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 BellaniIstituto di Geoscienze e Georisorse, CNR, Pisa, Italy

Giovanni ScandiffioENEL—Green Power, Pisa, Italy

Received 18 October 2001; revised 25 June 2002; accepted 23 August 2002; published 3 January 2003.

[1] The He isotope composition of paleofluids entrapped in fluid inclusions ofhydrothermal minerals is compared with the present-day fluid composition of theLarderello geothermal field. Almost constant values of (3He/4He)m/(

3He/4He)air (=R/Ra)over time indicate that no important changes have occurred in the deep source of gases, atleast during the last 3.8 million years. On a regional scale, a correlation has been foundbetween the R/Ra spatial distribution, heat flow, and Bouguer gravity anomaly. Highvalues of R/Ra and heat flow, and low Bouguer anomaly values indicate that theLarderello field is an area of preferential escape for mantle-derived fluids. A positivecorrelation has also been found between the R/Ra spatial distribution and a major seismicreflector named the ‘‘K horizon.’’ A deep magma source, refilled by periodic gasinput from the mantle, is the most likely source of 3He-enriched fluids and theanomalously high heat flow. The nearly constant value of R/Ra clearly indicates that inputof fresh mantle material has occurred up to recent times. Clear evidence of mixingbetween mantle and crustal fluids indicates that the high R/Ra is the lower limit of theactual mantle value, which is suggested to be similar to the subcontinental Europeanmantle. The decrease of R/Ra over time in the peripheral part of the Larderello fieldindicates that important changes in the feeding fracture system and/or cooling rate haveoccurred 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:10.1029/2001JB001590, 2003.

1. Introduction

[2] Larderello is one of the few vapor-dominated geo-thermal systems in the world that produce superheatedsteam. The system heat source has been postulated to be alarge igneous intrusion [see Gianelli et al., 1997a andreferences 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 downas 4.5 km. Granite and related metasomatic and contactmetamorphic rocks have been drilled in the deepest part ofthe field. Microthermometric and Raman data on fluidinclusions, trapped in both the primary minerals of theserocks, as well as in hydrothermal vein minerals, indicate acomplex paleocirculation of fluids of different origins (mag-

matic, contact metamorphic, meteoric) [see Cathelineau etal., 1989, 1994; Valori et al., 1992; Ruggieri et al., 1999]. Inthis paper, we present new data on the He isotope composi-tions of present-day fluids produced by geothermal wells, aswell as those of paleofluids entrapped in fluid inclusions ofselected 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 orderto extend it to the entire field; (2) to study the changes inR/Ra values at two different timescales: a short timeframe(50 years) in order to evaluate the effects of wastewaterreinjection on the R/Ra values and a long timeframe (mil-lions of years) to study the evolution of the fluids from earlyto present-day hydrothermal circulation; and (3) to correlatethe variation of R/Ra with some geophysical parameters(heat flow, depth of the seismic reflector ‘‘K,’’ Bouguergravity anomaly), with particular regard to the relationbetween R/Ra and heat flow.

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. B1, 2003, doi:10.1029/2001JB001590, 2003

Copyright 2003 by the American Geophysical Union.0148-0227/03/2001JB001590$09.00

ECV 3 - 1

[5] The ultimate objective is to identify the source of thedeep fluids and its evolution over time by means of the fluidHe isotope composition.

2. Geological Outlines

[6] The Larderello field, which can be considered as asingle, large hydrothermal system [Baldi et al., 1993], islocated in the pre-Apennine belt of Tuscany (central Italy),where extensional tectonics have been active since the lateMiocene. The reservoir is made up, from the top downward,of the carbonate and anhydrite formations of the TuscanNappe and underlying metamorphic units. According topermeability variations (higher in the shallower levels,lower at depth), two reservoirs can be identified: a shallowone at 500–1000 m below ground level (b.g.l.) and a deeperone at over 1500–2000 m b.g.l.[7] Although regional magmatic activity was present

from the late Miocene to the Quaternary, there are nooutcrops in the Larderello area. The nearest magmatic rockoutcroppings (Figure 1) are represented by the Roccastradaacidic 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 tobottom:— a cover of Neogene sediments (late Miocene to

Pliocene);— the allochtonous flysch formations (‘‘Ligurian units’’:

Jurassic to Eocene);— the Tuscan Nappe, with a predominantly carbonate-

evaporite sequence at the base and a terrigenous sequence atthe top (Late Triassic to Oligo-Miocene);— a complex of tectonic slices, which includes the lowest

formations of the Tuscan Nappe and part of the underlyingmetamorphic units (Paleozoic to Late Triassic);

— micaschists and gneisses (Paleozoic to Precambrian)intruded by granites with radiometric ages of between 1.0and 3.8 Ma [Gianelli et al., 1997a; Del Moro et al., 1982;Villa and Puxeddu, 1994; Villa et al., 2001; Gianelli andLaurenzi, 2001].[9] The chemical composition of the volcanic and plu-

tonic rocks is typical of the S-type, originating fromanatexis, [Van Bergen, 1983; Poli et al., 1989], the sourcerocks for which are micaschists. A minor mantle contribu-tion has been indicated by trace elements and rare earthelements (REE) studies [Giraud et al., 1986; Poli et al.,1989; Serri et al., 1993].

2.1. Fluid Geochemistry

[10] The dominant components of geothermal fluids areH2O, CO2, CH4, H2S, and N2, while noble gases are at partper million levels. Stable isotope data on H2O indicatesmeteoric water as the main source of the vapor [Craig,1963; Ferrara et al., 1965]. A simple mixing between twoendmembers could explain the isotopic composition of thesteam produced in the field [Panichi et al., 1995; Scandiffioet al., 1995]. The former endmember is a primary deepsteam having typical values of D = �40% and 18O = �2%,resulting from extensive water-rock interactions, whichshifted the original meteoric 18O toward higher values.The latter endmember is a secondary steam derived fromthe boiling of fresh waters, having D = �40% and 18O =�7%, the result of limited water-rock interactions due to ashort residence time in the reservoir.[11] An alternative explanation of the steam’s origin has

been proposed by D’Amore and Bolognesi [1994], whohypothesize the mixing of two endmembers, meteoricwaters, and magmatic fluids produced by the crystallizationof a deep-seated magma body. A ‘‘large subduction-relatedmagmatic contribution’’ in the Larderello geothermal gaseshas been suggested by these authors on the basis of thestable isotope variations in the fluids, as well as the existingR/Ra data and high N2/He and N2/Ar ratios, indicating alarge N2 excess.[12] The 3He/4He values (expressed as R/Ra, where Ra is

the typical air value of 1.38 � 10�6), ranging from 0.3 to3.2 [Polyak et al., 1979; Torgersen, 1980; Nuti, 1984;Hooker et al., 1985], strongly suggest the presence of amantle-derived, 3He-enriched fluid. Suggestions that thecrust is the only source of He [Mazor, 1978; D’Amoreand Truesdell, 1984] have been discounted by this Heisotope composition, which indicates the presence of 3Hemantle-enriched fluids. As stressed by Hooker et al. [1985],3He is one of the mantle-derived components clearlyresolved at the surface in the Tuscan magmatic province,where the Sr and O isotope compositions of the igneousrocks indicate melting of the continental crust as a majorsource of magma [Turi and Taylor, 1976].[13] Although the He isotope composition clearly indi-

cates the mantle as an important source of this gas, theactual sources of other inert gases (N2 and Ar) cannot beidentified on the basis of high N2/He and N2/Ar ratios alone,as supported by D’Amore and Bolognesi [1994]. Enrich-ment of the fumarole gas in heavy 15N in the Larderellofield and, more generally, in the gas manifestations ofcentral Italy, signals the heating of metasedimentary rocksas an important source of N2 [Minissale et al., 1997]. It

Figure 1. Overall geological map of the area. (1)Neoautochtonous sediments (late Miocene-Pliocene). (2)Igneous rocks (Pliocene-Quaternary). (3) Allochtonousflysch facies units (Cretaceous-Eocene). (4) Potentialreservoir formations (Tuscan Nappe, Tectonic Slices,Metamorphic Units, see text). The area under study isevidenced in the frame.

