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Igor M. Villa - One of the best experts on this subject based on the ideXlab platform.
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pliocene pleistocene ht lp metamorphism during multiple granitic intrusions in the southern branch of the Larderello geothermal field southern tuscany italy
Journal of the Geological Society, 2008Co-Authors: Federico Rossetti, Igor M. Villa, Fabrizio Balsamo, Mohamed Bouybaouenne, Claudio Faccenna, R FunicielloAbstract: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.
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geochronology and isotope transport systematics in a subsurface granite from the Larderello travale geothermal system italy
Journal of Volcanology and Geothermal Research, 2006Co-Authors: Igor M. Villa, Mariano Puxeddu, Giovanni Ruggieri, G BertiniAbstract:Abstract A monzogranite, here referred to as the Montieri pluton, was encountered along two depth profiles from ca. 2 to 4 km depth in wells of the Larderello–Travale geothermal system, Italy. We obtained high-resolution petrographic and microchemical data on seven fresh samples of the Montieri pluton. These data are required to correctly interpret the combined isotope data for the Rb–Sr and K–Ar systems. Biotite and K-feldspar from all samples were analyzed by Rb–Sr and 39Ar–40Ar. Rb–Sr analyses gave extremely variable two-point Rb–Sr apparent ages and Sr initials, indicating that biotite did not equilibrate with feldspar. Bulk dissolution of K-feldspars does not define a Rb–Sr alignment that could have proved a common origin of all K-feldspar samples from a single magma batch. The leachable fractions of K-feldspar separates have much more homogeneous 87Sr/86Sr ratios around 0.712, which may reflect the isotopic composition of a late-stage circulating fluid. Because of the anatectic origin of the granites from the Larderello–Travale geothermal system, it is likely that every K-feldspar separate contains at least three isotopically distinct feldspar generations: relics of the Hercynian gneiss basement, Pliocene magmatic minerals, and hydrothermal retrogression products. Such heterogeneity can be confirmed and quantified by petrographical observations and electron microprobe analyses. The 39Ar–40Ar age spectra of the biotite samples show some internal discordance. Because of deviations from stoichiometry, biotite discordance can be attributed both to chlorite intergrowths, as predicted from the active fluid circulation, and to the presence of multiple biotite generations, such as have already been documented from other micas from the Larderello–Travale geothermal system and as indeed confirmed by high-resolution petrography. Total K–Ar biotite ages on cluster around 3 Ma and broadly agree with a Rb–Sr age obtained by regressing only the biotite analyses. This suggests that the Montieri pluton, whose known volume is at least 25 km3, was emplaced at that time. The K-feldspar spectra are strongly discordant; no step age approaches the zero-age which would be predicted by Ar diffusivity modelling. The extraneous Ar in the feldspars is not excess Ar, but inherited Ar instead. Ar inheritance in the feldspar separates correlates with inheritance of Sr. Arrhenius trajectories of the apparent Ar diffusivities for all seven feldspars are astonishingly identical, despite the heterochemism amongst samples. However, the apparent diffusivity calculated from the step-heating experiments led to irreproducible models for the thermal history along the depth profile, as well as time-scales shorter than the historical record. This indicates that Ar diffusivities calculated from laboratory experiments must not be extrapolated to geological conditions.
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Geochronology of the Larderello geothermal field: new data and the “closure temperature” issue
Contributions to Mineralogy and Petrology, 1994Co-Authors: Igor M. Villa, Mariano PuxedduAbstract:The Larderello geothermal field is generally accepted to have been produced by a granite intrusion at 4–9 km depth. Hydrothermal parageneses and fluid inclusions always formed at temperatures greater than or equal to the current ones, which implies that the field has always undergone a roughly monotonic cooling history (fluctuations < 40 K) since intrusion of the granite at 4 Ma. The heat required to maintain the thermal anomaly over such a long period is supplied by a seismically anomalous body of ≈ 32000 km3 rooted in the mantle. Borehole minerals from Larderello are thus a unique well-calibrated natural example of thermally induced Ar and Sr loss under geological conditions and time spans. The observations (biotites retain Ar above 450°C) agree well with other, albeit less precise, geological determinations, but contrast with laboratory determinations of diffusivity from the literature. We therefore performed a hydrothermal experiment on two Larderello biotites and derived a diffusivity DLab(370°C)=5.3·10-18 cm2s-1, in agreement with published estimates of diffusivity in annite. From DLab and the rejuvenation of the K/Ar ages we calculate maximum survival times at the present in-hole temperatures. They trend smoothly over almost two orders of magnitude from 352 ka to 5.3 ka, anticorrelating with depth: laboratory diffusivities are inconsistent not only with geological facts, but also among themselves. From the geologically constrained lifetime of the thermal anomaly we derive a diffusivity DG(370°C)=3.81·1021 cm2s-1, 3±1 orders of magnitude lower than DLab. The cause of these discrepancies must be sought among various laboratory artefacts: overstepping of a critical temperature T*; enhanced diffusivities in “wet” experiments; presence of fast pathway (dislocation and pipe) diffusion, and of dissolution/reprecipitation reactions, which we imaged by scanning electron microscopy. These phenomena are minor in geological settings: in the absence of mineral transformation reactions, complete or near-complete resetting is achieved only by volume diffusion. Therefore, laboratory determinations will necessarily result in apparent diffusivities that are too high compared to those actually effecting the resetting of natural geochronometers.
