Helium isotopes in paleofluids and present-day fluids of the Larderello geothermal field: Constraints on the heat source

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. B1, 2003, doi: /2001jb001590, 2003 Helium isotopes in paleofluids and present-day fluids of the Larderello geothermal field: Constraints on the heat source Gabriella Magro, Giovanni Ruggieri, Giovanni Gianelli, and Stefano Bellani Istituto di Geoscienze e Georisorse, CNR, Pisa, Italy Giovanni Scandiffio ENEL Green Power, Pisa, Italy Received 18 October 2001; revised 25 June 2002; accepted 23 August 2002; published 3 January [1] The He isotope composition of paleofluids entrapped in fluid inclusions of hydrothermal minerals is compared with the present-day fluid composition of the Larderello geothermal field. Almost constant values of ( 3 He/ 4 He) m /( 3 He/ 4 He) air (=R/R a ) over time indicate that no important changes have occurred in the deep source of gases, at least during the last 3.8 million years. On a regional scale, a correlation has been found between the R/R a spatial distribution, heat flow, and Bouguer gravity anomaly. High values of R/R a and heat flow, and low Bouguer anomaly values indicate that the Larderello field is an area of preferential escape for mantle-derived fluids. A positive correlation has also been found between the R/R a spatial distribution and a major seismic reflector named the K horizon. A deep magma source, refilled by periodic gas input from the mantle, is the most likely source of 3 He-enriched fluids and the anomalously high heat flow. The nearly constant value of R/R a clearly indicates that input of fresh mantle material has occurred up to recent times. Clear evidence of mixing between mantle and crustal fluids indicates that the high R/R a is the lower limit of the actual mantle value, which is suggested to be similar to the subcontinental European mantle. The decrease of R/R a over time in the peripheral part of the Larderello field indicates that important changes in the feeding fracture system and/or cooling rate have occurred in these areas. INDEX TERMS: 1040 Geochemistry: Isotopic composition/chemistry; 3699 Mineralogy and Petrology: General or miscellaneous; 8045 Structural Geology: Role of fluids; 8130 Tectonophysics: Evolution of the Earth: Heat generation and transport; 8135 Tectonophysics: Evolution of the Earth: Hydrothermal systems (8424); KEYWORDS: Larderello, He isotopes, geothermal fluids, fluid inclusions, heat flow Citation: Magro, G., G. Ruggieri, G. Gianelli, S. Bellani, and G. Scandiffio, Helium isotopes in paleofluids and present-day fluids of the Larderello geothermal field: Constraints on the heat source, J. Geophys. Res., 108(B1), 2003, doi: /2001jb001590, Introduction Copyright 2003 by the American Geophysical Union /03/2001JB001590$09.00 [2] Larderello is one of the few vapor-dominated geothermal systems in the world that produce superheated steam. The system heat source has been postulated to be a large igneous intrusion [see Gianelli et al., 1997a and references therein]. [3] Over the last 20 years, a deep exploration of the field has been carried out by the field managing company (ERGA- ENEL), and geothermal wells have been drilled as far down as 4.5 km. Granite and related metasomatic and contact metamorphic rocks have been drilled in the deepest part of the field. Microthermometric and Raman data on fluid inclusions, trapped in both the primary minerals of these rocks, as well as in hydrothermal vein minerals, indicate a complex paleocirculation of fluids of different origins (magmatic, contact metamorphic, meteoric) [see Cathelineau et al., 1989, 1994; Valori et al., 1992; Ruggieri et al., 1999]. In this paper, we present new data on the He isotope compositions of present-day fluids produced by geothermal wells, as well as those of paleofluids entrapped in fluid inclusions of selected primary and hydrothermal minerals. [4] The main goals of the research are (1) to integrate the existing He isotope composition data [Polyak et al., 1979; Torgersen, 1980; Nuti, 1984; Hooker et al., 1985] in order to extend it to the entire field; (2) to study the changes in R/R a values at two different timescales: a short timeframe (50 years) in order to evaluate the effects of wastewater reinjection on the R/R a values and a long timeframe (millions of years) to study the evolution of the fluids from early to present-day hydrothermal circulation; and (3) to correlate the variation of R/R a with some geophysical parameters (heat flow, depth of the seismic reflector K, Bouguer gravity anomaly), with particular regard to the relation between R/R a and heat flow. ECV 3-1

2 ECV 3-2 MAGRO ET AL.: LARDERELLO HELIUM AND HEAT micaschists and gneisses (Paleozoic to Precambrian) intruded by granites with radiometric ages of between 1.0 and 3.8 Ma [Gianelli et al., 1997a; Del Moro et al., 1982; Villa and Puxeddu, 1994; Villa et al., 2001; Gianelli and Laurenzi, 2001]. [9] The chemical composition of the volcanic and plutonic rocks is typical of the S-type, originating from anatexis, [Van Bergen, 1983; Poli et al., 1989], the source rocks for which are micaschists. A minor mantle contribution has been indicated by trace elements and rare earth elements (REE) studies [Giraud et al., 1986; Poli et al., 1989; Serri et al., 1993]. Figure 1. Overall geological map of the area. (1) Neoautochtonous sediments (late Miocene-Pliocene). (2) Igneous rocks (Pliocene-Quaternary). (3) Allochtonous flysch facies units (Cretaceous-Eocene). (4) Potential reservoir formations (Tuscan Nappe, Tectonic Slices, Metamorphic Units, see text). The area under study is evidenced in the frame. [5] The ultimate objective is to identify the source of the deep fluids and its evolution over time by means of the fluid He isotope composition. 