ECV 3 - 2 MAGRO ET AL.: LARDERELLO HELIUM AND HEAT

follows that the high N2/Ar ratios at Larderello are moreindicative of crustal heating than true magmatic origins. Forinstance, the Gabbro area (Figure 2), characterized by thelowest R/Ra ratios (0.3–0.7) in the Larderello field, alsoexhibits the highest N2/Ar (from 99 to 449) and 40Ar/36Ar(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 coresamples from wells of the Larderello geothermal field, andregard both their contact metamorphic and hydrothermalminerals [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; Ruggieriand Gianelli, 1999; Ruggieri et al., 1999]. The results ofsuch studies suggest two main stages of hydrothermalactivity.[15] The first stage is related to the intrusion of granites at

a depth of >2000 m b.g.l., which were responsible forcontact metamorphism and metasomatic processes in thesurrounding rocks (gneiss, micaschist, and phyllite). High-temperature assemblages (consisting of biotite, andalusite,quartz, plagioclase, tourmaline, muscovite, and at somelocations, cordierite, corundum, and K feldspar) crystallizedduring this stage. Studies on the fluid inclusions in thequartz of granites and high-temperature assemblages revealthat two main types of fluids were present within andaround the intrusions: high-salinity, Na-Li-rich fluids ofmagmatic origins, and aqueous carbonic fluids resultingfrom 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 subcon-temporaneous and were trapped at temperatures of 690�–

425�C under lithostatic pressures of approximately 95–130MPa. On the basis of the radiometric ages of granite andcontact metamorphic minerals, Cathelineau et al. [1994]proposed that early high-temperature hydrothermal activitybegan 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, mus-covite, and titanite) filling veins at shallow-intermediatedepths (2500 m b.g.l.) or, in certain sites, replacing earlystage contact metamorphic or igneous minerals. During thisstage, low- to high-salinity aqueous fluids with vaporsproduced by fluid boiling were trapped, either in the lateminerals, or in the late-stage secondary inclusions in theearly quartz, at temperatures of roughly 150�–400�C, underhydrostatic pressures (<35 MPa) [Valori et al., 1992;Ruggieri et al., 1999; Ruggieri and Gianelli, 1999]. Thesefluids are interpreted as meteoric waters, whose composi-tion, salinity, and temperature were modified through water-rock interactions and fluid boiling, mixing, and cooling.[17] The d18O compositions of late-stage fluids (com-

puted using fluid inclusion temperatures from isotope dataon the carbonates, quartz, and chlorite in isotopic equili-brium with such fluids) are also compatible with thepresence of meteoric waters that have interacted withreservoir rocks at relatively low water/rock ratios [Petrucciet al., 1993; Gianelli et al., 1997b]. The final evolution ofthe hydrothermal system resulted in the development of thepresent-day vapor-dominated conditions.[18] Fluid inclusion studies were also carried out on some

hydrothermal veins outcropping in the peripheral parts ofthe Larderello geothermal field [Tanelli et al., 1991; Rug-gieri et al., 1993; Gianelli et al., 1997b]. The veins consistof quartz and/or carbonates with barite, gypsum, sulfides, Feoxides as a minor phase, and exhibit local Au enrichment.These minerals probably precipitated from low-to-moderatesalinity, low-temperature fluids, largely of meteoric origin,which circulated in shallow hydrothermal systems related tothe 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 Larderellogeothermal field for noble gas analyses (Table 1). Theprevious data published by Torgersen [1980], Nuti [1984],and Hooker et al. [1985] have been integrated with new dataon the geothermal fluids to cover a larger area of theLarderello field (Figure 2). The depths of the selected wellsvary from 500 to 2800 m b.g.l. Fluids from both shallow anddeep reservoirs have been sampled. Fluids from deep andshallow wells of the Valle Secolo (the northern part of thefield), where wastewaters from steam condensation havebeen reinjected since 1980, were sampled in order to estimatethe effects of such reinjection on R/Ra values.[20] In particular, the wells Miniera 3, Secolo 2, and

Casanova 2 and 2a, are fed by fluids present within themetamorphic basement, while wells 101 and 137 are heav-ily affected by water reinjection, as indicated by the valuesof the recovery coefficient, defined as the amount ofreinjected water recovered from productive wells near thereinjection sites, which ranged from 25 to 80% over a 20-

Figure 2. R/Ra distribution in the Larderello area; trianglesrepresent fumaroles, open circles mark the geothermalwells, and diamonds indicate fluid inclusion samples withtheir R/Ra values.

MAGRO ET AL.: LARDERELLO HELIUM AND HEAT ECV 3 - 3

year period [Panichi et al., 1995]. Changes (1980–2000) inR/Ra values over time were revealed in six wells (Gabbro 1,3, and 9, wells 155 and 162, and VC 10), for which thecomposition of the present-day fluid has been comparedwith that reported in the literature for the same wellssampled about 20 years ago.[21] The geothermal gases were sampled using Giggen-

bach’s [1975] method, i.e., by absorption of the conden-sable components (CO2, H2S, HCl, etc.) onto a 100 ccpreevacuated gas vial filled with 50 cc of a 5N NaOHsolution. The NaOH-vial sampling method drasticallyreduces the effects of accidental air contamination duringsampling because the concentration of the residual gases inthe NaOH vial is over a hundred times higher than theconcentration of incondensable gas in an empty one.

4.2. Fluid Extraction From Mineral Samples

[22] The minerals selected for extraction of the gasphases from fluid inclusions came from seven core samplesof geothermal wells SP2, BR1, MV7, CB11, MV2A,MV5A, CB11A, and two hydrothermal veins (MIC andSAS) (Figure 2, Table 2). The MIC sample is located in thevicinity 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 ofdifferent origins: meteoric and/or magmatic and/or contactmetamorphic (Table 2). Only one type of low-salinityaqueous liquid was found to be present in the inclusionsof the selected minerals of samples MIC (barite), SAS(calcite), and CB11A (calcite). A largely meteoric originfor this liquid is suggested by the relatively low salinity(<4.2 wt.% NaCl equiv.), the oxygen isotope data on thecalcite of SAS and CB11A, and the shallow formation depthof SAS and MIC [Ruggieri et al., 1993; Gianelli et al.,1997b]. A meteoric origin is also compatible with thecharacteristics of the fluids trapped in the inclusions ofsamples MV2A (quartz) and MV5A (calcite) [Ruggieri andGianelli, 1999; Ruggieri et al., 1999]. The only inclusionsfound to be present in the SP2 quartz sample were thosetrapped during early hydrothermal activity. In particular,both inclusions of magmatic-derived Na-Li-rich brines andaqueous-carbonic fluids formed during contact metamor-phism have been recognized [Cathelineau et al., 1994].[24] Early aqueous-carbonic fluids are observed in the

BR1 quartz sample. However, in this sample, inclusionsrelated to late-stage meteoric fluids also occur (G. Ruggieri,unpublished data, 2001). In the MV7 and CB11 quartzsamples, several populations are present and represent thevarious types of paleofluids that circulated in the Larderellogeothermal 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 fromthe other minerals of the samples by handpicking, andcleaned ultrasonically, first in water and then in ethylalcohol. After prior degassing for several hours under avacuum at 100�–150�C in order to avoid air contaminationon the crystal surface, the gases trapped in the fluidinclusions were vacuum extracted by crushing 0.7–3.1 gof the selected mineral fragments (grain size from 0.5 to 1mm). The crushing apparatus used consists of a stainless

steel container and rod, equipped with a pair of high-vacuum, all-metal valves. Crushing was performed byshaking the device for less than 5 min.

4.3. Rare Gas Mass Spectrometry

[26] Both the geothermal gases and minerals were pro-cessed on a stainless steel vacuum line equipped with cold(active charcoal at liquid N2 temperature) and hot traps (Tigetter) to separate the noble gases from the gaseous matrix.The extraction line was connected to both a magnetic massspectrometer (MMS = MAP 215-50) and a quadrupole massspectrometer (QMS = Spectralab 200, VG-Micromass). Theamount of He + Ne was checked via QMS before beingintroduced into the MMS. When the He partial pressuresexceeded the typical MMS inlet values (�10�6 mbar ofHe), which guarantees a low pressure (10�8–10�9 mbar)inside the MMS, the He and Ne fractions inside the secondinlet volume was expanded and the He and Ne partialpressures checked by QMS until suitable values for MMSmeasurements were reached. The 3He/4He ratios weredetermined by fitting the MMS with an ion counting device.The He/Ne ratios were determined via QMS for geothermalgases and via MMS for fluid inclusions. Resolution wasclose to 600 AMU for HD�3He at 5% of the peak. Typicalblanks were on the order of 0.6–1 � 10�9 cc STP for 4He,with a ratio close to that of air. No blank corrections wereapplied to the 3He/4He ratios of geothermal gases, as the Heconcentration was relatively high (a few hundred parts permillion), generally several orders of magnitude higher thanthe He blank (at parts per billion levels). Likewise, no blankcorrections were applied to 3He/4He of fluid inclusionsbecause the samples are in general characterized by an Hecontent of at least one order of magnitude greater than theblank. A standard volume of air at different pressures (from1013 to 10.13 mbar) was introduced into the extraction lineand treated in the same way as the samples. The air 3He/4Heratios exhibited a reproducibility of better than 10% overthe analysis period. The mass ratios 4/(20 + 22) measuredon air standards by QMS showed a reproducibility of morethan 5%.

