Torre Alfina Research Articles

Subduction‐Legacy and Olivine Monitoring for Mantle‐Heterogeneities of the Sources of Ultrapotassic Magmas: The Italian Case Study

AbstractThe origin of Italian kamafugites and lamproites is a matter of debate, not least due to their “crustal signature” displayed by trace element compositions and isotopic ratios, but also due to puzzling geodynamic significance. We combine in situ EMPA and LA‐ICP‐MS analyses with in situ analyses of oxygen isotopes (SIMS) on olivine from the Pleistocene San Venanzo kamafugites and Torre Alfina lamproites. Lamproitic olivine shows extremely high Mg# and Ni concentrations whereas Ca and Mn concentrations are low. Their δ18OV‐SMOW values are very high up to +11.5 ‰. In kamafugites we recognize three genetically different olivine groups: (a) phenocrystic one with high Mg#, very low Ni, high Ca and Mn. Values of δ18OV‐SMOW are up to +10.9 ‰; (b) melt‐related xenocrystic grains that compositionally resemble lamproitic olivine; (c) skarn‐related almost pure forsterite of extreme δ18OV‐SMOW ∼27 ‰, with negligible amounts of minor and trace elements. The melting and crystallization conditions of Italian kamafugites and lamproites indicate compositionally heterogeneous mantle sources on very small scales. Distinct geochemical features of the olivine macrocryst populations observed in kamafugite point to a range of processes occurring both within the magma storage and transport system. We suggest that the diversity of metasomatic agents was involved in mantle processes on local scales, coupled with magma mixing and/or the uptake of xenocrysts during magma ascend.

From landslide characterization to nature reserve management: The “Scialimata Grande di Torre Alfina” landslide Geosite (Central Apennines, Italy)

Italy is characterized by widespread geomorphological instability, among which landslides leave impressive marks on the landscape. Nevertheless, landslide bodies may represent key sites for thematic and educational itineraries, especially in protected areas, where their management becomes an important issue. Our study focuses on the “Monte Rufeno Nature Reserve” (Central Apennines, Italy), where iconic landslides are present. Here, the “Scialimata Grande di Torre Alfina” landslide (SGTA) is listed in the regional Geosite database. This work aims to propose a multiscale procedure for landslide analysis, in terms of both hazard sources but also educational and geoheritage enhancement opportunities in natural reserves. After performing a Landslide Susceptibility conditional Analysis (LSA) for the reserve territory, attention was focused on the SGTA, to define properly its features and morphodynamics. A multi-disciplinary approach was adopted, by applying both remote sensing (UAV structure from motion, Photointerpretation) and field survey (geomorphological and GPS monitoring). From the LSA, based on drainage density, curvature, and slope triggering factors, the road and trail susceptibility maps were derived, as base tools for future risk assessments and trail paths management within the reserve. At the SGTA scale, the monitoring showed a displacement of up to 23 m during the time interval between 2015 and 2018. The landslide dynamics seem to be driven by alternating dry and extremely wet periods; moreover, leaks from the aqueduct in the detachment area and piping effects through clays may have also decreased the substrate cohesion. The SGTA complex influence on the Paglia River valley geometry was also hypothesized, underlining the action of landslide through different spatial scales (on-site and off-site) and on different environment features (sediment connectivity, hydrology). Finally, the SGTA appears highly representative of the geomorphic dynamics within the Nature Reserve (i.e., scientific value) and it could be classified as an active geosite. Since the site was featured by a tourist trail, adequate management strategies must be adopted, considering the educational value and safety issues.

