Multi-stage fluid circulation in a hydraulic fracture breccia of the Larderello geothermal field (Italy)

Journal of Volcanology and Geothermal Research 90 Ž1999. 241–261 www.elsevier.comrlocatervolgeores

Multi-stage fluid circulation in a hydraulic fracture breccia of the Larderello geothermal field žItaly / Giovanni Ruggieri ) , Giovanni Gianelli Istituto Internazionale per le Ricerche Geotermiche CNR, Piazza Solferino 2, I-56126, Pisa, Italy Accepted 18 March 1999

Abstract The deep well MV5A, drilled in the western part of the Larderello geothermal field, crossed a 20-cm-thick hydraulic fracture breccia unit at a depth of 1090 m below ground level Žb.g.l... This breccia occurs in a fine-grained Triassic metasandstone and consists of angular to subangular clasts of up to some centimeters in size. Pervasive alteration has affected the breccia clasts and wall rock around the breccia, with the formation of Mg–Fe chlorite. After such alteration, hydrothermal circulation caused the precipitation of two generations of calcite cement. Then, ankerite partially replaced these two calcite generations. Ankerite also precipitated in late veinlets with chlorite. Late hydrothermal activity led to the crystallization of albite, quartz and finally, anhydrite. The calcite contains vapor-rich inclusions and two populations of liquid-rich ŽL1 and L2. inclusions. L1 inclusions are characterized by homogenization temperatures between 304 and 3618C and salinities from 7.4 to 11.6 wt.% NaCl equivalent; L2 inclusions revealed homogenization temperatures in the range of 189–2458C and salinities from 2.6 to 6.3 wt.% NaCl equivalent. The fluids contained in L2 inclusions were probably trapped coevally with some vapor-rich inclusions under boiling conditions after the L1 inclusions formed. Some of the abundant vapor-rich inclusions in calcite may also represent early, low-temperature inclusions affected by decrepitation andror stretching andror leaking during L1 trapping. The liquid-rich ŽL. inclusions trapped at later stages in ankerite, albite and anhydrite display, respectively, homogenization temperature ranges of 189–1988C, 132–1458C, and 139–1718C, and salinities ranging from 1.6 to 1.7 wt.% NaCl equivalent, 1.4 to 2.1 wt.% NaCl equivalent and 3.7 to 6.2 wt.% NaCl equivalent. The inclusions studied record the evolution, over time, of the fluids flowing in the breccia level: L1 inclusions capture high-temperature fluid Žabout 300 to 3508C. of high salinity Žaround 10 wt.% NaCl equivalent. at above-hydrostatic pressures Žup to about 150 bar.. The L2 inclusions in calcite and liquid-rich inclusions in ankerite and albite represent subsequent hydrothermal fluid evolution toward lower temperatures Žabout 250 to 1308C., pressures Ž45 to a few bar. and salinities Ž6.3 to 1.4 wt.% NaCl equivalent.. During this stage, boiling processes and infiltration of meteoric waters probably occurred. Finally, moderately saline fluids Žaround 5 wt.% NaCl equivalent. at a temperature Žabout 1608C. close to that of present-day in-hole measurements was trapped in the anhydrite inclusions. The liquids trapped in liquid-rich inclusions circulated at 41,000 years Žmaximum age of calcite. or later. This age represents an upper limit for the development of vapor-dominated condition, in this part of the geothermal system. The fluids circulating at the breccia level were probably meteoric andror connate waters. These fluids may have interacted with the anhydrite and carbonate bearing formations present in the Larderello area. The occurrence of the hot and saline fluids, trapped in L1 inclusions at above-hydrostatic

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0377-0273r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 7 - 0 2 7 3 Ž 9 9 . 0 0 0 3 0 - X

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pressure, suggests that similar fluids but with higher pressure ŽG 167 bar. and temperature ŽG 3608C. may have been responsible for rock fracturing. q 1999 Elsevier Science B.V. All rights reserved. Keywords: fluid inclusions; hydraulic fracturing; hydrothermal minerals; geothermal fields; Larderello, Italy

1. Introduction A hydraulic fracture breccia was recently found in well MV5A Ždrilled by the Italian Electric Authority, ENEL., located in the western part of the Larderello geothermal field ŽFig. 1.. The well MV5A is directional but the deviation from the vertical of the well started below the breccia unit. The breccia occurs at 1090 m below ground level Žb.g.l.. and is made up of angular to subangular clasts exhibiting a typical ‘jigsaw-puzzle’ texture. The in-hole temperature at 1090 m b.g.l. is about 1608C; no loss of circulation was recorded at this depth, whereas super-heated

steam is produced from the hole bottom at about 3500 m b.g.l., where a temperature of approximately 3308C was measured ŽGianelli and Bertini, 1993.. The breccia level was affected by hydrothermal circulation, as indicated by wall rock alteration and the presence of authigenic minerals cementing the breccia clasts. Several studies of the hydrothermal minerals ŽCavarretta et al., 1980, 1982; Bertini et al., 1985; Cavarretta and Puxeddu, 1990. and fluid inclusions at Larderello ŽBelkin et al., 1985; Cathelineau et al., 1989, 1994; Valori et al., 1992; Ruggieri and Gianelli, 1995. have documented the history of past

Fig. 1. Geological sketch map of the area around well MV5A, modified after Gianelli and Bertini Ž1993..

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hydrothermal fluids: a high-temperature early stage of circulation, dominated by thermo-metamorphic and magmatic-derived fluids, was followed by recent, lower temperature hydrothermal activity, which is characterized by fluids of predominately meteoric origin andror involving connate waters. The first study of fluid inclusions and isotopic composition of hydrothermal calcite samples found at Larderello ŽGianelli et al., 1997. furnished some data regarding the calcite in the breccia of well MV5A at 1090 m b.g.l. The study focused mainly on the origins of the fluid which deposited the calcite, though the different fluid types circulating in the breccia level and their evolution remained to be clarified. This paper presents the results of new microthermometric analyses of the fluid inclusions trapped in the hydrothermal minerals present in the breccia Žcalcite, ankerite, anhydrite and albite.. These minerals may contain fluid inclusions which have trapped fluids in the breccia during the different stages of their circulation. In addition, we investigate the paragenetic sequence of the hydrothermal minerals in the breccia and have performed electron microprobe analyses of some of these minerals. Based on the new findings and the results of previous fluid inclusion studies ŽGianelli et al., 1997., we have better defined some of the physical and chemical characteristics of the hydrothermal fluids which have circulated in the breccia, and thereby reconstructed their history. We also estimate the pressure and temperature conditions under which the breccia formed.