5. Results and Discussion

[27] The R/Ra ratios of all samples from the geothermalfluids (Table 1) and fluid inclusions (Table 2) fall within therange of 0.5–2.82, in agreement with the values of 0.6–3.2reported byHooker et al. [1985]. Figure 2 shows the locationof all the samples (paleofluids and present-day fluids),together with the distribution of the R/Ra values of thepresent-day geothermal fluids in the area of the Larderellofield. The contour lines in Figure 2 have been rendered usingboth the new and published data on geothermal gases(Table 1). At the scale of the field (approximately 10 � 15km, see Figure 2), the R/Ra distribution encompasses tworelative maxima elongated in the NE-SW direction. Arelative minimum follows the northeastward maximum andis located in the Gabbro area.

5.1. Short-Term Variation in the R/Ra Ratio in theLarderello Area

[28] The rationale for using data from samples collectedover a time span of approximately 20 years is based on thefact that nearly constant R/Ra values were found for two

ECV 3 - 4 MAGRO ET AL.: LARDERELLO HELIUM AND HEAT

wells, VC10 and 162, sampled before 1985 by Hooker et al.[1985] and Nuti [1984], respectively, and resampled in theperiod 1996–2000 (this paper). In fact, the R/Ra value of1.74 of the VC10 well fluid before 1985 is similar, withinthe limits of error, to that recorded in 1996 (R/Ra = 1.71) and

in 2000 (R/Ra = 1.76). A similar pattern is exhibited by well162 (1.09 in 1996 and 1.1 prior to 1984) [Nuti, 1984]. On amore limited timescale, small variations in the R/Ra and He/Ne ratios were found during a monitoring test of a singleproductive well (well 107; R/Ra from 1.4 to 1.6) conducted

Table 1. Helium Isotope Composition of Geothermal Wells of Larderello Field

Well Name Year Latitude Longitude Depth, m b.g.l. R/Ra SD He/Ne Referencea

Gabbro 1 2000 4791.6 1652.9 795 0.78 0.09 229 1Gabbro 1 1996 4791.6 1652.9 . . . 0.68 0.07 449 1Gabbro 1 b1980 4791.6 1652.9 . . . 0.44 . . . . . . 2Gabbro 3 1996 4790.6 1652.5 759 0.85 0.03 646 1Gabbro 3 b1980 . . . . . . . . . 0.39 . . . . . . 2Gabbro 6 b1985 4790.8 1653.2 771 0.66 . . . 253 3Gabbro 9 2000 4791.3 1652.5 958 0.69 0.02 483 1Bulera 4 b1985 4793.0 1652.5 857 0.81 . . . 470 357 b1980 4788.1 1653.1 486 0.50 . . . . . . 266 b1985 4786.2 1654.6 405 2.31 . . . . . . 380 b1985 4788.3 1653.3 307 1.18 . . . . . . 382 April 1992 4788.3 1651.8 471 2.29 0.27 67 182 Oct. 1992 . . . . . . . . . 2.03 0.05 49 185 b1980 4789.0 1652.4 . . . 1.32 . . . . . . 285 b1979 . . . . . . . . . 1.71 . . . . . . 488 b1985 4788.6 1652.5 . . . 2.10 . . . . . . 399 b1980 4787.5 1653.5 387 1.06 . . . . . . 2101 2000 4788.5 1651.3 432 1.64 0.07 3 1107 b1983 4789.3 1652.4 411 1.54 . . . 60 3137 2000 4788.6 1651.4 399 1.43 0.07 26 1145 b1985 4786.8 1650.3 752 1.95 . . . . . . 3151 2000 4788.0 1649.8 1198 1.70 0.05 218 1152 1981 4788.7 1653.5 855 1.80 . . . . . . 3155 1996 4789.9 1653.1 531 0.88 0.02 1253 1155 b1980 . . . . . . . . . 0.50 . . . . . . 2162 1996 4789.6 1653.5 720 1.09 . . . 647 1162 b1984 . . . . . . . . . 1.10 . . . 83 5Miniera 1 b1985 4788.9 1649.8 866 1.90 . . . . . . 3Miniera 3 2000 4789.4 1649.8 2824 1.82 0.08 180 1Secolo 2 1996 4789.0 1652.4 2498 2.13 0.21 993 1Casanova 2 2000 4788.3 1652.5 2190 2.29 0.07 533 1Casanova 2 1996 . . . . . . . . . 2.33 0.19 99 1Casanova 2a 2000 4788.2 1652.5 2305 2.60 0.11 146 1Fabiani b1980 4787.9 1653.3 . . . 1.24 . . . . . . 2S.Vincenzo 9 b1985 4789.0 1655.2 1085 1.44 . . . . . . 3VC 10 2000 4785.3 1648.2 1089 1.76 0.10 714 1VC 10 1996 . . . . . . . . . 1.74 0.07 305 1VC 10 b1985 . . . . . . . . . 1.74 . . . . . . 3Le prate 2a 1996 4783.1 1649.3 301 2.12 0.12 124 1Monterotondo 2 1996 4779.7 1650.7 . . . 0.96 0.10 162 1Puntone 1 1996 4779.5 1647.0 898 2.13 0.21 284 1Querciola 2 1996 4781.7 1646.9 1061 2.30 0.11 498 1Serrazzano.bcf3 1996 4784.9 1646.6 . . . 1.99 0.10 138 1S Martino 3 1996 4778.7 1648.9 608 2.82 0.42 238 1S.Martino 2 1996 4778.4 1649.4 831 2.57 0.23 168 1Colline 2 2000 4784.2 1649.8 . . . 2.18 0.08 592 1Capannini 2 2000 4777.5 1648.5 645 2.49 0.21 61 1LRA 2000 4781.4 1645.5 390 2.06 0.03 18 1Lumiera 3 1996 4777.7 1649.5 935 1.19 0.37 266 1San Pompeo b1983 4777.7 1647.5 2967 3.20 . . . 78 3Sasso 22 b1983 4781.5 1650.5 4092 1.87 . . . 95 3Carboli A b1985 4776.9 1648.3 900 2.56 . . . . . . 3Sesta 1 b1985 . . . . . . 553 1.92 . . . . . . 3Pozzaie 2 b1985 4785.7 1646.3 339 1.91 . . . . . . 3Camorsi 1 b1985 4784.1 1651.0 774 1.06 . . . . . . 3San Silvestro b1985 4782.1 1651.8 514 1.88 . . . . . . 3Zuccantine 1 b1985 4779.4 1646.4 614 2.81 . . . 22 3Villa Madama b1985 4779.1 1649.8 637 1.90 . . . . . . 3Lago Fum. 1995 4779.3 1647.2 0 1.63 . . . 0.55 6Sasso Fum. b1985 4781.2 1651.7 0 2.32 . . . . . . 3Le Prata fum. b1985 . . . . . . 0 2.30 . . . . . . 3Micciano Fum. 1995 4793.0 1651.8 0 1.21 . . . 15.2 6

aReferences: 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].bSampled before the reported year.