Exergo-economic and exergo-environmental analysis of a binary geothermal power plant with solar boosting

The exploitation of renewable energies is a solution to the energy, economic and environmental issues related to the massive use of fossil resources. Thus, investing in renewable technologies is essential to achieve the carbon-neutral scenario within 2050. In this framework, geothermal energy may have a key role. In particular, power plants with a closed binary cycle are suitable for harnessing geothermal resources with low and medium enthalpy levels. They are prone to be integrated with other renewable devices to increase the global power output.Geothermal fluid can be drawn constantly from underground throughout the day and seasons. Conversely, the availability and intensity of solar energy depend on weather conditions and the time of year. In Italy, geothermal energy is currently harvested for continuous electricity generation, while solar energy is mainly used for photovoltaic generation. For small-to-medium size plants, rated between 5 and 20 MWe, the geothermal and thermodynamic solar hybridization may lead to relevant benefits for the economic competitiveness regarding separate photovoltaic or thermodynamic solar systems.This article aims to investigate the economic and environmental aspects of geothermal power plants with a closed binary cycle coupled with a topper cycle fed by linear parabolic solar collectors. The system operation in both design and off-design conditions was analysed, and exergo-economic and exergo-environmental simulations were conducted. The application site was selected near Torre Alfina (Italy). It has a water-dominant reservoir with a pressure of 44 bar, a temperature of 140 °C, and content of non-condensable gases (NCGs) approximately equal to 2% by weight. At the design point, the net power is 8.4 MW and the first and second principle efficiencies are 9.31% and 18.45%, respectively. The exergo-economic and exergo-environmental analyses indicate that the components with the highest economic and environmental impact are the condenser, the field of solar collectors, the evaporator, and the low-pressure turbine. The levelized cost of electricity (LCOE) is equal to 14.19 c€/kWh.

The Carbon Dioxide Emission as Indicator of the Geothermal Heat Flow: Review of Local and Regional Applications with a Special Focus on Italy

We review the methods based on the measurement of CO2 emissions for the computation of geothermal heat flow, both at a local (hydrothermal sites, a few km2) and regional scale (hundreds km2). At the local scale, we present and discuss the cases of the Latera caldera and Torre Alfina (Italy) geothermal systems. At Torre Alfina and Latera, the convection process sustains a CO2 emission of ~1 kg s−1 and ~4 kg s−1, and heat flows of 46 MW and 130 MW, respectively. At the regional scale, we discuss the case of the central Apennine (Italy), where CO2 mass and enthalpy balances of regional aquifers highlights a wide and strong thermal anomaly in an area of low conductive heat flow. Notably, the CO2/heat ratios computed for the central Apennines are very similar to those of the nearby geothermal systems of Latium and Tuscany, suggesting a common source of CO2-rich fluids ascribed to the Tyrrhenian mantle.

Geothermal power plants with improved environmental performance: assessment of the potential for an Italian site

Geothermal energy is a clean resource, which could significantly contribute to the reduction of greenhouse and other gas emissions by replacing fossil fuels for power generation. In many geothermal sites, the resource contains substantial Non-Condensable Gases (NCGs: CO2 and contaminants), whose emissions can be limited to developing power plant schemes suitable for complete resource reinjection. Organic Rankine or other closed-loop cycles are definitely favored in this light. This work investigates a solution for complete NCG reinjection in the liquid-dominated reservoir conditions typical of the Monte Amiata area (Italy), referring to the specific site of Torre Alfina (IT) which presents a specific attractiveness because of its high pressurization. The solution considered avoids flashing the resource and thus presents an appealing environmental performance. The power plant models include energy and exergy balances, as well as exergo-environmental analysis. The overall environmental performance is evaluated by a simplified (preliminary) Life Cycle Analysis (LCA). Different solutions are compared, considering the possibility of sub- or super-critical power cycles.

