<0.1 nd nd

0.16 nd nd

0.01 nd nd

M. Angelone et al. / Applied Geochemistry 20 (2005) 317–340

Electrical conductivity and pH (Table 2a and 2b) indicate that some thermal and cold waters are strongly affected by the presence of a remarkably high CO2 content, which is, in fact, the main factor controlling water–rock interaction in those circuits. Using a Ludwig–Langelier diagram (Fig. 3), samples fall essentially in the field of Na–HCO3 and alkaline–Cl waters. Other waters, belonging to the ECF family, show an alkaline-earth SO4 chemistry. Therefore, it is too simplistic to explain, in such a dynamic environment, that the water chemistry is the result of a unique water–rock interaction process. In other words, the reaction kinetics cannot be separated from the fluid hydrodynamics. Additionally, the hydraulic conductivity of faults and the presence of hydraulic barriers, result in different fault segment behaviour, conditioning water transit time at depth, and the kinetic of the fluid-mineral reactions. Consequently, the thermal equilibrium between rocks

330

M. Angelone et al. / Applied Geochemistry 20 (2005) 317–340

Table 2b Chemical composition of groundwater N. Lab.

Family

N2 (%)

O2 (%)

Ar (%)

CO2 (%)

CH4 (%)

He (ppm)

C2H6 (ppm)

SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR

WCF TV LOG LOG GR GR WCF ECF WCF ECF ECF CF CF ECF WCF CF

83.52 79.58 4.07 1.81 80.26 87.35 72.27 88.51 86.07 26.84 33.60 87.96 72.31 66.14 55.96 81.99

3.28 13.36 1.04 0.15 10.53 9.12 3.00 8.47 4.11 9.76 11.06 4.90 5.13 12.79 14.75 12.61

1.49 1.29 0.05 0.09 1.35 1.52 1.78 1.25 1.83 1.04 0.60 1.28 1.42 1.67 0.93 1.61

9.17 4.68 94.78 97.94 0.99 1.05 20.77 1.08 5.86 62.12 54.46 4.65 20.78 17.74 29.18 3.62

0.00470 0.03630 0.00023 0.00059 2.03000 0.00055 0.05080 0.03740 0.00960 0.03000 0.00920 0.00220 0.00180 nd 0.04160 0.00074

989 730 4 3 345 102 599 1961 443 65 10 957 644 7 207 147

0.160 0.053 0.070 0.056 0.880 0.040 0.140 0.150 0.190 25.330 0.660 0.110 0.090 nd 0.170 0.160

3 5 6 7 9 10 11 13 15 18 19 24 25 30 33 36

Dissolved gases. nd: not determined.

Table 3 3 He/4He ratio and 4He/20Ne ratio of 10 selected Sardinia groundwater sites Sample

He (mol/g)a 10
12

3

±2r

4

SAR SAR SAR SAR SAR SAR SAR SAR SAR SAR

416 849 0,46 (a) 2.07 324 206 1129 8768 430 1120

0.26 0.28 2.89 2.06 0.83 0.62 0.07 0.22 0.67 0.30

0.010 0.010 0.680 0.080 0.010 0.010 0.003 0.005 0.010 0.005

57 118 3.20 1.45 49 5.80 64 176 48 214

a

5 6 6 (gas) 7 9 10 11 13 19 33

He/4He (R/Ra)

He/20Ne

3

He/4He (R/Ra

corr.)

0.26 0.28 3.04 2.32 0.83 0.60 0.06 0.22 0.67 0.30

Gas phase analysis expressed as moles/moles · 10
6.

and circulating fluids is strongly affected. In order to better discriminate the fluid circulation patterns throughout the different fault segments, Na–K–Mg data were reported in the Giggenbach (1988) diagram (Fig. 4). Data points indicate that none of the studied systems attained full equilibrium. The Casteldoria (SAR 46), the Sardara Bagno Antico and Terme (SAR 18, 19) and the Sandalia Acquacotta (SAR 33) samples show partial equilibrium, or could be considered as mixed. The geothermometric applications (Na–K, Na–Li and Li–Mg, silica is not available for this spring) indicate a reservoir temperature of about 160 C for the Casteldoria system. The Na–K geothermometer indicates for the Sardara and Sandalia-Acquacotta thermal systems a reservoir temperature of 170 and 130 C, respectively. These values are in

agreement with Loddo et al. (1982), that report a temperature of 150 C at 2000 m depth in the depo-centre of the Campidano basin. The silica geothermometer (chalcedony phase, with saturation indices close to zero), on the contrary, indicates for both waters a reservoir temperature of 60–65 C; it is very close to the outlet value and could not represent the real estimate of the deep temperatures. In fact, although the temperatures at the outflow are comprised between 40 and 60 C, Sardara and Sandalia-Acquacotta waters (SAR 18 and SAR 33, respectively) display a relatively low content of silica with respect to the other samples (Fig. 5). This suggests that during the movement toward the surface through the sedimentary formations or through the tectonic discontinuities, fluids cool due to mixing with shallow