MAGRO ET AL.: LARDERELLO HELIUM AND HEAT ECV 3 - 5

by Hooker et al. [1985] over a period of 17 months. As canbe seen in Table 1, no major changes occurred in the R/Ra

values during the period 1980–2000. The only exceptionconcerns the analyses reported by Torgersen [1980] forsome wells of the Gabbro area; all these values aresystematically lower than those reported by other authorsincluding the present study.[29] The few low R/Ra values reported by Torgersen

[1980] may represent the draining of radiogenic Heaccumulated in the reservoir rocks. A similar trend hasalso been recognized for other gases in the Larderello field;a decrease in radiogenic 40Ar was recorded in the 1950s and1960s, at which time the 40Ar/36Ar ratio decreased greatlyto values typical of air. This fact was linked to increasinggeothermal exploitation, which drained the radiogenic Araccumulated in the reservoir [Ferrara et al., 1963]. Ananalogous process of radiogenic He accumulation wouldlower the R/Ra ratio significantly and may explain the R/Ra

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/Ra

and well depth in the Larderello area. Such a conclusionseems to be confirmed by the new data from deep wells(more than 2000 m) in the Valle Secolo area and therelatively shallow (<1000 m) as well as deep (>2000 m)wells in the Monterotondo area (Figure 3), where relativelyhigh R/Ra values have been found.[31] Since 1984, the reinjection of wastewater has

become an important facet of the exploitation strategy. Itseffect on the noble gases (He, Ar) and nitrogen has beenevaluated using a very large data set that includes both preand postreinjection fluid compositions [Scandiffio et al.,1995; Panichi et al., 1995]. The conclusion is that mixingbetween a deep component and the meteoric one introducedby water reinjection could explain the shift of the relativeconcentrations of He, Ar, and N2 from mantle crustal valuesto air and/or air-saturated water ones (asw). For the mostpart, atmospheric N2 and Ar governed the change in inertgas composition, while He was influenced only slightly, orat not all, since this gas is by far more abundant in deep

Table 2. Helium Isotopic Composition of Fluid Inclusions in Hydrothermal Mineralsa

Sample Latitude LongitudeDepth,m b.g.l.

Trapped FluidOrigin

HostMineral

SampleWeight, g

He,ncc R/Ra SD He/Ne (R/Ra)c

MV2a 4783.9 1643.3 1871 met. quartz 2.73 35 1.24 0.12 0.8 1.37MV5a 4784.1 1642.0 1090 met. calcite 1.28 1139 2.17 0.14 265 2.17MV5a 4784.1 1642.0 1090 met. calcite 1.27 656 2.40 0.03 330 2.40MV7 4785.7 1641.0 3485 met. + mag + cont. metam. quartz 1.14 27 1.20 0.21 5.7 1.21BR1 4781.0 1653.6 3138 met + cont. metam. quartz 0.85 95 1.97 0.15 11 2.00SP2 4777.7 1647.5 2900 mag. + cont. metam. quartz 1.46 58 2.32 0.03 . . . . . .CB11A 4776.9 1648.3 1515 met. calcite 2.31 314 0.58 0.04 10 0.56CB11 4776.9 1648.3 3455 met. + mag + cont. metam. quartz 0.71 187 1.04 0.06 77 1.04Mic 4793.0 1651.8 . . . met. barite 1.38 94 1.89 0.03 3.7 1.97Sas 4790.9 1640.2 . . . met. quartz 3.10 5 1.87 0.65 0.5 . . .

aSamples from geothermal wells: MV2a, MV5a, and MV7, Monteverdi; BR1, Bruciano 1; SP2, San Pompeo 2; CB11A and CB11, Carboli. Samplesfrom surface veins: Mic, Micciano; Sas, La Sassa; met., meteoric; mag., magmatic; cont. metam., contact metamorphic. (R/Ra)c is the R/Ra ratio correctedfor atmospheric helium contamination, assuming Ne of air origin and according to Craig et al.’s [1978] formula: (R/Ra)c = (R/Ra � X � 1)/(X � 1), whereX = (He/Ne)m/(He/Ne)air.

Figure 3. Distribution of R/Ra values with well depth;symbols as in Figure 2. No clear correlation exists; the Heisotopic composition in space must be controlled by thenature of reservoir rocks. The R/Ra values of fluid inclusionsare, moreover, unrelated to well depth and fall within thesame range as present-day geothermal fluids.

Figure 4. Relation between R/Ra and He/Ne ratios inwells, fumarolic gases, and fluid inclusions of the Larderellogeothermal field. The mixing curves have been calculatedusing Craig’s formula from Craig et al. [1978]. Symbols aresame as in Figure 2.

ECV 3 - 6 MAGRO ET AL.: LARDERELLO HELIUM AND HEAT

fluids than in the secondary steam produced by thereinjected waters [Panichi et al., 1995]. The high He/Neratios of all the samples, particularly those from theshallower wells 101 and 137 located in the reinjection area,reveal that the He content as well as the He isotopecomposition are unaffected by atmospheric He frommeteoric and reinjection water recharge (Figure 4). TheR/Ra range is mainly the result of different mixing betweena 3He-enriched fluid, derived from the mantle, and a 4He-enriched fluid originating in the crust and most likely storedin the geothermal reservoir rocks.

5.2. Long-Term Variation in R/Ra: Paleofluids in FluidInclusions

[32] The R/Ra values of the paleofluids extracted frominclusions range between 0.58 and 2.4, though in most casesthey are >1 (Table 2). The variations in R/Ra are unrelated tothe depth of the well samples (Figure 3). The He/Ne ratiosrange between 0.5 and 330, suggesting a variable contribu-tion of the atmospheric component (between 0.1 and 56%)to the trapped fluids (Figure 4). The R/Ra values correctedfor the atmospheric contribution are not very different fromthe measured values, with the exception of the SAS sample.[33] The R/Ra of the paleofluids fall within the range of

present-day fluid values (0.39–3.2). This suggests that Hein fluid inclusions could have reequilibrated with He of thepresent-day fluids due to He diffusion into the hostminerals. In particular, a high He diffusion rate, favoredby high temperatures, could have affected the samples coredin the deepest part of the Larderello geothermal field (CB11,SP2, MV7, and BR1), which were, in fact, found atrelatively high temperatures (330�–450�C). However, theR/Ra values of the paleofluids do not generally match thepresent-day fluids spatial distribution of the R/Ra (Figure 2).In addition, discrepancies between the R/Ra of thepaleofluids and present-day fluids in the same well or thesame sampling area is clearly demonstrated by severalsamples (CB11A, CB11, SP2, and MIC), for which the R/Ra

values are available for both paleofluids and present-dayfluids (Tables 1 and 2). In particular, R/Ra of the fluidinclusions of samples CB11A and CB11, (0.58 and 1.04,respectively) are distinctly lower than those of the reservoirfluids produced by wells Carboli A (2.56) and Capannini 2(2.49), both located near the Carboli 11 and 11A wells. Asimilar situation is revealed by comparing the paleofluid insample SP2 (R/Ra = 2.3), with the present-day fluid of thesame well (R/Ra = 3.1–3.2). Differences in R/Ra values havealso been found between the fluid inclusions of the MICsample and the present-day fumarolic gases gushing out inthe same area. However, in this case, the R/Ra of thepaleofluid (1.97) is higher than the fumarolic one (1.21). Allthese differences indicate that He of the paleofluids has notreequilibrated 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 inhydrothermally altered rocks of The Geysers geothermalfield, also found considerable evidence that, despite the highdiffusion rate of He at reservoir temperatures, the trappedpaleofluids have not reequilibrated with present-dayreservoir fluids and therefore represent a record of pastconditions.

[35] Since one or more fluids of distinct origin (meteoric,contact metamorphic, magmatic) are trapped in the studiedsamples, the R/Ra of the bulk fluid extracted from inclusionsis the result of either a single fluid or a mixing of fluids withdifferent origin. All the samples (MV2A, MV5A, CB11A,MIC, and SAS) containing only meteoric-derived fluids arecharacterized by R/Ra values different from typical air and/or asw values (R/Ra = 1).[36] In addition, the He/Ne ratios of all samples contain-

ing meteoric-derived fluids are considerably higher than thetypical air and asw values (0.28 and 0.24, respectively),ranging between 0.5 and 330, hence suggesting a variablecontribution (between 0.1 and 56%) of atmospheric He.The highest values of atmospheric He (35–56%) werefound in one of the vein samples from outcrops (SAS) thatformed at shallow depths and in the MV2A sample. For theother samples (MIC, MV5A, and CB11), the He/Ne ratiosindicate relatively small amounts (0.1–7.6%) of atmos-pheric He, which does not significantly change the R/Ra.[37] The lowest R/Ra (0.58) for fluid inclusions is

exhibited by the CB 11A sample, which contains onlyfluids of meteoric origin. The relatively low R/Ra can beexplained by assuming a long geothermal circulation andstorage in the reservoir, and the stripping of radiogenic 4He(produced in situ from the U and Th decay chain), whichcauses a decrease of R/Ra and an increase in the He/Ne ratio.Therefore, in general, the He isotope composition of themeteoric-derived paleofluids, similar to present-daygeothermal fluids, is not strongly affected by a meteoricHe component, but is instead related to nonatmosphericsources.[38] The gases extracted from samples SP2, BR1, CB11,