Giovanni Gianelli - One of the best experts on this subject based on the ideXlab platform.
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data integration and conceptual modelling of the Larderello geothermal area italy
Energy Procedia, 2017Co-Authors: Gianluca Gola, Giovanni Gianelli, Andrea Brogi, Andrea Dini, G Bertini, Marco Bonini, S Botteghi, Roberto De Franco, Assunta Donato, D LiottaAbstract:Abstract 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.
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Geological structure of a long-living geothermal system, Larderello, Italy
Terra Nova, 2006Co-Authors: Giovanni Bertini, Giovanni Gianelli, Michele Casini, Enrico PandeliAbstract:We have examined the data of 600 geothermal wells and re- interpreted approximately 500 km seismic profiles through the field of Larderello, Italy. We conclude that the two main seismic reflectors present below the geothermal area host two differ- ent fluids: (1) superheated steam in the upper H-horizon (reached by drillholes) and (2) supercritical fluid in the deeper K-horizon (reached by few unproductive or damaged wells). The superheated steam has the physical and chemical connotation of the geothermal fluid exploited so far at Larderello, whereas the supercritical fluid represents a potential unconventional deep-seated resource still to be assessed. The high tempera- tures existing in correspondence of the K-horizon suggest that the silica-rich rocks are close to a plastic state and the fluids should remain confined in a medium sealed to the confining rocks, unless occasional fluid overpressure and abrupt high strain rates occur.
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origin and evolution of pliocene pleistocene granites from the Larderello geothermal field tuscan magmatic province italy
Lithos, 2005Co-Authors: Andrea Dini, Giovanni Gianelli, Mariano Puxeddu, Giovanni RuggieriAbstract:Extensive, mainly acidic peraluminous magmatism affected the Tuscan Archipelago and the Tuscan mainland since late Miocene, building up the Tuscan Magmatic Province (TMP) as the Northern Apennine fold belt was progressively thinned, heated and intruded by mafic magmas. Between 3.8 and 1.3 Ma an intrusive complex was built on Larderello area (Tuscan mainland) by emplacement of multiple intrusions of isotopically and geochemically distinct granite magmas. Geochemical and isotopic investigations were carried out on granites cored during drilling exploration activity on the Larderello geothermal field. With respect to the other TMP granites the Larderello intrusives can be classified as two-mica granites due to the ubiquitous presence of small to moderate amounts of F-rich magmatic muscovite. They closely resemble the almost pure crustal TMP acidic rocks and do not show any of the typical petrographic features commonly observed in the TMP hybrid granites (enclaves, patchy zoning of plagioclase, amphibole clots). On the basis of major and trace elements, as well as REE patterns, two groups of granites were proposed: LAR-1 granites (3.8–2.3 Ma) originated by biotite-muscovite breakdown, and LAR-2 granites (2.3–1.3 Ma) generated by muscovite breakdown. At least three main crustal sources (at 14–23 km depth), characterized by distinct eNd(t) and 87Sr/86Sr values, were involved at different times, and the magmas produced were randomly emplaced at shallow levels (3–6 km depth) throughout the entire field. The partial melting of a biotite-muscovite-rich source with low eNd(t) value (about −10.5) produced the oldest intrusions (about 3.8–2.5 Ma). Afterwards (2.5–2.3 Ma), new magmas were generated by another biotite-rich source having a distinctly higher eNd(t) value (−7.9). Finally, a muscovite-rich source with high eNd(t) (about −8.9) gave origin to the younger group of granites (2.3–1.0 Ma). The significant Sr isotope disequilibrium recorded by granites belonging to the same intrusion is interpreted, as due to the short residence time of magmas in the source region followed by their rapid transfer to the emplacement level. Partial melting was probably triggered by multiple, small-sized mafic intrusions, distributed over the last 3.8 Ma that allowed temporary overstepping of biotite- and muscovite-dehydration melting reactions into an already pre-heated crust. Dilution in time of the magmatic activity probably prevented melt mingling and homogenization at depth, as well as the formation of a single, homogeneous, hybrid pluton at the emplacement level. Moreover the high concentrations of fluxing elements (B, F, Li) estimated for the LAR granites modified melt properties by reducing solidus temperatures, decreasing viscosity and increasing H2O solubility in granite melts. The consequences were a more efficient, fast, magma extraction and transfer from the source, and a prolonged time of crystallization at the emplacement level. These key factors explain the long-lived hydrothermal activity recorded in this area by both fossil (Plio-Quaternary ore deposits) and active (Larderello geothermal field) systems.
