Larderello Geothermal Field Research Articles

Heat mining from super-hot horizons of the Larderello geothermal field, Italy

Super-hot rock geothermal is an emerging source of renewable and carbon-free energy. This paper is the first attempt to explore fluid and heat flow dynamics in the reservoir-wellbore coupled system, to assess the power generation performance of a super-hot (>450 °C) enhanced geothermal system (EGS). We developed a high-performance code and built a 3-D wellbore-reservoir coupled model based on data from a recently completed deep-drilling project at Larderello, Italy. The general pattern of the super-hot EGS is characterized by a significant temperature plummet (>60 °C), after which the production fluid evolves from steam to a two-phase mixture till the end of the operation period. Reservoir pressure emerges as a key parameter to determine the temperature of the two-phase mixture. By realistically capturing phase transitions driven by coupled thermo-hydraulic processes during operations, our numerical model predicts a lower power generation efficiency compared to previous attempts based on ultra-simplified models. Although finalized at assessing the thermodynamic viability of a specific system, this modeling approach provides general information on fundamental thermo-hydraulic processes in the Earth crust that might be applied for the design of similar EGS projects elsewhere.

CO2 and heat energy transport by enhanced fracture permeability in the Monterotondo Marittimo-Sasso Pisano transfer fault system (Larderello Geothermal Field, Italy)

Carbon dioxide is one of the most important gasses naturally released from geothermal systems. Establishing the processes and pathways that regulate the CO2 diffuse degassing can provide valuable information for exploration and exploitation purposes of geothermal reservoirs. In this work, a high-resolution CO2 flux (with records up to 2927 g m–2 d–1) and soil temperature (with records up to 98.8 °C) survey was carried out along with detailed fracture parameters measurements in a selected area of the Monterotondo Marittimo-Sasso Pisano transfer fault (Larderello geothermal system, Tuscany, Italy). The main aim was to define the behavior of diffuse CO2 through the fault system and investigate how the soil CO2 flux and steam change with respect to the architecture of the fault damage zone (i.e., volumetric fracture intensity, permeability, and persistence of the fractures). The presence of multiple populations of CO2 flux suggested that three different transport mechanisms control soil degassing: i) purely diffusive, ii) mixed diffusive-advective, and iii) purely advective, characterized by efflux values of <20, between 20 and 300 and >300 g m–2 d–1, respectively. The spatial distribution of these fluxes well agrees with the fracture distribution and features of the Jurassic radiolarite (Diaspri Fm) dissected by NNE-striking faults. Areas with high CO2 emissions are indeed able to reveal major upflow areas from deep reservoirs through deep-reaching permeable fault zones. The interaction between pre-existing fractures and fracture-related fault-zone locally enhances the secondary rock permeability as highlighted by the correlation between Discrete Fracture Network (DFN) modeling and advective flux. Eventually, by normalizing the CO2 output to the fault strip (1350 m2), a release of CO2 equal to ∼155 t d–1 km–2 was estimated.

Origin of continuous hydrogen flux in gas manifestations at the Larderello geothermal field, Central Italy

The back arc extensional area of Tuscany is characterized by anomalously high heat flow as a result of lithospheric thinning facilitated by locally intruded granite, which ultimately led to the occurrence of numerous fumaroles and other manifestations of geothermal activity. The present study builds up on molecular and isotopic geochemical analyses of fumarolic gases discharging near Larderello geothermal field in order to report on their origin and generation mechanisms. The fumarole gases are rich in CO2 (15.65–92.5 vol%), poor in CH4 (0.08–3.70 vol%) and highly-depleted in C2+ (< 0.1 vol%) whereas H2 content is rather constant (~4.5 vol%). The δ13CCH4 (from −16.7 to −29.5‰VPDB) and δDCH4 (from −155 to −185.6‰SMOW) suggest an abiotic origin of methane. The heavy δ13CCO2 values (>−4.2‰VPDB) reflect thermo-metamorphic of CO2, however magmatic-volcanic contribution cannot be ruled out.This work tests various hypotheses of the origin of H2: the intruded granite, the ophiolite and/or a deeper source. Petrographic and electron microbeam investigations performed on the Ligurian ophiolite and plutonic granite, which outcrops in the vicinity of the study area, revealed serpentinization of ophiolite (in contact with geothermal fluids at 200 °C) as the most probable source of the measured H2. Hydrogen that forms deeper, potentially near or within the granitic intrusion, at higher temperatures, is likely consumed to form abiotic methane.

