Contact Metamorphic Aureole High T ism

Evidence of a contact metamorphic aureolewith high-temperature metasomatism in thedeepest part of the active geothermal field

of Larderello, Italy

Giovanni Gianelli*, Giovanni Ruggieri

Institute of Geosciences and Earth Resources - CNR, Area della Ricerca, Via Moruzzi 1, I-56124 Pisa, Italy

Received 26 July 2000; accepted 2 October 2001

Abstract

The deep part (1.5–4.5 km) of the Larderello geothermal field (Tuscany, Italy) consists ofrocks that were metamorphosed by the Hercynian and Alpine orogenies, and which were

thermally metamorphosed in the same place during the emplacement of granite intrusions of3.8–1.0 Ma age. These rocks are potential deep-seated reservoirs and could be the target offuture exploitation. The petrographic, geochemical and fluid inclusion data indicate thatthermally metamorphosed phyllite, micaschist, gneiss, amphibolite and carbonates underwent

a recrystallisation at temperatures of 425–670 �C, under a lithostatic pressure regime of 95-130 MPa. Li–Na-rich fluids of magmatic origin, and aqueous-carbonic fluids with varyingproportions of H2O and CO2 that formed during the contact metamorphism, were present

during this stage. The fluids present during the contact metamorphic event were responsiblefor a widespread B-metasomatism and local F-metasomatism. In some cases, high-tempera-ture metamorphism of graphitic schists can control the composition of the aqueous-carbonic

fluids. A late-stage, lower temperature hydrothermal activity was responsible for both thepropylitic and sericitic alterations and for the replacement of the high-temperature mineralassemblage. Stable isotope (�18O and �D) data on the thermally metamorphosed rocks andgranites indicate that these rocks underwent depletion in the heavy isotopes. Magma degassing

and dehydration metamorphic reactions can explain the isotopic values of these rocks.Late-hydrothermal fluid of meteoric origin may also have contributed to the depletion of theheavy isotopes from the rocks. Under contact metamorphism conditions the rocks were

Geothermics 31 (2002) 443–474

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E-mail address: [email protected] (G. Gianelli).

plastic and impermeable. A transition from plastic to brittle conditions occurred as a con-

sequence of the cooling of the system at depths of <4 km. The brittle-plastic transition atLarderello now occurs at a depth of 4.5–5 km, where present-day temperatures are in therange 400–450 �C. Deep-seated fluids probably occur in this zone, as suggested by the geo-

physical data. # 2002 CNR. Published by Elsevier Science Ltd. All rights reserved.

Keywords: Deep-seated geothermal reservoir; Contact metamorphism; Fluid inclusions; Stable isotopes;

Larderello; Italy

1. Introduction

The feasibility of extracting heat from rocks of very low permeability and hightemperature has attracted a great deal of attention in the last few years. A project ofenhanced heat recovery from the high-temperature zone (260–370 �C) of The Geysersfield (California) is currently under scientific evaluation (Nielson et al., 2001). Thedeep (3729 m) well WD-1a at Kakkonda (Japan) reached a granite with temperatureof more than 500 �C, and methods for extracting heat from these deep-seated geo-thermal reservoirs (DSGR) are under study in Japan (Muraoka et al., 1998; Hashida etal., 2001). In Iceland the presence of DSGR with supercritical temperatures has beenaffirmed by drilling into rocks of basaltic composition (Fournier, 1991; Fridleifsson etal., 2001). The characteristics of the DSGR at Kakkonda and The Geysers are thoseof low-permeability, thermally metamorphosed rocks at the contact with younggranite intrusions. Their mechanical behaviour is considered to be transitionalbetween brittle and plastic (Fournier, 1991; Nielson and Moore, 2000) and the per-meating fluid possibly in a supercritical state. At Larderello a number of wells havereached rocks at depths of 3–4.5 km, which are considered potential reservoirs, withtemperatures over 300 �C, and in some cases over 400 �C (Fig. 1).

The geological units at 1.5–4.5 km depth (below ground level) consist of variousmetamorphic rocks (phyllite, micaschist, amphibolite, gneiss, marble), showingmultiple crystallization phases and a final hydrothermal alteration. In addition,granite, pegmatite and aplite have been found at the bottom of several wells (DelMoro et al., 1982; Villa and Puxeddu, 1994; Gianelli and Laurenzi, 2001). High-temperature interaction of the country rocks and magma-derived fluids occurredduring the early stage of the hydrothermal circulation and was followed by latehydrothermal alteration processes, which are probably still active today. The studyof the thermally metamorphosed rocks and the reconstruction of the fluid associatedwith them could therefore improve our knowledge of the DSGR at Larderello.

In order to define the P–T–X conditions of the system (rock+fluid) and the evolutionof the hydrothermal fluids, our study was based on new and existing petrographic,mineralogical, stable isotope (�18O and �D) and fluid inclusion data of samplesfrom several deep wells (Fig. 1), focussing on the high-temperature (HT), low-pressure (LP) metamorphism of the wall rock intruded by granites of 3.8–1.0 Maage.

444 G. Gianelli, G. Ruggieri / Geothermics 31 (2002) 443–474

2. Previous studies

2.1. Geological units

The sequence of the geological units (Fig. 2a) has been reconstructed using infor-mation from many deep geothermal wells, data on regional geology, and regionalcorrelation studies (Elter and Pandeli, 1990; Pandeli et al., 1994, and related biblio-graphy). Geochemical data have also been used to distinguish between differentmetapelites of similar composition but different stratigraphic position (Gianelli andPuxeddu, 1978; Puxeddu et al., 1984; Gianelli and Rossini, 1991). The field is madeup of eight main geological units from Neogene to Paleozoic or unknown age(Gianelli et al., 1978; Bagnoli et al., 1979; Pandeli et al., 1994). From top to bottom,these units are:

(1) Neogene marine and lacustrine sediments: sands, clays, marls and evaporitesof Late Miocene to Pliocene age;

(2) Ligurian units: Jurassic-to-Eocene ophiolite and flysch sequences;(3) Tuscan Nappe: Upper Triassic to Oligo-Miocene siliciclastic, carbonate and

evaporitic sequences;

Fig. 1. Temperature (�C) distribution 3000 m below ground level in the Larderello area (after Barelli et

al., 2000) and location of the studied wells (black dots). Well names: (1) San Pompeo 2; (2) Sasso 22; (3)

Selva 4; (4) Dolmi 4; (5) VC11; (6) San Martino 1; (7) Capannoli 2b; (8) Padule 2; (9) Sperimentale Ser-

razzano; (10) Bruciano 1; (11) Canneto 4; (12) Badia 1b; (13) Montecerboli 1; (14) Lumiera 1b; (15)

Monteverdi 7; (16) Monteverdi 5 and Monteverdi 5a; (17) Carboli Cbis; (18) Carboli 11a; (19) Radi-

condoli 30; (20) Radicondoli 29; (21) Radicondoli 26.

G. Gianelli, G. Ruggieri / Geothermics 31 (2002) 443–474 445

Fig. 2. (a) Sequence of geologic units at Larderello; (b) schematic model of the Larderello geothermal

area through a 40 km E-W cross-section (after Gianelli et al., 1997a, modified). (1) Neogene units; (2)

Jurassic to Tertiary ophiolite and flysch units; (3) Triassic to Tertiary, mainly carbonate units; (4) Triassic

dolostones; (5) Triassic, Paleozoic and pre-Paleozoic units; (6) contact aureole, granite, aplite and peg-

matite; (7) granite; (8) magma and /or partial melting.


(4) Verrucano, a geologic formation including quartzite and phyllite, comparableto analogous rocks of Upper Triassic age. Quartz, chlorite, muscovite and, inplaces, chloritoid and kyanite are the main minerals found in this unit;

(5) Metagreywacke and graphite phyllite of Permian and Carboniferous age. Insome places limestones are interbedded with these siliciclastic rocks;

(6) Quarziti e filladi superiori, Quarziti e filladi inferiori, Scisti porfirici: quartz-phyllite and phyllite, metagraywake associated with metabasite or meta-rhyolite and dolomitic limestone. These rocks are considered to be of pre-Hercynian age on the basis of regional correlations with similar lithotypes incentral Italy and Sardinia;

(7) Complesso dei Micascisti: plagioclase-garnet-micaschist with amphibolite(Early Paleozoic?). This unit is correlated with an analogous NorthernApennine formation (Ricci, 1968) present in both an outcrop and anexploratory oil well (Pontremoli 1 well; Lat. 44�2200100N, Lon. 9�5104800E);

(8) Complesso degli Gneiss (Early Paleozoic?-Precambrian?): paragneiss withamphibolite and minor calc-silicate and siliceous marble layers. Gneiss con-sists of quartz, plagioclase, biotite and garnet as major components. Amphi-bolite consists of brown-green hornblende, plagioclase, quartz and titanite.Calc-silicate rocks are described in Section 3.