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the crystalline units of the middle upper crust of the Larderello geothermal region southern tuscany italy new data for their classification andtectono metamorphic evolution
Bollettino della Società Geologica Italiana. Volume speciale, 2005Co-Authors: Enrico Pandeli, Giovanni Gianelli, Marco MorelliAbstract:New structural and mineralogical-petrographical studies were conducted on the crystalline units (Micaschist Complex and the underlying Gneiss Complex) present at depth in the Larderello geothermal region. These data, which have been compared with others derived from similar outcropping (Cerreto Pass) and drilled (Pontremoli 1 well) metamorphic units in the Tuscan-Emilian sector of the Northern Apennines, allow the refinement of the structural-metamorphic evolution of the upper-middle crust in southern Tuscany. A low-medium to high grade regional metamorphism (e.g. zoned garnets, oligoclase-andesine blasts) was recognized in both complexes, overprinted by a 285 Ma thermometamorphic event, during which andalusite+muscovite crystallised. Only the rocks of the Micaschist Complex were then affected by traspositive polyphase tectono-rnetamorphism in the greenschist facies. Finally, both complexes underwent a low- to high-grade contact metamorphism (crystallisation of poikilitic andalusite, cordierite, biotite, muscovite and, in places, corundum with sanidine rims) due to the emplacement of Pliocene-Quaternary granitoids and hydrothermalism, which locally obliterated the previous textures and mineral assemblages. In spite of the lack of radiometric dates for the foliations, comparison of the structural and mineralogical framework of the Larderello micaschist.s and gneisses with those of other dated metamorphic, Alpine and pre-Alpine successions of the Northern Apennines (e.g. Cerreto Pass) and northern Sardinia, allow us to hypothesize that the older, up to high-grade Barrovian framework (well-preserved in the Gneiss Complex) can be related to the Variscan Orogeny. In the Micaschist Complex, the following pervasive greenschist facies tectono-metamorphism was due to the Alpine events. The Micaschist Complex and the Gneiss Complex, now tectonically superposed in the Northern Apennines nappe pile, probably represent contiguous sectors of the original Variscan middle crust of the Adriatic Plate. Notwithstanding the evident local strong Pliocene-Quaternary contact metamorphic imprint, these rocks can mostly be classified as regional metamorphic rocks rather than hornfels. This classification allows a better reconstruction of the structural framework of the middle-upper crust in the Larderello geothermal area along the CROP 18 seismic profile.
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helium isotopes in paleofluids and present day fluids of the Larderello geothermal field constraints on the heat source
Journal of Geophysical Research, 2003Co-Authors: G Magro, Giovanni Gianelli, Giovanni Ruggieri, Stefano Bellani, Giovanni ScandiffioAbstract:[1] The He isotope composition of paleofluids entrapped in fluid inclusions of hydrothermal minerals is compared with the present-day fluid composition of the Larderello geothermal field. Almost constant values of (3He/4He)m/(3He/4He)air (=R/Ra) over time indicate that no important changes have occurred in the deep source of gases, at least during the last 3.8 million years. On a regional scale, a correlation has been found between the R/Ra spatial distribution, heat flow, and Bouguer gravity anomaly. High values of R/Ra and heat flow, and low Bouguer anomaly values indicate that the Larderello field is an area of preferential escape for mantle-derived fluids. A positive correlation has also been found between the R/Ra spatial distribution and a major seismic reflector named the “K horizon.” A deep magma source, refilled by periodic gas input from the mantle, is the most likely source of 3He-enriched fluids and the anomalously high heat flow. The nearly constant value of R/Ra clearly indicates that input of fresh mantle material has occurred up to recent times. Clear evidence of mixing between mantle and crustal fluids indicates that the high R/Ra is the lower limit of the actual mantle value, which is suggested to be similar to the subcontinental European mantle. The decrease of R/Ra over time in the peripheral part of the Larderello field indicates that important changes in the feeding fracture system and/or cooling rate have occurred in these areas.
Giovanni Ruggieri - One of the best experts on this subject based on the ideXlab platform.
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geochronology and isotope transport systematics in a subsurface granite from the Larderello travale geothermal system italy
Journal of Volcanology and Geothermal Research, 2006Co-Authors: Igor M. Villa, Mariano Puxeddu, Giovanni Ruggieri, G BertiniAbstract:Abstract A monzogranite, here referred to as the Montieri pluton, was encountered along two depth profiles from ca. 2 to 4 km depth in wells of the Larderello–Travale geothermal system, Italy. We obtained high-resolution petrographic and microchemical data on seven fresh samples of the Montieri pluton. These data are required to correctly interpret the combined isotope data for the Rb–Sr and K–Ar systems. Biotite and K-feldspar from all samples were analyzed by Rb–Sr and 39Ar–40Ar. Rb–Sr analyses gave extremely variable two-point Rb–Sr apparent ages and Sr initials, indicating that biotite did not equilibrate with feldspar. Bulk dissolution of K-feldspars does not define a Rb–Sr alignment that could have proved a common origin of all K-feldspar samples from a single magma batch. The leachable fractions of K-feldspar separates have much more homogeneous 87Sr/86Sr ratios around 0.712, which may reflect the isotopic composition of a late-stage circulating fluid. Because of the anatectic origin of the granites from the Larderello–Travale geothermal system, it is likely that every K-feldspar separate contains at least three isotopically distinct feldspar generations: relics of the Hercynian gneiss basement, Pliocene magmatic minerals, and hydrothermal retrogression products. Such heterogeneity can be confirmed and quantified by petrographical observations and electron microprobe analyses. The 39Ar–40Ar age spectra of the biotite samples show some internal discordance. Because of deviations from stoichiometry, biotite discordance can be attributed both to chlorite intergrowths, as predicted from the active fluid circulation, and to the presence of multiple biotite generations, such as have already been documented from other micas from the Larderello–Travale geothermal system and as indeed confirmed by high-resolution petrography. Total K–Ar biotite ages on cluster around 3 Ma and broadly agree with a Rb–Sr age obtained by regressing only the biotite analyses. This suggests that the Montieri pluton, whose known volume is at least 25 km3, was emplaced at that time. The K-feldspar spectra are strongly discordant; no step age approaches the zero-age which would be predicted by Ar diffusivity modelling. The extraneous Ar in the feldspars is not excess Ar, but inherited Ar instead. Ar inheritance in the feldspar separates correlates with inheritance of Sr. Arrhenius trajectories of the apparent Ar diffusivities for all seven feldspars are astonishingly identical, despite the heterochemism amongst samples. However, the apparent diffusivity calculated from the step-heating experiments led to irreproducible models for the thermal history along the depth profile, as well as time-scales shorter than the historical record. This indicates that Ar diffusivities calculated from laboratory experiments must not be extrapolated to geological conditions.