2. Geological Outlines [6] The Larderello field, which can be considered as a single, large hydrothermal system [Baldi et al., 1993], is located in the pre-apennine belt of Tuscany (central Italy), where extensional tectonics have been active since the late Miocene. The reservoir is made up, from the top downward, of the carbonate and anhydrite formations of the Tuscan Nappe and underlying metamorphic units. According to permeability variations (higher in the shallower levels, lower at depth), two reservoirs can be identified: a shallow one at m below ground level (b.g.l.) and a deeper one at over m b.g.l. [7] Although regional magmatic activity was present from the late Miocene to the Quaternary, there are no outcrops in the Larderello area. The nearest magmatic rock outcroppings (Figure 1) are represented by the Roccastrada acidic volcanites (SE of Larderello), which are 2.5 Ma old [Innocenti et al., 1992 and references therein]. [8] Hundreds of wells drilled at Larderello have enabled the following stratigraphic setting to be defined, from top to bottom: a cover of Neogene sediments (late Miocene to Pliocene); the allochtonous flysch formations ( Ligurian units : Jurassic to Eocene); the Tuscan Nappe, with a predominantly carbonateevaporite sequence at the base and a terrigenous sequence at the top (Late Triassic to Oligo-Miocene); a complex of tectonic slices, which includes the lowest formations of the Tuscan Nappe and part of the underlying metamorphic units (Paleozoic to Late Triassic); 2.1. Fluid Geochemistry [10] The dominant components of geothermal fluids are H 2 O, CO 2, CH 4,H 2 S, and N 2, while noble gases are at part per million levels. Stable isotope data on H 2 O indicates meteoric water as the main source of the vapor [Craig, 1963; Ferrara et al., 1965]. A simple mixing between two endmembers could explain the isotopic composition of the steam produced in the field [Panichi et al., 1995; Scandiffio et al., 1995]. The former endmember is a primary deep steam having typical values of D = 40% and 18 O= 2%, resulting from extensive water-rock interactions, which shifted the original meteoric 18 O toward higher values. The latter endmember is a secondary steam derived from the boiling of fresh waters, having D = 40% and 18 O= 7%, the result of limited water-rock interactions due to a short residence time in the reservoir. [11] An alternative explanation of the steam s origin has been proposed by D Amore and Bolognesi [1994], who hypothesize the mixing of two endmembers, meteoric waters, and magmatic fluids produced by the crystallization of a deep-seated magma body. A large subduction-related magmatic contribution in the Larderello geothermal gases has been suggested by these authors on the basis of the stable isotope variations in the fluids, as well as the existing R/R a data and high N 2 /He and N 2 /Ar ratios, indicating a large N 2 excess. [12] The 3 He/ 4 He values (expressed as R/R a, where R a is the typical air value of ), ranging from 0.3 to 3.2 [Polyak et al., 1979; Torgersen, 1980; Nuti, 1984; Hooker et al., 1985], strongly suggest the presence of a mantle-derived, 3 He-enriched fluid. Suggestions that the crust is the only source of He [Mazor, 1978; D Amore and Truesdell, 1984] have been discounted by this He isotope composition, which indicates the presence of 3 He mantle-enriched fluids. As stressed by Hooker et al. [1985], 3 He is one of the mantle-derived components clearly resolved at the surface in the Tuscan magmatic province, where the Sr and O isotope compositions of the igneous rocks indicate melting of the continental crust as a major source of magma [Turi and Taylor, 1976]. [13] Although the He isotope composition clearly indicates the mantle as an important source of this gas, the actual sources of other inert gases (N 2 and Ar) cannot be identified on the basis of high N 2 /He and N 2 /Ar ratios alone, as supported by D Amore and Bolognesi [1994]. Enrichment of the fumarole gas in heavy 15 N in the Larderello field and, more generally, in the gas manifestations of central Italy, signals the heating of metasedimentary rocks as an important source of N 2 [Minissale et al., 1997]. It

3 MAGRO ET AL.: LARDERELLO HELIUM AND HEAT ECV 3-3 Figure 2. R/R a distribution in the Larderello area; triangles represent fumaroles, open circles mark the geothermal wells, and diamonds indicate fluid inclusion samples with their R/R a values. follows that the high N 2 /Ar ratios at Larderello are more indicative of crustal heating than true magmatic origins. For instance, the Gabbro area (Figure 2), characterized by the lowest R/R a ratios ( ) in the Larderello field, also exhibits the highest N 2 /Ar (from 99 to 449) and 40 Ar/ 36 Ar (close to 320) ratios [Mazor, 1978; Magro et al., 1998]. 3. Fluid Inclusions in the Larderello Area [14] A number of studies have been conducted on core samples from wells of the Larderello geothermal field, and regard both their contact metamorphic and hydrothermal minerals [Cavarretta et al., 1982; Bertini et al., 1985; Cavarretta and Puxeddu, 1990; Gianelli and Ruggieri, 2000] as well as the fluid inclusions encountered in minerals [Cathelineau et al., 1994; Valori et al., 1992; Petrucci et al., 1993; Gianelli et al., 1997b; Magro et al., 1998; Ruggieri and Gianelli, 1999; Ruggieri et al., 1999]. The results of such studies suggest two main stages of hydrothermal activity. [15] The first stage is related to the intrusion of granites at a depth of >2000 m b.g.l., which were responsible for contact metamorphism and metasomatic processes in the surrounding rocks (gneiss, micaschist, and phyllite). Hightemperature assemblages (consisting of biotite, andalusite, quartz, plagioclase, tourmaline, muscovite, and at some locations, cordierite, corundum, and K feldspar) crystallized during this stage. Studies on the fluid inclusions in the quartz of granites and high-temperature assemblages reveal that two main types of fluids were present within and around the intrusions: high-salinity, Na-Li-rich fluids of magmatic origins, and aqueous carbonic fluids resulting from the heating of the Paleozoic rocks (locally C-rich) during the contact metamorphism [Valori et al., 1992; Cathelineau et al., 1994]. These early fluids