Energy, exergy, and cost comparison of Goswami cycle and cascade organic Rankine cycle/absorption chiller system for geothermal application

Combined cooling and power production can improve the overall efficiency of a geothermal energy conversion system. Most systems have complex structures, characterized by a large number of devices resulting in a greater heat loss, low energy conversion efficiency and higher costs. This paper presents a geothermal system based on a modified Goswami cycle, aimed to produce cooling and power simultaneously in one loop. The proposed system has a simple structure with few components, and the heat rejected to the environment by the rectifier is internally recovered to preheat the strong solution, involving lower heat losses and higher conversion efficiencies. In order to investigate the exergy destructions and losses, the overall performances, and the total absolute component cost, the modeling of the proposed system is established in detail. The Goswami system performance is evaluated for the conditions of an actual application, the geothermal reservoir of Torre Alfina (Italy), and the results compared with those of a cascade system based on organic Rankine cycle coupled with a water/lithium bromide absorption chiller. Results show that for the same power output produced in both systems, the best scenario is the Goswami cycle system with an exergy efficiency of 35.53% against 32.77% of the cascade system case. In addition, the component cost analysis showed that the highest total absolute component cost is obtained for the cascade system case with a total of 14.6 M€, while the Goswami case showed a lower value with a total of 11.8 M€.

Geothermal fluid circulation in a caldera setting: The Torre Alfina medium enthalpy system (Italy)

The Torre Alfina medium enthalpy geothermal field is located about 10 km north of the Bolsena caldera (Italy). The reservoir is a buried structural high consisting of fractured Meso-Cenozoic carbonate sequences and sealed by clayey flysch successions and Pliocene marine clays. We performed TOUGH2 numerical simulations, testing different model designs based on contrasting conceptual models.Results indicate that deep circulation is forced by the geometry of the reservoir and by the applied T and P gradients. We interpret the Torre Alfina field as a "blind" system, mostly recharged by lateral advection of heat and fluids from the Bolsena caldera deep high-enthalpy system, through the permeable caldera faults.

Microseismicity analysis in the geothermal area of Torre Alfina, Central Italy

The geothermal field of Torre Alfina is located in central Italy at the northern edge of the Vulsini Volcanic District, the northernmost area of the so-called Quaternary Roman Co-Magmatic Province. In the framework of a medium-enthalpy geothermal exploitation project, INGV installed a local seismic network close to the future geothermal production site for monitoring natural local seismicity. In this paper, we show the results of a study of the microseismicity recorded from June 2014 to May 2016 in a small area of about 10 km2 around Torre Alfina. Analyzing seismic signals recorded by a local temporary network of ten short-period stations and by four permanent stations of the INGV national seismic network, we detected 846 local earthquakes. Then, we accurately relocated 799 events using HypoDD code. Our results show that the region of Torre Alfina is characterized by intense microseismicity, with hypocentral depths between 3 and 7 km and with moderate magnitudes between Md = 0.1 and ML = 2.8. Moreover, more than half of the earthquakes are grouped into six main swarm-like clusters each lasting few days. Furthermore, we computed 36 well-constrained fault plane solutions, which show a clear transtensional deformation regime in the whole study area. Three main tectonic directions have been evidenced from the focal mechanisms analysis: E-W, WSW-ENE, and NW-SE. The understanding of the seismogenic structural setting of the Torre Alfina geothermal field, and the study of its background natural seismicity can be of great importance in recognizing any possible future seismicity induced by the exploitation of the field.

Thermodynamic Model of Geothermal Resources for Low-medium Temperatures Energy Conversion Process Optimization

Environmental and economic concerns are motivating manufacturers and public entities towards the use of renewable energy, which is continuously increasing its market penetration. Among the possible renewable energy resources, geothermal is particularly attractive compared to others such as solar and wind, mainly because of its continuity and dispatchability. Most studies in geothermal energy conversion systems model the resource as pure water or steam, while in reservoir simulations it is common practice to apply advanced geochemical modelling to estimate the long-term productivity. The presence of CO2 and saline equilibria may determine power plant optimisation conditions that differ from the ones which assume the resource as pure water or steam. The thermodynamic properties of the geothermal resource with high carbon dioxide (CO2) contents (1 to 8% in mass) within the 298 – 473 K temperature and 15 bar pressure are examined. The model applied focuses on the Equations of State (EoS) to be used in the calculation of the physical/thermodynamic properties of a mixture. The obtained results are validated through the comparison of different commercial software (UNISIM®, EES®, REFPROP®, TREND®). As an example, the Torre Alfina geothermal resource data (Middle Italy) are considered and the effects on the performance of a binary (ORC) geothermal power plant producing are examined in terms of energy and exergy efficiency. A sub-critical Iso-butane and a supercritical R134a power cycle are compared, and the advantages of the supercritical solution are demonstrated.