M. Angelone et al. / Applied Geochemistry 20 (2005) 317–340

331

Fig. 3. Ludwig–Langelier diagram of the Sardinia Island analysed groundwater. Owing to their geographical and geo-structural homogeneity, sampling points have been subdivided into different families: CA, Casteldoria; LOG, Logudoro; TV, Tirso Valley; WCF, Western Campidano Fault; GR, Central Campidano Graben; ECF, Eastern Campidano Fault; CF, Capoterra Fault; PCB, palaeozoic crystalline basement.

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M. Angelone et al. / Applied Geochemistry 20 (2005) 317–340

Fig. 4. Distribution of sampled waters in a Na–K–Mg diagram (Giggenbach, 1988). None of the studied systems attained full equilibrium (see the text). tkn and tkm are isotherms calculated using the K–Na and the K–Mg geothermometers, respectively.

waters, causing silica precipitation, and/or re-equilibrate with host rocks at lower temperatures. The Na–Li and Li–Mg geothermometers give unreliable high values, probably due to the Na–HCO3 chemistry of these waters. The high Na content of the outliers SAR 5 and SAR 13 can be explained by water circulation in environments completely confined and geochemically closed to the atmosphere (Helgeson, 1969). To explain these outliers, two contemporaneous mechanisms may be evoked. The relatively low Mg content displays the labile geochemistry of this ion. It could be incorporated into secondary minerals (chlorite or other silicates) filling faults and veins and reducing fluid movement. Moreover, because no other sources of CO2 are present along these fault segments, atmospheric and biogenic CO2 components are exhausted, the water–rock interaction slows down and the chemical composition of water reaches a steady-state with Na and K feldspars, which is a function of velocity of water percolation and reaction rates (Henley

et al., 1984). The high percentage of Mg in all the remaining samples places them in the compositional area of immature or highly diluted waters; this chemical disequilibrium in the water–rock system is in agreement with the ‘‘non equilibrium’’ segment of the 222Rn cumulative curve (Fig. 6(a)). Radon content in groundwater is used as a sound methodology to discover both buried fault systems and convective fluid circulation conditions in different structural and lithological settings (Mancini et al., 2000; Quattrocchi et al., 1999; Pizzino et al., 2002, 2004a,b). In the 222Rn cumulative diagram plot (Fig. 6(a)), 3 segments are clearly discriminated. The first segment clusters fast circulating groundwater, which have not reached equilibrium with the leached U–Th-rich rocks (up to 21 Bq/L); the second one clusters groundwater samples which are in equilibrium with host rocks (up to 51 Bq/L). The third segment clusters 222Rn-enriched groundwater samples, due to granite leaching, locally associated with the CO2 uprising. In particular, leaching of U and Th bearing minerals, with negligible CO2 input

M. Angelone et al. / Applied Geochemistry 20 (2005) 317–340

333

Fig. 5. Dissolved silica content vs. temperature.

(Fig. 6(b)), occurs along the Capoterra NW–SE fault segment (where mainly granitic sands or fractured granites are exposed), where 222Rn reaches values of 251 Bq/ L at the SAR 36 site. Along the Casteldoria fault system values up to 125 Bq/L have been found. The latter are similar to those of the WCF (sample SAR 2, 15, 32). The CF 222Rn anomaly reported in the Rn map of the CB (Fig. 7), could be used as a reference point for further Rn-indoor risk assessment (Pizzino et al., 2002). The area of Villacidro (up to 75 Bq/L) is also the seat of a strong 222Rn anomaly, in connection with the horst of Soglia di Siliqua (Pala et al., 1982a). The 222Rn – temperature binary diagram (Fig. 8) differentiates the ECF from the WCF segments as well as the CF family, strongly suggesting the segmentation of the graben, confirming once more that the real control on the Rn input in groundwater is linked both to the rock permeability, deep gas carrier and thus to the tectonic framework. The rapid circulation of fluids through limited sectors of regional fault systems enhances water– rock interaction processes associated with different degrees of convective circulation and gas leakage. Radon input is a very powerful tool to discriminate these two factors and the association of both (very high anomalies, e.g., the upper part of the third segment in Fig. 6(a)). Nevertheless, the relationship between 222Rn and CO2 in groundwater is complex (Pizzino et al., 2002),