are subcontemporaneous and were trapped at temperatures of C under lithostatic pressures of approximately MPa. On the basis of the radiometric ages of granite and contact metamorphic minerals, Cathelineau et al. [1994] proposed that early high-temperature hydrothermal activity began between 3.8 and 1.5 Ma ago. [16] The second hydrothermal stage was characterized by precipitation of lower-temperature mineral assemblages (quartz, chlorite, epidote, adularia, calcite, anhydrite, muscovite, and titanite) filling veins at shallow-intermediate depths (2500 m b.g.l.) or, in certain sites, replacing early stage contact metamorphic or igneous minerals. During this stage, low- to high-salinity aqueous fluids with vapors produced by fluid boiling were trapped, either in the late minerals, or in the late-stage secondary inclusions in the early quartz, at temperatures of roughly C, under hydrostatic pressures (<35 MPa) [Valori et al., 1992; Ruggieri et al., 1999; Ruggieri and Gianelli, 1999]. These fluids are interpreted as meteoric waters, whose composition, salinity, and temperature were modified through waterrock interactions and fluid boiling, mixing, and cooling. [17] The d 18 O compositions of late-stage fluids (computed using fluid inclusion temperatures from isotope data on the carbonates, quartz, and chlorite in isotopic equilibrium with such fluids) are also compatible with the presence of meteoric waters that have interacted with reservoir rocks at relatively low water/rock ratios [Petrucci et al., 1993; Gianelli et al., 1997b]. The final evolution of the hydrothermal system resulted in the development of the present-day vapor-dominated conditions. [18] Fluid inclusion studies were also carried out on some hydrothermal veins outcropping in the peripheral parts of the Larderello geothermal field [Tanelli et al., 1991; Ruggieri et al., 1993; Gianelli et al., 1997b]. The veins consist of quartz and/or carbonates with barite, gypsum, sulfides, Fe oxides as a minor phase, and exhibit local Au enrichment. These minerals probably precipitated from low-to-moderate salinity, low-temperature fluids, largely of meteoric origin, which circulated in shallow hydrothermal systems related to the Pliocene-Quaternary intrusions [Tanelli et al., 1991]. 4. Sampling and Analytical Procedures 4.1. Present-Day Geothermal Fluid [19] Twenty-seven wells were sampled in the Larderello geothermal field for noble gas analyses (Table 1). The previous data published by Torgersen [1980], Nuti [1984], and Hooker et al. [1985] have been integrated with new data on the geothermal fluids to cover a larger area of the Larderello field (Figure 2). The depths of the selected wells vary from 500 to 2800 m b.g.l. Fluids from both shallow and deep reservoirs have been sampled. Fluids from deep and shallow wells of the Valle Secolo (the northern part of the field), where wastewaters from steam condensation have been reinjected since 1980, were sampled in order to estimate the effects of such reinjection on R/R a values. [20] In particular, the wells Miniera 3, Secolo 2, and Casanova 2 and 2a, are fed by fluids present within the metamorphic basement, while wells 101 and 137 are heavily affected by water reinjection, as indicated by the values of the recovery coefficient, defined as the amount of reinjected water recovered from productive wells near the reinjection sites, which ranged from 25 to 80% over a 20-

4 ECV 3-4 MAGRO ET AL.: LARDERELLO HELIUM AND HEAT year period [Panichi et al., 1995]. Changes ( ) in R/R a values over time were revealed in six wells (Gabbro 1, 3, and 9, wells 155 and 162, and VC 10), for which the composition of the present-day fluid has been compared with that reported in the literature for the same wells sampled about 20 years ago. [21] The geothermal gases were sampled using Giggenbach s [1975] method, i.e., by absorption of the condensable components (CO 2, H 2 S, HCl, etc.) onto a 100 cc preevacuated gas vial filled with 50 cc of a 5N NaOH solution. The NaOH-vial sampling method drastically reduces the effects of accidental air contamination during sampling because the concentration of the residual gases in the NaOH vial is over a hundred times higher than the concentration of incondensable gas in an empty one Fluid Extraction From Mineral Samples [22] The minerals selected for extraction of the gas phases from fluid inclusions came from seven core samples of geothermal wells SP2, BR1, MV7, CB11, MV2A, MV5A, CB11A, and two hydrothermal veins (MIC and SAS) (Figure 2, Table 2). The MIC sample is located in the vicinity of the fumarolic gas manifestation at Micciano. [23] Most of the samples contain more than one type of fluid inclusion, characterized by distinct fluid compositions. In some samples, the inclusion types contain fluids of different origins: meteoric and/or magmatic and/or contact metamorphic (Table 2). Only one type of low-salinity aqueous liquid was found to be present in the inclusions of the selected minerals of samples MIC (barite), SAS (calcite), and CB11A (calcite). A largely meteoric origin for this liquid is suggested by the relatively low salinity (<4.2 wt.