Energy and exergy analysis of cold and power production from the geothermal reservoir of Torre Alfina

Geothermal power plants can provide clean and renewable energy and can be proposed as integrated units for simultaneous production of cooling and power. This paper presents a cascade arrangement of an organic Rankine cycle (ORC) and a water/lithium bromide (LiBr) absorption chiller (ABS). Starting from a literature reference layout which is taken as benchmark, some improvements are proposed at system level.To assess the performance of the system, a thermodynamic model is developed in EES and the energy and exergy balance is calculated. The proposed system is re-evaluated with reference to resource conditions corresponding to a planned power plant in central Italy, Torre Alfina (TA). A sensitivity analysis is performed in order to investigate the operating range of the plant and the possibility of adapting its design to the requirements of the customers. Under optimized conditions, the TA Case (targeted on a 5 MW power output) showed an energy utilization factor (EUF) of 46.2% and an exergy efficiency of 27.7%, neglecting the brine reinjection loss. The highest exergy destructions occur in the ORC economizer (8.6%), in the ABS generator (6.3%) and absorber (5.5%). The good resource conditions in TA case drive the design optimization to production of power rather than cold.

The seismic sequence of 30th May–9th June 2016 in the geothermal site of Torre Alfina (central Italy) and related variations in soil gas emissions

In the framework of a medium-enthalpy geothermal exploitation project, seismicity and soil gas emissions have been monitored in the area of Castel Giorgio–Torre Alfina (central Italy) since 2014. A dedicated local seismic network, called ReMoTA, allows deepening of the knowledge of the natural local seismicity in terms of source mechanisms, high-quality event localization and magnitude estimation. From November 2014 to May 2016, ReMoTA recorded 846 seismic events with a magnitude range from Md 0.1 to ML 2.8 and with hypocentres between 4 and 8 km depth. Most of these events occurred in six short lasting clusters. On the 30th May 2016 a Mw 4.3 earthquake occurred near Castel Giorgio, followed by almost 1700 aftershocks. The moment tensor solution for the main shock depicts a WNW-ESE oriented normal fault with small right-lateral strike component. An overview of the epicentral distribution of the different clusters recorded since 2014 highlights that the active tectonic structures are orientated principally along the NE–SW and WNW-ESE directions. The relocation of the 1957 Me 4.9 earthquake suggests that this event occurred in the same fault system as of the 2016 seismic sequence. In the study area, there is only one natural emergence (Solfanare) emitting a CO2 dominated gas, having the same chemical and isotopic composition of the gas contained in a cap at the summit of the geothermal reservoir. Apart from small perturbations, no significant compositional variations were recorded during the 2016 seismic sequence in the gas of the Solfanare vents that was analyzed continuously by an automatic gas-chromatographic station. The diffuse soil CO2 flux is monitored since 2013 in six target areas located around the future production and reinjection wells, in order to assess the level of background natural degassing. In all target areas the maximum value of soil CO2 flux has been recorded during the 2016 seismic sequence. However, values remained relatively low (maximum 112 g ⋅m−2 ⋅d−1) and the values of δ13C of the emitted CO2 (−25.25 to −24.22‰ vs. PDB) indicated a shallow biological origin of the gas (by soil respiration). Only at Solfanare high values of diffuse soil CO2 flux were recorded up to a maximum of 20125 g ⋅m−2 ⋅d−1). All the seismicity of the May–June 2016 sequence is located above the ML4.1 main event and is distributed on small distinct faults (such as at San Lorenzo Nuovo and Acquapendente) and triggered by the main shock. The source mechanism provided by the full moment tensor indicates that rupture processes at depth probably deviate from a pure normal fault. The significant contribution of CLVD and isotropic components suggest a possible opening of fluid cracks below the geothermal reservoir hosted in fractured Mesozoic limestones. In spite of the increase of the CO2 flux, no significant changes have been observed in the chemical and isotopic composition of the Solfanare gas. The seismo-tectonic scenario indicates that the Solfanare fault was not activated. Kinematics and orientation of the activated faults suggest a relationship with the Bolsena caldera collapse.