as a consequence of the CO2 flux threshold, above which the ‘‘stripping effect’’ of the gaseous rising phase prevails on the carrier role of CO2. Where the CO2 flux is too high, as in the LOG group and in the Sardara thermal springs (ECF group), 222Rn escapes from groundwater (Fig. 6(b)). It is expected to be high in soil gases, which will be the object of a future study. On the other hand, fluids with a low content of both gases, as in sample SAR 13 (Su Campu), confirm that the main control over the depth to which advective processes penetrate may be rock permeability. Total dissolved CO2 data (Table 2a and 2b) show two zones characterised by huge and deep fluid leakage: one corresponds to the above mentioned horst of Soglia di Siliqua, seat of crossing fault systems, the second is related to the area near Sardara, S.Gavino and Sanluri (ECF, Fig. 6(b)), where the maximum heat flow anomalies have been recognised (Loddo et al., 1982). In the classical diagram (Fig. 9) N2/100–Ar–100 He (Giggenbach, 1980) water samples fall along the mixing line between shallow meteoric waters equilibrated with the atmosphere (air saturated water: ASW pole in Fig. 9) and the crustal-radiogenic component (He-pole in Fig. 9). The latter may be explained only by the significant presence of radioactive elements (U–Th series) inside the mineralogical matrix of the aquifers. A magmatic component is absent. The ASW–He line highlights the presence

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Fig. 6. (a) Cumulative frequency diagram of groundwater Rn content. See the text for the explanation of segments 1–3. (b) Binary diagram 222Rn versus total CO2.

of two groups of fluid, confirming the conclusions reached by the interpretation of water chemistry data and summarised in the Ludwig–Langelier diagram (Fig. 3):

 CO2 dominated, poor in He and Rn, with 3He/4He ratio relatively high (up to R/Ra = 2.32 in Abbarghente sample, see Table 3), associated with Na–

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Fig. 7. Map of dissolved Rn in groundwater of the Campidano Basin. LOG and CA waters have been excluded. See Fig. 1(a) for the lithologic legend. CF, Capoterra Fault; CB, Campidano Basin.

HCO3–Cl cold groundwater. The fault zones characterised by similar fluids are: the LOG fault interaction zone, the ECF Sardara fault segment, the WCF segment and, locally, the GR sector.  N2 and He rich, poor in CO2 and Rn, with low 3 He/4He ratio, associated to alkaline (Cl and HCO3) thermal waters. Similar gas components characterise fluids discharging at the border of the ECF (Su Campu segment) and along the TV faults. In both cases fluids have a meteoric origin with no peculiar shift towards geothermal processes, as testified by the H and O isotopic data (Bertorino et al., 1982a,b; Caboi et al., 1986, 1989, 1993; Cidu and Mulas, 2003). It should be noted that the dominantly CO2-rich gas fluxes, are not homogeneous, but are related to (1) the different fault segments and to their permeability (role of barrier instead of role of fluid pathways), (2) to outcrops of recent Quaternary extensional basalt volcanism, or (3) to the presence of MCP at depth. The joint interpretation of seismological and geochemical data allows strengthening the hypothesis that

in the Sardinia region seismicity cannot be generated at the typical seismogenic depths (10–20 km). This is mainly due to the strongly suggested presence of magma intrusion near the surface, able to produce thermo-metamorphic CO2 with He mantle signature. The presence of a clear mantle signature and a high geogenic CO2 flux in the northern sector of the island hint at the presence of a ductile layer near the surface, that does not allow significant seismic activity (Dragoni et al., 1996). The depicted geochemical versus structural framework of the Sardinian fluids may give some suggestions for solving one of the most important challenges for environmental management on a global scale: the deep geologic disposal of fossil fuel derived CO2. For this purpose, the preferred disposal should be located in porous and permeable reservoirs capped by a low permeability seal, at depths of around 800 m or more, where the CO2 will be injected and stored in a supercritical phase. In other words, the potential geological disposals to trap man-made CO2 must be reservoirs sealed to fluid circulation for inadvertent leakage to the surface as the result of over-pressuring, or those characterised by