% NaCl equiv.), the oxygen isotope data on the calcite of SAS and CB11A, and the shallow formation depth of SAS and MIC [Ruggieri et al., 1993; Gianelli et al., 1997b]. A meteoric origin is also compatible with the characteristics of the fluids trapped in the inclusions of samples MV2A (quartz) and MV5A (calcite) [Ruggieri and Gianelli, 1999; Ruggieri et al., 1999]. The only inclusions found to be present in the SP2 quartz sample were those trapped during early hydrothermal activity. In particular, both inclusions of magmatic-derived Na-Li-rich brines and aqueous-carbonic fluids formed during contact metamorphism have been recognized [Cathelineau et al., 1994]. [24] Early aqueous-carbonic fluids are observed in the BR1 quartz sample. However, in this sample, inclusions related to late-stage meteoric fluids also occur (G. Ruggieri, unpublished data, 2001). In the MV7 and CB11 quartz samples, several populations are present and represent the various types of paleofluids that circulated in the Larderello geothermal field [Valori et al., 1992; Ruggieri and Gianelli, 1995]. [25] The study samples were gently disaggregated. Then the quartz, calcite, and barite were carefully separated from the other minerals of the samples by handpicking, and cleaned ultrasonically, first in water and then in ethyl alcohol. After prior degassing for several hours under a vacuum at C in order to avoid air contamination on the crystal surface, the gases trapped in the fluid inclusions were vacuum extracted by crushing g of the selected mineral fragments (grain size from 0.5 to 1 mm). The crushing apparatus used consists of a stainless steel container and rod, equipped with a pair of highvacuum, all-metal valves. Crushing was performed by shaking the device for less than 5 min Rare Gas Mass Spectrometry [26] Both the geothermal gases and minerals were processed on a stainless steel vacuum line equipped with cold (active charcoal at liquid N 2 temperature) and hot traps (Ti getter) to separate the noble gases from the gaseous matrix. The extraction line was connected to both a magnetic mass spectrometer (MMS = MAP ) and a quadrupole mass spectrometer (QMS = Spectralab 200, VG-Micromass). The amount of He + Ne was checked via QMS before being introduced into the MMS. When the He partial pressures exceeded the typical MMS inlet values (10 6 mbar of He), which guarantees a low pressure ( mbar) inside the MMS, the He and Ne fractions inside the second inlet volume was expanded and the He and Ne partial pressures checked by QMS until suitable values for MMS measurements were reached. The 3 He/ 4 He ratios were determined by fitting the MMS with an ion counting device. The He/Ne ratios were determined via QMS for geothermal gases and via MMS for fluid inclusions. Resolution was close to 600 AMU for HD 3 He at 5% of the peak. Typical blanks were on the order of cc STP for 4 He, with a ratio close to that of air. No blank corrections were applied to the 3 He/ 4 He ratios of geothermal gases, as the He concentration was relatively high (a few hundred parts per million), generally several orders of magnitude higher than the He blank (at parts per billion levels). Likewise, no blank corrections were applied to 3 He/ 4 He of fluid inclusions because the samples are in general characterized by an He content of at least one order of magnitude greater than the blank. A standard volume of air at different pressures (from 1013 to mbar) was introduced into the extraction line and treated in the same way as the samples. The air 3 He/ 4 He ratios exhibited a reproducibility of better than 10% over the analysis period. The mass ratios 4/( ) measured on air standards by QMS showed a reproducibility of more than 5%. 5. Results and Discussion [27] The R/R a ratios of all samples from the geothermal fluids (Table 1) and fluid inclusions (Table 2) fall within the range of , in agreement with the values of reported by Hooker et al. [1985]. Figure 2 shows the location of all the samples (paleofluids and present-day fluids), together with the distribution of the R/R a values of the present-day geothermal fluids in the area of the Larderello field. The contour lines in Figure 2 have been rendered using both the new and published data on geothermal gases (Table 1). At the scale of the field (approximately km, see Figure 2), the R/R a distribution encompasses two relative maxima elongated in the NE-SW direction. A relative minimum follows the northeastward maximum and is located in the Gabbro area Short-Term Variation in the R/R a Ratio in the Larderello Area [28] The rationale for using data from samples collected over a time span of approximately 20 years is based on the fact that nearly constant R/R a values were found for two

5 MAGRO ET AL.: LARDERELLO HELIUM AND HEAT ECV 3-5 Table 1. Helium Isotope Composition of Geothermal Wells of Larderello Field Well Name Year Latitude Longitude Depth, m b.g.l. R/R a SD He/Ne Reference a Gabbro Gabbro Gabbro 1 b Gabbro Gabbro 3 b Gabbro 6 b Gabbro Bulera 4 b b b b April Oct b b b b b b b b Miniera 1 b Miniera Secolo Casanova Casanova Casanova 2a Fabiani b S.Vincenzo 9 b VC VC VC 10 b Le prate 2a Monterotondo Puntone Querciola Serrazzano.bcf S Martino S.Martino Colline Capannini LRA Lumiera San Pompeo b Sasso 22 b Carboli A b Sesta 1 b Pozzaie 2 b Camorsi 1 b San Silvestro b Zuccantine 1 b Villa Madama b Lago Fum Sasso Fum. b Le Prata fum. b Micciano Fum a References: 1, this work; 2, Torgersen [1980]; 3, Hooker et al. [1985]; 4, Polyak et al. [1979]; 5, Nuti [1984]; 6, Minissale et al. [1997, 2000]. b Sampled before the reported year. wells, VC10 and 162, sampled before 1985 by Hooker et al. [1985] and Nuti [1984], respectively, and resampled in the period (this paper). In fact, the R/R a value of 1.74 of the VC10 well fluid before 1985 is similar, within the limits of error, to that recorded in 1996 (R/R a = 1.71) and in 2000 (R/R a = 1.76). A similar pattern is exhibited by well 162 (1.09 in 1996 and 1.1 prior to 1984) [Nuti, 1984]. On a more limited timescale, small variations in the R/R a and He/ Ne ratios were found during a monitoring test of a single productive well (well 107; R/R a from 1.4 to 1.6) conducted