A complex of tectonic slices (incorporating the lowest formations of unit 3 as wellas units 4, 5 and 6) is usually present below unit 3 (Gianelli et al., 1978). Units 5–8are found in the geothermal wells and in some places show evidence of HT-LPcontact metamorphism in a depth range of approximately 1.5–4.5 km. The graniteintrudes units 7 and 8. The Rb-Sr, 40Ar-39Ar and K-Ar ages of both granite andcontact metamorphic minerals range from 1.0 to 3.8 Ma (Del Moro et al., 1982;Villa and Puxeddu, 1994; Villa et al., 1997, 2001; Gianelli and Laurenzi, 2001). Thethickness of the contact metamorphic aureole is estimated to be up to 600 m(Cavarretta and Puxeddu, 1990). The size of the contact metamorphic aureoleprobably corresponds to the gravity, thermal, seismic and conductive anomaliespresent below the Larderello geothermal area (Gianelli et al., 1997a). The graniteintrusions have a regional extension and thickness of several km, as deduced fromgeophysical data (Gianelli et al., 1997a; Manzella et al., 1998). The conceptualmodel of the field, shown in Fig. 2b, is based on well data down to approximately4.5 km below ground level, as well as on geophysical data.

2.2. Regional metamorphism

It has been recognised that the metamorphic units of Tuscany, on a regional scale,received a major imprint from the Alpine orogeny (Carmignani and Kligfield, 1990).Structural and mineral relics of older phases (garnet, kyanite, staurolite) are clearlyrecognisable in many samples, and are related to Hercynian or pre-Hercynianevents. In the northern Apennines, the range of P–T conditions of the Alpine meta-morphism is approximately 200–600 MPa and 300–450 �C (Deschamps, 1980;Dechomets, 1983; Puxeddu et al., 1984; Franceschelli et al., 1986). Jolivet et al.


(1998) hypothesise pressures up to the surprisingly high value of 1500 MPa, on thebasis of the recent finding of carpholite-bearing rocks. The schistose or gneissic texturesof the metamorphic rocks at Larderello were also formed by the regional Alpinetectonic and metamorphic event. Clear evidence of a post-tectonic HT–LP meta-morphic event is present in many rock samples. The succession of metamorphicevents is summarised in Table 1.

According to Elter and Pandeli (1990), the gneiss complex represents the Hercynianbasement of the Northern Apennines weakly affected by the Alpine orogeny. On thecontrary, Franceschini (1998) rejects the existence of Hercynian gneissic rocks andconsiders all the HT–LP mineral assemblages present in unit 8 to be related to contactmetamorphism. This latter interpretation, however, is not supported by texturalstudies (Elter and Pandeli, 1990). For example, a Hercynian metamorphic layeringin the gneiss unit is made up of Amphibolite Facies minerals, such as muscovite,quartz, biotite, plagioclase and hornblende. A pre-Hercynian mineral assemblage ofthe Amphibolite Facies is also present in units 7 and 8. The Hercynian age of theHT–LP paragenesis is supported by one Rb-Sr age (285�11 Ma) of a muscovite co-existing with an andalusite deformed by the Alpine schistosity (Del Moro et al.,1982; Pandeli et al., 1994).

2.3. Contact metamorphism

The first, clear example of contact metamorphic rocks at Larderello was found inthe geothermal well San Pompeo 2, which penetrated a micaschist with post-tectonicandalusite, biotite and tourmaline. A spontaneous blow-out of the well occurredwhen a fluid of over 420 �C and at a pressure greater than 24 MPa was encountered at2900 m depth (below ground level), damaging the well (Batini et al., 1983). Further

Table 1

Metamorphic events and textures in the metamorphic rocks of Larderello

Main

texture

Pre-Hercynian

Mineral

assemblage 1

Hercynian

HT-LP

Mineral

assemblage 2

Alpine

MT-MP

Mineral

assemblage 3

Contact

metamorphism

Mineral

assemblage 4

T1 Q, Pl, Gt, Ms, Bt, St, (Chl?) And, Bt, (Crd?) Q, Ab, Chl, Ms Bt, And Crd, Co, Kfl, Tur

T2 Q, Pl, Gt, Ms, Bt, St And, Bt, (Crd?) Q, Ab, Chl, Ms Bt, And, (Sill?), Crd, Co,

Kfl, Tur

T2 ? Hbl-1 Chl, Act Hbl-2

T3 Cc, Dol, Anh ? ? ? Ol, Phl, Cpx

T1: granolepidoblastic texture with minerals orientated along an Alpine (Tortonian?) schistosity; T2:

granoblastic, lepidoblastic or nematoblastic, mm thick layers of leucocratic to melanocratic layers; T3:

granoblastic texture of calc-silicate rocks. Q: quartz; Pl: plagioclase (anorthite>20%); Gt: garnet; St:

staurolite; Chl: Chlorite; And: andalusite; Bt: biotite; Crd: cordierite; Ab: albite; Ms: muscovite; Kfl: K-

feldspar; Tur: tourmaline; Co: corundum; Sill: non-fibrolitic (>0.1 mm) sillimanite; Hbl-1: green parga-

sitic hornblende; Hbl-2: colourless to green grunerite; Act: actinolite; Cc: calcite; Dol: dolomite; Anh:

anhydrite; Ol: olivine; Phl: phlogopite; Cpx: diopsiditic to hedenbergitic clinopyroxene.


evidence of contact metamorphic rocks and granite has been found in a number ofwells. Mineralogical, petrographic and geological studies have allowed us to reachestimates of the P–T conditions for the HT–LP metamorphism at Larderello (Puxedduet al., 1984; Cavarretta and Puxeddu 1990; Gianelli et al., 1997a; Manzella et al.,1998). Pressure conditions can be constrained by the following geological data: (1)the contact metamorphic rocks are mostly found at approximately 2.5–4.0 kmdepth; at greater depths the most common rock found is granite; (2) an uplift rate ofapproximately 0.2 mm/year has been estimated in the geothermal area by Del Moroet al. (1982). The contact metamorphism therefore occurred in a pressure range ofapproximately 85–120 MPa, assuming an average rock density of 2.6 g/cm3 andlithostatic conditions.

The tourmaline composition in the contact metamorphic rocks of Larderello rangesfrom shorl-dravite to shorl-elbaite (Cavarretta and Puxeddu, 1990; Ruggieri andGianelli, 1995). Colopietro and Friberg’s (1987) geothermometer (based on the Mg/Fe partitioning coefficient between tourmaline and biotite) has yielded temperaturesin the range of 400–625 �C for four samples.

Metamorphic feldspars in pelitic contact metamorphic rocks at Larderello havebeen studied in detail in the ‘‘San Pompeo 2’’ well (Cavarretta et al., 1983). Feldspargeothermometry for two samples indicates a minimum temperature of approxi-mately 480–530 �C for a core sample at 2580 m depth, and 570–630 �C for a sampleat 2900 m depth. The co-existing K-feldspar and albite in a core sample of the samewells at 2389 m depth exhibit an intermediate structural state. These two mineralsalso coexist with oligoclase, indicating temperatures near 480 �C.

In some samples (e.g., well Sasso 22 at 1600 m depth), wollastonite is a widespreadcontact metamorphic mineral (Cavarretta et al., 1982), and its presence suggestshigh temperatures (>500 �C).

Additional temperature estimates are based on oxygen isotope data for HT–LPmineral assemblages. Temperatures as high as 640 �C for the quartz-muscovite andquartz-biotite pairs have been computed by Petrucci et al. (1994).