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from top to bottom, a phyllite–quartzite complex, a mica schist complex, and a gneiss complex. In some deep wells, granite intrusions and dikes, with ages between 1.3 and 3.8 Ma, and contact metamorphic rocks, genetically related to the emplacement of granites, have also been found ŽCavarretta et al., 1980; Batini et al., 1983; Cavarretta and Puxeddu, 1990.. The surface geology of the area around the MV5A well and the well stratigraphy are illustrated in Figs. 1 and 2, respectively. The hydraulic fracture breccia occurs in the low-grade metamorphic siliciclastic

2. Geological setting The subsurface geology of the Larderello area has been reconstructed using data from many geothermal deep wells ŽGianelli et al., 1978; Bagnoli et al., 1979; Pandeli et al., 1991; Baldi et al., 1995.; from top to bottom it consists of: a cover of Neogene marine and lacustrine sediments ŽLate Miocene to Pliocene sands, clays, marls and evaporites.; the Ligurian units ŽJurassic to Eocene ophiolite and flysch sequences.; the Tuscan Nappe ŽLate Triassic to Oligo-Miocene siliciclastic, carbonate and evaporitic sequences.; a complex of tectonic slices Žincorporating the lowest formations of the Tuscan Nappe and the uppermost part of the metamorphic units.; and lastly, the Paleozoic metamorphic rocks, including,

Fig. 2. Stratigraphy and measured in-hole temperature data for well MV5A, modified after Gianelli and Bertini Ž1993.. Ž1. Cretaceous to Paleocene flysch sequences of the Ligurian units; Ž2. Tuscan Nappe Žmainly Upper Triassic siliciclastic sequences. and tectonic slices complex; Ž3. Silurian to Ordovician phyllites; Ž4. Paleozoic mica schist group; Ž5. Paleozoic gneiss. The position of the breccia level is also shown.

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sequence Žthe Early–Middle Triassic ‘Verrucano’ formation. of the Tuscan Nappe. 3. Reservoir rocks and permeability The Larderello geothermal field produces superheated steam from a vapor-dominated reservoir ŽFacca and Tonani, 1961.. The heat source at Larderello is thought to be one or more granite intrusions ŽMarinelli, 1963.. The uppermost productive horizons Žshallow reservoir. are mostly within the Triassic carbonate and evaporite–dolomite sequences Žthe ‘Calcare Cavernoso’ and ‘Anidriti di Burano’ formations. and, subordinately, in the Late Triassic metaconglomerates of the ‘Verrucano’ formation. Important productive horizons also occur in some permeable levels of the metamorphic units, which have been drilled extensively in the western and eastern portions of the geothermal field to depths ranging from 2500 to 4500 m b.g.l. Bertani and Cappetti Ž1995. have reported a permeability of about 10y1 5 m2 for the reservoir rocks in the metamorphic units, while Calore et al. Ž1981. have estimated values of 10y1 4 to 10y1 5 m2 for the shallow reservoir of the Larderello geothermal field. The productive horizons’ permeability may result from rock fracturing. In particular, Baldi et al. Ž1979. noted that circulation losses mainly occur in correspondence of cataclastic rocks, tectonic breccias and overthrust planes. Rock fracturing at Larderello is mostly related to extensional tectonic activity. Such activity has affected southern Tuscany Žwhere the geothermal field is located. and, more generally, the inner part of the Northern Apennines, since the Early Miocene ŽElter et al., 1975; Bertini et al., 1994.. Crustal extension was accompanied by formation of NW–SE-trending depressions which were filled by the Neogene marine and lacustrine sediments. The many geological sections available for much of the Larderello area have also demonstrated that most of the faults are normal and steeply dipping. In addition, formation microscanner logging and mesostructural analyses of oriented core samples from the western part of the field reveal the presence of subvertical extensional fractures ŽBaldi et al., 1995.. The origins of rock permeability in the shallow reservoir may also be attributable to dissolution of the Triassic anhydrite

horizons in the presence of hot saline solutions, with the consequent collapse and rupture of the overlying carbonate rocks ŽBaldi et al., 1979.. Apart from the tectonic faulting, hydraulic fracturing could have also increased the rock permeability of the reservoirs at Larderello, as suggested by the occurrence of the hydraulic fracture breccia found in well MV5A, by the presence of some phreatic explosion craters, in the geothermal area, and by the observation of a phreatic explosion, described in middle-age chronicles, and reported by Marinelli Ž1967.. Hydraulic fracturing is, in fact, a significant process in the maintaining and enhancing rock permeability in active geothermal systems ŽHulen and Nielson, 1988.. This process has been recognized in several geothermal areas, as documented by direct observations and the occurrence of hydrothermal breccias, for example, at Broadlands ŽGrindley and Browne, 1976., Waiotapu ŽHedenquist and Henley, 1985a., The Geysers ŽMoore et al., 1989., and Valles Caldera ŽHulen and Nielson, 1988.. 4. Description of the breccia and hydrothermal minerals The breccia occurs in a fine-grained metasandstone Žquartz q white mica. unit of the Triassic ‘Verrucano’ formation and forms a 20-cm-thick unit in a 9-m-long core sample from 1090 m depth ŽGianelli and Bertini, 1993.. The breccia dips at an angle of 30–408 to vertical and consists of angular to subangular metasandstone clasts, characterized by a small displacement from their original position Ž‘jigsaw-puzzle texture’.. The clasts do not show evidence of rotations and their dimensions range from millimeters up to centimeters ŽFig. 3A.. Indications of shearing were not observed in the breccia. Fractures Žup to 1 cm wide. occur around the brecciated area and some cross the clasts. The core sample also crosses a system of sub-vertical, quartzcarbonate metamorphic veins and lenses of Alpine age. The paragenetic sequence of hydrothermal mineral deposition is shown in Fig. 4. Around the brecciated zone, the quartz and white mica of the sandstone have been dissolved and replaced by chlorite. Strong chloritization affects the breccia clasts, with the smallest fragments completely replaced by chlo-

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Fig. 3. Photographic plates of the breccia and hydrothermal minerals. ŽA. Core samples from the MV5A well showing the hydraulic fracture breccia Žscale bar is 20 cm.; chlorite, which can be distinguished by its dark color, rims the large clasts or completely replaces the smallest clasts. ŽB. Euhedral and anhedral calcite Žcc. crystals and late interstitial ankerite Žank.. ŽC. Late subhedral albite Žab. crystal in calcite. ŽD. Late anhedral anhydrite Žanh. crystal in calcite. ŽB., ŽC. and ŽD. are photomicrographs taken in transmitted light, parallel polars ŽB., and crossed polars ŽC, D..

rite. Minor amounts of hematite are associated with chlorite. Chlorite also precipitated with calcite in open spaces between the breccia clasts, and, together with ankerite, in some small, late veins Žsee below.. The later veins are not surrounded by pervasive

chloritization. Representative electron microprobe analyses ŽWDS. of hydrothermal chlorite are reported in Table 1. The Mg–Fe chlorite in the altered wall rock and that associated with calcite show relatively similar compositions ŽTable 1..