6 ECV 3-6 MAGRO ET AL.: LARDERELLO HELIUM AND HEAT Table 2. Helium Isotopic Composition of Fluid Inclusions in Hydrothermal Minerals a Sample Latitude Longitude Depth, m b.g.l. Trapped Fluid Origin Host Mineral Sample Weight, g He, ncc R/R a SD He/Ne (R/R a )c MV2a met. quartz MV5a met. calcite MV5a met. calcite MV met. + mag + cont. metam. quartz BR met + cont. metam. quartz SP mag. + cont. metam. quartz CB11A met. calcite CB met. + mag + cont. metam. quartz Mic met. barite Sas met. quartz a Samples from geothermal wells: MV2a, MV5a, and MV7, Monteverdi; BR1, Bruciano 1; SP2, San Pompeo 2; CB11A and CB11, Carboli. Samples from surface veins: Mic, Micciano; Sas, La Sassa; met., meteoric; mag., magmatic; cont. metam., contact metamorphic. (R/R a )c is the R/R a ratio corrected for atmospheric helium contamination, assuming Ne of air origin and according to Craig et al. s [1978] formula: (R/R a )c = (R/R a X 1)/(X 1), where X = (He/Ne)m/(He/Ne)air. by Hooker et al. [1985] over a period of 17 months. As can be seen in Table 1, no major changes occurred in the R/R a values during the period The only exception concerns the analyses reported by Torgersen [1980] for some wells of the Gabbro area; all these values are systematically lower than those reported by other authors including the present study. [29] The few low R/R a values reported by Torgersen [1980] may represent the draining of radiogenic He accumulated in the reservoir rocks. A similar trend has also been recognized for other gases in the Larderello field; a decrease in radiogenic 40 Ar was recorded in the 1950s and 1960s, at which time the 40 Ar/ 36 Ar ratio decreased greatly to values typical of air. This fact was linked to increasing geothermal exploitation, which drained the radiogenic Ar accumulated in the reservoir [Ferrara et al., 1963]. An analogous process of radiogenic He accumulation would lower the R/R a ratio significantly and may explain the R/R a value of 0.35 reported by Torgersen [1980]. [30] As previously pointed out by Hooker et al. [1985], there is no evidence of a simple correlation between R/R a and well depth in the Larderello area. Such a conclusion seems to be confirmed by the new data from deep wells (more than 2000 m) in the Valle Secolo area and the relatively shallow (<1000 m) as well as deep (>2000 m) wells in the Monterotondo area (Figure 3), where relatively high R/R a values have been found. [31] Since 1984, the reinjection of wastewater has become an important facet of the exploitation strategy. Its effect on the noble gases (He, Ar) and nitrogen has been evaluated using a very large data set that includes both pre and postreinjection fluid compositions [Scandiffio et al., 1995; Panichi et al., 1995]. The conclusion is that mixing between a deep component and the meteoric one introduced by water reinjection could explain the shift of the relative concentrations of He, Ar, and N 2 from mantle crustal values to air and/or air-saturated water ones (asw). For the most part, atmospheric N 2 and Ar governed the change in inert gas composition, while He was influenced only slightly, or at not all, since this gas is by far more abundant in deep Figure 3. Distribution of R/R a values with well depth; symbols as in Figure 2. No clear correlation exists; the He isotopic composition in space must be controlled by the nature of reservoir rocks. The R/R a values of fluid inclusions are, moreover, unrelated to well depth and fall within the same range as present-day geothermal fluids. Figure 4. Relation between R/R a and He/Ne ratios in wells, fumarolic gases, and fluid inclusions of the Larderello geothermal field. The mixing curves have been calculated using Craig s formula from Craig et al. [1978]. Symbols are same as in Figure 2.

7 MAGRO ET AL.: LARDERELLO HELIUM AND HEAT ECV 3-7 fluids than in the secondary steam produced by the reinjected waters [Panichi et al., 1995]. The high He/Ne ratios of all the samples, particularly those from the shallower wells 101 and 137 located in the reinjection area, reveal that the He content as well as the He isotope composition are unaffected by atmospheric He from meteoric and reinjection water recharge (Figure 4). The R/R a range is mainly the result of different mixing between a 3 He-enriched fluid, derived from the mantle, and a 4 Heenriched fluid originating in the crust and most likely stored in the geothermal reservoir rocks Long-Term Variation in R/R a : Paleofluids in Fluid Inclusions [32] The R/R a values of the paleofluids extracted from inclusions range between 0.58 and 2.4, though in most cases they are >1 (Table 2). The variations in R/R a are unrelated to the depth of the well samples (Figure 3). The He/Ne ratios range between 0.5 and 330, suggesting a variable contribution of the atmospheric component (between 0.1 and 56%) to the trapped fluids (Figure 4). The R/R a values corrected for the atmospheric contribution are not very different from the measured values, with the exception of the SAS sample. [33] The R/R a of the paleofluids fall within the range of present-day fluid values ( ). This suggests that He in fluid inclusions could have reequilibrated with He of the present-day fluids due to He diffusion into the host minerals. In particular, a high He diffusion rate, favored by high temperatures, could have affected the samples cored in the deepest part of the Larderello geothermal field (CB11, SP2, MV7, and BR1), which were, in fact, found at relatively high temperatures ( C). However, the R/R a values of the paleofluids do not generally match the present-day fluids spatial distribution of the R/R a (Figure 2). In addition, discrepancies between the R/R a of the paleofluids and present-day fluids in the same well or the same sampling area is clearly demonstrated by several samples (CB11A, CB11, SP2, and MIC), for which the R/R a values are available for both paleofluids and present-day fluids (Tables 1 and 2). In particular, R/R a of the fluid inclusions of samples CB11A and CB11, (0.58 and 1.04, respectively) are distinctly lower than those of the reservoir fluids produced by wells Carboli A (2.56) and Capannini 2 (2.49), both located near the Carboli 11 and 11A wells. A similar situation is revealed by comparing the paleofluid in sample SP2 (R/R a = 2.3), with the present-day fluid of the same well (R/R a = ). Differences in R/R a values have also been found between the fluid inclusions of the MIC sample and the present-day fumarolic gases gushing out in the same area. However, in this case, the R/R a of the paleofluid (1.97) is higher than the fumarolic one (1.21). All these differences indicate that He of the paleofluids has not reequilibrated with He of present-day fluids. [34] It is worth noting that Moore et al. [2001], who