Important data on the fluids circulating in the Larderello geothermal system duringthe contact metamorphism have been provided by earlier fluid inclusion studies.Fluid inclusions from re-crystallized quartz lenses, quartz veins in samples displayingHT parageneses, and magmatic quartz in leucogranite, found in the deepest part ofthe Larderello geothermal field (>2500 m depth), have already been studied byValori et al. (1992), Cathelineau et al. (1994) and Ruggieri and Gianelli (1995).These studies document the occurrence of several generations of high-temperaturefluids present during the early hydrothermal circulation: (a) aqueous–carbonic(H2O+CO2�CH4�N2) vapours and liquids; (b) aqueous vapours, containing LiCl,with variable salinity; (c) aqueous brines (Li–Na–Cl-rich), often halite- over-saturated; (d) complex brines always oversaturated at room temperature in two ormore salts. These fluids are interpreted as recording the interaction, at temperaturesof 425–670 �C under lithostatic pressures of approximately 95–130 MPa, betweenLi-bearing fluids of magmatic origin and aqueous-carbonic fluids resulting from theheating of Paleozoic rocks (locally C-rich) during the contact metamorphism (Valoriet al., 1992; Cathelineau et al., 1994).


2.4. Hydrothermal metamorphism

Reservoir rocks at Larderello (i.e. the metamorphic and igneous units below thecap rocks represented by the Ligurian units, see Fig. 2a) exhibit basically two typesof late hydrothermal alteration (Cavarretta et al., 1982; Pandeli et al., 1994):

(1) propylitic-type alteration, characterised by veins of epidote, chlorite, quartz,calcite, K-feldspar, titanite, actinolite, anhydrite, albite and pyrite in variableproportions;

(2) sericitic-type, with K-mica, chlorite and quartz in different proportions.

At Larderello propylitic-type alterations occurred when a nearly neutral-pHsolution at partial pressures of CO2 from 0.2 to 8 MPa permeated the reservoir rocks(Cavarretta et al., 1982). The sericitic-type alteration is less common and has beenfound in cordierite-bearing contact metamorphic rocks and granite. The stability ofK-mica instead of K-feldspar suggests a pH below neutrality.

Fluid inclusion studies indicate that fluids with different compositions were pre-sent during the late hydrothermal alterations: (1) liquids (H2O+NaCl�CO2) withlow-to-moderate salinity, (2) relatively high-salinity waters (H2O+NaCl+CaCl2),(3) high-salinity solutions (H2O+NaCl) produced by local boiling with steam loss,(4) low-density vapours (H2O�CO2) derived from boiling, and (5) nearly pure H2Oresulting from condensation of vapours issued from boiling (Belkin et al., 1985;Valori et al., 1992; Gianelli et al., 1997b; Ruggieri et al., 1999; Ruggieri and Gianelli,1999). Late-hydrothermal fluids were trapped at temperatures varying from 150 to400 �C, under hydrostatic pressures (<35MPa), in late quartz, carbonates, K-feldspar,albite and anhydrite, and in early quartz as late secondary inclusions. These fluidsare interpreted as being meteoric-derived, and to have changed composition, salinityand temperature through water–rock interactions (in particular with evaporite layers),fluid boiling, mixing and cooling.

The final evolution of the hydrothermal system resulted in the development of thepresent-day vapour-dominated conditions. Present-day temperatures of about 200–400 �C are consistent with the stability of the hydrothermal minerals.

3. Petrographic description of the studied samples

In the present study more than 100 thin sections of thermometamorphic rocks coredin the Larderello field have been revised or analysed for the first time. The petro-graphic characteristics of the contact metamorphic mineral assemblages are describedfor the different lithotypes (phyllite, micaschist, gneiss, amphibolite and marbles)that were thermally metamorphosed.

Thermometamorphism on phyllites produced the re-crystallization of muscovite andthe crystallization of a post-tectonic biotite, mimetic on previous folds and crenula-tions, andalusite, tourmaline (Fig. 3a) and, in some cases, cordierite. Metagreywackesand albite-schists become plagioclase-bearing rocks. The schistose fabric of the


Fig. 3. Features of the contact metamorphic rocks at Larderello: (a) tourmaline (Tur), quartz (upper left)

and white K-mica (bottom right) assemblage in a thermometamorphic phyllite, Padule 2 well, 2782 m

depth; (b) post-tectonic, decussate texture of biotite (Bt) associated with tourmaline (Tur) and a muscovite

(Ms) mimetic growth on a pre-existing mica, Sperimentale Serrazzano well, 2243 m depth; (c) relict garnet

(Gr) and post-tectonic crystallization of biotite (Bt) and andalusite (And), Sperimentale Serrazzano well,

2432 m depth; (d) thermo-metamorphic amphibolite: co-existing hornblende (Hbl), biotite (Bt) and gru-

nerite (Gru), Lumiera 1bis well, 3110 m depth. The groundmass is quartz and plagioclase; (e) calc-silicate

rock made up of an assemblage of carbonates (calcite and dolomite, Cc in the microphotograph), olivine

(Ol), diopsidic pyroxene (Cpx) and phlogopite (Phl), Selva 4 well, 3374 m depth; (f) thermo-metamorphic

micaschist: (A) pre-tectonic texture with schistose planes (vertical in this picture), formed by quartz and

biotite mimetic on an existing mica; (B) biotite and tourmaline assemblage growing along a shear zone

cross-cutting the schistosity, Carboli 11 well, 3533 m depth.


rocks gives way to a granoblastic texture, sometimes with triple-point grain bound-aries, although not pervasively diffuse.

Thermally metamorphosed micaschists show a similar post-tectonic mineralassemblage (Fig. 3b and c). In these rocks a relict almandine-rich garnet is oftenpresent (Fig. 3c), and is clearly distinguished by less common post-tectonic spessartine-and grossular-rich garnet. In the pelitic hornfelses, K-feldspar, plagioclase andminor corundum can be present. Mg–Fe-chlorite is usually present as a relictmineral, transformed into biotite. However, some Mg–Fe-chlorite is texturally anewly-formed mineral after contact metamorphic biotite.

The thermometamorphic mineral assemblage in the gneissic rocks consists ofpost-tectonic biotite with decussate texture, poikiloblastic cordierite and andalusite,frequent crystallization of tourmaline, and re-crystallization of plagioclase andfeldspar.

Thermally metamorphosed amphibolites are characterised by a granoblastic texturein which the original oriented fabric has more or less disappeared. These rocksconsist of a post-tectonic grunerite crystallised after a green pargasitic hornblende,plagioclase (An10–85), biotite, quartz and Fe–Ti-oxide (Fig. 3d).

Rare marbles and calc-silicate contact metamorphic rocks have been found in afew geothermal wells. In particular, in the Selva 4 well (3370 m depth), the mineralassemblages consist of: (i) calcite, dolomite, diopside, forsterite, tremolite and phlogo-pite (Fig. 3e), and (ii) calcite, dolomite, anhydrite, phlogopite, plagioclase (oligo-clase-andesine) and diopside. The texture is granoblastic and triple-point boundariesbetween minerals indicate that crystallization occurred under static conditions.Antigorite after olivine is present in places.

In addition, HT mineral assemblages (quartz, tourmaline, biotite and plagioclase)were also found sporadically along hydrothermal veins cross-cutting micaschist andgneiss (Fig 3f).

4. Analytical methods

Metamorphic rock samples have been selected from 21 geothermal wells (Fig. 1).Electron microprobe (EMP) analyses of the minerals (biotite, cordierite, amphibole,feldspar, olivine and pyroxene) present in contact metamorphic rocks were performedwith a wave-dispersion Jeol-FXA-8600 machine at the University of Florence underthe following operating conditions: acceleration voltage of 15 kV, counting time 15 s,excitation current 10 nA, and execution of the correction program by Bence andAlbee (1968). The present study has also included EMP analyses of garnets in order tofurther clarify the equilibrium/disequilibrium conditions with the contact metamorphicassemblages. Selected representative EMP analyses and well names and depths ofcore samplings are reported in Table 2a–g. EDS (Energy Dispersive Spectrometer)analyses were also used to identify a number of mineral phases, in particular thesulphides and oxides.

New isotope analyses (�18O and �D of whole rocks and micas) were performed byGeochron Laboratories, Cambridge, Massachusetts (Table 3).