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Fig. 4. Paragenesis of the hydrothermal minerals formed in the breccia at 1090 m b.g.l. in well MV5A.

Because of their small size, it was not possible to obtain reliable microprobe analyses of the chlorite grains in the late veinlets. Small apatite crystals form rare euhedral crystals in contact with chlorite. Microprobe analyses reveal that these are fluoroapatite. After wall rock chloritization, carbonate cement filled the fractures and voids between the breccia clasts. Textural relationships reveal two main generations of carbonate minerals ŽFig. 3B.: the first was characterized by the precipitation of calcite, while ankerite ŽMgrŽFe q Mn. F 4. formed during the second stage. Two types of calcite have been recognized: early calcite formed anhedral to subhedral crystals, while late calcite is characterized by euhedral crystals occurring in the inner parts of the filled fractures and voids. Microprobe analyses show that late calcite is slightly enriched in Fe, Mn and Mg with respect to early calcite ŽTable 1.. Preliminary 230 Thr234 U dating of bulk calcite separates Žearly q late calcite. indicate a maximum age 41,000 years for deposition of this mineral ŽBertini et al., 1996.. Chlorite, quartz and small hematite crystals are often associated with early calcite. Both types of calcite show evidence of dissolution and replacement by later ankerite. Ankerite forms anhedral crystals precipitated in the interstices between the calcite and fills the late

Žfew millimeters wide. veinlets. In the latter case, ankerite is often accompanied by chlorite and small quartz crystals. Microprobe analyses show that the ankerite is characterized by its variable composition. Mg, Mn and Fe variations may be due to replacement of Mg and Mn by Fe ŽTable 1.; the ankerite in the late veins is usually iron-rich. After ankerite deposited, rare subhedral to euhedral albite crystals ŽFig. 3C. and anhedral anhydrite interstitial crystals ŽFig. 3D. precipitated and some replaced the ankerite and calcite. Microprobe analyses show that this albite has a nearly stoichiometric composition ŽTable 1.. Albite and anhydrite have never been found in contact, and the temporal relationship between them is therefore unknown. However, the occurrence of anhydrite exclusively as anhedral crystals suggests that this mineral was the last phase that precipitated.

5. Fluid inclusion studies 5.1. Methods The fluid inclusions were investigated in doubly polished wafers Ž100–300 mm thick. of vein and breccia core samples. Microthermometric measure-

Oxides

Chlorite in altered wall-rock

Chlorite in altered wall-rock

Chlorite associated to calcite

Oxides

Early calcite

Late calcite

Ankerite Ž1.

Ankerite Ž1.

Ankerite Ž2.

Oxides

Albite

SiO 2 TiO 2 Al 2 O 3 FeO MnO MgO CaO Na 2 O K 2O Total

29.63 0.03 19.55 15.81 0.15 21.78 0.06 0.04 0.00 87.05

29.77 0.04 20.01 13.43 0.36 23.46 0.03 0.00 0.03 87.13

29.45 0.02 18.52 15.81 0.49 22.44 0.07 0.00 0.00 86.80

FeO MnO MgO CaO SrO Total

0.08 0.53 0.06 52.69 0.07 53.43

0.47 1.09 0.66 52.52 0.00 54.74

6.70 2.41 15.79 29.50 0.07 54.47

10.42 1.12 10.75 33.40 0.04 55.73

9.62 0.73 11.81 32.23 0.05 54.44

SiO 2 TiO 2 Al 2 O 3 FeO MnO MgO CaO Na 2 O K 2O Total

67.51 0.00 20.42 0.39 0.03 0.71 0.33 11.46 0.04 100.89

0.28 0.03 0.52 1.16 0.00

0.26 0.02 0.58 1.14 0.00

Number of cations based on 28 oxygens Si 5.93 5.89 Al 4.62 4.67 Fe 2.65 2.22 Mn 0.03 0.06 Mg 6.50 6.92 Ti 0.00 0.01 Ca 0.01 0.01 Na 0.02 0.00 K 0.00 0.01

Number of cations based on six oxygens Fe 0.00 0.01 Mn 0.02 0.03 Mg 0.00 0.03 Ca 1.98 1.92 Sr 0.00 0.00

5.94 4.40 2.67 0.08 6.74 0.00 0.02 0.00 0.00

Ž1. s Ankerite replacing calcite or precipitated between calcite crystals. Ž2. s Ankerite filling late veinlets.

0.18 0.06 0.75 1.01 0.00

Number of cations based on 32 oxygens Si 11.74 Ti 0.00 Al 4.19 Fe 0.06 Mn 0.00 Mg 0.18 Ca 0.06 Na 3.86 K 0.01

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Table 1 Representative microprobe analyses of some hydrothermal minerals in the hydraulic breccia. Microprobe analyses were performed with a JEOL-8600 probe at the CRMGA of Florence. Analytical conditions were: 15 kV accelerating voltage, 15 to 40 s counting time, 10 nA excitation current, and beam width - 5 mm