Tracing Ancient Carbon Dioxide Emission in the Larderello Area by Means of Historical Boric Acid Production Data

The impact of natural CO2 emissions in the development of geothermal areas is presently gaining more attention than ever before. In Italian geothermal fields, a reduction in the natural CO2 emissions has been observed. This paper reviews and provides an analysis of the historical production data of boric acid from 1818 to 1867 used to calculate the natural emissions of CO2 associated with boric acid production that pre-dates the use of geothermal resources for power production, which started in 1913. Boric acid was already being extracted from the natural geothermal fluids in geysers and natural ponds emitting steam and gases. After 1827 the ‘lagone coperto’ (covered lake) equipment optimized production, and the drilling of shallow wells (20–30 m) starting in 1836, which further increased the quantity of its extraction. The first geothermal reservoir was developed at the turn of the century and the Larderello geothermal field began to grow. The use of deep wells, keeping pace with the power production, led to the gradual disappearance of the natural ponds and the ‘lagoni’ (lakes) in the historical area, so the residual natural emission of CO2 is presently restricted to diffuse soil emission. Comparisons of the ancient CO2 emissions with those of the Geothermal Power Plant (GPP) in the Larderello area show that both amounts are in the same order of magnitude, suggesting a balance between the depletion of natural emissions and geothermal activity.

Drilling super-hot horizons of the Larderello geothermal field: insights from noble gases

In the frame of EU-funded DESCRAMBLE project, the Venelle 2 well was deepened down to 2900 m b.g.l. to test novel drilling techniques, and to improve the knowledge of physical-chemical conditions of super-hot horizons in the Larderello geothermal field, Central Italy. The major seismic reflectors present in the area were the target of the drilling, to verify their potential as geothermal reservoirs of supercritical fluids. No significant fluid entries were observed during the operations. Noble gases were analysed in the emerging drilling fluids to determine whether deep-seated fluids with a different signature than those currently discharged by productive wells entered the borehole. Despite the predominant atmospheric signature of the noble gas suite, 3He/4He values up to 1.59 Ra were measured, and the contribution of a deep component with ~4 Ra was hypothesised by extrapolation of binary mixing trends.

Carbon isotopic signature of interstitial soil gases reveals the potential role of ecosystems in mitigating geogenic greenhouse gas emissions: Case studies from hydrothermal systems in Italy

Volcanic and hydrothermal areas largely contribute to the natural emission of greenhouse gases to the atmosphere, although large uncertainties in estimating their global output still remain. Nevertheless, CO2 and CH4 discharged from hydrothermal fluid reservoirs may support active soil microbial communities. Such secondary processes can control and reduce the flux of these gases to the atmosphere. In order to evaluate the effects deriving from the presence of microbial activity, chemical and carbon (in CO2 and CH4) isotopic composition of interstitial soil gases, as well as diffuse CO2 fluxes, of three hydrothermal systems from Italy were investigated, i.e. (i) Solfatara crater (Campi Flegrei), (ii) Monterotondo Marittimo (Larderello geothermal field) and (iii) Baia di Levante in Vulcano Island (Aeolian Archipelago), where soil CO2 fluxes up to 2400, 1920 and 346 g m−2 day−1 were measured, respectively. Despite the large supply of hydrothermal fluids, 13CO2 enrichments were observed in interstitial soil gases with respect to the fumarolic gas discharges, pointing to the occurrence of autotrophic CO2 fixation processes during the migration of deep-sourced fluids towards the soil-air interface. On the other hand, (i) the δ13C-CH4 values (up to ~48‰ vs. V-PDB higher than those measured at the fumarolic emissions) of the interstitial soil gases and (ii) the comparison of the CO2/CH4 ratios between soil gases and fumarolic emissions suggested that the deep-sourced CH4 was partly consumed by methanotrophic activity, as supported by isotope fractionation modeling. These findings confirmed the key role that methanotrophs play in mitigating the release of geogenic greenhouse gases from volcanic and hydrothermal environments.