measured the He isotope composition of fluid inclusions in hydrothermally altered rocks of The Geysers geothermal field, also found considerable evidence that, despite the high diffusion rate of He at reservoir temperatures, the trapped paleofluids have not reequilibrated with present-day reservoir fluids and therefore represent a record of past conditions. [35] Since one or more fluids of distinct origin (meteoric, contact metamorphic, magmatic) are trapped in the studied samples, the R/R a of the bulk fluid extracted from inclusions is the result of either a single fluid or a mixing of fluids with different origin. All the samples (MV2A, MV5A, CB11A, MIC, and SAS) containing only meteoric-derived fluids are characterized by R/R a values different from typical air and/ or asw values (R/R a =1). [36] In addition, the He/Ne ratios of all samples containing meteoric-derived fluids are considerably higher than the typical air and asw values (0.28 and 0.24, respectively), ranging between 0.5 and 330, hence suggesting a variable contribution (between 0.1 and 56%) of atmospheric He. The highest values of atmospheric He (35 56%) were found in one of the vein samples from outcrops (SAS) that formed at shallow depths and in the MV2A sample. For the other samples (MIC, MV5A, and CB11), the He/Ne ratios indicate relatively small amounts ( %) of atmospheric He, which does not significantly change the R/R a. [37] The lowest R/R a (0.58) for fluid inclusions is exhibited by the CB 11A sample, which contains only fluids of meteoric origin. The relatively low R/R a can be explained by assuming a long geothermal circulation and storage in the reservoir, and the stripping of radiogenic 4 He (produced in situ from the U and Th decay chain), which causes a decrease of R/R a and an increase in the He/Ne ratio. Therefore, in general, the He isotope composition of the meteoric-derived paleofluids, similar to present-day geothermal fluids, is not strongly affected by a meteoric He component, but is instead related to nonatmospheric sources. [38] The gases extracted from samples SP2, BR1, CB11, and MV7 yield R/R a values between 1.0 and 2.3 and He/Ne values between 5.7 and 77. Such values result from the mixing of two or more fluid types (magmatic brines and/or contact metamorphic fluids and/or meteoric-derived fluids) present in different proportions in these samples. Even if the R/R a of each single-fluid type cannot be determined because of the fluid extraction technique adopted, the He/Ne values indicate that the atmospheric He component of the gas mixtures is low (<5%). Consequently, the variations in the R/R a values in all these samples can be ascribed for the most part to variable contributions of mantle-derived 3 He and crustal radiogenic 4 He. The highest R/R a value is displayed by sample SP2, wherein the extracted fluid represents a mixture of early stage fluids of magmatic and contact metamorphic origins. We can expect the R/R a ratio of the gas present in the magmatic-derived fluid of this sample to be higher than that of the contact metamorphic fluid produced during the heating of the Paleozoic to Precambrian crustal units. Thus the R/R a value of 2.3 can be regarded as the lower limit of the actual value in the magmatic fluids and clearly indicates that significant amounts of mantle-derived 3 He have been present in the fluids since the first stage of hydrothermal activity, which began about 3.8 Ma ago and continued until recent times [Cathelineau et al., 1994; Gianelli and Laurenzi, 2001]. [39] From the R/R a values reported by Moore et al. [2001], a long-standing 3 He contribution from the mantle is also clear at The Geysers geothermal field. In fact, the R/R a values of present-day fluids range between 6.3 and 8.3, whereas the He isotope composition of paleofluids trapped

8 ECV 3-8 MAGRO ET AL.: LARDERELLO HELIUM AND HEAT Figure 5. Regional comparison among R/R a, HFD, and Bouguer gravity anomaly data. HFD and Bouguer anomaly contours redrawn after Baldi et al. [1995]. in fluid inclusions, formed either during the liquiddominated stage (from 1.2 to 1.1 Ma) or at the initial development of vapor-dominated conditions (around Ma), are for the most part in the range of Relationship Between R/R a and Geophysical Data [40] Since the He isotope composition of the wells is fairly constant over time (see Table 1) and unaffected by the wastewater reinjection, the helium isotopic variation in space must be governed by the nature of the reservoir rocks and the physical-chemical conditions existing in different parts of the field. Figure 5 shows the iso-contour lines of R/R a, constructed by adding the R/R a values of western Tuscan gas manifestations to the geothermal well data [data from Minissale et al., 1997, 2000]. [41] Comparing the values of R/R a, the surface heat flow (HFD) and the Bouguer gravity anomaly on a regional scale reveal a correspondence between the highest R/R a with the highest HFD and the lowest Bouguer gravity anomaly values (Figure 5). The gravity minimum (<15 Mgal) falls within the 250 mw/m 2 HFD isoline, which roughly encompasses the Larderello geothermal field. The finding of a gravity minimum together with the HFD maxima in this area points to the presence of a large intrusive body at depth [Gianelli et al., 1997a]. [42] Resistivity values greater than few hundred ohm meters have never been found [Fiordelisi et al., 1995; Manzella et al., 1995; Gianelli et al., 1996] and the electrical conductivity anomaly reaches its maximum in correspondence to the seismic velocity anomaly revealed by tomography and extends to a depth of km. Therefore the conductivity structure of the Larderello geothermal field