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ments were performed with a Chaixmeca heatingfreezing stage ŽPoty et al., 1976.. The stage was calibrated using synthetic organic and inorganic melting point standards and natural pure CO 2 inclusions. In order to assess the presence of non-condensable gases in the inclusions, we carried out a number of crushing tests Žcf. Roedder, 1970.. Fluid inclusion isochores were calculated by means of the MacFlinCor computer software ŽBrown and Hagemann, 1994. using the data from Zhang and Frantz Ž1987.. 5.2. Description of fluid inclusions The fluid inclusions were mostly found and studied in early and late calcite, in albite and, more rarely, in anhydrite and ankerite crystals. The sizes of the analyzed inclusions were usually between 5 and 50 mm. Inclusions in quartz were too small to be analysed. Optical observations at room temperature reveal that both types of calcite contain abundant vapor-rich inclusions ŽV. ŽFig. 5A,B. and more rarely, liquidrich inclusions ŽL. ŽFig. 5C,D.. L inclusions are always two-phase Žaqueous liquid plus vapor., while most V inclusions appear to be a single phase Žvapor., though small amounts of liquid, which is not visible under the optical microscope, may be present in one-phase V inclusions. Many of the observed inclusions are of unknown origin. However, some euhedral calcite crystals contain texturally primary inclusions. Distinct planes of secondary or pseudosecondary L and V inclusions were also found in calcite. Two types of L inclusions can be distinguished in calcite on the basis of their microthermometric data: L1 inclusions ŽFig. 5C. are characterized by their higher salinities and higher homogenization temperatures; and L2 inclusions ŽFig. 5D. which show lower salinities and lower homogenization temperatures. L1 inclusions are rare and were seen in only four crystals. L2 inclusions occur both as primary or secondary inclusions. Gianelli et al. Ž1997. reported that L1 inclusions Žcalled L2 inclusions in their paper. occur along pseudosecondary or secondary trails. However, few L1 inclusions with primary texture Žisolated inclusions. were found in the present study.

Two groups of V inclusions were distinguished on the basis of the presence ŽV1 inclusions. or absence ŽV2 inclusions. of a clathrate during freezing experiments. V2 inclusions ŽFig. 5B. are very abundant and were observed both with primary features and along secondary trails. V1 inclusions display variable liquid-to-vapor ratios, suggesting that they were affected by post-entrapment processes Ži.e., necking down andror stretching. or heterogeneous trapping phenomena Ži.e., two-phase trapping.. Since V1 inclusions do not represent any of original homogeneous fluid present at the time of trapping, their microthermometric were not reported. Because of the poor optical quality of ankerite, only a few fluid inclusions in this mineral could be studied; they are one- or two-phase, vapor-rich inclusions ŽV., and two-phase, liquid-rich ŽL. inclusions ŽFig. 5E.. Some L and V inclusions show primary features. Both anhydrite and albite have trapped two-phase, liquid-rich ŽL. inclusions ŽFig. 5F,G.. In albite, single-phase vapor inclusions ŽV. were also found. L and V inclusions in this mineral often occur along a crystallographic direction ŽFig. 5G. suggesting that they are of primary origin; L inclusions were also observed along trails Žsecondary or pseudosecondary inclusions.. In anhydrite L inclusions occur in trails but few isolated primary inclusions were also found. 5.3. Microthermometric data Homogenization ŽTh . and final ice melting ŽTmi . temperatures were systematically measured only on liquid-rich inclusions. Microthermometric measurements Žabout 300. made on the L2, L1 and L inclusions trapped in the studied minerals are summarized in Table 2 and presented in Figs. 6 and 7. These data include the analyses obtained by Gianelli et al. Ž1997. on L1 and L2 inclusions Žcalled L2 and L1 inclusion, respectively in their paper. in calcite. The first ice melting temperature ŽTe . was measured in a few L2 inclusions in calcite. The microthermometric data of the L inclusions of unknown origin and occurring along trails in ankerite, albite and anhydrite are similar to those for the primary L inclusions within the same minerals, suggesting that each mineral trapped only one type of liquid. Microthermometric

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Fig. 5. Photomicrographs of fluid inclusions. ŽA. Vapor-rich ŽV2. inclusions in calcite. ŽB. Enlargement of ŽA. showing two-phase, vapor Žv. q liquid Žl., V2 inclusions. ŽC. Trail of L1 inclusions in calcite. ŽD. Isolated L2 inclusion in calcite. ŽE. L inclusions in ankerite. ŽF. L inclusion in anhydrite. ŽG. L and V inclusions in albite.

measurements Žabout 10. could only be performed on a limited number of V2 inclusions in calcite; thus, these data are simply described below.

5.3.1. Calcite In one-phase V2 inclusions in calcite no phase transitions could be clearly detected by microther-

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Table 2 Summary of fluid inclusion microthermometric data from hydrothermal minerals of the hydraulic breccia of well MV5A ŽLarderello, Italy. Mineral

Fluid inclusion type

Tmi Ž8C. range

Tmi Ž8C. average

Th Ž8C. range

T h Ž8C. average

Calcite Calcite Ankerite Albite Anhydrite

L1 L2 L L L

y4.7ry 7.9 Ž22. y1.5ry 3.9 Ž81. y0.9ry 1.0 Ž6. y0.8ry 1.2 Ž31. y2.2ry 3.8 Ž18.

y6.6 y2.4 y1.0 y1.0 y2.8

304r361 Ž18. 189r245 Ž75. 189r198 Ž6. 132r145 Ž25. 139r171 Ž13.

335 221 193 137 150

Abbreviations: Tmi s final ice melting temperature, Th s homogenization temperature. Number of measurements are given in parentheses.

mometry. In the few two-phase V2 inclusions, Tmi was y1.28C, while T h of eight V2 inclusions ranged between about 225 and 2958C. However, the T h data must be regarded with caution, as T h measurements made on vapor-rich inclusions usually suffer from large errors ŽRoedder, 1984; Sterner, 1992.. Tmi of L1 and L2 inclusions ranged respectively from y4.7 to y7.98C ŽFig. 7., with an average value of y6.68C, and from y1.5 to y3.98C ŽFig. 7., with an average value of y2.48C. Te could be detected only in L2 inclusions between approximately y30 and y478C. The Th range for L1 was from 304 to 3618C, with most values between 320 and 3408C ŽFig. 6., while for L2 inclusions it was

between 189 and 2458C, with most of the measurements in the 210–2308C range ŽFig. 6.. 5.3.2. Ankerite Microthermometry data of L inclusions in this mineral are uniform: Tmi was between y0.9 and y1.08C ŽFig. 7. and T h ranged from 189 to 1988C ŽFig. 6.. 5.3.3. Albite L inclusions in albite exhibited Tmi between y0.8 and y1.28C ŽFig. 7., with an average of y1.08C, while T h was in the range of 132–1458C, most values falling between 132 and 1408C ŽFig. 6..

Fig. 6. Histogram of the homogenization temperatures ŽTh . of the studied inclusions.

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Fig. 7. Plot of the homogenization temperature ŽTh . vs. final ice melting temperature ŽTmi . of the studied inclusions.