and MV7 yield R/Ra values between 1.0 and 2.3 and He/Nevalues between 5.7 and 77. Such values result from themixing of two or more fluid types (magmatic brines and/orcontact metamorphic fluids and/or meteoric-derived fluids)present in different proportions in these samples. Even if theR/Ra of each single-fluid type cannot be determined becauseof the fluid extraction technique adopted, the He/Ne valuesindicate that the atmospheric He component of the gasmixtures is low (<5%). Consequently, the variations in theR/Ra values in all these samples can be ascribed for the mostpart to variable contributions of mantle-derived 3He andcrustal radiogenic 4He. The highest R/Ra value is displayedby sample SP2, wherein the extracted fluid represents amixture of early stage fluids of magmatic and contactmetamorphic origins. We can expect the R/Ra ratio of thegas present in the magmatic-derived fluid of this sample tobe higher than that of the contact metamorphic fluidproduced during the heating of the Paleozoic to Precam-brian crustal units. Thus the R/Ra value of 2.3 can beregarded as the lower limit of the actual value in themagmatic fluids and clearly indicates that significantamounts of mantle-derived 3He have been present in thefluids since the first stage of hydrothermal activity, whichbegan about 3.8 Ma ago and continued until recent times[Cathelineau et al., 1994; Gianelli and Laurenzi, 2001].[39] From the R/Ra values reported by Moore et al.

[2001], a long-standing 3He contribution from the mantle isalso clear at The Geysers geothermal field. In fact, the R/Ra

values of present-day fluids range between 6.3 and 8.3,whereas the He isotope composition of paleofluids trapped

MAGRO ET AL.: LARDERELLO HELIUM AND HEAT ECV 3 - 7

in fluid inclusions, formed either during the liquid-dominated stage (from 1.2 to 1.1 Ma) or at the initialdevelopment of vapor-dominated conditions (around 0.28 �0.25 Ma), are for the most part in the range of 6–10.7.

6. Relationship Between R/Ra and GeophysicalData

[40] Since the He isotope composition of the wells isfairly constant over time (see Table 1) and unaffected by thewastewater reinjection, the helium isotopic variation inspace must be governed by the nature of the reservoir rocksand the physical-chemical conditions existing in differentparts of the field. Figure 5 shows the iso-contour lines ofR/Ra, constructed by adding the R/Ra values of westernTuscan gas manifestations to the geothermal well data [datafrom Minissale et al., 1997, 2000].

[41] Comparing the values of R/Ra, the surface heat flow(HFD) and the Bouguer gravity anomaly on a regional scalereveal a correspondence between the highest R/Ra with thehighest HFD and the lowest Bouguer gravity anomalyvalues (Figure 5). The gravity minimum (<15 Mgal) fallswithin the 250 mW/m2 HFD isoline, which roughlyencompasses the Larderello geothermal field. The findingof a gravity minimum together with the HFD maxima in thisarea 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 theelectrical conductivity anomaly reaches its maximum incorrespondence to the seismic velocity anomaly revealed bytomography and extends to a depth of 12–15 km. Thereforethe conductivity structure of the Larderello geothermal field

Figure 5. Regional comparison among R/Ra, HFD, and Bouguer gravity anomaly data. HFD andBouguer anomaly contours redrawn after Baldi et al. [1995].

ECV 3 - 8 MAGRO ET AL.: LARDERELLO HELIUM AND HEAT

is also consistent with the occurrence of a partially moltengranite body. All these geophysical anomalies are correlatedwith a thin crust over a large sector of Tuscany, as revealedby the Moho depth, as shallow as 20–25 km [Calcagnileand Panza, 1979].[43] Seismic tomography [Batini et al., 1995] and

teleseismic travel time residuals reveal the presence of athick body at a depth of 8–10 km, which tends to widentoward the bottom and is characterized by a low seismic Pwave velocity [Block et al., 1991; Foley et al., 1992]. Onthe basis of teleseismic data, the volume of the anomalousbody (partially solidified granite) is estimated to be 18,000–20,000 km3 [Gianelli and Puxeddu, 1992; Manzella et al.,1995]. The top of the igneous intrusion should be at a depthof between 4 and 7 km, approximately 1–2 km below amajor seismic reflector, the K horizon [Foley et al., 1992;Manzella et al., 1995]. This horizon is postulated to be astructure related to emplacement of the granites, and someauthors have interpreted it as stemming from the presence offluids inside the contact metamorphic aureole or thesolidified carapace [Batini et al., 1983].[44] A good correlation exists between the depth to the K

horizon and the R/Ra areal distribution (Figure 6). Theculmination of the reflector at a depth of about 3000–3500m matches the highest R/Ra contour lines very well; thismay identify an area where rapid uplift of fluids of mantleorigin allows the existence of relatively high R/Ra values ina typical crustal melting environment.

6.1. Constraints on the 3He Source

[45] The highest R/Ra values (3 ± 0.2) of Larderellofluids, clearly indicating the presence of mantle fluids, arelower than typical mantle values, such as MORB (8.5 ± 0.5,N. Atlantic MORB) [Kurz et al., 1982], the mantle values insubduction areas (6 < R/Ra < 8) [Poreda and Craig, 1989]and also the European subcontinental mantle (R/Ra = 6.1 ±0.7) [Dunai and Baur, 1995]. Low R/Ra ratios are foundthroughout the world in both fumarole gases and pheno-crysts in basalts, and are explained either by degassing ofsubducted sediments or continental crust or by the

assimilation of crustal material by magma stored in anintracrustal condition [Hilton et al., 1993a, 1993b]. Assum-ing the MORB value as representative of the mantle beneathTuscany,Hooker et al. [1985] calculated that 40% of the totalHe in Larderello geothermal fluids derives from the mantle.Obviously, the He mantle fraction estimate is biased by thechoice of an R/Ra value representative of the mantle.[46] This signature of the mantle beneath Italy is, in fact,

a matter of great debate. The R/Ra in fluids increases fromNorth to South and is considered to reflect the differentassimilation by the mantle of crustal material involved inthe subduction caused by collision of the African andEuropean plates [Marty et al., 1994; Tedesco, 1997]. Withinthis rather regular geographical trend, Larderello representsthe exception; its highest values (up to 2.8–3.2) exceed therange (0.4 < R/Ra < 0.6) of the nearby Mt. Amiata and Mt.Vulsini volcanic areas, approaching the values (2 < R/Ra < 3)found in the active Campanian volcanic area (PhlaegreanFields and Mt. Vesuvius).[47] A metasomatized mantle with a He isotope signature

close to the interval of 2 < R/Ra < 2.8 was suggested byGraham et al. [1993] on the basis of the results obtainedfrom phenocrysts of the least differentiated lavas at Mt.Vesuvius. If this value range is assumed to be representativeof the mantle beneath Larderello, the estimated mantle Hefraction in the geothermal fluids would approach 100% ofthe total He. This estimation contrasts with the presence ofgranitic bodies and related contact metamorphic rocks,which provides clear evidence of the melting and degassingof the crust.[48] Therefore we can only conclude that the He isotope

composition of paleofluids and present-day geothermalfluids is the result of the mixing of a variable fraction ofmantle and crust fluids. Thus the highest R/Ra values (2.3R/Ra < 3.2) could, at most, approach the lower limits of theR/Ra values representative of the mantle beneath thegeothermal area. Taking such a low value of R/Ra (from 2to 3) to be representative of the mantle beneath Tuscanycould be misleading.[49] We suggest that the typical range of the subcon-

tinental European mantle (R/Ra = 6 ± 0.7) would bereasonably representative of the mantle beneath Italy. It isworth noting that the presence of this type of mantle isconsidered to be the source of magmas for the volcano Etna[Marty et al, 1994].

6.2. Evolution of 3He Source Over Time

[50] The evolution of paleofluids to present-day geother-mal fluids suggests that a constant and long lasting (>1 Ma)mantle contribution to the geothermal fluid at Larderello hasbeen active for at least the last 3.8�1.5 Ma. A long-livedheat and fluid source is necessary to keep the R/Ra constantover such a long period of time. A single magma chamber,without any recharge from a deep magma source, could notsustain such a constant R/Ra because the strong uraniumenrichment of the degassed magma relative to He wouldcause a radiogenic He increase, which would, in turn,strongly affect the primary mantle signature of He, with adecrease in R/Ra for a period in excess of 104–105 years[Zindler and Hart, 1986].[51] Recent numerical calculations [Cathles and Erendi,

1997] have shown that a single episode of intrusion could

Figure 6. Correlation between depth to the K horizon andR/Ra areal distribution. K horizon isobaths (m b.g.l.)redrawn after Barelli et al. [2000]. Full dots, geothermalwells; full triangles, fumaroles; open diamonds, fluidinclusions.