Table 2

a–g Representative electron microprobe analyses found in thermally metamorphosed rocks

2a: Garneta 2b: Cordieritea

Well depth(m)

Rad303050

MC2916

MC2916core

MC2916)

MC2916)

MC2916rim

SM12700

SM12700

Well depth(m)

Bad1B3654

MV53405

SiO2 48.46 49.02SiO2 36.44 36.90 37.37 36.91 37.34 37.28 37.34 37.26 TiO2 0.02 0.04TiO2 0.08 0.06 0.14 0.11 0.05 0.06 0.13 0.09 Al2O3 33.64 32.81Al2O3 21.66 20.85 20.90 20.53 21.30 21.14 21.01 20.91 FeOtot 7.72 9.54Cr2O3 0.00 0.00 0.00 0.05 0.00 0.00 0.00 0.00 MnO 0.38 0.43FeOtot 28.53 34.16 32.67 34.52 37.83 37.91 29.99 31.36 MgO 7.96 7.80MnO 0.37 1.66 1.92 1.53 0.78 0.59 4.75 4.18 CaO 0.08 0.00MgO 1.13 1.49 1.35 1.64 1.80 1.81 1.22 1.40 Na2O 0.37 0.35CaO 11.78 5.13 5.64 4.97 2.68 2.32 5.76 5.18 K2O 0.07 0.00Total 99.99 100.25 99.99 100.26 101.78 101.11 100.20 100.38 Total 98.70 99.99

apfu on the basis of 24 oxygens apfu based on 18 oxygensSi 5.84 5.97 6.02 5.97 5.96 5.98 6.01 6.00 Si 4.97 5.00AlIV 0.17 0.03 0.00 0.03 0.04 0.02 0.00 0.01 AlIV 1.03 1.00AlVI 3.92 3.84 3.97 3.89 3.97 3.98 3.99 3.96 Ti 0.00 0.00Fe+3 0.08 0.16 0.03 0.11 0.03 0.02 0.02 0.04 AlVI 3.03 2.95Cr+3 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 Fe+2 0.66 0.81Fe+2 3.75 4.56 4.37 4.58 5.02 5.07 4.02 4.18 Mn 0.03 0.04Mg 0.27 0.36 0.33 0.40 0.43 0.43 0.29 0.34 Mg 1.22 1.19Mn 0.05 0.23 0.26 0.21 0.11 0.08 0.64 0.57 Ca 0.01 0.00Ca 2.02 0.89 0.97 0.86 0.46 0.40 0.99 0.89 Na 0.07 0.07Ti 0.01 0.01 0.02 0.01 0.01 0.01 0.02 0.01 K 0.01 0.00

2c: Biotiteb 2d: Amphiboleb

Well depth(m)

Bruc-12443

Lum1b3110

Bad1b3654

Can4A2419

Selv43370

Pad22782

Carb113533

Well depth(m)

Cap2b3170

Lum1b3110 A

Lum1b3110 B

S223279

SiO2 36.76 36.05 35.24 34.84 39.28 35.39 36.72TiO2 1.98 3.27 3.67 1.28 0.77 1.58 1.10 SiO2 38.37 42.98 52.34 52.90Al2O3 16.35 15.62 19.68 21.82 16.25 19.41 17.92 TiO2 0.22 0.99 0.17 1.60FeOtot 19.05 23.40 18.53 19.05 4.61 20.90 17.71 Al2O3 11.57 13.12 0.71 0.00MnO 0.53 0.08 0.17 0.10 0.10 0.41 0.31 FeOtot 27.58 21.80 31.28 19.20MgO 12.61 9.08 8.45 9.12 21.42 8.34 11.62 MnO 0.23 0.25 0.56 0.39CaO 0.02 0.05 0.00 0.05 0.00 0.00 0.00 MgO 4.30 7.51 12.88 11.70Na2O 0.10 0.18 0.19 0.26 0.48 0.14 0.20 CaO 11.53 11.29 0.89 12.20K2O 9.69 9.31 9.34 9.10 9.79 8.65 9.53 Na2O 0.85 1.31 0.06 0.11BaO 0.14 0.45 0.26 0.32 nd 0.00 0.00 K2O 3.29 0.74 0.04 0.04Cl 0.17 0.09 nd 0.02 nd 0.11 0.31 Cl 3.68 nd nd ndF 0.62 nd 0.33 0.28 nd 0.35 1.60 F 0.00 nd nd ndTotal 98.02 97.58 95.86 96.24 92.70 95.42 97.01 Total 101.62 99.99 98.93 98.14

apfu based on 22 oxygens apfu based on 23 oxygensSi 5.50 5.50 5.34 5.25 5.72 5.76 5.55 Si 6.11 6.40 7.87 7.80AlIV 2.50 2.50 2.66 2.75 2.28 2.25 2.45 AlIV 1.89 1.61 0.13 0.20AlVI 0.39 0.31 0.85 1.12 0.51 1.26 0.75 AlVI 0.28 0.70 0.00 0.07Fe+2 2.39 2.99 2.35 2.40 0.56 2.68 2.24 Fe+3 0.63 0.14 0.00 0.12

(continued on next page)


Table 2 (continued)

Mn 0.07 0.01 0.02 0.01 0.01 0.05 0.04 Fe+2 3.05 2.58 3.93 2.24Mg 2.82 2.07 1.91 2.05 4.65 1.91 2.62 Mn 0.03 0.03 0.07 0.05Ti 0.22 0.38 0.42 0.15 0.08 0.18 0.13 Ti 0.03 0.11 0.02 0.00Ca 0.00 0.01 0.00 0.01 0.00 0.00 0.00 Ca 1.97 1.80 0.14 1.93Na 0.03 0.05 0.06 0.08 0.14 0.04 0.06 Mg 1.02 1.67 2.88 2.57K 1.85 1.81 1.80 1.75 1.82 1.69 1.84 Na 0.26 0.38 0.02 0.03Ba 0.01 0.03 0.02 0.02 nd 0.00 0.00 K 0.67 0.14 0.01 0.01Cl 0.04 0.02 nd 0.01 nd 0.03 0.08 Cl 1.00 nd nd ndF 0.29 nd 0.16 0.13 nd 0.17 0.77 F 0.00 nd nd ndOH 3.66 3.98 3.84 3.86 4.00 3.80 3.16 OH 1.01 2.00 2.00 2.00

2e: Plagioclasec 2f: Olivinec 2g: clinopyroxenec

Welldepth(m)

Cap2b3170

Cap2b2494

Lum1b3110

Bad1b3654

Bruc12443

Can4A3523

Dol43521

Sel43370

Welldepth(m)

Sel43174

Welldepth(m)

Sel43174

SiO2 46.92 67.73 53.80 61.46 45.43 55.95 68.57 61.21 SiO2 41.53 SiO2 53.60TiO2 0.00 0.01 0.03 0.03 0.00 0.02 0.00 0.00 TiO2 0.00 TiO2 0.12Al2O3 34.73 21.49 29.85 24.20 35.97 28.94 20.32 24.69 Al2O3 0.19 Al2O3 1.30FeOtot 0.41 0.03 0.28 0.00 0.00 0.10 0.10 0.00 FeOtot 6.21 FeOtot 4.06CaO 17.44 1.14 12.10 5.29 18.80 10.27 0.15 5.79 MnO 0.17 MnO 0.15Na2O 1.62 10.62 4.68 7.89 0.78 5.84 11.89 7.99 MgO 52.21 MgO 15.77K2O 0.03 0.04 0.08 0.36 0.00 0.18 0.08 0.00 CaO 0.02 CaO 24.85Total 101.15 101.06 100.82 99.23 100.98 101.30 101.11 99.68 Na2O 0.02 Na2O 0.29

K2O 0.00 K2O 0.00Total 100.35 Total 100.14

apfu based on 8 oxygens apfu based on4 oxygens

apfu based on6 oxygens

Si 2.13 2.93 2.42 2.74 2.07 2.49 2.97 2.72 Si 1.00 Si 1.97Ti 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 Al 0.01 AlIV 0.04Al 1.86 1.10 1.58 1.27 1.93 1.52 1.04 1.29 Ti 0.00 Ti 0.00Fe+3 0.02 0.01 0.01 0.00 0.00 0.00 0.00 0.00 Fe+2 0.13 AlVI 0.02Ca 0.85 0.05 0.58 0.25 0.92 0.49 0.01 0.28 Mn 0.00 Fe+2 0.13Na 0.14 0.89 0.41 0.68 0.07 0.50 1.00 0.69 Mg 1.87 Mn 0.01K 0.00 0.00 0.01 0.02 0.00 0.01 0.00 0.00 Ca 0.00 Mg 0.86

Na 0.00 Ca 0.98K 0.00 Na 0.02

K 0.00

a apfu: atoms per formula unit; Rad30: Radicondoli 30 well, garnet schist; MC: Montecerboli 1 well,micaschist; SM1: San Martino 1 well, micaschist; Bad1b: Badia 1b well, micaschist; MV5: Monteverdi 5well, granite

b apfu: atoms per formula unit; nd: not detected; Bruc1: Bruciano 1 well, phyllite; Bad1b: Badia 1b well,micaschist; Can4A: Canneto 4 well, biotite schist; Selv4: Selva 4 well, forsterite-diopside-phlogopite mar-ble; Pad2: Padule 2 well, biotite-tourmaline schist; Carb11: Carboli 11 well, biotite-tourmaline vein; Cap2b:Capannoli 2 well, amphibolite containing blue-green hornblende (K-pagasite); Lum1b: Lumiera 1b well,amphibolite containing green edenite (A) coexisting with colourless grunerite (B); S22: Sasso 22 well,amphibolite containing actinolite.

c apfu: atoms per formula unit; Cap2b: Capannoli 2 well, amphibolite; Cap2b: Capannoli 2b well, gar-net micaschist; Lum1b: Lumiera 1b well, amphibolite; Bad1b: Badia 1b well, micaschist; Can4A: Canneto 4well, micaschist; Dol4: Dolmi 4 well, albite phyllite; Sel4–3370: Selva 4 well, micaschist; Sel4–3174: Selva 4well, siliceous marble.