Determining the influence of pressure and temperature on the elastic constants of anisotropic rock samples using ultrasonic wave techniques

Measuring rock elastic constants and anisotropy parameters and understanding their changes in response to changes in pressure and temperature is crucial for modeling and interpreting seismic measurements. The rapid advance in seismic exploration and characterization of conventional and unconventional reservoirs requires a thorough understanding of variations in seismic velocities in response to variations of the rock elastic constants and anisotropy parameters. In attempt to develop this understanding, we measured variations in the elastic constants of three rock samples in the lab as a function of temperature (up to 600 ∘C) and pressure (up to 150 MPa) to simulate the in-situ conditions using a triaxial ultrasonic pulse transmission method. The rock samples used in this study came from a metamorphic rock core sample collected from a geothermal reservoir at the Larderello Geothermal Field in Italy. The rock samples have different chemical and structural composition and inherently contain both vertical transverse isotropy (VTI) and orthorhombic isotropy. We used two different methods to calculate the elastic stiffness constants and corresponding anisotropic parameters including VTI and orthorhombic isotropy. Our results showed that increasing the pressure leads to an increase in the orthorhombic constants while increasing the temperature leads to a decrease in these constants. The lagging decrease of the elastic constants with respect to the decrease in the pressure is explained by the hysteresis phenomenon. These observations and results will promote a better understanding and interpretation of reservoirs within the seismic domain.

Fluid inclusion evidence on the direct exsolution of magmatic brines from a granite intrusion beneath the eastern sector of Larderello geothermal field (Italy)

This work is based on a fluid inclusion investigation on two core samples (sampled at 3017m and 4150m of depth) of a granite intrusion drilled by the Travale 1 sud geothermal well, in the eastern sector of Larderello geothermal field. Fluid inclusion data demonstrate that the earliest fluids circulating in the intrusion were high-temperature brines exsolved directly from the crystallizing magma during its ascent in the shallow crust. The progressive release of magmatic fluids from the crystallizing granitic magma probably occurred under confining lithostatic pressure of about 200 MPa. Two phase fluid inclusions represent the hydrothermal meteoric water-dominated fluids that penetrated in the granite after opening of the system, induced by cooling and consequent transition from ductile behaviour to brittle fracturing. Type 1b two phase fluid inclusions, having temperature of homogenization that agree well with down-hole measured temperatures, may be considered to approximate the fluid that is actually circulating in the granite in recent times.

Discovering geothermal supercritical fluids: a new frontier for seismic exploration

Exploiting supercritical geothermal resources represents a frontier for the next generation of geothermal electrical power plant, as the heat capacity of supercritical fluids (SCF),which directly impacts on energy production, is much higher than that of fluids at subcritical conditions. Reconnaissance and location of intensively permeable and productive horizons at depth is the present limit for the development of SCF geothermal plants. We use, for the first time, teleseismic converted waves (i.e. receiver function) for discovering those horizons in the crust. Thanks to the capability of receiver function to map buried anisotropic materials, the SCF-bearing horizon is seen as the 4km-depth abrupt termination of a shallow, thick, ultra-high (>30%) anisotropic rock volume, in the center of the Larderello geothermal field. The SCF-bearing horizon develops within the granites of the geothermal field, bounding at depth the vapor-filled heavily-fractured rock matrix that hosts the shallow steam-dominated geothermal reservoirs. The sharp termination at depth of the anisotropic behavior of granites, coinciding with a 2 km-thick stripe of seismicity and diffuse fracturing, points out the sudden change in compressibility of the fluid filling the fractures and is a key-evidence of deep fluids that locally traversed the supercritical conditions. The presence of SCF and fracture permeability in nominally ductile granitic rocks open new scenarios for the understanding of magmatic systems and for geothermal exploitation.

Data integration and conceptual modelling of the Larderello geothermal area, Italy

In the frame of the Integrated Method for Advanced Geothermal Exploration (IMAGE) Project, a reliable exploration and resource assessment workflow was implemented on the basis of an integrated and multidisciplinary approach. Our study addressed to a better understanding of the thermal structure of the deepest part of the Larderello geothermal field (Southern Tuscany, Italy) by integrating structural, geological, geochemical, geochronological, petrological and geophysical data. With the aim to characterize the reservoir located nearby an important seismic reflector (the K-horizon), we systematized the available data and, successively, we applied a numerical thermal modelling approach to test our hypotheses and concepts.