MAGRO ET AL.: LARDERELLO HELIUM AND HEAT ECV 3 - 9

sustain hydrothermal circulation and near-surface geother-mal activity only for about 800,000 years, even if theintrusion were large. Thus long-lived (>1 Ma) hydrothermalactivity normally suggests multiple pulses of magma.Recent geophysical and petrological modeling of theLarderello field [Mongelli et al., 1998; Gianelli andLaurenzi, 2001] has suggested that at least three magmaticpulses during the last 3 Ma has generated the presentthermal state of the field, following the initial emplacementof 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 bodyidentified by drill holes cannot be envisaged as the onlysource of gases. The He isotope composition at The Geysersis consistent with degassing from an active magma chamberrecharged over time with magma from the underlyingmantle [Kennedy and Truesdell, 1996]. Like The Geysers,the Larderello geothermal field cannot be sustained by asingle, insulated magmatic body. The constancy of R/Ra

over a time span of over 1 Ma suggests that mantle-derivedmagma is still degassing.

6.3. 3He as a Tracer of Paleo—Heat Flow Distribution:Some Evidence

[53] A relationship between helium isotope ratios andheat flow has been found for a large variety of tectonicsettings [Polyak and Tolstikhin, 1985]. However, in mostcases, there is no simple correlation between 3He and heatflow [Torgersen, 1993 and references therein].[54] As noted above, the R/Ra ratios of the fluid extracted

from the inclusions may differ from the distribution ofpresent-day R/Ra values. In particular, the R/Ra values ofMIC, MV5A, and SAS, within the limits of error, are higherthan the measured or extrapolated R/Ra of the reservoirfluids. As discussed for geothermal gases, the R/Ra

distribution in present-day geothermal gas correlates withthe heat flow, Bouguer anomaly, and K horizon. Assumingthat this relation was valid in the past as well, the presenceof higher R/Ra values in paleofluids than in present-dayfluids could indicate that the hydrothermal circulation onceextended across a different, possibly wider area than thepresent one, located between well MV5A and the SAS andMIC mineralizations (Figure 2). The eastward migration ofthe feeding system of the deep heat source and/or thecooling in different parts of the field, depending on localconditions, could explain the observed differences inpaleofluids and present-day fluids.[55] Evidence of changes over time and space of the deep

feeding system could be envisaged in the different coolingage of minerals of granitic bodies reached by deep drilling.The radiometric ages of the granitic bodies minerals andcontact aureole range from 3.8 to 1.0 Ma and testify to adiachronic emplacement of the apical part of the batholith,most likely governed by the main fracture systems acting atemplacement time [Mongelli et al., 1998].[56] An analogous behavior could be expected for mantle

fluids rising to the surface, so that the relative depletion overtime of 3He-enriched fluids could possibly depend onchanges in the feeding fracture system. Progressive coolingof the system would also provide a reasonable explanation of

the variations in time of R/Ra in paleofluids and present-dayfluids. Slow monotonic cooling was suggested to have beenactive from 4 Ma ago up to the present day [Villa andPuxeddu, 1994, and references therein], while a completecooling followed by recent reheating was excluded on thebasis of hydrothermal mineral assemblage evolution andfluid inclusion microthermometry [Cathelineau et al., 1994;Mongelli et al., 1998]. Of course, the cooling rate coulddiffer from one part of the field to the other, since it isinfluenced by the local geological setting. For example, theefficiency of the convection heat transfer due to a high rateof meteoric water recharge and circulation at depth couldplay 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 coolingrate in the area between the La Sassa, Micciano, andMonteverdi areas (Figure 2), due to extensive invasion ofmeteoric waters into the hydrothermal system, may explainthe differences between the R/Ra values of paleofluids andpresent-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 byR/Ra values higher than present-day fluids and containmainly fluids of presumed meteoric origin, in contrast topresent-day fluids.[59] Further data are necessary to confirm this hypothesis

and find a clear relationship between heat flow and 3Heflow in the past, taking into account the substantial differ-ences existing in the diffusion rate of these two parameters.Heat diffuses faster than helium, so the maximum release ofheat could occur before the maximum release of He.

7. Conclusions

[60] A comparison of the paleofluids with the present-daygeothermal fluids in the Larderello field indicates that along-lived heating and a volatile source must be presentbeneath the Larderello field, and more generally beneathTuscany. The R/Ra values in the range of 0.5–3.2 indicatethat He, and most likely some other volatile elements, werederived mainly from a mantle source. The R/Ra distributionof present-day fluids matches the distribution of somegeophysical data such as the Bouguer gravity anomaly, theheat flow density distribution, and depth to the seismicreflector ‘‘K.’’ The mantle is the main source of heat and3He enriched fluids. Fluids enriched in 4He are added tothe mantle fluids during their ascent to the surface due tocrust heating, indicated by the presence of granitic bodiesand the thermo-metamorphic aureole in Larderello deepwells.[61] The He mantle fraction in fluid inclusions cannot be

quantitatively estimated due to the limitations of the fluidextraction technique (vacuum crushing). In fact, within theselected samples, the magmatic-derived brines, which pre-sumably contain the largest amounts of mantle 3He, arealways associated with trapped fluids (of meteoric and/orcontact metamorphic origins) bearing some crustal 4He.[62] The selected hydrothermal mineral samples trapped

each type of fluid inclusion, and so the crustal and mantle

ECV 3 - 10 MAGRO ET AL.: LARDERELLO HELIUM AND HEAT

fractions in fluid inclusions cannot be quantitatively esti-mated because of the fluid extraction technique used (vac-uum crushing).[63] Thus the highest R/Ra values (2.3–3.2) found in

present-day (R/Ra from 2.8 to 3.2) and paleofluids (R/Ra =2.3–2.4) must be considered to be a lower limit. Thiscaution must also apply to adopting anomalously low ratiosof R/Ra, in the range of 2–3, as representative of the mantlebeneath Tuscany.[64] The presence of meteoric water-derived fluids (nat-

ural recharge and/or reinjection) in present-day geothermalfluids and paleofluids (natural recharge) does not signifi-cantly change the isotopic composition of He because of thehigher He content of deep fluids as compared with Hederived from asw. The higher R/Ra values, with respect topresent-day fluids, found in the paleofluids of some samplesfrom the relatively external parts of the Larderello fieldcould be related to a different cooling rate and/or a differentsetting of a deep fracture feeding system. In this regard, wesuggest utilizing the 3He of paleofluids as a tracer for theheat flow conditions existing at the time of the paleofluidinclusions’ formation.

[65] Acknowledgments. We are grateful to Alfred Truesdell, ananonymous reviewer, and the Associate Editor Don Baker for instructivecomments, which helped us to improve the manuscript. Thanks to LorenzoGori for his skilled help in artworks and to Antonio Caprai for his help infield gas sampling. Thanks are due to Enel Green-Power for the permissionto publish data. This work was partly supported by CNR Deep GeothermalReservoirs Project.

ReferencesBaldi, P., G. Bertini, and A. Ceccarelli, Geothermal fields of central Italy,Resour. Geol., 16, 69–81, 1993.

Baldi, P., S. Bellani, A. Ceccarelli, A. Fiordelisi, P. Squarci, and L. Taffi,Geothermal anomalies and structural features of southern Tuscany, inProceedings of the World Geothermal Congress, 1995: Florence, Italy18–31 May 1995, edited by E. Barbier et al., pp. 693–696, Elsevier Sci.,New York, 1995.

Barelli, A., G. Bertini, G. Buonasorte, G. Cappetti, and A. Fiordelisi, Re-cent deep exploration results at the margins of the Larderello-Travalegeothermal system, in Proceedings of the World Geothermal Congress,2000, Kyushu-tohoku, Japan, 28 May-10 June 2000, edited by E. Iglesiaset al., pp. 965–970, Elsevier Sci., New York, 2000.

Batini, F., G. Bertini, G. Gianelli, E. Pandeli, and M. Puxeddu, Deepstructures of the Larderello field, contribution from recent geophysicaland geological data, Mem. Soc. Geol. Ital., 25, 219–235, 1983.