5. Analytical results

5.1. Mineral chemistry

New mineral chemistry data show that relict garnet is a solid solution betweenalmandine, pyrope, spessartine and grossular, with almandine as the main compo-nent (representative analyses in Table 2a). There is an antithetic relationshipbetween the almandine and the grossular mole fractions (Fig. 4).

In most of the analysed samples cordierite shows retrograde replacement by a veryfine-grained white K-mica. When unaltered (as in the sample from well Badia 1b at3654 m depth, Table 2b), the Mg/(Mg+Fe) ratio is approximately 0.64.

A detailed study on the composition of biotite, found in contact metamorphicrocks and an HT hydrothermal vein (well Carboli 11 at 3533 m depth), crossed inthe deepest part of the geothermal field, revealed highly variable F contents (Table 2cand Fig. 5). The Mg/(Mg+Fe) and F/(F+OH) ratios of these biotites are comparedwith those of the biotites in granites (Gianelli and Ruggieri, 2000). Biotite frompelitic contact metamorphic rocks shows Mg/(Mg+Fe) and F/(F+OH) values in

Fig. 4. Compositional variation (EMP analyses) of garnets from micaschist cored from three geothermal

wells at Larderello. Horizontal axis=molar fraction of almandine garnet; vertical axis=molar fraction of

grossular garnet. Legend: diamonds, well Radicondoli 30, 3050 m depth; triangles, well San Martino 1,

2700 m depth; squares, well Montecerboli 1, 2916 m depth.


the range 0.30–0.60 and 0–0.20, respectively (Fig. 5). In the hydrothermal vein ofwell Carboli 11 (3533 m depth) these values are 0.55–0.60 and 0.15–0.35, respec-tively.

The composition of the amphiboles from three amphibolites is reported inTable 2d and Fig. 6. In the sample from well Lumiera 1b (3110 m depth), an Na–Kgreen amphibole (edenite, according to the classification of the IMA protocol; seeLeake et al., 1997) co-exists with a colourless Na–K-poor amphibole (grunerite).The hornblende with intermediate composition is the pre-existing amphibole re-equilibrated under thermal metamorphic conditions, and coexisting with a newlyformed Na–K-poor phase. The sample from well Capannoli 2b (3170 m depth) is anamphibolite with only the re-crystallised pargasite present. The sample from wellSasso 22 (3279 m depth) is similar to the Lumiera sample, but the Na–K-pooramphibole is actinolite.

The plagioclase composition of micaschist and metagreywacke passes from albiticto intermediate or even calcic plagioclase (Table 2e), whereas the plagioclasecomposition of the gneisses remains more or less the same.

Fig. 5. F/(F+OH) vs. Mg/(Mg+Fe) diagram for biotites present in granites, contact metamorphic rocks

and a hydrothermal vein cored in the Larderello geothermal field. Legend: diamonds=granites; open

circles=contact metamorphic rocks; asterisks=hydrothermal vein.


The chemical compositions of the olivine and pyroxene from the calc-silicate rockof well Selva 4 correspond to almost pure forsterite and diopside-hedenbergite-johannsenite solid solution, respectively (Table 2f and g).

5.2. Stable isotopes

Fig. 7 and Table 3 show new stable isotope analyses (�D and �18O) for granite(three whole rock and two mineral samples), rocks of the thermo-metamorphicaureole (six whole rock samples), and one micaschist, as a reference term, from thePontremoli 1 well (Northern Apennines), which does not show evidence of post-Alpine contact metamorphism. Pontremoli 1 well is, in fact, approximately 130 kmNW far from the geothermal area, and shows only Alpine and pre-Alpine mineralassemblages and structures.

Fig. 6. Amphibole compositions in three geothermal wells in an AlIV vs. Na+K diagram. Values are

atoms per formula unit. Open circles are theoretical end members, full symbols (diamonds) are EMP

analyses. See text for explanation.


6. Discussion

6.1. Physical-chemical conditions of the contact metamorphism

A combined review of the old and new data has provided us with a better defini-tion of the P–T–X conditions during the contact metamorphism. Geological data,rock chemistry, fluid-inclusion and stable isotope data will all be used in an inte-grated interpretation.

6.1.1. GeothermometryBiotite is the most common mineral that developed during contact metamorphism

at Larderello. A minimum temperature of 400 �C can be assumed for a contactmetamorphic event, on the basis of the first appearance of biotite in the presence ofquartz, K-feldspar, chlorite and muscovite. The attainment of the cordierite iso-grade, with the disappearance of chlorite and muscovite and the crystallisation ofbiotite in metapelites, can occur at temperatures of about 500 �C (Bucher and Frey,1994).

These temperatures are in agreement with the temperatures estimated at 100 MPafrom the P–T equilibrium lines (Fig. 8) computed for the KMASH and KFASHsystems using the PeRpLeX code from Connolly (1995a,b) and thermodynamic

Fig. 7. �D vs. �18O diagram for granite, metamorphic rocks and one micaschist (reference micaschist)

sampled in the Pontremoli 1 well (Northern Apennines) far from the geothermal area. The isotopic com-

position fields of phyllite and micaschist reported by Petrucci et al. (1993) are also shown.


Table 3

�18O and �D values of different rocks and minerals from the Larderello geothermal wells, and computed values for the H2O in equilibrium with the rock at

500–650 �C

Well depth (m) Lithology �18O �D �18O-500 �D-500 �18O-600 �D-600 �18O-650 �D-650

Pontremoli-3529 Micaschist (oil well, Northern Apennines) Whole rock 13 �42 – – – – – –

Sperimentale Serrazzano-2076 Thermo-metamorphic micaschist

(Bt, And, Crd)

Whole rock 7.5 �72 6.01 �30 6.78 �38.1 7.1 �41.2

Sperimentale Serrazzano-2158 Thermo-metamorphic micaschist (Bt) Whole rock 5.45 �71 3.96 �29 4.73 �37.1 5.05 �40.2

San Pompeo 2–2900 Thermo-metamorphic phyllite (Bt, Cor) Whole rock 10.7 �71 9.21 �29 9.98 �37.1 10.3 �40.2

Sperimentale Serrazzano-2242 Thermo-metamorphic micaschist

(Bt, And, Crd)

Whole rock 8.25 �73 6.76 �31 7.53 �39.0 7.85 �42.2

Sasso 22–3529 Gneiss Whole rock 12.6 �70 11.11 �28 11.88 �36.1 12.1 �39.2

VC11–3191 Gneiss Whole rock 12.6 �69 11.11 �27 11.88 �35.1 12.1 �38.2

Monteverdi 7–3486 Granite Whole rock 6.4 �77 4.91 �26.4 5.68 �34.5 6 �37.6

Radicondoli 26B-4600 Granite Whole rock 11.5 �78 10.01 �27.4 10.78 �35.5 11.1 �38.6

Radicondoli 29 Granite Biotite 8.35 �81 10.76 �30.4 10.9 �38.5 11 �41.6

CarboliCbis-4200 Granite Whole rock 13.1 �77 11.61 �26.4 12.38 �34.5 12.6 �37.6

Monteverdi 5A Granite Mica 7.3 – – – – – – –

Bt: biotite; And: andalusite; Crd: cordierite; �18O-500, �18O-600, �18O-650, �D-500, �D-600, �D-650: computed �18O and �D of the fluid in equilibrium with the

rock at 500, 600 and 650 �C.