Assessment of the risk of pollution from Arsenic on human health as a function of its speciation in intestinal fluids

Investigating the risk on human health arising from exposure to As-polluted drinking water is a very important task since many area of the world has to face with this problem. Actually, chronic poisoning by arsenic carried by water has been well documented in many European and extra-European countries. Pollution of water with arsenic develops generalized symptoms of poisoning and carcino genesis. It is well known that inorganic As is rapidly absorbed after oral exposure, but little information is still available about the fate of arsenic after oral ingestion. In particular, the mechanism which affects the speciation of this element in the gastrointestinal tract is not clear, especially when other trace elements are present. In this paper we present an estimate of the health risk due to the ingestion of water containing micro-pollutants, such as As and B, using two computational codes: PHREEQC, which allowed us to determine the effect of boron on arsenic speciation in equilibrium with the fluids of the gastrointestinal tract; SADA software for assessing the risk of exposure to contaminated matrices. The use of software in the pre sent case study is mandatory since it is quite impossible to obtain experimental data. We focused on a sample area, the geothermal field of Larderello (Italy), where anomalous concentrations of arsenic and boron were found in the drinking water. The results obtained in this study prove that the risk to human health depends on the speciation of As, which is strictly related to the pH values reached at equilibrium with intestinal fluids. The pH values change as a function of the concentration of boric acid in drinking water. In waters characterized by high contents of B, the risk factor is amplified by the increase of the species As 3+ in intestinal fluids. Specifically, this study suggests that interactions between two elements in solution, such as boric acid and arsenic, both in solution in drinking water, can have a negative impact on human health when balanced with intestinal fluids, since the effects of one may magnify the toxicity of the other.

Identifying sources of B and As contamination in surface water and groundwater downstream of the Larderello geothermal – industrial area (Tuscany–Central Italy)

Summary A study on the upper reaches of the Cecina River (Tuscany–Central Italy) and the associated unconfined aquifer was carried out from September 2007 to August 2008. The study aimed to identify the sources of B and As contamination in stream water and groundwater, and record contamination levels. The study area, which comprises a northern sector of the Larderello geothermal field, has in time been contaminated by both surface geothermal manifestations (now thought to have ceased) and anthropogenic activity. The latter refers to the disposal of spent geothermal fluids and borogypsum sludge, by-product of colemanite treatment with sulphuric acid, which until the late ‘70s were discharged in the Larderello area into the Possera Creek, a southern tributary of the Cecina River. A network of 22 stream sections and 9 observation wells was defined. Stream discharge (16 sites), well water levels and chemical concentrations (mainly B, As and anions) in water were measured monthly. Together, discharge and chemical concentrations were used to define the source of contamination by calculating the contaminant load in successive sections of the river network. Due to the stream’s intermittent flow, only 50% of the performed monthly surveys could be used in comparing the contaminant load at different sections. Both contaminant loads (referring to median to high flow conditions) and chemical concentrations suggest that B mainly derives from the leakage of a concentrated Na–SO4 water rich in B, SO4, NO3 likely from a small aquitard located in the Larderello area. The B load from this area is about 2 kg/h and increases to approximately 2.7 kg/h in the final section of the study area, likely due to contribution of groundwater. As mainly derives from dissolution and adsorption–desorption processes involving water and As-rich stream bed sediments. Of the total 15 g/h As load measured at the end section, only about 3 g/h derive from the Larderello area. Further to stream bed, As-rich sediments are also found at shallow depths in the area of the Cecina–Possera confluence and in the upper part of the aquifer skeleton. These sediments contribute to increase up to about 76 μg/L the As content of groundwater of the Cecina–Possera confluence area which, draining water from the Possera Creek, represents the aquifer root zone. This zone determines the B and As contents of groundwater which flows more or less parallel to the Cecina River, undergoes progressive dilution during its westward flow and locally supplies the same river. Most of the study stream water and groundwater in the study area cannot be exploited because mean B and As contents (respectively in the range 1.2–15.6 mg/L and 1.1–75.9 μg/L), are often well above the permissible limits for drinking water (1 mg/L for B, 10 μg/L for As).