Batini, F., A. Fiordelisi, F. Graziano, and M. Nafi Toksoz, Earthquaketomography in the Larderello geothermal area, in Proceedings of theWorld Geothermal Congress, 1995: Florence, Italy 18–31 May1995, edited by E. Barbier et al., pp. 817–820, Elsevier Sci., NewYork, 1995.

Bertini, G., G. Gianelli, E. Pandeli, and M. Puxeddu, Distribution of hydro-thermal minerals in Larderello-Travale and Mt. Amiata geothermal fields(Italy), Trans. Geotherm. Resour. Counc., 9, 261–266, 1985.

Block, L. V., M. N. Toksoz, and F. Batini, Crustal velocity structure of theLarderello geothermal field determined from local earthquake arrival timedata, paper presented at 53rd Meeting and Technical Exhibit, Eur. Assoc.of Explor. Geophys., Florence, Italy, 1991.

Calcagnile, C., and C. F. Panza, Crustal and upper mantle structures beneaththe Apennines region as inferred from the study of Rayleigh waves,J. Geophys. Res., 45, 319–327, 1979.

Cathelineau, M., J. Dubessy, Ch. Marignac, A. Valori, G. Gianelli, and M.Puxeddu, Pressure-temperature-composition changes from magmatic topresent-day stages in the Larderello geothermal field (Italy), in Proceed-ings of the Sixth International Symposium on Water-Rock Interaction,Malvern, U.K., edited by D. L. Miles, pp. 137–140, A. A. Balkema,Brookfield, Vt., 1989.

Cathelineau, M., Ch. Marignac, M. C. Boiron, G. Gianelli, and M. Pux-eddu, Evidence for Li-rich brines and early magmatic fluid-rock interac-tion in the Larderello geothermal system, Geochim. Cosmochim. Acta,58, 1083–1099, 1994.

Cathles, L. M., and A. H. J. Erendi, How long can a hydrothermal systembe sustained by a single event?, Econ. Geol., 92, 766–771, 1997.

Cavarretta, G., and M. Puxeddu, Schorl-dravite ferridravite tourmalinesdeposited by hydrothermal magmatic fluids during the early evolutionof the Larderello geothermal field, Italy, Econ. Geol., 85, 1236–1251,1990.

Cavarretta, G., G. Gianelli, and M. Puxeddu, Formation of authigenicminerals and their use as indicators of physical-chemical parameters ofthe fluid in the Larderello-Travale geothermal field, Econ. Geol., 77,1071–1084, 1982.

Craig, H., The isotope geochemistry of water and carbon in geothermalareas, in Nuclear Geology on Geothermal Areas, edited by E. Tongiorgi,Pisa, Lab. di Geol. Nucl., Cons. Naz. Delle Ric., Pisa, 1963.

Craig, H., J. E. Lupton, and Y. Horibe, A mantle helium component incircum-pacific volcanic gases: Hakone, Marianas and Mt. Lassen, inTerrestrial Rare Gases: Proceedings of the U.S.-Japan Seminar on RareGas Abundance and Isotopic Constraints on the Origin and Evolution ofthe Earth’s Atmosphere, edited by E. G. Alexander Jr. and M. Ozima, pp.3–16, Bus. Cent. for Acad. Soc. Jpn., Tokyo, 1978.

D’Amore, F., and L. Bolognesi, Isotopic evidence for a magmatic contribu-tion to fluids of the geothermal systems of Larderello, Italy, and theGeysers, California, Geothermics, 23, 21–32, 1994.

D’Amore, F., and A. Truesdell, Helium in the Larderello geothermal fluid,Geothermics, 3, 227–239, 1984.

Del Moro, A., M. Puxeddu, F. Radicati di Brozolo, and I. Villa, Rb-Sr andK-Ar ages on minerals at temperature of 300–400�C from deep wells inthe Larderello geothermal field (Italy), Contrib. Mineral. Petrol., 81,340–349, 1982.

Dunai, T. J., and H. Baur, Helium, neon and argon systematics of theEuropean subcontinental mantle: Implications for its geochemical evolu-tion, Geochim. Cosmochim. Acta, 59, 2767–2783, 1995.

Ferrara, G., R. Gonfiantini, and P. Pistoia, Isotopic composition of Argonfrom steam jets of Tuscany, in Nuclear Geology on Geothermal Areas,edited by E. Tongiorgi, Pisa, Lab. di Geol. Nucl., Cons. Naz. Delle Ric.,Pisa, 1963.

Ferrara, G. C., R. Gonfiantini, and C. Panichi, La composizione isotopicadel vapore di alcuni soffioni di Larderello e dell’acqua di alcune sorgentie mofete della Toscana, Atti Soc. Tosc. Sc. Nat., 72, 3–21, 1965.

Fiordelisi, A., R. L. Mackie, T. Madden, A. Manzella, and S. A. Rieven,Application of the magnetotelluric method using a remote-remote refer-ence system for characterizing deep geothermal system, in Proceedings ofthe World Geothermal Congress, 1995: Florence, Italy 18–31 May 1995,edited by E. Barbier et al., pp. 893–897, Elsevier Sci., New York, 1995.

Foley, J. E., M. N. Toksoz, and F. Batini, Inversion of teleseismic traveltime residuals for velocity structure in the Larderello geothermal field,Italy, Geophys. Res. Lett., 19, 5–8, 1992.

Gianelli, G., and M. A. Laurenzi, Age and cooling rate of the geothermalsystem of Larderello, Trans. Geotherm. Res. Counc., 25, 731–735, 2001.

Gianelli, G., and M. Puxeddu, Geological comparison between Larderelloand The Geysers geothermal fields, paper presented at 28th InternationalGeological Congress, Kyoto, Japan, 1992.

Gianelli, G., and G. Ruggieri, Contact metamorphism in the LarderelloGeothermal System, in Proceedings of the World Geothermal Congress,2000, Kyushu-tohoku, Japan, 28 May-10 June 2000, edited by E. Iglesiaset al., pp. 1163–1168, Elsevier Sci., New York, 2000.

Gianelli, G., A. Manzella, and M. Puxeddu, Crustal models of the geother-mal areas of Larderello and Mt. Amiata, Italy, Trans. Geotherm. Resour.Counc., 20, 287–293, 1996.

Gianelli, G., A. Manzella, and M. Puxeddu, Crustal models of the geother-mal areas of southern Tuscany (Italy), Tectonophysics, 281, 221–239,1997a.

Gianelli, G., G. Ruggieri, and M. Mussi, Isotopic and fluid inclusion studyof hydrothermal and metamorphic carbonates in the Landerello geother-mal field and surrounding areas, Italy, Geothermics, 26, 393–417,1997b.

Giggenbach, W. F., A simple method for the collection and analysis ofvolcanic gas samples, Bull. Volcanol., 39, 132–145, 1975.

Giraud, A., C. Dupuy, and J. Dostal, Behaviour of trace elements duringmagmatic processes in the crust: Application to acidic volcanic rocks ofTuscany (Italy), Chem. Geol., 57, 269–288, 1986.

Graham, D. W., P. Allard, C. R. J. Kilburn, F. J. Spera, and J. E. Lupton,Helium isotopes in some historical lavas from Mt. Vesuvius, J. Volcanol.Geotherm. Res., 58, 359–366, 1993.

Hilton, D. R., K. Hammerschmidt, G. Loock, and H. Friedrichsen, Heliumand argon isotopic systematics of the central Lau basins and Valu FaRidge: Evidences of crust/mantle interactions in a back-arc basin, Geo-chim. Cosmochim. Acta, 57, 2819–2841, 1993a.

Hilton, D. R., K. Hammerschmidt, S. Teufel, and H. Friedrichsen, Heliumisotopes characteristics of Andean geothermal fluids and lavas, EarthPlanet. Sci. Lett., 120, 265–282, 1993b.

MAGRO ET AL.: LARDERELLO HELIUM AND HEAT ECV 3 - 11

Hooker, P. J., R. Bertrami, S. Lombardi, R. K. O’Nions, and E. R. Oxburgh,Helium-3 anomalies and crust-mantle interaction in Italy, Geochim. Cos-mochim. Acta, 49, 2505–2513, 1985.

Innocenti, F., G. Serri, G. Ferrara, P. Manetti, and S. Tonarini, Genesis andclassification of rocks of the tuscan magmatic province: Thirty years afterMarinelli’s model, Acta Vulcanol., 2, 247–265, 1992.

Kennedy, B. M., and A. H. Truesdell, The northwest geysers high-tempera-ture reservoir: Evidence for active magmatic degassing and implicationsfor the origin of The Geysers geothermal field, Geothermics, 25, 365–387, 1996.