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stable isotope and noble gas isotope compositions of inclusion fluids from Larderello geothermal field italy constraints to fluid origin and mixing processes
Journal of Volcanology and Geothermal Research, 2005Co-Authors: Luigi Dallai, G Magro, Eleonora Petrucci, Giovanni RuggieriAbstract:Noble gas and N 2 compositions have been determined in paleofluids of core samples from geothermal wells in the Larderello area, Italy. The results were coupled with hydrogen and carbon isotope compositions of similar fluids from an independent batch of samples to provide constraints on the origin of the fluids. The fluids trapped during early-stage hydrothermal circulation are characterized by relatively high 4 0 Ar/ 3 6 Ar and N 2 /Ar ratios (332≤ 4 0 Ar/ 3 6 Ar≤564 and 248≤N 2 /Ar≤4197, respectively) and indicate the presence of radiogenic 4 0 Ar and excess N 2 when compared to air/asw (air saturated water). The fluid inclusions related to late-stage hydrothermal activity show low N 2 /Ar and 4 0 Ar/ 3 6 Ar ratios (309≤ 4 0 Ar/ 3 6 Ar≤354 and 47
thermal fluids. The 6D values of the inclusion fluids range from -53‰ to -85‰, indicating that the fluids were derived from mixing of water-rich components, probably from a shallow reservoir similar in composition to present-day meteoric waters, and fluids from a deeper reservoir. The R/R a ratios (up to 3.2) of the paleofluids and present-day fluids suggest a deep He contribution at Larderello. The δ 1 3 C values for CO 2 trapped during early-stage activity (-3.8%o to 0.5%o) were produced by carbonate country rock volatilisation; δ 1 3 C values as low as -17‰ were measured in inclusion fluids related to late-stage circulation, and were likely produced by oxidation of organic matter at shallow depths. -
origin and evolution of pliocene pleistocene granites from the Larderello geothermal field tuscan magmatic province italy
Lithos, 2005Co-Authors: Andrea Dini, Giovanni Gianelli, Mariano Puxeddu, Giovanni RuggieriAbstract:Extensive, mainly acidic peraluminous magmatism affected the Tuscan Archipelago and the Tuscan mainland since late Miocene, building up the Tuscan Magmatic Province (TMP) as the Northern Apennine fold belt was progressively thinned, heated and intruded by mafic magmas. Between 3.8 and 1.3 Ma an intrusive complex was built on Larderello area (Tuscan mainland) by emplacement of multiple intrusions of isotopically and geochemically distinct granite magmas. Geochemical and isotopic investigations were carried out on granites cored during drilling exploration activity on the Larderello geothermal field. With respect to the other TMP granites the Larderello intrusives can be classified as two-mica granites due to the ubiquitous presence of small to moderate amounts of F-rich magmatic muscovite. They closely resemble the almost pure crustal TMP acidic rocks and do not show any of the typical petrographic features commonly observed in the TMP hybrid granites (enclaves, patchy zoning of plagioclase, amphibole clots). On the basis of major and trace elements, as well as REE patterns, two groups of granites were proposed: LAR-1 granites (3.8–2.3 Ma) originated by biotite-muscovite breakdown, and LAR-2 granites (2.3–1.3 Ma) generated by muscovite breakdown. At least three main crustal sources (at 14–23 km depth), characterized by distinct eNd(t) and 87Sr/86Sr values, were involved at different times, and the magmas produced were randomly emplaced at shallow levels (3–6 km depth) throughout the entire field. The partial melting of a biotite-muscovite-rich source with low eNd(t) value (about −10.5) produced the oldest intrusions (about 3.8–2.5 Ma). Afterwards (2.5–2.3 Ma), new magmas were generated by another biotite-rich source having a distinctly higher eNd(t) value (−7.9). Finally, a muscovite-rich source with high eNd(t) (about −8.9) gave origin to the younger group of granites (2.3–1.0 Ma). The significant Sr isotope disequilibrium recorded by granites belonging to the same intrusion is interpreted, as due to the short residence time of magmas in the source region followed by their rapid transfer to the emplacement level. Partial melting was probably triggered by multiple, small-sized mafic intrusions, distributed over the last 3.8 Ma that allowed temporary overstepping of biotite- and muscovite-dehydration melting reactions into an already pre-heated crust. Dilution in time of the magmatic activity probably prevented melt mingling and homogenization at depth, as well as the formation of a single, homogeneous, hybrid pluton at the emplacement level. Moreover the high concentrations of fluxing elements (B, F, Li) estimated for the LAR granites modified melt properties by reducing solidus temperatures, decreasing viscosity and increasing H2O solubility in granite melts. The consequences were a more efficient, fast, magma extraction and transfer from the source, and a prolonged time of crystallization at the emplacement level. These key factors explain the long-lived hydrothermal activity recorded in this area by both fossil (Plio-Quaternary ore deposits) and active (Larderello geothermal field) systems.