The oxygen isotope exchange between carbon dioxide and water in the Larderello geothermal field (Italy) during fluid reinjection

The oxygen isotope compositions of CO 2 and water vapor samples collected from Larderello geothermal wells after the start of the fluid reinjection program suggest that if the oxygen isotope exchange in the vapor phase does, in fact, exist, it is a very slow process when compared with the residence time of the fluids in the geothermal reservoir. This is because carbon dioxide and water vapor phases could not have equilibrated significantly in the vapor-dominated reservoir. This conclusion implies that the oxygen isotope composition of carbon dioxide may possibly be used as a tool in geothermal exploration for revealing the presence of liquid water in deep geothermal systems. Based on the interpretation of the oxygen isotope data of the CO 2, I propose that the origin of the low oxygen isotope ratios of carbon dioxide at Larderello is the high-temperature exchange with liquid water in the lower reservoir. In Larderello, the liquid water–rock interaction in the lower reservoir may have increased the 18O/ 16O ratio of the recharge meteoric component. By contrast, lack of high-temperature liquid water in the upper reservoir suggests that the large “δ 18O shift” described for the upper-reservoir steam during the last decades reflects varying degrees of dilution of the lower-reservoir fluid by the low- 18O vaporized liquid water of meteoric origin that recharges the field at shallow depth, with local contribution from still deeper high- 18O water vapor of magmatic origin. The low oxygen isotope composition of the Mesozoic carbonaceous rocks that form the upper reservoir, consequently, likely represents a “fossil” record of the past hot-water geothermal stage.

Migration of geothermal fluids in extensional terrains: the ore deposits of the Boccheggiano-Montieri area (southern Tuscany, Italy)

An integrated study based on fluid inclusion, δ18O composition and structural analyses was carried out on a Pliocene fossil hydrothermal system, located to the South of the present active Larderello geothermal field, in the Boccheggiano-Montieri area. The study area is typified by mineralized cataclastic levels related to Late Oligocene–Early Miocene thrust surfaces, and to the following two generations of normal faults of Miocene and Pliocene ages, respectively. Within the damage zone of the Pliocene Boccheggiano fault, the mineralization is mainly made up of quartz and pyrite. Quartz + Pb–Zn sulfides, or quartz + Pb–Zn sulfides + fluorite + carbonates assemblages occur instead in the older cataclastic levels. Two generations of liquid-rich fluid inclusions were recognized in quartz and fluorite: the first one, with homogenization temperatures ranging between 172 and 331°C and salinity between 0.0 and 8.8 wt.% NaClequiv., records the early stage of hydrothermal activity. The second generation of fluid inclusions documents a later stage, with homogenization temperature from 124 to 288°C and salinity from 0.2 to 1.9 wt.% NaClequiv.. Fluid inclusions analyses also indicate that mixing of fluid with distinct salinities and/or temperatures was a widespread process during the early stage, and that fluid temperatures decreased moving from the Boccheggiano fault toward the more distal and older cataclastic levels. The δ18O values of water in equilibrium with hydrothermal quartz, which range from −5.7 to −0.1‰, are related to the circulation of meteoric water mixed with saline water that leached the evaporite level and enriched in δ18O through water–rock interaction, and/or with magmatically derived fluids. Results indicate that the damage zone of the Pliocene Boccheggiano fault represented the main channel for the flow of meteoric water, which was heated at depth, then mixed with high salinity fluids, and finally ascend to infiltrate along the older cataclastic levels. Our results, based on fluid inclusions, oxygen isotopic compositions and structural analyses indicate that a single fluid flow path run through the damage zone of the Boccheggiano fault and the older cataclasites, which were thus hydraulically connected.

CO2 Degassing over Seismic Areas: The Role of Mechanochemical Production at the Study Case of Central Apennines

Field observations coupled with experimental results show that CO2 can be produced by mechanical energy applied to carbonate rocks becoming an unexpected additional gas source besides that degassed from the mantle or produced by thermometamorphism. The evidence that a large amount of carbon dioxide associated with radiogenic-type helium (R/Ra as low as 0.01–0.08) is released through continental areas, denotes the absence of a contribution from the mantle or from mantle-derived fluids. Data collected during the seismic crisis which struck the Central Apennines in 1997–98 have shown an enhanced CO2 flux not associated with the presence of mantle or thermometamorphic-derived fluids. On the other hand, new experimental results highlight the possibility of producing CO2 by mechanical energy that acts on the calcite crystalline lattice. While the CO2 released over the geothermal areas (e.g., Larderello Geothermal Field) is obviously derived by mantlederived activities, this is not the case of the huge amount of CO2 released over the seismically active areas where the presence mantle-derived products is ruled out. We propose that mechanical energy, e.g., released during seismic events, microseismicity or creeping processes is a possible additional energy source able to produce CO2 and thus could explain the presence of CO2 degassing over tectonic areas where the influence of the mantle is low.