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Fig. 8. 222Rn versus temperature binary diagram differentiates the ECF from the WCF segments as well as the CF family. The real control on the Rn input in groundwater is linked to the rock permeability and thus to the tectonic framework.

geochemical conditions with maximum pH buffering capacity of the rocks and scarce presence of deep CO2 (Bergman and Winter, 1995; Bachu, 2002). The fault systems should have the role of barriers, both hydrodynamic and geochemical, avoiding preferential pathways

of gas leaks towards the surface (Hitchon et al., 1999; Klusman, 2003). The tectonic setting and geology of Sardinia, the geothermal regime of the rift and the hydrodynamic regime of circulating groundwater, indicate the PCB as an ideal

Fig. 9. Triangular plot of dissolved gases: N2/100–Ar–100 Æ He. Water samples line up along the mixing line between waters equilibrated with the atmosphere (ASW pole) and the crustal-radiogenic component (He-pole). Available data do not suggest any magmatic component.

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storage reservoir for fossil fuel produced CO2. The sedimentary silicate-rich cap filling the CB (Fig. 1(b)), suggests the best geological and geochemical storage conditions for CO2, taking into account the pH-buffering capacity of silicates, having different behaviour with respect to limestone and dolomites lithologies (Quattrocchi et al., 2004a). In particular, the S. Gavino Basin (GR group of waters) could be ideal for geological CO2 sequestration also considering the vicinity of industrial districts. In particular, considering that the chemistry of circulating waters and rock mineralogy play an important role in CO2 sequestration, two different geochemical processes can be foreseen for Sardinia to enhance the potential disposal of CO2: (1) Gunter et al. (1997, 2000 and unpublished data discussed at the GHGT-7 conference in Vancouver, Canada) have grouped solubility/mineral trapping into different types in the framework of CO2 storage. The silicate–carbonate mineral assemblage has the highest buffer capacity. Solubility trapping is the first water–rock interaction process foreseen. Reactions described by the above Authors in a silico-clastic environment (e.g., 2 Albite + 2H2O + 2CO2 ! 2HCO3
+ 2Na + Paragon. + 6 quartz) can occur when CO2 dissolves in water and is held in the aqueous phase as HCO3. Protons, resulting from dissociation, allow rock attack, under acid conditions, on the silicate minerals. The attack results in free dissolved ions released from the mineralogical matrix, and produces calcite or other carbonates. This reaction forms the theoretical basis for the sequestering of CO2 as mineral. (2) Carbon dioxide mineral traps are most effective when the aquifer matrix contains minerals that are proton sinks such as the feldspars and clay minerals. Silico-clastic aquifers are favoured over carbonate aquifers. The mineralogical matrix acts like a base and facilitates CO2 storage giving origin mainly to Na–HCO3 waters, as observed along the above discussed fault segments. In the specific Sardinia case, it would be not advantageous to trap CO2 in the northern part of the island (LOG), where the MCP basement is dominant and where the high pressure of CO2 would not allow a real sealed system. The seismicity of the area and the recognised presence of seismogenic segments are further factors to be considered either in the future management of CO2 geological sequestration or to deepen the comprehension of the role of fluids (industrially sequestered or naturally stored at depth), e.g., in triggering the seismicity itself (Quattrocchi, 1999 and references herein). Moreover, the range of PCB suitable for CO2 sequestration increases significantly if other criteria like acces-

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sibility, infrastructure and cost of CO2 capture and injection are taken into account. In fact, the foreseen industrial development and thus CO2 production would be located in the Campidano area, minimising in this way the economic and socio-political impacts.