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helium isotopes in paleofluids and present day fluids of the Larderello geothermal field constraints on the heat source
Journal of Geophysical Research, 2003Co-Authors: G Magro, Giovanni Gianelli, Giovanni Ruggieri, Stefano Bellani, Giovanni ScandiffioAbstract:[1] The He isotope composition of paleofluids entrapped in fluid inclusions of hydrothermal minerals is compared with the present-day fluid composition of the Larderello geothermal field. Almost constant values of (3He/4He)m/(3He/4He)air (=R/Ra) over time indicate that no important changes have occurred in the deep source of gases, at least during the last 3.8 million years. On a regional scale, a correlation has been found between the R/Ra spatial distribution, heat flow, and Bouguer gravity anomaly. High values of R/Ra and heat flow, and low Bouguer anomaly values indicate that the Larderello field is an area of preferential escape for mantle-derived fluids. A positive correlation has also been found between the R/Ra spatial distribution and a major seismic reflector named the “K horizon.” A deep magma source, refilled by periodic gas input from the mantle, is the most likely source of 3He-enriched fluids and the anomalously high heat flow. The nearly constant value of R/Ra clearly indicates that input of fresh mantle material has occurred up to recent times. Clear evidence of mixing between mantle and crustal fluids indicates that the high R/Ra is the lower limit of the actual mantle value, which is suggested to be similar to the subcontinental European mantle. The decrease of R/Ra over time in the peripheral part of the Larderello field indicates that important changes in the feeding fracture system and/or cooling rate have occurred in these areas.
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contact metamorphism in the Larderello geothermal system
2000Co-Authors: Giovanni Gianelli, Giovanni RuggieriAbstract:The potential reservoir of the steam-heated Larderello geothermal field consists of various metamorphic rocks and granite. These rocks have been found in the deepest part of the field in wells reaching depths close to 4.5 km. The intrusion of granite into the wall rocks, the circulation of magmatic fluids and their evolution to the present-day conditions of superheated steam involve complex processes which are reconstructed by fluid-inclusion and mineral studies. The results obtained through a study of core samples of granite and contact metamorphic rock indicate that: 1) the granitic rocks are highly differentiated, and their solidus temperature may be as low as 600°C; 2) the thermal metamorphism developed at temperatures of 400-600°C and pressures of approximately 100-120 MPa and the fluids permeating the contact aureole evolved from magmatic to meteoric; 3) the chemistry of the early magmatic stage is characterised by the presence of elements such as F, Cl, Li and B, derived from the anatectic magma and therefore of crustal origin; 4) at least part of the He and CO2, and probably other gases as well, have a deep origin, possibly from the Earth’s mantle. These findings depict a complex scenario for the deepest part of the Larderello geothermal field. When deep exploration is successful, the well produces superheated steam. However, projects for deep exploration, in particular those in the vicinity of the major seismic reflector present throughout the geothermal region, should take into consideration the possibility of encountering very hot fluids, with pressures greater than hydrostatic and unusual and undesired chemistry. Moreover, the prospect of finding melted rock (possibly as dykes of differentiated granite) should also be considered.
Andrea Brogi - One of the best experts on this subject based on the ideXlab platform.
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data integration and conceptual modelling of the Larderello geothermal area italy
Energy Procedia, 2017Co-Authors: Gianluca Gola, Giovanni Gianelli, Andrea Brogi, Andrea Dini, G Bertini, Marco Bonini, S Botteghi, Roberto De Franco, Assunta Donato, D LiottaAbstract:Abstract 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.
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migration of fluids in the boccheggiano montieri southern tuscany italy fossil geothermal system insights for the Larderello high enthalpy active geothermal field
70th EAGE Conference and Exhibition - Workshops and Fieldtrips, 2008Co-Authors: Andrea Brogi, Paolo Fulignati, Domenico Liotta, A Dini, G Ruggieri, A SbranaAbstract:Understanding the migration of hydrothermal fluids represents a continuous task for successful exploration of geothermal resources. Contributions to better constrain the hydrogeological models in geothermal areas can derive from field and laboratory studies on fossil geothermal systems, evidenced by the concentration of ore deposits in wide areas. This work presents an integrated study based on fluid inclusion and structural analyses on a Pliocene-Pleistocene fossil hydrothermal system, located to the south of the present active Larderello geothermal field. Mineralization, mainly made up of quartz and pyrite, is widespread distributed in the damage zone of the Pliocene Boccheggiano normal fault and, far from it, in the older cataclastic levels, deriving from previous deformational events.