Pliocene–Pleistocene HT–LP metamorphism during multiple granitic intrusions in the southern branch of the Larderello geothermal field (southern Tuscany, Italy)

This work presents the results of a multidiscipinary study carried out in the southern branch of the Larderello geothermal field (Travale–Montieri area), which was based on the integration of field data with information on the deeper structures as derived from interpretation of seismic reflection profiles and the P – T – t history of the metamorphic substratum as reconstructed from borehole data. Our data document that the structural and metamorphic signature of the metamorphic substratum is chiefly related to a prograde HT–LP metamorphic overprint, ranging in age from c . 2.8 to 0.7 Ma. The metamorphic climax was attained within the K-feldspar zone and equilibrated at P – T conditions of about 0.2 GPa and 650 °C. We interpret these new findings as evidence of Pliocene–Pleistocene thermal metamorphism associated with multiple granite intrusions at shallow levels in the crust. These results (1) add strength to the interpretations emphasizing the contribution of Neogene thermal metamorphism to the tectonometamorphic signature of the basement rocks found at depth in the Larderello geothermal field, and (2) provide insights on the thermal structures of the crustal section hosting the geothermal field and on the modes through which the long-lived positive thermal anomaly operated in southern Tuscany.

Thermal modelling of the Larderello geothermal field (Tuscany, Italy)

We present a 3-D thermal model of the Larderello geothermal field (Tuscany) to evaluate (1) the extent and contribution of the heat transfer mechanisms (conduction vs. convection) at the intermediate-upper crust levels, (2) the variability of the heat and mass fluxes entering from below and (3) the crucial role of the formation permeability. The model, composed by three main layers, considers the upper 10 km of the crust to better constrain the simulations with experimental data from borehole, fluid inclusion studies and hypocentral distributions. Several sets of simulations were carried out with different bottom boundary temperatures and different formation permeabilities for the two deeper layers. The results indicate that the present temperature (T) and pressure distributions in the Larderello field require deep reservoir rocks with higher permeability than the overlying capping units and underlying intermediate crust. Permeability values of 1 mDarcy for the reservoir rocks are enough to allow fluid convection, if the temperature at 10 km depth is as high as 500 ± 50°C. The presence of localized zones with formation permeability 50–100 times higher than the surrounding rocks strongly favours the migration of over-pressurized fluids, which episodically break through the overburden, feeding the presently exploited geothermal fields.

Active contact metamorphism and CO 2–CH 4 fluid production in the Larderello geothermal field (Italy) at depths between 2.3 and 4 km