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values from Holland and Powell’s data bank (1990, updated to 1998) (Holland andPowell, 1990, 1998).

The occurrence of grunerite in the amphibolite, together with the labradoriticcomposition of plagioclase, indicates the attainment of the amphibolite zone (Seki etal., 1969) and temperatures possibly over 600 �C. Similar conclusions can be drawnfor the sample from well Capannoli 2b (3170 m depth) of amphibolite, in particular,on the basis of the high AlIV content of the green hornblende.

6.1.2. Fluid chemistryThe composition of four fluids related to the contact metamorphic processes

(Table 4) has been determined from Raman analyses, microthermometric data andthe volumetric fraction of the vapour phase of fluid inclusions, after Cathelineau etal. (1994) and unpublished data.

Fig. 8. Equilibria for reactions in the systems KFASH (1 and 3) and KMASH (2 and 4); the reactions are:

3chlorite+8K-feldspar=5biotite+3muscovite+9quartz+4H2O (1,2) and chlorite+muscovite+2quartz

=cordierite+biotite+4H2O (3,4).


Table 4

Reconstructed compositions of contact metamorphic fluids from fluid inclusions and theoretical fluid compositions computed assuming equilibrium with gra-

phite and different mineral equilibria

Computed COHNS fluid composition of fluid inclusions

Well depth (meters) n XH2O XCO2XCH4

XH2S XN2XNaCl

m s m s m s m s m s m s

VC11–2946 3 0.448 0.094 0.51 0.03 0.023 0.003 nd 0.018 0.008 0 0

MV7–3485 7 0.467 0.047 0.498 0.004 0.012 0.013 0.002 0.004 0.011 0.014 0.01 0.016

S22–4027 6 0.838 0.055 0.091 0.036 0.058 0.015 <0.001 0.012 0.006 0

SP2–2900 2 0.782 0.008 0.154 0.003 0.062 0.007 <0.001 0.002 0.002 0

Theoretical COH fluid composition at 100 MPa, assuming equilibrium with graphite

T(�C) XH2O XCO2XCH4

XCO XH2

VC11–2946 425 0.443 0.551 0.005 0.0002 4E-04 1

MV7–3485 490 0.474 0.512 0.012 0.0007 0.0012 2

MV7–3485 490 0.419 0.57 0.009 0.0008 0.0011 3

S22–4027 630 0.614 0.185 0.18 0.0039 0.0167 4

SP2–2900 670 0.581 0.198 0.19 0.007 0.024 5

(1) Bt, Pl-schist.Reaction defining fH2O: 2.91Fls(Ab10) + Clin=3 Q + 0.246 Ab + 1.67 Phl + Mica (Ms95) +1.33H2O.

(2) Granite. Reaction defining fH2O: 2.5 Q + 0.33Phl+ Ms=1.33 Kfs + 0.5 Crd + H2O.

(3) Granite. Reaction defining fH2O: 5.5 Q + 4 And +Clin=2.50 Crd + 4 H2O.

(4) Gneiss. Reaction defining fH2O: maximum H2O for a graphite COH system.

(5) Graphite-bearing Bt-micaschist. Reaction defining fH2O: maximum H2O for a graphite COH system.

m: Mean; s: standard deviation; n: number of samples; MV7: Monteverdi 7 well; S22: Sasso 22 well; SP2: San Pompeo 2 well; Ab: albite, NaAlSi3O8; And:

andalusite, Al2SiO5; Clin: clinochlore, Mg5Al2Si3O10(OH)8; Crd: cordierite, Mg2Al4Si5O18; Cor: corundum, Al2O3; Fls(Ab10): K0.9Na0.1AlSi3O8; Ms: musco-

vite, KAl3Si3O10(OH)2; Mica(Ms95): K0.95Na0.05Al3Si3O10(OH)2; Phl: phlogopite, KMg3AlSi3O10(OH)2; Q: quartz, SiO2.

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For the metapelitic rocks of Larderello, the redox conditions corresponding tographite–oxygen equilibrium were proposed by Cathelineau et al. (1994) on the basisof the abundance of graphite-bearing phyllite and micaschist in the Larderellometamorphic complexes. Other possible sources of CO2 are mantle degassing,thermo-metamorphic de-carbonation and carbonate hydrolysis (Gianelli, 1985;Magro and Ruggieri, 1999). The theoretical composition of the contact meta-morphic fluids present at the site of the fluid inclusion samples was computed onthe basis of suitable dehydration reactions in the KMASHOC system in the pre-sence of a graphite-buffered COH fluid (Table 4). The calculations were performedfollowing the procedure proposed by Connolly and Cesare (1993) and Connolly(1995a), using their MRK equation-of-state for COHS fluids buffered by graphiteat the temperature estimated on the basis of fluid inclusion studies (Cathelineau etal., 1994).

For two samples (Monteverdi 7, 3485 m depth, granite; VC11, 2946 m depth,micaschist with biotite and cordierite), the theoretical composition is comparable tothat of the fluid trapped in fluid inclusion, assuming that the fluid is controlled bymetamorphic reactions producing biotite or cordierite in the CO2+H2O-dominatedpart of the ternary C–O–H system (see Connolly, 1995a,b). For these samples, wesuggest that the carbonic fluids originated by contact metamorphism of graphite-bearing micaschist and phyllite, via the reactions reported in Table 4, are consistentwith the mineral assemblages present in our samples.

For two samples (Sasso 22, 4027 m depth, gneiss; San Pompeo 2, 2900 m depth,graphite-bearing micaschist), the computed XH2O values of the fluid inclusions aregreater than the maximum admissible water content for stable graphite at tempera-tures of 630 and 670 �C (e.g., Ohmoto and Kerrick, 1977; Labotka, 1991). Anexternal source for CO2 could be hypothesised for these fluids, as was proposed forother contact metamorphic fluids (see Labotka, 1991, and references therein).

A more complex system than a simple COH system could also lead to somewhatdifferent theoretical results. Calculations have not been made for a COHNS system,as evidence of buffers for the nitrogen and sulphur species is lacking, so only a fewremarks can be made on this point:

(1) Moine et al. (1994) computed the XH2O isopleths in a ternary N2–CH4–CO2

diagram at 600 �C and 200 MPa for a fluid in equilibrium with graphite andan NH+

4 -bearing mica. For a theoretical fluid with the same relativeproportions of these species as found in sample Sasso 22, 4027 m depth,(Table 4), the XH2O is between 0.8 and 0.85, in good agreement with theestimate made on the basis of the computed fluid inclusion composition. We,therefore, cannot exclude that the nitrogen may be buffered by a mineralphase.

(2) The pair pyrite–pyrhotite could also act as a sulphur buffer. However, theseminerals co-exist in the contact metamorphic rocks in only a few samples. Atthe pyrite–pyrhotite equilibrium, the fH2

=fH2S ratio should range fromapproximately 0.17 to 0.02 at 450 and 650 �C, respectively, for pressures of100 MPa.


Some information on the composition of the fluid present during the formation ofthe calc-silicate rocks can be obtained from mineral equilibria. The mineral assem-blages found in the calc-silicate rock of Selva 4 well at 3177 m depth, with calciteand dolomite co-existing with diopside, forsterite and phlogopite (with sporadictremolite), indicate intermediate-to-high XCO2

(near the pseudo-invariant: point A inFig. 9, corresponding to an XCO2

around 0.50 and a temperature of 545 �C at a

Fig. 9. Stability fields of the mineral assemblages and T-XCO2equilibrium conditions at 100 MPa for calc-

silicate rocks at Larderello. A: pseudo-invariant point describing the condition of Selva 4 well (3177 m

depth), with calcite and dolomite coexisting with diopside, forsterite, phlogopite and sporadic tremolite.