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upper crust boudinage during post collisional miocene extension in tuscany insights from the southern part of the Larderello geothermal area northern apennines italy
Geodinamica Acta, 2007Co-Authors: Andrea Brogi, Alessio CerboneschiAbstract:A geological study carried out in the southern part of the Larderello geothermal area (Northern Apennines) provides new information on the development mechanism and timing of the earlier extensional structures that formed during the Miocene post-collisional tectonics which affected the orogen. Staircase low-angle normal faults (LANFs) affected a multilayered thickened upper crust after the collisional stage, producing the lateral segmentation of the Tuscan Nappe, the deeper non-metamorphic tectonic unit of the Northern Apennines in the Tuscan area. The tectonic history recorded in two Tuscan Nappe discontinuous bodies revealed that the LANFs took place during the Middle–Late Miocene, displacing collisional structures developed from the Late Oligocene. These Tuscan Nappe bodies are delimited by detachment faults located at the base, within the Tuscan evaporites, and at the top within the Ligurian Units. Their western and eastern margins coincide with east-dipping ramps. These structures and the Tuscan Napp...
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crustal structures in the geothermal areas of southern tuscany italy insights from the crop 18 deep seismic reflection lines
Journal of Volcanology and Geothermal Research, 2005Co-Authors: Andrea Brogi, Domenico Liotta, A Lazzarotto, Giorgio RanalliAbstract:Abstract 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.
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insights into the Larderello geothermal field structural setting and distribution of thermal and 3 he anomalies
World Geothermal Congress 2005, 2005Co-Authors: S Bellani, Andrea Brogi, A Lazzarotto, G Magro, D LiottaAbstract:In the Larderello geothermal field, similarly to the other geothermal areas in the world (i.e. Yellowstone, The Geysers, Rhinegraben), heat flow at a regional scale anomaly corresponds to the presence of 3 He enriched fluids. At least in the last one million years in a typical crustal melting setting, high R/Ra values (ranging from 0.5 to 3.2) in present and inclusion fluids indicate mantle as the main source of the thermal and 3 He anomaly. The combination of thermal and He data provide information on transport mechanism in upper and lower crust, which is strongly controlled by fault systems. A targeted geological section of the Larderello field, integrating field, borehole and reflection seismic surveys data correspond to shear zones, where upward displacement of isotherms correspond to the NE-dipping normal fault system. At surface, higher R/Ra values correspond to heat flow maxima, though slightly biased in space in the order of 1 - 2 km, and indicate that fault systems act as preferential pathways for mantle-derived fluids. The similar evolution in time and space of heat and 3 He anomalies, even though they differ by at least 1-3 orders of magnitude in diffusion time constants, indicate that they share sources and transfer mechanisms. Mantle-He transfer through the crust requires fluid advection or diffusion via fluid filled conduits while heat is more efficiently transferred by conductivity through the bulk rocks, enhanced by fluid circulation in the upper brittle crust. The local addition of an advective component to the regional background thermal anomaly caused by increased permeability along faults could explain the presence of maxima of thermal and 3 He anomaly at Larderello.
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geochronology and isotope transport systematics in a subsurface granite from the Larderello travale geothermal system italy
Journal of Volcanology and Geothermal Research, 2006Co-Authors: Igor M. Villa, Mariano Puxeddu, Giovanni Ruggieri, G BertiniAbstract:Abstract A monzogranite, here referred to as the Montieri pluton, was encountered along two depth profiles from ca. 2 to 4 km depth in wells of the Larderello–Travale geothermal system, Italy. We obtained high-resolution petrographic and microchemical data on seven fresh samples of the Montieri pluton. These data are required to correctly interpret the combined isotope data for the Rb–Sr and K–Ar systems. Biotite and K-feldspar from all samples were analyzed by Rb–Sr and 39Ar–40Ar. Rb–Sr analyses gave extremely variable two-point Rb–Sr apparent ages and Sr initials, indicating that biotite did not equilibrate with feldspar. Bulk dissolution of K-feldspars does not define a Rb–Sr alignment that could have proved a common origin of all K-feldspar samples from a single magma batch. The leachable fractions of K-feldspar separates have much more homogeneous 87Sr/86Sr ratios around 0.712, which may reflect the isotopic composition of a late-stage circulating fluid. Because of the anatectic origin of the granites from the Larderello–Travale geothermal system, it is likely that every K-feldspar separate contains at least three isotopically distinct feldspar generations: relics of the Hercynian gneiss basement, Pliocene magmatic minerals, and hydrothermal retrogression products. Such heterogeneity can be confirmed and quantified by petrographical observations and electron microprobe analyses. The 39Ar–40Ar age spectra of the biotite samples show some internal discordance. Because of deviations from stoichiometry, biotite discordance can be attributed both to chlorite intergrowths, as predicted from the active fluid circulation, and to the presence of multiple biotite generations, such as have already been documented from other micas from the Larderello–Travale geothermal system and as indeed confirmed by high-resolution petrography. Total K–Ar biotite ages on cluster around 3 Ma and broadly agree with a Rb–Sr age obtained by regressing only the biotite analyses. This suggests that the Montieri pluton, whose known volume is at least 25 km3, was emplaced at that time. The K-feldspar spectra are strongly discordant; no step age approaches the zero-age which would be predicted by Ar diffusivity modelling. The extraneous Ar in the feldspars is not excess Ar, but inherited Ar instead. Ar inheritance in the feldspar separates correlates with inheritance of Sr. Arrhenius trajectories of the apparent Ar diffusivities for all seven feldspars are astonishingly identical, despite the heterochemism amongst samples. However, the apparent diffusivity calculated from the step-heating experiments led to irreproducible models for the thermal history along the depth profile, as well as time-scales shorter than the historical record. This indicates that Ar diffusivities calculated from laboratory experiments must not be extrapolated to geological conditions.