Structural and chemical variations in phlogopite from lamproitic rocks of the Central Mediterranean region

Micas from mafic ultrapotassic rocks with lamproitic affinity from several localities of the Central Mediterranean region were studied through single-crystal X-ray diffraction (SC-XRD), electron microprobe analysis (EMPA) and Secondary Ion Mass Spectrometry (SIMS); Mössbauer Spectroscopy (MöS), when feasible, was also applied to minimise the number of unknown variables and uncertainties. Analysed lamproitic samples cover the most important Central Mediterranean type localities, from Plan d'Albard (Western Alps) to Sisco (Corsica), Montecatini Val di Cecina and Orciatico (Tuscany, Italy) and Torre Alfina (Northern Latium, Italy). The studied crystals show distinctive chemical and structural features; all of them belong to the phlogopite-annite join and crystallise in the 1M polytype, except for micas from Torre Alfina, where both 1M and 2M1 polytypes were found. Studied micas have variable but generally high F and Ti contents, with Mg/(Mg+Fe) ranging from ~0.5 to ~0.9; 2M1 crystals from Torre Alfina radically differ in chemical composition, showing high contents of Ti and Fe as well as of Al in both tetrahedra and octahedra, leading to distinctive structural distortions, especially in tetrahedral sites.SIMS data indicate that studied micas are generally dehydrogenated with OH contents ranging from ~0.2 apfu (atoms per formula unit) for Orciatico and Torre Alfina to ~1.4 for Plan d'Albard crystals; this feature is also testified by the length of the c parameter, which decreases with the loss of hydrogen and/or the increase of the F→OH substitution. Chemical and structural data suggest that the entry of high charge octahedral cations is mainly balanced by an oxy mechanism and, to a lesser extent, by a M3+,4+-Tschermak substitution. Our data confirm that Ti preferentially partitions into the M2 site and that different Ti and F contents, as well as different K/Al values, are both dependant upon fH2O and the composition of magma rather than controlled by P and T crystallisation conditions.The obtained data help to discriminate among lamproite-like rocks formed within a complex geodynamic framework but still related to a destructive tectonic margin and evidence different trends for micas from the youngest Torre Alfina (Northern Latium) lamproites, referred to the Apennine orogeny and those of the older lamproites from Orciatico, Montecatini Val di Cecina (Tuscany), Western Alps, and Corsica, the latter referred to the Alpine orogeny. Phlogopite crystals from the older lamproites fall within the compositional and structural field of worldwide phlogopites from both within-plate and subduction-related settings. Phlogopite from the Plio-Pleistocene lamproite-like occurrence in Tuscany and Northern Latium, despite crystals with low Mg# of the Torre Alfina rock plot well within the general field of the other crystals in less evolved samples, follows a different evolution trend similar to that of shoshonites from Tuscany and Northern Latium. On this basis, we argue that the observed differences are inherited by slight differences in the magma compositions that are related to different genetic and evolution pathways.