Several zones can be distinguished in the Larderello geothermal field (southern Tuscany, Italy) based on the nature of the fluid, mineralogical assemblages and the fluid flow as a function of depth. The deepest levels of the field are characterized by mineralogical assemblages produced by currently active thermal metamorphism that is thought to have started a few million years ago, e.g., during or later than the main intrusions that are dated from 1.6 to 3.8 Ma. At depths between 2 and 4 km below ground level (b.g.l.), the rocks are characterized by the assemblage quartz–biotite–tourmaline. This zone is characterized by present-day productive layers of superheated steam. Fluid inclusions from these deep zones are dominated by C–H–O–N and occur as CO 2–CH 4–N 2–H 2O vapors, with CO 2 as the dominant gas. The CO 2–CH 4 ratio and the water content, which are not affected by immiscibility or mixing processes, are compatible with values expected from calculations of water–graphite equilibrium. A thermal metamorphic origin is suggested for the genesis of the aqueous-carbonic fluid, especially the volatiles (CO 2–CH 4). Such fluids may be considered as the dominant type, compared to other fluid types in the deeper levels, especially within the seismic high-amplitude reflector called K horizon. The water component of the aqueous-carbonic vapors has a rather low salinity (< 5 wt.% eq. NaCl) and may correspond either to water produced by dehydration reactions or to waters that have extensively equilibrated with the metamorphic host rocks. Three other fluid sources may also contribute to the variety of fluids found in the biotite zone: (i) deep Li-rich brines considered as having a magmatic origin, (ii) brines resulting from water–halite interactions within the evaporites, (iii) meteoric derived fluids, which penetrated downwards. The carbonic vapors were generated and trapped under lithostatic conditions of 100 to 120 MPa and high temperatures between 600 and 420 °C, depending on the depth and the distance to the granitic intrusives. These fluids may be considered as representative of the dominant fluid at depth within the pressurized zones that have been inferred by geophysical investigations (K horizon). After cooling, and in many cases decompression, aqueous fluid inclusions were trapped at pressures fluctuating between lithostatic and hydrostatic. The estimated pressure corresponds to depths between 2 and 4 km, which is in agreement with the transition from lithostatic to hydrostatic conditions. The end of the proposed fluid path corresponds to an evolution towards the present-day temperature and hydrostatic pressure conditions.

Heat flow, heat transfer and lithosphere rheology in geothermal areas: Features and examples

Surface heat flow measurements over active geothermal systems indicate strongly positive thermal anomalies. Whereas in “normal“ geothermal settings, the surface heat flow is usually below 100–120 mW m − 2 , in active geothermal areas heat flow values as high as several watts per meter squared can be found. Systematic interpretation of heat flow patterns sheds light on heat transfer mechanisms at depth on different lateral, depth and time scales. Borehole temperature profiles in active geothermal areas show various signs of subsurface fluid movement, depending on position in the active system. The heat transfer regime is dominated by heat advection (mainly free convection). The onset of free convection depends on various factors, such as permeability, temperature gradient and fluid properties. The features of heat transfer are different for single or two-phase flow. Characteristic heat flow and heat transfer features in active geothermal systems are demonstrated by examples from Iceland, Italy, New Zealand and the USA. Two main factors affect the rheology of the lithosphere in active geothermal areas: steep temperature gradients and high pore fluid pressures. Combined with lithology and structure, these factors result in a rheological zonation with important consequences both for geodynamic processes and for the exploitation of geothermal energy. As a consequence of anomalously high temperature, the mechanical lithosphere is thin and its total strength can be reduced by almost one order of magnitude with respect to the average strength of continental lithosphere of comparable age and thickness. The top of the brittle/ductile transition is located within the upper crust at depths less than 10 km, acts as the root zone of listric normal faults in extensional environments and, at least in some cases, is visible on seismic reflection lines. These structural and rheological features are well illustrated in the Larderello geothermal field in Tuscany.

Crustal structures in the geothermal areas of southern Tuscany (Italy): Insights from the CROP 18 deep seismic reflection lines

Two deep seismic reflection lines, named CROP 18A and 18B and roughly NNW–SSE-oriented, were acquired to investigate the crust between the Larderello and the Mt. Amiata geothermal areas, both located in southern Tuscany. Since the Early–Middle Miocene, southern Tuscany has been affected by extensional tectonics, and since the Pliocene, by widespread magmatism. Presently, extension is demonstrated by NE-dipping normal faults and brittle shear zones which affect the crust down to the brittle/ductile transition. In the Larderello area, these shear zones are seismically evidenced by NE-dipping weak reflections and by the loss of the K-horizon reflectivity, a regional mid-crustal high amplitude reflector, located at the top of the brittle/ductile transition. The CROP 18A and 18B lines were reprocessed down to 10 s TWT, down to the crust/mantle boundary. Their reflectivity results are closely related to the direction of the survey: a clear decrease in reflectivity occurs when the survey plane intersects the crustal shear zones along strike. Nevertheless, both lines display segments with low reflectivity and homogeneous acoustic impedance which indicate intrusive magmatic bodies at different crustal levels. A wide, partially crystallised granitoid may be sited at about 7–8 km depth underneath the Larderello geothermal field. In the lower part of the crust, the CROP 18A and 18B transects show group of bright reflections, mainly related to the occurrence of pressurised fluids and/or lithological differences. The base of the crust is located at about 20 km depth.