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origin and evolution of pliocene pleistocene granites from the Larderello geothermal field tuscan magmatic province italy
Lithos, 2005Co-Authors: Andrea Dini, Giovanni Gianelli, Mariano Puxeddu, Giovanni RuggieriAbstract:Extensive, mainly acidic peraluminous magmatism affected the Tuscan Archipelago and the Tuscan mainland since late Miocene, building up the Tuscan Magmatic Province (TMP) as the Northern Apennine fold belt was progressively thinned, heated and intruded by mafic magmas. Between 3.8 and 1.3 Ma an intrusive complex was built on Larderello area (Tuscan mainland) by emplacement of multiple intrusions of isotopically and geochemically distinct granite magmas. Geochemical and isotopic investigations were carried out on granites cored during drilling exploration activity on the Larderello geothermal field. With respect to the other TMP granites the Larderello intrusives can be classified as two-mica granites due to the ubiquitous presence of small to moderate amounts of F-rich magmatic muscovite. They closely resemble the almost pure crustal TMP acidic rocks and do not show any of the typical petrographic features commonly observed in the TMP hybrid granites (enclaves, patchy zoning of plagioclase, amphibole clots). On the basis of major and trace elements, as well as REE patterns, two groups of granites were proposed: LAR-1 granites (3.8–2.3 Ma) originated by biotite-muscovite breakdown, and LAR-2 granites (2.3–1.3 Ma) generated by muscovite breakdown. At least three main crustal sources (at 14–23 km depth), characterized by distinct eNd(t) and 87Sr/86Sr values, were involved at different times, and the magmas produced were randomly emplaced at shallow levels (3–6 km depth) throughout the entire field. The partial melting of a biotite-muscovite-rich source with low eNd(t) value (about −10.5) produced the oldest intrusions (about 3.8–2.5 Ma). Afterwards (2.5–2.3 Ma), new magmas were generated by another biotite-rich source having a distinctly higher eNd(t) value (−7.9). Finally, a muscovite-rich source with high eNd(t) (about −8.9) gave origin to the younger group of granites (2.3–1.0 Ma). The significant Sr isotope disequilibrium recorded by granites belonging to the same intrusion is interpreted, as due to the short residence time of magmas in the source region followed by their rapid transfer to the emplacement level. Partial melting was probably triggered by multiple, small-sized mafic intrusions, distributed over the last 3.8 Ma that allowed temporary overstepping of biotite- and muscovite-dehydration melting reactions into an already pre-heated crust. Dilution in time of the magmatic activity probably prevented melt mingling and homogenization at depth, as well as the formation of a single, homogeneous, hybrid pluton at the emplacement level. Moreover the high concentrations of fluxing elements (B, F, Li) estimated for the LAR granites modified melt properties by reducing solidus temperatures, decreasing viscosity and increasing H2O solubility in granite melts. The consequences were a more efficient, fast, magma extraction and transfer from the source, and a prolonged time of crystallization at the emplacement level. These key factors explain the long-lived hydrothermal activity recorded in this area by both fossil (Plio-Quaternary ore deposits) and active (Larderello geothermal field) systems.
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an oxygen isotope study of silicates in the Larderello geothermal field italy
Geothermics, 1994Co-Authors: Eleonora Petrucci, Giovanni Gianelli, Mariano Puxeddu, Paola IacuminAbstract:Abstract Stable-isotope analyses were carried out on hydrothermal minerals sampled from the deep metamorphic units at Larderello, Italy. The ∂ 18 O values obtained for the most retentive minerals, quartz and tourmaline, are from + 12.0‰ to + 14.7‰ and 9.9‰ , respectively, and indicate deposition from an 18 O-rich fluid. Calculated ∂ 18 O values for these fluids range from + 5.3‰ to + 13.4‰ . These values, combined with available fluid inclusion and petrographic data, are consistent with the proposed existence of an early thermal fluid of probable magmatic origin and a late meteoric water. Mixing between these two fluids occurred locally.