5.3.4. Anhydrite Tmi of L inclusions trapped in this mineral were between y2.2 and y3.88C ŽFig. 7., with an average of y2.88C; T h ranged from 139 to 1718C, with most values in the 140–1508C range ŽFig. 6.. 5.4. Crushing tests and estimation of fluid inclusion CO2 content Crushing tests were performed successfully only on L2 and V inclusions in calcite and a single L inclusion in anhydrite. For the remaining inclusions, crushing experiments were inconclusive because of their small size and the poor optical quality of ankerite and albite. During crushing, the bubbles slowly expanded in most of the L2 inclusions, but only in some did the bubble completely fill the cavity. In these latter cases, however, the bubble did not seem to expand beyond the size of the inclusion. In a few L2 inclusions, the bubble shrank. Bubble expansion indicates

the presence of pressurized ŽG 1 bar. gasŽes. in the vapor bubble. These gases are probably made up mainly of CO 2 , as this is the dominant gas in the present-day geothermal fluid at Larderello ŽD’Amore and Truesdell, 1979.. Bubble expansion with incomplete cavity filling indicates that the CO 2 concentration in most L2 inclusions Žassuming an average Th of 2218C. is less than 0.15 mol% of CO 2 ŽSasada, 1985.. For the few L2 inclusions exhibiting bubble shrinkage, the CO 2 content is below 0.08 mol%, while for the crushed inclusions in which the bubbles expanded, completely filling the cavities, the CO 2 is equal to or greater than 0.15 mol% ŽSasada, 1985.. However, in these latter inclusions, the CO 2 concentration is not likely to be much greater than 0.15 mol%, as the gas bubble does not appear to expand beyond the cavity when broken. V2 inclusions were characterized by different responses to crushing: in some inclusions the bubble either did not apparently change in volume, or it filled the cavity and may have expanded slightly beyond it, while in others the bubble shrank and

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sometimes disappeared completely. For the V2 inclusions whose bubbles did not change during crushing, the CO 2 concentration Žassuming an average Th of 2608C. is equal to 0.04 mol%, while for the inclusions exhibiting bubble expansion or shrinking, the CO 2 contents are clearly above and below this value, respectively. Finally, bubble disappearance upon crushing of the L inclusion in anhydrite indicates the absence of pressurized gases in it.

the maximum overestimation of the salinity of these inclusions would be 0.3 wt.% NaCl equivalent. The salinities calculated for the inclusions in anhydrite are only related to the presence of dissolved salts in the trapped fluids, as significant amounts of CO 2 were absent in L inclusions, as indicated by crushing experiments. The Tmi values of V2 inclusions yield a salinity of 2.1 wt.% NaCl equivalent. If V2 inclusions trapped only steam, they would display a salinity close to 0.0 wt.% NaCl equivalent. Thus, the relatively high salinity values Žlow Tmi . of such inclusions may be due to either the effects of CO 2 dissolved in the liquid phase ŽHedenquist and Henley, 1985b. or else to the heterogeneous trapping, or both.

5.5. Salinity estimations of the fluid inclusions The salinity of L inclusions ŽTable 3., expressed in wt.% of NaCl equivalent, were calculated from Tmi values by using the equation proposed by Bodnar and Vityk Ž1994.. The calculated salinities of L1 and L2 inclusions in calcite range from 7.4 to 11.6 wt.% NaCl equivalent and from 2.6 to 6.3 wt.% NaCl equivalent, respectively. L inclusions in albite and ankerite are characterized by relatively low salinity ranges: 1.6–1.7 wt.% NaCl equivalent for inclusions in ankerite and 1.4–2.1 wt.% NaCl equivalent for albite hosted inclusions. L inclusions in anhydrite show higher salinities: from 3.7 to 6.2 wt.% NaCl equivalent. Because of the possible presence in the inclusion fluid of dissolved CO 2 , which can depress the Tmi value by as much as 1.58C, the salinity calculated from Tmi may have been overestimated by up to 2.5 wt.% NaCl equivalent ŽHedenquist and Henley, 1985b.. However, as shown above, the CO 2 content in most of the crushed L2 inclusions in calcite is near or below 0.15 mol% Žcorresponding to 0.09 mol of CO 2 .. For such a concentration the reduction in Tmi related to the presence of CO 2 would be only 0.158C ŽHedenquist and Henley, 1985b., and thus,

5.6. Fluid inclusion trapping history The trapping time sequence of L1 and L2 inclusions is not obvious, as both types of fluid inclusions occur both in primary inclusions and along pseudosecondary or secondary trails. Gianelli et al. Ž1997. suggest that L2 inclusions probably postdate L1 inclusions in calcite. In fact, if L2 inclusions were formed first, during the trapping of high temperature L1 inclusions, the overpressure in L2 inclusions Žinternal inclusion pressure minus the external pressure. could induce inclusion stretching or decrepitation with fluid leakage. Fig. 8 shows the representative isochores of L1 and L2 inclusions and the corresponding liquid–vapor curves. The overpressure generated in L2 inclusions during L1 inclusion trapping can be evaluated from the pressure difference between the L2 and L1 isochores at any given temperature. For example, if L1 inclusions were

Table 3 Estimated salinity, temperature and pressure of the fluids trapped in the fluid inclusions Mineral

Fluid inclusion type

Salinity Žwt.% NaCl equivalent. range

Salinity Žwt.% NaCl equivalent. average

Trapping temperature Ž8C.

Trapping pressure Žbars.

Calcite Calcite Ankerite Albite Anhydrite

L1 L2 L L L

7.4r11.6 2.6r6.3 1.6r1.7 1.4r2.1 3.7r6.2

9.9 4.0 1.7 1.8 4.7

304r-360 189r254 189r198 132r145 144r177

- 167 23r45 11r13 2r3 F 110

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Fig. 8. Pressure vs. temperature diagram showing the hypothetical overpressures generated in already formed L2 inclusions during L1 inclusion trapping at 3358C. The representative isochores of L1 and L2 inclusions were calculated for H 2 O–NaCl fluids on the basis of the average values of T h Ž2218C for primary L2 inclusions and 3358C for L1 inclusions. and salinities Ž4.0 wt.% NaCl equivalent for L2 inclusions and 10 wt.% NaCl equivalent for L1 inclusions.. The effects of CO 2 are disregarded Žsee text for details..