The sealing capacity of the cap rock above the Torre Alfina geothermal reservoir (Central Italy) revealed by soil CO2 flux investigations

Torre Alfina is a medium enthalpy (T=140°C) geothermal field in Central Italy, hosted in buried fractured Mesozoic limestones, extensively explored in the 1970s and 1980s, but which so far has not been exploited. A detailed diffuse soil CO2 flux investigation (1336 measurements over a surface of 12.6km2) and the periodic monitoring of soil CO2 flux from target areas indicate that in most of the area, even above a pressurized gas cap existing at the reservoir top, the soil CO2 flux is low and mostly within the background threshold (48gm−2 day−1) and is likely generated by biological soil respiration. Anomalous values (up to 30.250gm−2 day−1) are found only in the proximity of a small zone with gas vents whose composition is identical to that of the gas produced by the geothermal wells tapping the reservoir. This is the only zone where a fault connecting the deep geothermal reservoir with the surface does exist. These data are compared with those of the near Latera high enthalpy geothermal field (T=210°C), where anomalous soil CO2 flux is recorded above the productive reservoir. The difference of soil CO2 release in the two fields is attributed to the differences in the impervious cover (allochthonous flysch deposits) above the carbonate reservoir, which is thin and locally lacking at Latera, and continuous and thick (>400m) at Torre Alfina. Results demonstrate that soil CO2 flux investigations are useful in geothermal exploration, but only high flux values likely indicate the presence of a geothermal reservoir at depth, whereas low flux values can indicate either the lack of an active geothermal reservoir at depth or the presence of a very effective impervious cover above the reservoir.

Structural compartmentalisation of a geothermal system, the Torre Alfina field (central Italy)

Abstract Recent surging of renewed industrial interest in the exploration of low and medium enthalpy geothermal fields makes the accurate assessment of the geothermal potential essential to minimise uncertainties during both exploration and exploitation. The Torre Alfina field is a case of abandoned, but promising, geothermal field of central Italy where the roles of the internal structural setting and of the recharge areas on the hydrothermal circulation are largely unconstrained. In this paper, field structural data integrated with geomorphic lineament analysis document the occurrence of post-orogenic deformation structures controlling the compartmentalisation of the Torre Alfina geothermal field. Strike-slip and subordinate normal fault systems (with associated network fractures) cut and dislocate the internal architecture of the reservoir and prevent its hydraulic connection with Mount Cetona, considered to be the recharge area and where hydrothermal manifestation, including travertine deposition, occurs. 230Th/234U radiometric dating of superposed travertine units gives 200, 120 and 90 ka respectively, inferred to correspond to the age of the fossil hydrothermal circulation during tectonic activity. The results have been used for illustrating a new geological conceptual model for the Torre Alfina area where the geothermal system is composed of different compartments. Tectonic structures define the main boundaries between compartments, helping the understanding of why productive and non-productive wells were found in apparently similar structural settings within the Torre Alfina field.

Revision, calibration, and application of the volume method to evaluate the geothermal potential of some recent volcanic areas of Latium, Italy

The volume method is used to evaluate the productive potential of unexploited and minimally exploited geothermal fields. The distribution of P CO2 in shallow groundwaters delimits the geothermal fields. This approach is substantiated by the good correspondence between zones of high CO 2 flux, and the areal extension of explored geothermal systems of high enthalpy (Monte Amiata and Latera), medium enthalpy (Torre Alfina) and low enthalpy (Viterbo). Based on the data available for geothermal fields either under exploitation or investigated by long-term production tests, a specific productivity of 40 t h −1 km −3 is assumed. The total potential productivity for the recent volcanic areas of Latium is about 28 × 10 3 t h −1, with 75% from low-enthalpy geothermal fields, 17% from medium-enthalpy systems, and 8% from high-enthalpy reservoirs. The total extractable thermal power is estimated to be 2220–2920 MW, 49–53% from low-enthalpy geothermal fields, 28–32% from medium-enthalpy systems, and 19–20% from high-enthalpy reservoirs.