The diagram assumes saturation with calcite, dolomite and H2O. Abbreviations: Atg=antigorite,

Cc=calcite, Di=diopside, Dol=dolomite, Fo=forsterite, Kfs=potassium feldspar, Phl=phlogopite,

Q=quartz, Tc=talc, Tr=tremolite. The reactions are: (1) Atg+20Dol=34Fo+20Cc+31H2O+20CO2,

(2) 17Tr+107Dol+107H2O=4Atg+141Cc+73CO2, (3) 8.5Tc+22.5Dol+22.5H2O=Atg+22.5Cc+

22.5CO2, (4) 2Tc+3Cc=Tr+Dol+H2O +CO2, (5) Tr+11Dol=8FO +13Cc+H2O +9CO2, (6)

3Dol+4Q+H2O=Tc+3Cc+3CO2, (7) 8Q+ 5Dol+H2O=Tr+3Cc+3CO2, (8) Tr+3Cc=4Di+Dol+

H2O+CO2, (9) Di+Dol=Fo +Cc+CO2, (10) Kfs+3Dol +H2O=Phl+3Cc+CO2, (11) 2Q+Dol=

Di+2CO2.


pressure of 100 MPa). The presence of NaCl as a component of the metamorphicfluid would modify the stability of the mineral assemblages, particularly if the P–T–X conditions coincide with the immiscibility field of the CO2–H2O–NaCl fluids(Bowers and Helgeson, 1983a,b). However, for pressures of approximately 100 MPaand fluids with a NaCl wt.% less than 6.0, the XCO2

temperature estimates for theSelva 4 mineral assemblages do not change significantly (compare Fig. 9, this paper,with Figs. 9 and 10 in Bowers and Helgeson, 1983a, and Fig. 10 in Bowers andHelgeson, 1983b). The partial re-crystallization of forsterite into antigorite indicatesa late metamorphism in the presence of CO2-poor fluids.

Other carbonate layers are made up of mineral assemblages that include wollas-tonite, a mineral that is stable under very low XCO2

(Greenwood, 1967), so we mustconclude that the contact metamorphism of carbonates occurred under differentCO2 concentrations. The retrograde metamorphic re-crystallisation of the lime-stones and dolostones found in the Larderello contact aureole are also characterisedby minerals that are stable only under very low XCO2

conditions, such as prehnite,epidote (Liou et al., 1982) and serpentine minerals.

Boron and fluorine probably occurred in significant amounts in the fluids presentduring contact metamorphism. The widespread crystallization of tourmaline and thepresence of some biotite with relatively high F content, in the contact metamorphicaureole, testifies to B- and F-metasomatism at Larderello. The F/(F+OH) values ofthe biotite in these rocks range from 0.0 to about 0.2. The lowest values are found inbiotite of gneissic rocks unaffected by a significant contact metamorphism. Thehighest values are related to a local F concentration, as suggested by the very high Fcontent (F/F+OH up to 0.57) in biotites occurring in a hydrothermal vein in theCarboli 11 well at 3533 m depth.

Fluorine is of magmatic origin, as shown by the F content of 0.06 to 0.2 wt.% inthe granites found in the geothermal wells and by the relatively high fluorine contentin the magma (at least 1 wt.%) estimated by Gianelli et al. (2001) from magmaticbiotite compositions using the Icenhower and London (1997) method. The inter-pretation is that F-rich fluid of magmatic origin circulated in the hydrothermal vein,while the contact metamorphic rocks probably underwent moderate-to-minor or noF-metasomatism.

One method for estimating the fHF in the contact metamorphic fluid is based onthe biotite composition, according to the equation proposed by Munoz and Swen-son (1981) and Munoz (1984):

2100=Tþ 1:523 XPhl þ 0:416 XAnn þ 0:2 XSid ¼ log fH2O=fHF

� �þ log XF=XOHð Þ

where T is the temperature in Kelvin, XPhl, XAnn and XSid are the mole fractions ofthe biotite components annite, phlogopite and siderophyllite, respectively, fH2O/fHF

is the ratio between the fugacities of water and HF, and XF/XOH is the ratio betweenF and OH (atoms per formula units) in biotite. The calculation assumes a totalpressure of 100 MPa and temperatures of 415 �C (fH2O=28.5 MPa) and 475 �C(fH2O=41.5 MPa), estimated for the contact metamorphic rocks of wells Padule 2 at


2782 m depth and Carboli 11 at 3533 m depth. The fHF values obtained are 0.5 and 6kPa, respectively.

6.1.3. General P–T evolutionsThe P–T conditions for contact and late-hydrothermal metamorphism at Larder-

ello estimated from previous fluid inclusion data are reported in Fig. 10. Two fieldscan be distinguished:

(1) The high-pressure field (P=95–130 MPa, T=425–670 �C) corresponds to theearly phase and is characterised by Li-bearing fluids of magmatic origin and aqu-eous-carbonic fluid that developed during contact metamorphism. The high-pres-sure field is compatible with the P–T conditions for the stability of cordierite+biotiteand corundum in metapelitic hornfelses, and for the estimated temperature values forthe calc-silicate rock of the well Selva 4. The field is intersected by the solidus curve forgranitoids enriched in volatile elements such as F, B and Li. The granite solidus shiftcaused by the presence of these elements has been estimated, for the most evolvedLarderello granitoids, by Gianelli et al. (1997a) on the basis of existing experimentaldata (Pichavant and Manning, 1984; London, 1995) and of the composition of theLarderello granites, which is assumed to be comparable with volatile-rich pegmatiticmagmas.

(2) The low-pressure field shown in Fig. 10 corresponds to the late hydrothermalfluids responsible for the formation of both the propylitic and sericitic alterations,and for the replacement of high-temperature minerals with low-temperature ones.These fluids were characterized by mixing and boiling processes. They show variablesalinities, and gas contents (largely CO2, with minor CH4) that are higher than in thepresent-day geothermal fluid (possibly because of the accumulation of gas in theupper part of the reservoir). Late hydrothermal fluid circulated for the most partunder hydrostatic pressure and at temperatures (<400 �C) that are lower than thoseof the early magmatic and contact-metamorphic derived fluids. This is consistentwith the replacement of high-temperature minerals with low-temperature ones.

6.1.4. Rheologic conditionsFournier (1991) argues that in active geothermal fields the potential reservoir

rocks should reach quasi-plastic behaviour when deformed at 450 �C. This seemseven more plausible for the early stages of geothermal activity, when granite intru-sions were emplaced at very shallow depths. On the other hand, brittle deformation,enhanced by processes of fluid overpressures and low strain rates, is supported byevidence of vein minerals that formed at 500 �C (Gianelli, 1994).

After granite emplacement, a general monotonic cooling of the system is indicatedby late sericitic and propylitic alterations and fluid-inclusion data. Hydrothermalveins indicate brittle conditions at depths of up to 4 km during this stage of coolingand decompression of the system. This part of the system is considered a potentialdeep reservoir.

Below this depth, temperatures over 400–450 �C measured in some wells atLarderello suggest that brittle–ductile transition should be present. Hydraulic


Fig. 10. P–T conditions present during different stages in the geothermal system of Larderello. The fields

outlined by dashed lines were determined from fluid inclusion data (Belkin et al., 1985; Valori et al., 1992;

Cathelineau et al., 1994; Ruggieri and Gianelli, 1995; Gianelli et al., 1997b; Ruggieri et al., 1999) and new

data presented here. The high-temperature, high-pressure conditions developed during an early stage

characterised by magmatic and contact metamorphic fluids. Cordierite, biotite and corundum within

rocks of pelitic compositions are mineral markers of this water-rock interaction stage. The P–T conditions

assumed for the contact metamorphism in the carbonate rocks of Selva 4 well are also reported. Lower

temperatures and pressures characterise the hydrothermal circulation of the metamorphic fluids of

prevalently meteoric origin. Present-day conditions are steam-dominated.


fracturing, however, can sporadically occur in response to violent explosions causedby the expansion of deep fluids. Magmatic-derived and/or contact metamorphicfluids could be still present in the deepest part of the geothermal field, as indicatedby geophysical data, and in particular by the presence of a major reflecting horizonof the bright-spot type at 4–6 km depth, and by the strongly conductive crustalanomaly below the geothermal area (Batini et al., 1983; Manzella et al., 1998). Theoverpressure may derive from the tremendous volume increase of a fluid phase whenmagma crystallizes (Burnham, 1979).

6.2. Water-rock interaction: stable isotope data

Studies conducted by Petrucci et al. (1993, 1994) indicate that:

(1) The computed �18O values of the fluid in equilibrium with contact meta-morphic minerals (quartz, biotite, muscovite and amphibole) range from 5.3 to13.4%. The highest values indicate re-crystallisation in a closed system, whereasthe lowest values indicate that biotite and amphibole reacted with a meteoric fluidduring a hydrothermal stage after the contact metamorphism.