trapped at 3358C Žthe L1 isochore starting point on the liquid–vapor curve., then the overpressure in L2 inclusions would be around 1600 bar. Such a high overpressure greatly exceeds that Žaround 1000 bar. required for decrepitation of the strongest inclusions in calcite of size G 5 mm Žsee fig. 4.13 of Goldstein and Reynolds, 1994.. This supports the hypothesis that L2 inclusions could not precede the formation of L1 inclusions. It was also noted that many L2 inclusions, which underwent overheating during microthermometry, decrepitated or stretched below 3008C; at such temperatures the internal pressure of L2 inclusions is about 1200 bar ŽFig. 8.. This indicates that L2 inclusions cannot resist great overpressures. Therefore, the apparently primary L2 inclusions were probably trapped after the L1 inclusions. A possible model to explain the occurrence in the same sample of apparently primary L2 inclusions together with L1 inclusions along trails is the following: calcite precipitates from a fluid to form primary

inclusions with relatively low Th Žperhaps similar to L2 inclusions.. Then, input of a hotter fluid, trapped in L1 inclusions trails, causes decrepitation and leaking or stretching of all the earlier primary inclusions, some of which are refilled with the high temperature fluid and resealed. Subsequent circulation of low-Th L2 fluid refills a part of the decrepitated primary inclusions and forms some planes of late inclusions. In this model, V ŽV1 and V2. inclusions can represent either Žearly. liquid-rich, primary inclusions affected by stretching andror leaking, or an original Žearly. vapor phase trapped in primary inclusions during calcite formation, or a Žlate. vapor phase which refilled the decrepitated fluid inclusion which then sealed again. The very abundant one-phase V2 inclusions which do not show any phase transition may also be decrepitated Žopen. inclusions. The occurrence of some V2 inclusions along secondary trails suggests that some of the apparently primary V2 inclusions may have been the result of a late refilling Žlike L2 inclusions. of the previously

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decrepitated inclusions. It is therefore possible that L2 and these V2 inclusions were coevally trapped during a boiling event: L2 inclusions represent the liquid phase and the V2 inclusions, the vapor phase. The lower Th values of V2 inclusions, which are in the Th range of L2 inclusions, support such a hypothesis. The higher Th of other V2 inclusions may represent heterogeneous fluid trapping, which is a common process during boiling ŽRoedder, 1984.. The boiling fluid probably contained a moderate amount of CO 2 , as evidenced by the presence of CO 2 in both the V2 and L2 inclusions demonstrated by crushing tests. The L2 and V2 inclusions that exhibit bubble shrinkage on crushing trapped a Žlate?. degassed fluid. V1 inclusions may represent either once early liquid-rich inclusions affected by great stretching andror partial leaking during overheating, or were originally vapor-rich inclusions in which the variable liquid-to-vapor ratio may be due to two-phase trapping. After the formation of the inclusions in calcite, the evolution of the hydrothermal fluid is recorded by the trapping of L inclusions in ankerite, then in albite and finally in anhydrite. Ankerite and albite also contains one-phase V inclusions, some of which are primary, suggesting that the hydrothermal fluid was boiling during deposition of these minerals. In this case, if V and L inclusions were coevally trapped, they must show the same Th . However, the small amount of liquid which should be present in the former is not visible with the optical microscope, and their Th could not therefore be measured.

6. Discussion 6.1. Estimates of hydrothermal fluid pressure and temperature The geothermal activity at Larderello probably started between 1.5 and 3.8 Ma ago ŽCathelineau et al., 1994.. Previous fluid inclusion studies indicate that early fluid circulation occurred under lithostatic pressure, whereas during the recent hydrothermal stage, the pressure decreased to hydrostatic values ŽValori et al., 1992; Cathelineau et al., 1994.. During

the recent stage, transient pressure increases above hydrostatic levels may have at times occurred due to self-sealing phenomena. Finally, the pressure evolved to the present-day vapor-static conditions. Nevertheless, the blow out of well San Pompeo 2 indicates that fluids at above-hydrostatic pressure may still be present in the deepest parts of the geothermal field ŽBatini et al., 1983.. The young age of calcite ŽF 41,000 years. suggests that mineral deposition in the breccia level occurred during the recent stage of hydrothermal activity characterized by hydrostatic conditions. 6.1.1. Fluids trapped in L inclusions in ankerite and albite, and L2 inclusions in calcite Boiling under hydrostatic conditions during inclusion trapping is evidenced by the presence of vaporrich inclusions in most of the studied minerals. In particular, L2 inclusions in calcite and L inclusions in ankerite and albite could have formed coevally with some V2 and V inclusions during the boiling. The temperatures of such trapped fluids is given by the inclusion Th ŽTable 3.. A finding worthy of comment is the presence in the breccia unit of pure albite containing primary inclusions trapped at around 1408C. This indicates that the Na-feldspar could have been formed at temperatures significantly lower than those Ž) 3008C. at which this mineral has usually been found at Larderello Že.g., Bertini et al., 1985.. The occurrence of authigenic albite over a relatively wide range of temperatures has also been reported in other geothermal areas; this mineral was found, for example, at temperatures between about 50 and 3008C at Djibouti ŽZan et al., 1990. and Valles Caldera ŽGoff and Gardner, 1994.. Fluid pressures at boiling, calculated using the data from ŽHaas, 1976., on the basis of the T h values, are between 11 and 13 bar for L inclusions in ankerite, and between 2 and 3 bar for L inclusions in albite ŽTable 3.. L2 inclusions contain some CO 2 , and the presence of even small quantities of this gas in the boiling fluids can contribute significantly to the total fluid pressure ŽHenley et al., 1994.. Thus, the incipient boiling pressure for fluid trapped in these inclusions has been computed here by adding the CO 2 partial pressure Žconsidering an average CO 2 content of 0.09 mol in the boiling fluid. to the

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partial pressure of the aqueous fluid of the appropriate salinity. The CO 2 partial pressures were obtained using the Henry’s law coefficient Žfor the average fluid inclusion salinity. reported by Ellis and Golding Ž1963.. The calculations indicate a boiling pressure between 23 and 45 bar for the Th range of the L2 inclusions ŽTable 3.. 6.1.2. Fluid trapped in L inclusions in anhydrite For L inclusions in anhydrite, there is no evidence of their trapping under boiling conditions. Thus, the trapping temperatures of these inclusions have been calculated from their isochores assuming a hydrostatic pressure regime. The young age of calcite ŽF 41,000 years. suggests that the depth of inclusion trapping was close to the present depth of the breccia. At 1090 m b.g.l., a cold water column will produce a pressure of about 110 bar. For such a pressure, the maximum trapping temperature of L

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inclusions in anhydrite ranged from 144 to 1778C ŽTable 3.. 6.1.3. Fluid trapped in L1 inclusions in calcite and the fluid responsible for hydraulic fracturing Microthermometric data indicate that L1 inclusions were not trapped under hydrostatic pressures. In fact, from Fig. 9, it can be seen that: Ž1. the Th and pressures at Th of all L1 inclusions except one exceed the temperature and pressure values of a boiling fluid with a salinity of 10 wt.% NaCl at a depth of 1090 m below the piezometric surface under hydrostatic condition; Ž2. the pressures at T h of most L1 inclusions were above the hydrostatic pressure of a cold water column at a depth of 1090 m. Thus, L1 inclusions record the presence of a fluid with a pressure above hydrostatic value in an environment generally characterized by hydrostatic conditions.