Fluxes of deep CO 2 in the volcanic areas of central-southern Italy

Both the shallow (organic-derived) and deep (mantellic–magmatic–metamorphic) fluxes of CO 2 [ Φ CO 2 , mass time −1] and specific fluxes of CO 2 [ ϕ CO 2 mass time −1 surface −1] dissolving in the shallow groundwaters of the volcanic areas of Amiata, Vulsini-Vico-Sabatini, Albani, Roccamonfina, Vesuvio, Vulture, and Etna were evaluated by partitioning the composed population of total dissolved inorganic carbon in two individual populations and subsequent subtraction of local background population. The flux of deep CO 2 released from the geothermal fields of Piancastagnaio (Amiata), Torre Alfina, Latera, Marta, Bracciano south, Cesano, and Mofete and from the Overall Northern Latium Hydrothermal Reservoir were also evaluated by means of the total surface heat flux and the enthalpy and CO 2 molality of the single liquid phase circulating in each geothermal reservoir. These data suggest that the ϕ CO 2 released to the atmosphere varies from 9.5×10 6 to 3.0×10 6 mol year −1 km −2, over the geothermal fields of Bracciano south and Cesano, respectively, and that a total Φ CO 2 of 3.8×10 8 mol year −1 is cumulatively released from the geothermal fields of Torre Alfina, Latera and Cesano extending over an area of only 66 km 2. In addition, a flux of ∼2.2×10 11 to 3.8×10 11 mol year −1 of gaseous CO 2 entering the atmosphere is obtained for the entire anomalous area of central Italy, extending from the Tyrrhenian coastline to the Apennine chain (45,000 km 2). Thus terrestrial CO 2 emission in central-southern Italy appears to be a significant carbon source.

Anisotropic seismic properties of the upper mantle beneath the Torre Alfina area (Northern Apennines, Central Italy)

Central Italy is an active tectonic area that has been recently studied by several regional mantle, Pn and SKS, studies which revealed the presence of a strong regional anisotropy. In this paper, we present the first petrophysical results on the only mantle xenoliths from Central Italy, which place new constraints on the upper mantle structures of this region. The Torre Alfina mantle xenoliths are very small in size, from few millimetres to about 1.5 cm. They are mainly dunites and harzburgites, with subordinate lherzolites and wehrlites. Since olivine and spinel are always present, they should have crystallised in the spinel-bearing lherzolite field. Their mineralogical composition is ol+spl±opx±cpx. Both olivines and pyroxenes are present as porphyroclasts and as neoblasts. The xenoliths show different degrees of recrystallization. Geothermobarometry on these xenoliths give a temperature range of 1040±40 °C and a pressure estimate of about 1.5 GPa, corresponding to 50 to 60 km depth. Previous seismic studies have estimated the Moho to be at 20 to 25 km in this region, hence the xenoliths come from a hot mantle, probably asthenospheric, below a lithosphere of about 25 to 40 km in thickness below the Moho. We measure the crystallographic preferred orientation (CPO) of olivines and pyroxenes using a SEM and the Electron Back Scattered Diffraction (EBSD) technique. The CPO shows all three axes of olivine are tightly clustered: [100] axis is typically more tightly clustered than [010] and [001] is the most widely distributed axis. The fabric strength expressed by the integral J index, varies from 4.5 to 25.9, and decreases with the degree of recrystallization. We use CPO data to calculate anisotropic seismic properties of the xenoliths. They are very homogenous and probably statistically representative of the mantle below the Torre Alfina area. Vp ranges from 8.4 to 9.1 km/s, Vs 1 from 4.8 to 5.0 km/s. The seismic anisotropy is more variable; AVp ranges from 9.8% to 19.3% and AVs from 7.3% to 13.4%. The majority of the xenoliths display an orthorhombic seismic symmetry, but xenoliths with a transverse isotropic behaviour have also been observed. We consider four geodynamic models for the source region of the xenoliths (extension, shear, upwelling, slab tilted), defined by different orientations of the structural reference frame, and we calculated for each model the variation of the seismic properties with temperature, pressure and volume fraction of orthopyroxene. After comparing this variation of calculated seismic parameters with seismic observations from the region, we form the hypothesis that the xenoliths come from either an extensional tectonic zone (lineation X and foliation plane XY horizontal) or transcurrent shear zone (lineation X horizontal and foliation plane XY vertical) and that the mantle beneath Torre Alfina is composed by 70% olivine and 30% orthopyroxene forming an anisotropic layer of about 160 or 110 km in thickness, respectively.