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evidence for li rich brines and early magmatic fluid rock interactionin the Larderello geothermal system
Geochimica et Cosmochimica Acta, 1994Co-Authors: Michel Cathelineau, Giovanni Gianelli, Christian Marignac, Mariechristine Boiron, Mariano PuxedduAbstract:Abstract The geochemical features of fluids accompanying the first stages of geothermal activity linked to magmatic intrusions have been documented for the Larderello geothermal system (Italy). Deep drilling has provided samples which preserve evidence of this early geothermal activity. Four wells (San Pompeo 2, Monteverdi 7, Sasso 22, and Serrazzano, VC 11) penetrated the deeper parts of the Larderello system, located in a metamorphic basement underlying the Tertiary nappe complex which constitutes the main aquifer at Larderello. The drill holes terminated close to the inferred roof of a granitic complex thought to be responsible for geothermal activity. Fluid inclusion data were obtained from recrystallized quartz lenses and quartz veins in samples displaying high temperature assemblages (plagioclase-actinolite-biotite-tourmaline; clinopyroxene ± andradite-wollastonite) and also from magmatic quartz in a leucogranite dike. The inclusions are mainly secondary in origin, oriented in fluid inclusion planes (FIP) related to hydrothermal circulation in the Larderello system. Several generations of high temperature fluids were trapped and include: 1. (1) H2OCO2 dominated vapors displaying variable but significant contents of CH4 and N2; 2. (2) aqueous vapors containing LiCl, with variable salinity; 3. (3) aqueous LiCl brine, often oversaturated with respect to halite at room temperature; 4. (4) complex brine, always oversaturated at room temperature with respect to two (halite and sylvite) or more (n ≤ 4) salts. The presence of LiCl was confirmed by identification of the salt hydrate (LiCl5H2O) at very low temperature using Raman spectroscopy. Bulk salinities could be roughly estimated at around 30 wt% eq. LiCl for the LiCl brine. Geometric and chronologic relationships between FIP reveal close relationships and mutual contamination between the H2OCO2 vapors and LiCl brine, indicating synchronism in their trapping. These fluids were generated and trapped at pressures of 100–130 MPa, nearly 23 MPa above the estimated present-day lithostatic pressure. This implies a denudation rate between 0.2 and 0.5 mrn· a−1 since the onset of hydrothermal activity, compatible with the setting of Larderello in a young (Tortonian) collision belt. Fluid inclusion trapping temperatures (425–650°C) show a monotonous increase towards the inferred granite, and are around 100–200°C higher than the highest present-day temperatures. The results are interpreted as recording the interaction between magmatic and contact metamorphic fluids in the early Larderello system. The H2OCO2 vapors resulted from the reheating of the basement metamorphic series (often C-rich) under relatively high temperatures during contact metamorphism. Lirich fluids expelled from an underlying Li-rich leucogranite migrated through the metamorphic series and the already cooled granite dikes and experienced local boiling. The fluid inclusion data demonstrate the involvement of magmatic fluids during the initial development of this high energy geothermal field.
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Geochronology of the Larderello geothermal field: new data and the “closure temperature” issue
Contributions to Mineralogy and Petrology, 1994Co-Authors: Igor M. Villa, Mariano PuxedduAbstract:The Larderello geothermal field is generally accepted to have been produced by a granite intrusion at 4–9 km depth. Hydrothermal parageneses and fluid inclusions always formed at temperatures greater than or equal to the current ones, which implies that the field has always undergone a roughly monotonic cooling history (fluctuations < 40 K) since intrusion of the granite at 4 Ma. The heat required to maintain the thermal anomaly over such a long period is supplied by a seismically anomalous body of ≈ 32000 km3 rooted in the mantle. Borehole minerals from Larderello are thus a unique well-calibrated natural example of thermally induced Ar and Sr loss under geological conditions and time spans. The observations (biotites retain Ar above 450°C) agree well with other, albeit less precise, geological determinations, but contrast with laboratory determinations of diffusivity from the literature. We therefore performed a hydrothermal experiment on two Larderello biotites and derived a diffusivity DLab(370°C)=5.3·10-18 cm2s-1, in agreement with published estimates of diffusivity in annite. From DLab and the rejuvenation of the K/Ar ages we calculate maximum survival times at the present in-hole temperatures. They trend smoothly over almost two orders of magnitude from 352 ka to 5.3 ka, anticorrelating with depth: laboratory diffusivities are inconsistent not only with geological facts, but also among themselves. From the geologically constrained lifetime of the thermal anomaly we derive a diffusivity DG(370°C)=3.81·1021 cm2s-1, 3±1 orders of magnitude lower than DLab. The cause of these discrepancies must be sought among various laboratory artefacts: overstepping of a critical temperature T*; enhanced diffusivities in “wet” experiments; presence of fast pathway (dislocation and pipe) diffusion, and of dissolution/reprecipitation reactions, which we imaged by scanning electron microscopy. These phenomena are minor in geological settings: in the absence of mineral transformation reactions, complete or near-complete resetting is achieved only by volume diffusion. Therefore, laboratory determinations will necessarily result in apparent diffusivities that are too high compared to those actually effecting the resetting of natural geochronometers.