Fig. 9. Pressure vs. temperature diagram showing the pressure–temperature Ž P–T . conditions at homogenization for L1 inclusions. The fluid pressure required for hydraulic fracturing, calculated through the equation of Hubbert and Willis Ž1957., is also shown. Note that most of the studied L1 inclusions are characterized by pressure values, at homogenization, above the hydrostatic pressure at 1090 m b.g.l. Žsee text for details..

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The absence of significant brecciation during and after calcite precipitation suggests that the pressure at which the fluids were trapped in L1 inclusions was not sufficient to promote hydraulic fracturing. Under extensional tectonic regimes, as at Larderello, the fluid pressure required for hydraulic fracturing is: Pfracturing ( Ž P lithostatic q 2 Phydrostatic .r3 ŽHubbert and Willis, 1957.. At a depth of about 1090 m b.g.l., the lithostatic pressure Žconsidering an average density of 2.6 grcm3 for the overlying rock. is around 280 bar, and the hydrostatic pressure Žfor a cold water column. is 110 bar. From these values and the above equation, we estimate that the pressure required for hydraulic fracturing is about 167 bar. This pressure corresponds to a temperature of about 3608C for a fluid containing 10 wt.% of NaCl ŽFig. 9.. Thus, the fluid trapped in the L1 inclusions should have circulated at temperatures and pressures below 3608C and 167 bar, respectively. The Th and pressure at T h of all except one of the L1 inclusions are in agreement with these pressure and temperature limits. The inclusion with a pressure at Th higher than the fracturing pressure may be a necked inclusion. Barring this, the highest T h of the L1 inclusions is around 3508C, which corresponds to a pressure of about 150 bar on the liquid–vapor curve ŽFig. 9.. These values are slightly lower than the fracturing pressure and temperature, and can therefore be considered close to the upper pressure and temperature limits for L1 inclusion trapping ŽTable 3.. The lower limits of the trapping conditions for L1 inclusions are given by the inclusions exhibiting the minimum Th ŽTable 3.. L1 inclusions in calcite indicate that fluids with a pressure above hydrostatic values have at times circulated in the breccia unit and, more generally, that these types of fluid can occur at about 1 km depths in the Larderello geothermal field. Brecciation in well MV5A at 1090 m b.g.l. may have been promoted by a fluid similar in composition to the saline fluid trapped in L1 inclusions, but with higher temperatures ŽG 3608C. and pressures ŽG 167 bar.. 6.2. Characteristics and eÕolution of hydrothermal fluids Based on the above data, it is possible to outline the fluid history at the level of the breccia in well

MV5A. In particular, we have reconstructed some of the physical–chemical characteristics of the fluids present at different times in the breccia. The temporal evolution of temperature, pressure and salinity are schematized in Fig. 10. 6.2.1. Breccia formation The relatively young age of the calcite cement suggests that the hydrothermal fluid that deposited it in the breccia level in well MV5A was probably related to the recent activity of the Larderello geothermal field. The fluid, which caused hydraulic fracturing, was hot ŽG 3608C. and saline Ž( 10% NaCl equivalent., and with a pressure of at least 167 bar. Fluid inclusions from deep wells Ž3–4 km b.g.l.. record the presence of aqueous fluids with temperatures around 350–4008C and variable salinity, characterized by boiling and mixing processes during the late hydrothermal stage of the Larderello field ŽCathelineau et al., 1989; Valori et al., 1992.. Extensional tectonic processes may have promoted the formation of fractures allowing these hot fluids to rise to relatively shallow depths in the area of well MV5A. If these fluids ascended quickly, their temperatures would not have changed greatly; at a depth of 1090 m b.g.l., the fluid pressure was thus sufficient to promote hydraulic fracturing. It is likely that after brecciation the system was open to the surface and characterized by hydrostatic conditions and boiling. Hydraulic brecciation, at shallow depth, is evidenced by the presence of some phreatic explosion craters in the geothermal area. In addition, a phreatic explosion observed during historic times, and the fluids with pressures above hydrostatic values, sporadically found in some of the deep wells, suggest that recent hydraulic fracturing processes occurred and may be continuing even today in the Larderello geothermal system. The presence of paleo-geothermal fluids with greater-than-hydrostatic pressures have been also recorded by fluid inclusions at the Valles Caldera ŽHulen and Nielson, 1988. and at The Geysers ŽHulen et al., 1997. geothermal systems. 6.2.2. Chloritization After brecciation, wall rock chloritization affected the breccias. During chloritization, the dissolution of quartz and mica testifies to a loss of silica and

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Fig. 10. Temperature, pressure and salinity of the fluids present during the different stages of hydrothermal circulation in the breccia level. The numbers indicate the stages of hydrothermal circulation: Ž1. hydraulic fracturing and formation of the hydraulic fracture breccia, Ž2. wall rock chloritization, Ž3. calcite precipitation, Ž4. trapping of L1 inclusions in calcite, Ž5. trapping of L2 inclusions in calcite, Ž6. precipitation of ankerite and trapping of L inclusions, Ž7. precipitation of albite and trapping of L inclusions, Ž8. precipitation of anhydrite and trapping of L inclusions, Ž9. current conditions. The pressure of hydraulic fracturing was obtained from the equation of Hubbert and Willis Ž1957.; the corresponding hydraulic fracturing temperature was calculated on the basis of the pressure from the liquid–vapor curve for a 10 wt.% NaCl fluid; temperatures for chloritization and calcite precipitation were inferred from the chlorite geothermometer of Cathelineau Ž1988.; present-day conditions Žblack rectangle. were obtained from in-hole measurements; all other values were obtained or inferred from fluid inclusion data.

potassium ŽGianelli and Bertini, 1993.. The precipitation of Mg–Fe chlorite and hematite indicate a relatively high Mg, Fe and O 2 activity in the fluid which circulated at this stage and during the formation of early calcite. There is no evidence in inclusions of the fluid responsible for chlorite deposition. The little information that is available has been

provided by application of the chlorite geothermometer of Cathelineau Ž1988. to the chlorites in the wall rock, which yields an average temperature of about 280 " 208C. Such estimates must, however, be regarded with caution, as the validity of this geothermometer has been questioned ŽDe Caritat et al., 1993..