7. Conclusions The fluid geochemistry of Sardinia, with peculiar emphasis on the Sardinian rift and the Campidano Graben, sheds new light on the following results, part of which have also been discussed by previous authors: 1. Faults must be considered as always active circuits controlling mobility for water and gases in cratonic areas. Past ‘‘active’’ areas or ‘‘quiescent’’ areas are still productive areas of hot water and deep gases. Mostly where recent sporadic earthquakes have occurred, the possible triggered ‘‘fault valve mechanism’’ and mechano-chemical reactions are able to cut sealing barriers, as a fluid geochemistry survey may demonstrate. As a consequence, cratons cannot be considered as ‘‘dead’’ massifs: the production of geogenic CO2 drives the upward fluid movement, in a carrier role, and restricts the self-sealing of discontinuities in some sectors of the tectonic systems. The presence of a clear mantle signature and high geogenic CO2 flux in the northern sector of the island hints at the presence of a ductile layer close to the surface, possibly causing the observed low seismicity level. 2. The fault systems of Sardinia, and in particular those of the Campidano Graben, are strongly segmented, allowing (i) exclusion of a unique geothermal reservoir and (ii) discrimination of different fluids associated with specific circuits, outlining ‘‘Geochemically Active Fault Zones’’ (definition of GAFZ in Salvi et al., 2000). In particular, the authors have discriminated two kind of fluids: (i) CO2 dominated, poor in He and Rn, with a relatively high 3He/4He ratio, associated with Na–HCO3–Cl cold groundwater; (ii) N2 and He rich, poor in CO2 and Rn, with low 3 He/4He ratio, associated with alkaline thermal waters. 3. The geochemical framework of the Sardinian fluids gives some suggestions for solving the problem of CO2 geological sequestration. In order to reduce the atmospheric CO2 excess, the best geological disposal sites are reservoirs with low hydraulic conductivity, sealed to fluid movement, like the Palaeozoic Crystalline Basement covered by the Campidano sedimentary silicate-rich deposits. 4. The 222Rn anomalies like those reported for the Campidano Graben, may be used, also, as a reference point for Rn-indoor risk assessment, e.g., the Capoterra Fault sector.

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In Italy, it is of strategic importance to assess both the seismic and geochemical risks (CO2 leakage and micro-seepage along fault segments) before geological storage of CO2. This has resulted in a wide and embarassing debate in Italy (e.g., CSLF – Carbon Sequestration Leadership Forum activities). In Italy, the CO2 geological storage sites (CCS sites) will be selected not only on the basis of (i) the proximity to CO2 capture/hydrogen carrier plants or (ii) silico-clastic rock availability (sedimentary deep basins) having high pH-buffering capacity, but mostly contemplating the seismotectonical factor e.g., how far from a seismogenic and/or a CO2-leacking fault segment is the site located. By understanding the fluid geochemistry behavior in a fault system framework characterized by CO2 open/ closed sub-systems, the authors wish to inform the policy makers and stakeholders (NGO entities, environmental policy, public acceptance of the CO2 storage industrial process) in the framework of the growing scientific and managerial Capture-Storage community. Acknowledgements This work was performed within the framework of the Geochemical Seismic Zonation EC Program, funded under Contract ENV4-CT96-0291, Project leader Prof. Salvatore Lombardi. We thank the owners of the thermal and mineralised springs throughout Sardinia Island. We thank the student Stefano Bellini who completed a thesis on Sardinia Island and particularly thank R. Cidu and F. Frau of the University of Cagliari. References Bachu, S., 2002. Sequestration of CO2 in geological media in response to climate change: road map for site selection using the transform of the geological space into the CO2 phase space. Energy Convers. Manage. 43, 87–102. Balia, R., Carrozzo, M.T., Chirenti, A., Loddo, M., Luzio, D., Margiotta, C., 1983a. Carta gravimetrica della Sardegna. In: Proc. 2nd GNGTS Meeting. CNR-Rome, pp. 461–470. Balia, R., Fais, S., Trudu, R., 1983b. Studio Gravimetrico del Campidano meridionale, prime note. In: Proc. 2nd GNGTS Meeting. CNR-Rome, pp. 471–487. Balia, R., Ciminale, M., Loddo, M., Pecorini, G., Ruina, G., Trudu, R., 1984. Gravity survey and interpretation of Bouguer anomalies in the Campidano geothermal area (Sardinia, Italy). Geothermics 13, 333–347. Balia, R., Ciminale, M., Loddo, M., Patella, D., Pecorini, G., Tramacere, A., 1991. A new geophysical contribution to the study of the Campidano geothermal area (Sardinia, Italy). Geothermics 20, 147–163. Beccaluva, L., Macciotta, G., Siena, F., Zeda, O., 1989. Harzburgite-lherzolite xenoliths and clinopyroxene megacrysts of alkaline basic lavas from Sardinia (Italy). Chem. Geol. 77, 331–345.

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