Kurz, M. D., W. J. Jenkins, J. G. Schilling, and S. R. Hart, Helium isotopicvariations in the mantle beneath the central North Atlantic Ocean, EarthPlanet. Sci. Lett., 58, 1–14, 1982.

Magro, G., G. Ruggieri, C. Panichi, G. Scandiffio, M. C. Boiron, and M.Cathelineau, Noble gases and carbon isotopic composition of fluid inclu-sions from Larderello geothermal field (Italy): Preliminary data, Min.Mag., 62A, 939–940, 1998.

Manzella, A., G. Gianelli, and M. Puxeddu, Possible model of thedeepest part of the Larderello geothermal field, in Proceedings ofthe World Geothermal Congress, 1995: Florence, Italy 18–31 May1995, edited by E. Barbier et al., pp. 1279–1282, Elsevier Sci., NewYork, 1995.

Marty, B., T. Trull, P. Lussiez, I. Basile, and J.-C. Tanguy, He, Ar, O, Sr andNd isotope constraints on the origin and evolution of Mount Etna mag-matism, Earth Planet. Sci. Lett., 126, 23–39, 1994.

Mazor, E., Noble gases in a section across the vapor dominated geothermalfield of Larderello, Italy, Pure Appl. Geophys., 117, 262–275, 1978.

Minissale, A., W. Evans, G. Magro, and O. Vaselli, Multiple source com-ponents in gas manifestations from north-central Italy, Chem. Geol., 142,175–192, 1997.

Minissale, A., G. Magro, G. Martinelli, O. Vaselli, and G. F. Tassi, Fluidgeochemical transect in the Northern Apennines (center-northern Italy):Fluid genesis and migration and tectonic implications, Tectonophysics,319, 199–222, 2000.

Mongelli, F., F. Palumbo, M. Puxeddu, I. M. Villa, and G. Zito, Interpreta-tion of the geothermal anomaly of Larderello, Italy, Mem. Soc. Geol. Ital.,52, 305–318, 1998.

Moore, J. N., D. I. Norman, and B. M. Kennedy, Fluid inclusion gas compo-sitions from an active magmatic-hydrothermal system: A case study of TheGeysers geothermal field, USA, Chem. Geol., 173, 3–30, 2001.

Nuti, S., Elementary and isotopic compositions of noble gases in geother-mal fluids of Tuscany, Italy, Geothermics, 13, 215–226, 1984.

Panichi, C., G. Scandiffio, and F. Baccarin, Variation of geochemical para-meters induced by reinjection in the Larderello area, in Proceedings of theWorld Geothermal Congress, 1995: Florence, Italy 18–31 May 1995,edited by E. Barbier et al., pp. 1845–1849, Elsevier Sci., New York,1995.

Petrucci, E., S. M. F. Sheppard, and B. Turi, Water/rock interaction in theLarderello geothermal field (southern Tuscany, Italy): An 18O/16O and D/H isotope study, J. Volcanol. Geotherm. Res., 59, 145–160, 1993.

Poli, G., P. Manetti, and S. Tommasini, A petrological review on Miocene-Pliocene intrusive rocks from Southern Tuscany and Tyrrhenian Sea(Italy), Period. Mineral., 58, 109–126, 1989.

Polyak, B. G., and I. N. Tolstikhin, Isotopic composition of the earth’shelium and the problem of the motive forces of tectogenesis, Chem. Geol.Isot. Geosci., 52, 9–33, 1985.

Polyak, B. G., E. M. Prasolov, G. I. Buachidze, V. I. Kononov, B. A.Mamyrin, I. I. Surovtseva, L. V. Khabarin, and V. S. Yudenich, Isotopecomposition of He and Ar in fluids of the Alpine-Apennine region and itsrelation with volcanic activity, Dokl. Akad. Nauk, 247, 1220–1225,1979.

Poreda, R., and H. Craig, Helium isotope ratios in circum-Pacific volcanicarcs, Nature, 338, 473–478, 1989.

Ruggieri, G. and G. Gianelli, Fluid inclusion data from the Carboli 11 well,Larderello geothermal field, Italy, paper presented at World GeothermalCongress, Florence, Italy, 1995.

Ruggieri, G., and G. Gianelli, Multi-stage fluid circulation in a hydraulicfracture breccia of the Larderello geothermal field (Italy), J. Volcanol.Geotherm. Res., 90, 241–261, 1999.

Ruggieri, G., P. Lattanzi, and G. Tanelli, Carbonate-hosted gold mineraliza-tion in southern Tuscany (Italy): The examples of Frassine and La Cam-pigliola, in Current Research in Geology Applied to Ore Deposits, pp.551–554, Soc. for Geol. Appl. to Mineral. Deposits, Spain, 1993.

Ruggieri, G., M. Cathelineau, M. Ch. Boiron, and Ch. Marignac, Boilingand fluid mixing in the chlorite zone of the Larderello geothermal system,Chem. Geol., 154, 237–256, 1999.

Scandiffio, G., C. Panichi, and M. Valenti, Geochemical evolution of fluidsin the Larderello geothermal field, in Proceedings of the World Geother-mal Congress, 1995: Florence, Italy 18–31 May 1995, edited by E.Barbier et al., pp. 1839–1843, Elsevier Sci., New York, 1995.

Serri, G., F. Innocenti, and P. Manetti, Geochemical and petrological evi-dence of the subduction and delaminated Adriatic continental lithospherein the genesis of Neogene—Quaternary magmatism of central Italy, Tec-tonophysics, 223, 117–147, 1993.

Tanelli, G., P. Lattanzi, G. Ruggieri, and F. Corsini, Metallogeny of gold inTuscany, Italy, in Brazil Gold’91, edited by E. A. Ladeira, pp. 109–114,A. A. Balkema, Brookfield, Vt., 1991.

Tedesco, D., Systematic variation in the 3He/4He ratio and carbon of fu-marolic fluids from active volcanic areas in Italy: Evidence for radiogenic4He and crustal carbon addition by subducting African plate?, EarthPlanet. Sci. Lett., 151, 255–269, 1997.

Torgersen, T., Controls on pore-fluid concentration of 4He and 222Rn andthe calculation of 4He/222Rn ages, J. Geochem. Explor., 13, 57–75,1980.

Torgersen, T., Defining the role of magmatism in extensional tectonics:Helium 3 fluxes in extensional basins, J. Geophys. Res., 98, 16,257–16,269, 1993.

Turi, B., and H. P. Taylor Jr., Oxygen isotope studies of potassic rocks ofthe roman province, central Italy, Contrib. Mineral. Petrol., 55, 1–31,1976.

Valori, A., M. Cathelineau, and Ch. Marignac, Early fluid migration in adeep part of the Larderello geothermal field: A fluid inclusion study ofthe garnite sill from well Monteverdi 7, J. Volcanol. Geotherm. Res., 51,115–131, 1992.

Van Bergen, M. J., Polyphase metamorphic sedimentary xenoliths from Mt.Amiata volcanics (Central Italy): Evidence for a partially disrupted con-tact aureole, Geol. Rundsch., 72, 637–662, 1983.

Villa, I., and M. Puxeddu, Geocronology of the Larderello geothermal field:New data and the ‘‘closure temperature’’ issue, Contrib. Mineral. Petrol.,115, 415–426, 1994.

Villa, I. M., G. Ruggieri, and M. Puxeddu, Geocronology of magmatic andhydrothermal micas from the Larderello geothermal field, Italy, Proceed-ings of the 10th International Symposium on Water-Rock Interaction,Villasimius, Italy, edited by R. Cidu, pp. 1589–1592, A. A. Balkema,Brookfield, Vt., 2001.

Zindler, A., and S. Hart, Helium: Problematic primordial signals, EarthPlanet. Sci. Lett., 79, 1–8, 1986.

�����������������������S. Bellani, G. Gianelli, G. Magro, and G. Ruggieri, Istituto di Geoscienze

e Georisorse, CNR (National Research Council of Italy), Via G. Moruzzi 1,56124 Pisa, Italy. (g.magro@igg.cnr.it; ruggieri@igg.cnr.it; gianelli@igg.cnr.it; sbellani@igg.cnr.it)G. Scandiffio, ENEL Green-Power, Via Andrea Pisano 120, 56122 Pisa,

Italy. (scandiffio.giovanni@enel.it)

ECV 3 - 12 MAGRO ET AL.: LARDERELLO HELIUM AND HEAT