9 MAGRO ET AL.: LARDERELLO HELIUM AND HEAT ECV 3-9 Figure 6. Correlation between depth to the K horizon and R/R a areal distribution. K horizon isobaths (m b.g.l.) redrawn after Barelli et al. [2000]. Full dots, geothermal wells; full triangles, fumaroles; open diamonds, fluid inclusions. is also consistent with the occurrence of a partially molten granite body. All these geophysical anomalies are correlated with a thin crust over a large sector of Tuscany, as revealed by the Moho depth, as shallow as km [Calcagnile and Panza, 1979]. [43] Seismic tomography [Batini et al., 1995] and teleseismic travel time residuals reveal the presence of a thick body at a depth of 8 10 km, which tends to widen toward the bottom and is characterized by a low seismic P wave velocity [Block et al., 1991; Foley et al., 1992]. On the basis of teleseismic data, the volume of the anomalous body (partially solidified granite) is estimated to be 18,000 20,000 km 3 [Gianelli and Puxeddu, 1992; Manzella et al., 1995]. The top of the igneous intrusion should be at a depth of between 4 and 7 km, approximately 1 2 km below a major seismic reflector, the K horizon [Foley et al., 1992; Manzella et al., 1995]. This horizon is postulated to be a structure related to emplacement of the granites, and some authors have interpreted it as stemming from the presence of fluids inside the contact metamorphic aureole or the solidified carapace [Batini et al., 1983]. [44] A good correlation exists between the depth to the K horizon and the R/R a areal distribution (Figure 6). The culmination of the reflector at a depth of about m matches the highest R/R a contour lines very well; this may identify an area where rapid uplift of fluids of mantle origin allows the existence of relatively high R/R a values in a typical crustal melting environment Constraints on the 3 He Source [45] The highest R/R a values (3 ± 0.2) of Larderello fluids, clearly indicating the presence of mantle fluids, are lower than typical mantle values, such as MORB (8.5 ± 0.5, N. Atlantic MORB) [Kurz et al., 1982], the mantle values in subduction areas (6 < R/R a <8)[Poreda and Craig, 1989] and also the European subcontinental mantle (R/R a = 6.1 ± 0.7) [Dunai and Baur, 1995]. Low R/R a ratios are found throughout the world in both fumarole gases and phenocrysts in basalts, and are explained either by degassing of subducted sediments or continental crust or by the assimilation of crustal material by magma stored in an intracrustal condition [Hilton et al., 1993a, 1993b]. Assuming the MORB value as representative of the mantle beneath Tuscany, Hooker et al. [1985] calculated that 40% of the total He in Larderello geothermal fluids derives from the mantle. Obviously, the He mantle fraction estimate is biased by the choice of an R/R a value representative of the mantle. [46] This signature of the mantle beneath Italy is, in fact, a matter of great debate. The R/R a in fluids increases from North to South and is considered to reflect the different assimilation by the mantle of crustal material involved in the subduction caused by collision of the African and European plates [Marty et al., 1994; Tedesco, 1997]. Within this rather regular geographical trend, Larderello represents the exception; its highest values (up to ) exceed the range (0.4 < R/R a < 0.6) of the nearby Mt. Amiata and Mt. Vulsini volcanic areas, approaching the values (2 < R/R a <3) found in the active Campanian volcanic area (Phlaegrean Fields and Mt. Vesuvius). [47] A metasomatized mantle with a He isotope signature close to the interval of 2 < R/R a < 2.8 was suggested by Graham et al. [1993] on the basis of the results obtained from phenocrysts of the least differentiated lavas at Mt. Vesuvius. If this value range is assumed to be representative of the mantle beneath Larderello, the estimated mantle He fraction in the geothermal fluids would approach 100% of the total He. This estimation contrasts with the presence of granitic bodies and related contact metamorphic rocks, which provides clear evidence of the melting and degassing of the crust. [48] Therefore we can only conclude that the He isotope composition of paleofluids and present-day geothermal fluids is the result of the mixing of a variable fraction of mantle and crust fluids. Thus the highest R/R a values (2.3 R/R a < 3.2) could, at most, approach the lower limits of the R/R a values representative of the mantle beneath the geothermal area. Taking such a low value of R/R a (from 2 to 3) to be representative of the mantle beneath Tuscany could be misleading. [49] We suggest that the typical range of the subcontinental European mantle (R/R a = 6 ± 0.7) would be reasonably representative of the mantle beneath Italy. It is worth noting that the presence of this type of mantle is considered to be the source of magmas for the volcano Etna [Marty et al, 1994] Evolution of 3 He Source Over Time [50] The evolution of paleofluids to present-day geothermal fluids suggests that a constant and long lasting (>1 Ma) mantle contribution to the geothermal fluid at Larderello has been active for at least the last Ma. A long-lived heat and fluid source is necessary to keep the R/R a constant over such a long period of time. A single magma chamber, without any recharge from a deep magma source, could not sustain such a constant R/R a because the strong uranium enrichment of the degassed magma relative to He would cause a radiogenic He increase, which would, in turn, strongly affect the primary mantle signature of He, with a decrease in R/R a for a period in excess of years [Zindler and Hart, 1986]. [51] Recent numerical calculations [Cathles and Erendi, 1997] have shown that a single episode of intrusion could