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6.2.3. Precipitation of hydrothermal minerals The characteristics of the fluid responsible for the subsequent calcite precipitation are also unknown, as the primary fluid inclusions in this mineral probably do not contain the fluid originally present during their formation. However, the decrepitation and stretching phenomena which probably affected these inclusions indicate that they originally trapped a relatively cool fluid that may be similar to the lower temperature, moderately saline fluid trapped in L2 inclusions. The chlorite geothermometer of Cathelineau Ž1988. indicates an average temperature of about 265 " 108C for the chlorite associated with this calcite. After calcite precipitated, the breccia level was affected by circulation of the higher temperature Žabout 300–3508C., high salinity Žaround 10 wt.% NaCl equivalent. fluids trapped in L1 inclusions. As shown above, the maximum pressure of such fluids was probably around 150 bar, and certainly lower than the value Ž167 bar. required for hydraulic fracturing. Then, the breccias was invaded by a series of fluids characterized by decreasing temperatures Žfrom about 250 to approximately 1308C., pressures Žfrom 45 to a few bar. and salinity Žfrom 6.3 to 1.4 wt.% NaCl equivalent., which were trapped by L2 inclusions in calcite and L inclusions in ankerite and albite ŽFig. 10.. Boiling Žrecorded by V inclusions. probably occurred at this time. The general temperature and pressure decline recorded by L2 inclusions Žin calcite. to L inclusions Žin albite. under boiling conditions may result from a lowering of the piezometric surface. The contemporaneous decrease in salinity suggests progressive exhaustion of the moderate salinity reservoir fluid which was replaced by a lower salinity fluid, perhaps linked to direct circulation of meteoric water at the depth of the breccia level. This circulation, however, did not prevent the boiling process. The final stage of hydrothermal circulation in the breccia was characterized by inflow of cooler Žabout 145 to 1758C., moderate salinity Ž3.7 to 6.2 wt.% NaCl equivalent. fluid from which anhydrite precipitated. There is no evidence that this fluid was boiling. Consequently, the fluid pressure cannot be reliably estimated. The measured current in-hole temperature Ž1608C. is within the range of the trapping

temperatures of L inclusions in anhydrite, suggesting that the thermal conditions in this part of the reservoir have not changed significantly since the anhydrite precipitated. 6.2.4. Composition and origin of the hydrothermal fluids The relatively low values of Te of the L2 inclusions indicate that, in addition to NaCl, dissolved CaCl 2 andror MgCl 2 are present in the solutions ŽCrawford, 1981.. The presence of Ca in the hydrothermal fluid flowing in the breccia is consistent with the precipitation of calcite, ankerite and anhydrite. The geothermal fluid may have become enriched in Ca andror Mg by interacting with the Triassic dolomite-anhydrite sequences Ž‘Anidriti di Burano’. and marine carbonates present in the Tuscan Nappe. Based on calcite isotopic data, Gianelli et al. Ž1997. have also suggested that the geothermal water Žcharacterized by a d18 O of 2.33‰. in equilibrium with the calcite in well MV5A is a meteoric fluid which has been enriched in d18 O by interacting with carbonates of the Tuscan Nappe.

7. Conclusions Fluid inclusion and mineral paragenesis studies of the hydraulic fracture breccia in well MV5A at 1090 m b.g.l. indicates that a series of fluids have circulated, and authigenic minerals have precipitated during different stages of hydrothermal activity. Based on these studies, it has been possible to reconstruct the evolution of the hydrothermal fluid. Fluid inclusion data prove that a saline Žaround 10 wt.% NaCl equivalent., high temperature Žabout 300 to 3508C. fluid, with greater-than-hydrostatic pressures has, at times, been present in the breccia. A similar saline fluid at temperatures and pressures of at least 3608C and 167 bar, respectively, may have been responsible for hydraulic fracturing at the depth of breccia unit, during extensional tectonic activity. This fluid could have ascended from the deepest part of the Larderello geothermal system in response to formation of extensional fractures. After hydraulic fracturing, the rock around the breccia and breccia clasts were partially replaced by Mg–Fe chlorite. Then, chlorite, hematite and two

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generations of calcite progressively filled the fractures and voids between the breccia clasts; these minerals were followed by ankerite. Finally, small amounts of albite, quartz and anhydrite deposited. Calcite probably precipitated from a relatively low temperature fluid. Then, the high temperature saline fluid circulated into the breccia and presumably caused post-entrapment phenomena on the inclusions trapped during calcite formation. This latter fluid was followed by input of boiling liquids Žtrapped in calcite, ankerite and albite. at progressively lower temperatures Žfrom about 250 to 1308C., pressures Žfrom 45 to a few bar. and salinity Žfrom 6.3 to 1.4 wt.% NaCl equivalent.. Decreases in pressure and temperature may be the result of the lowering of the piezometric surface over time, while the variation in salinity was possibly linked to infiltration of meteoric water into the breccia. The final fluid Žtrapped in anhydrite. was moderately saline Žabout 4.7 wt.% NaCl equivalent. with temperatures between 145 and 1758C. The current temperature Ž1608C. at the depth of the breccia level is within this range. Liquid-rich inclusions in the minerals of the breccia record a relatively recent circulation of hydrothermal liquids as indicated by the maximum age of calcite Ž41,000 years.; this is an upper age limit for the creation of vapor-dominated conditions in the area of the breccia unit. The hydrothermal fluids may be meteoric waters or connate waters undergoing modification of their composition through boiling, mixing and water–rock interactions with anhydrite levels and carbonates present in the Larderello area. The hydraulic breccia of well MV5A testifies that rock fracturing due to hydrothermal activity occurred at Larderello. Fluid inclusion geothermometry demonstrates that the presence of a fluid with greater-than-hydrostatic pressure at the depth of the breccia was not restricted to the brecciation event alone. Moreover, hydraulic fracturing processes are potentially active at present, since a phreatic explosion was observed in historic times and fluid with pressure above hydrostatic values still occurs in some deep parts of the field. This suggests that hydraulic brecciation in the Larderello geothermal system may be a recurring phenomenon which has probably contributed, and continues to contribute, to the reservoir permeability.

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Acknowledgements This work was financially supported by CNR ŽNational Research Council of Italy., the 5-year research contract of G.R. is financed by CNR ŽCROP project.. The Electron-Microprobe Laboratory of CRMGA of Florence is financed by CNR-GNV. We acknowledge ENEL for providing the core sample. The authors would like to thank P.R.L. Browne and F. Goff for their helpful reviews.

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