(2) The stable isotope values of rocks (phyllite and micaschist, with evidence ofhydrothermal alteration and relics of contact metamorphism in places) found in twogeothermal wells at Larderello range from 2.9 to 10.7% for �18O and from �76 to�64% for �D, indicating depletion in the heavy isotopes. The computed isotopiccomposition of the fluid in equilibrium with quartz and chlorite in the temperaturerange 260–366 �C (corresponding to the trapping temperatures of fluid inclusionsfound in the samples) ranges from 3.4 to 14.6% and �42 to �31% for �18O and �D,respectively.

Petrucci et al. (1993) interpreted these findings as indicating that the isotopiccomposition of the geothermal fluid is a mixture of magmatic and meteoric waters.Their model is also confirmed by D’Amore and Bolognesi (1994), who argued thatthe stable isotope values of a few deep wells do not support a simple interactionbetween rocks and meteoric waters, but rather a mixing between meteoric and localmagmatic waters, whose �18O and �D values are assumed to be in the ranges 11 to15% and �35 to �15%, respectively. D’Amore and Bolognesi (1994) explain thesevalues by allowing for degassing of the acidic intrusions and isotopic fractionationbetween the magma and separated fluid.

It seems noteworthy to recall here that the micaschists are considered to be possiblesources of the anatectic Tuscan granites and vulcanites (Van Bergen, 1983).

The new �18O and �D values of the thermo-metamorphic rocks and granites indi-cate that these rocks underwent depletion in the heavy isotopes. The highest �18Ovalues of the granites are consistent with the values reported by Taylor and Turi(1976) for the acidic rock of the Tuscan Magmatic Province.

The fluid present during the granite emplacement and contact metamorphismshould have the computed isotopic values reported in Table 3. The �18O values of theH2O in equilibrium with the rock at 500–650 �C were computed using the fractionation


equation for quartz, feldspar and anorthite and considering the real normativecompositions for the granites, and modal composition for the metamorphic rocks.The following equation was used for �D calculations:�(Bt�water)=�22.4(106/T2)+28.2+C, with C=�32.7 for metamorphic rocks

and �41.3 for granite; T, temperature in Kelvin. These C values correspond to bio-tites with mole fraction ratios Al:Mg:Fe equal to 0.12:0.43:0.46 and 0.18:0.22:0.60,respectively, and correspond to the most common values found in the analysedrocks.

The temperature values of 500–650 �C correspond to those of the contact meta-morphic processes at Larderello, as well as to the solidus of the most differentiatedacidic rocks found in the wells (Gianelli et al., 1997a). The fluids in isotopic equili-brium at 500–650 �C with rocks consisting of quartz, K-feldspar, plagioclase andbiotite have compositions compatible with magmatic fluids. Furthermore, assumingan initial magma �D near the value of the original melted rock (i.e., approximately�42%), a fractionation of the order of 30–40% must also be hypothesised to explainthe values of the four granites analysed (Table 3). This fractionation occurred duringmagma degassing, according to the model proposed by Taylor (1992) and others.The same process can explain the difference between the isotope values of the referencemicaschist and similar rocks, which underwent thermal metamorphism at Larderello.In this case, the fractionation should be linked to processes of metamorphic dehy-dration (Valley, 1986).

The granite intrusions and the contact metamorphism have radiometric agesranging from 3.8 to 1.0 Ma. There is some doubt, however, as to whethermagmatic fluids, or fluid derived fromHT dehydration of metapelites, can circulate inthe system for such a long period of time. If we do not admit this possibility, and weassume that the contact metamorphic aureole and crystallized igneous rocks reactwith meteoric water under low water/rock ratios at temperatures of 250–400 �C,then we obtain a result analogous to that of Petrucci et al. (1993). However, fluid-inclusion and mineralogical data clearly indicate the occurrence of a magmaticphase, at least during the early stages of the hydrothermal system.

6.3. Garnet incompatibility with the contact metamorphic conditions

The relic texture of garnet is clearly present in a number of samples (Fig. 3c). In afew samples garnet is in equilibrium with a biotite whose textural features indicate apre-Alpine age. The garnet-biotite geothermometer of Ferry and Spear (1978) hasbeen applied, despite the quite high AlVI content of the mica, which renders theresulting temperature estimates of 450–500 �C less reliable. The relict texture of thegarnet and its composition indicate that this mineral was formed during a regionalpre-Alpine metamorphic event, likely under transitional low-to-medium grade con-ditions. The incompatibility of an almandine-rich garnet with the P–T conditionsestimated for the contact metamorphic aureole (approximately 100 MPa, see below)is also supported by the AFM projections in Fig. 11a and b. Garnet could only bepresent in the micaschists and gneisses of the chemical composition found in thegeothermal area for pressures of at least 400 MPa.


Fig. 11. Al2SiO5-annite-phlogopite projections at the P–T conditions of contact (a) and regional meta-

morphism (b), and compositional field of the garnet-bearing units of the contact aureole present at Lar-

derello (17 micaschists and 8 gneiss, after Gianelli, 1998). Q=quartz, Mu=muscovite, Sill=sillimanite,

And=andalusite, Ann=annite, Phl=phlogopite, Crd=Mg-cordierite, Sud=sudoite, Fstd=ferro-staur-

olite, Alm=almandine, Clin=Mg-chlorite, Mcss=Fe-Mg-cordierites, Py10=garnet solid solution with

10% pyrope, Cl42 to Cl85=chlorite solid solutions with 42 to 85 mol% of Mg-chlorite, Ph20, Ph36, biotites

with 20 and 36 mol% phlogopite, Cr29=cordierite with 29 mol% of Mg-cordierite.


7. Conclusions

The deep-seated geothermal reservoir (DSGR) at Larderello consists of bothgranite and a related contact metamorphic aureole, whose characteristics have beendescribed in this paper. The petrographic, geochemical and fluid inclusion dataindicate that the thermally metamorphosed rocks underwent a recrystallisation attemperatures over 425 �C and up to approximately 670 �C, under a lithostatic pres-sure regime. Magma-derived Li–Na-rich fluids and contact metamorphic aqueous-carbonic fluids were present during this high-temperature event. These fluids wereresponsible for B and F metasomatism. Fluid inclusion data and computed theore-tical compositions of the contact metamorphic fluids indicate the occurrence of bothfluids, with CO2 as the major component (XCO2

=0.50–0.55), and of an aqueous-richfluid, in which the amounts of CO2 (XCO2

=0.06–0.20) and CH4 are comparable. Anexternal source can be hypothesised for the carbonic component of the latter type offluid trapped in fluid inclusions. On the contrary, the carbonic-rich fluid has acomposition consistent with an equilibrium with graphite-metapelites. No evidencehas been found for an external origin of other gases, such as methane, which mayoriginate in the contact metamorphic aureole through gas-rock equilibrium reac-tions.

The fluids present during the granite emplacement and contact metamorphismhave isotopic compositions compatible with magmatic fluids. Isotopic data indicatethat the granite and thermo-metamorphic rocks are depleted in �18O and �D withrespect to their original composition, which is represented by the Pontremoli 1reference micaschist. Magma degassing processes and metamorphic dehydrationcould explain the isotopic fractionation between the granite and fluids, and betweenthe thermally metamorphosed micaschist and fluids, respectively. Late-hydro-thermal fluid of meteoric origin may also have contributed to the depletion of heavyisotopes from the rocks.

Under the P–T conditions of contact metamorphism the mechanical behaviour ofthe rock was plastic, and the permeability extremely low at depths of 2.5–4 km.After this stage, a generally monotonic cooling of the system is indicated by the latesericitic and propylitic alterations and fluid inclusion data. However, at depths of4.5–5 km the present-day temperatures at Larderello are still in the 400–450 �Crange. This high-temperature zone probably corresponds to a ‘‘transition zone’’ inwhich the rocks pass from plastic to brittle mechanical behaviour when sporadic,violent explosions related to deep-seated magmatic-derived or contact metamorphicfluid occur.

The deepest part of the Larderello geothermal field can thus be considered aDSGR, and consequently a target for further exploration and exploitation.

Acknowledgements

The authors would like to thank Enel Green Power Company for providing sam-ples and data on the Larderello geothermal field. The work was carried out as part of a


Special Project of the Italian National Research Council. This research formed part ofthe activity of the international working group on Deep Geothermal Resources co-ordinated by the International Energy Agency. A part of these results was presentedat the World Geothermal Congress held in Japan in 2000. Thanks are also extendedto Dr. Keiji Kimbara for his efforts in coordinating the IEA task group.

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