10 ECV 3-10 MAGRO ET AL.: LARDERELLO HELIUM AND HEAT sustain hydrothermal circulation and near-surface geothermal activity only for about 800,000 years, even if the intrusion were large. Thus long-lived (>1 Ma) hydrothermal activity normally suggests multiple pulses of magma. Recent geophysical and petrological modeling of the Larderello field [Mongelli et al., 1998; Gianelli and Laurenzi, 2001] has suggested that at least three magmatic pulses during the last 3 Ma has generated the present thermal state of the field, following the initial emplacement of the batholith 3.8 Ma ago. [52] The vapor-dominated Larderello field shows some striking similarities with The Geysers geothermal field [Gianelli and Puxeddu, 1992; D Amore and Bolognesi, 1994]. In the case of The Geysers, the acidic intrusive body identified by drill holes cannot be envisaged as the only source of gases. The He isotope composition at The Geysers is consistent with degassing from an active magma chamber recharged over time with magma from the underlying mantle [Kennedy and Truesdell, 1996]. Like The Geysers, the Larderello geothermal field cannot be sustained by a single, insulated magmatic body. The constancy of R/R a over a time span of over 1 Ma suggests that mantle-derived magma is still degassing He as a Tracer of Paleo Heat Flow Distribution: Some Evidence [53] A relationship between helium isotope ratios and heat flow has been found for a large variety of tectonic settings [Polyak and Tolstikhin, 1985]. However, in most cases, there is no simple correlation between 3 He and heat flow [Torgersen, 1993 and references therein]. [54] As noted above, the R/R a ratios of the fluid extracted from the inclusions may differ from the distribution of present-day R/R a values. In particular, the R/R a values of MIC, MV5A, and SAS, within the limits of error, are higher than the measured or extrapolated R/R a of the reservoir fluids. As discussed for geothermal gases, the R/R a distribution in present-day geothermal gas correlates with the heat flow, Bouguer anomaly, and K horizon. Assuming that this relation was valid in the past as well, the presence of higher R/R a values in paleofluids than in present-day fluids could indicate that the hydrothermal circulation once extended across a different, possibly wider area than the present one, located between well MV5A and the SAS and MIC mineralizations (Figure 2). The eastward migration of the feeding system of the deep heat source and/or the cooling in different parts of the field, depending on local conditions, could explain the observed differences in paleofluids and present-day fluids. [55] Evidence of changes over time and space of the deep feeding system could be envisaged in the different cooling age of minerals of granitic bodies reached by deep drilling. The radiometric ages of the granitic bodies minerals and contact aureole range from 3.8 to 1.0 Ma and testify to a diachronic emplacement of the apical part of the batholith, most likely governed by the main fracture systems acting at emplacement time [Mongelli et al., 1998]. [56] An analogous behavior could be expected for mantle fluids rising to the surface, so that the relative depletion over time of 3 He-enriched fluids could possibly depend on changes in the feeding fracture system. Progressive cooling of the system would also provide a reasonable explanation of the variations in time of R/R a in paleofluids and present-day fluids. Slow monotonic cooling was suggested to have been active from 4 Ma ago up to the present day [Villa and Puxeddu, 1994, and references therein], while a complete cooling followed by recent reheating was excluded on the basis of hydrothermal mineral assemblage evolution and fluid inclusion microthermometry [Cathelineau et al., 1994; Mongelli et al., 1998]. Of course, the cooling rate could differ from one part of the field to the other, since it is influenced by the local geological setting. For example, the efficiency of the convection heat transfer due to a high rate of meteoric water recharge and circulation at depth could play an important role in modifying the cooling rate. [57] Fluid inclusion studies have documented that only fluids of largely meteoric origin were trapped in MIC, SAS, and MV5A samples [Tanelli et al., 1991; Gianelli et al., 1997b]. We therefore suggest that a relatively rapid cooling rate in the area between the La Sassa, Micciano, and Monteverdi areas (Figure 2), due to extensive invasion of meteoric waters into the hydrothermal system, may explain the differences between the R/R a values of paleofluids and present-day fluids in MIC, SAS, and MV5A samples. [58] It is worth noting that the paleofluids in samples from the Monteverdi, La Sassa, and Micciano areas (peripheral to the Larderello field) are characterized by R/R a values higher than present-day fluids and contain mainly fluids of presumed meteoric origin, in contrast to present-day fluids. [59] Further data are necessary to confirm this hypothesis and find a clear relationship between heat flow and 3 He flow in the past, taking into account the substantial differences existing in the diffusion rate of these two parameters. Heat diffuses faster than helium, so the maximum release of heat could occur before the maximum release of He. 7. Conclusions [60] A comparison of the paleofluids with the present-day geothermal fluids in the Larderello field indicates that a long-lived heating and a volatile source must be present beneath the Larderello field, and more generally beneath Tuscany. The R/R a values in the range of indicate that He, and most likely some other volatile elements, were derived mainly from a mantle source. The R/R a distribution of present-day fluids matches the distribution of some geophysical data such as the Bouguer gravity anomaly, the heat flow density distribution, and depth to the seismic reflector K. The mantle is the main source of heat and 3 He enriched fluids. Fluids enriched in 4 He are added to the mantle fluids during their ascent to the surface due to crust heating, indicated by the presence of granitic bodies and the thermo-metamorphic aureole in Larderello deep wells. [61] The He mantle fraction in fluid inclusions cannot be quantitatively estimated due to the limitations of the fluid extraction technique (vacuum crushing). In fact, within the selected samples, the magmatic-derived brines, which presumably contain the largest amounts of mantle 3 He, are always associated with trapped fluids (of meteoric and/or contact metamorphic origins) bearing some crustal 4 He. [62] The selected hydrothermal mineral samples trapped each type of fluid inclusion, and so the crustal and mantle