The effect of crustal contamination on ultrapotassic magmas with lamproitic affinity: mineralogical, geochemical and isotope data from the Torre Alfina lavas and xenoliths, Central Italy

Torre Alfina, a Quaternary volcano located in the northernmost part of the Roman Province, is characterized by two main types of mafic ultrapotassic lavas with slightly different chemical, isotopic, and mineralogic characteristics. Mica-rich enclaves, peridotitic and granulitic xenoliths, and upper crustal xenoliths are hosted by the lavas. Both of the lavas have the assemblage olivine+clinopyroxene+sanidine+phlogopite with mafic phases showing insufficient Si+Al cations to completely fill the tetrahedral sites, as is typical of lamproitic minerals. Other features are also transitional between lamproite mineralogy and Roman-type mineralogy. Chemically, the host lavas have high Mg/Mg+Fe values (71–77) and compatible trace element contents (e.g., Ni=245–249 ppm), as well as high K 2O (5.7–7.5 wt.%) and incompatible trace element abundances. Furthermore, they have high initial 87 Sr/ 86 Sr (0.7159–0.7165) and low initial 143 Nd/ 144 Nd (0.51211–0.51212) values. Passing from the most primitive through the more evolved lavas, the Sr isotopic ratios, (Tb/Yb) N ratios, and SiO 2 and Al 2O 3 contents increase, whereas the abundances of both the compatible and incompatible trace elements decreases. Crustal xenoliths are represented by shale, sandstone, gneiss, micaschist, and granulitic rock fragments. They have variable chemical compositions with 87 Sr/ 87 Sr and 143 Nd/ 144 Nd values, calculated at the time they were trapped by the magma, in the ranges 0.7136–0.7176 and 0.51204–0.51209, respectively. Digestion of crustal rocks, such as those included in the lavas, is not able to drive the magmas from Roman-type to lamproitic compositions. Mass-balance calculations show that extensive crustal contamination of lamproitic parental magma can produce the observed chemical, isotopic, and mineralogical characteristics of the Torre Alfina rocks. Since no crustal rocks with incompatible trace element abundances as high as those of lamproitic rocks are common at the earth's surface, lamproitic magmas assimilating crustal rocks become diluted in incompatible trace elements, and enriched in silica and alumina.

In situ Measurement of Oxygen Isotope Ratios by Laser Ablation Mass Spectrometry — an Example from the Torre Alfina Volcano, Central Italy

The Torre Alfina volcano is located in the southern part of the Tuscan Magmatic Province, Central Italy. The rocks of the volcano have a rather constant mineralogical and geochemical composition and are slightly porphyric with olivine microphenocrysts set in a microcrystalline groundmass consisting of biotite and feldspar. They belong to the ultrapotassic rock family (Peccerillo, 1994). Corresponding to the classification of Foley et al. (1987) the Torre Alfina rocks are transitional rocks between lamproites and Roman-type rocks. Frequently mantle xenoliths and crustal xenoliths are embedded. Lamproitic magma modified by crustal assimilation is assumed as source material for the Torre Alfina transitional rocks (Peccerillo, 1994). The genesis of lamproitic magma is considered by partial melting of a refractory harzburgitic mantle, which was metasomatized by a melt with a composition similiar to pelitic sediments (Conticelli and Peccerillo, 1992). The metasomatism of the mantle probably occurs in the formation of mica rich veins (Foley, 1992). Since the late 80th's laser microprobe techniques have been developed to measure oxygen isotope ratios in single minerals or in small rock sections. Refractory minerals like olivine and garnet can be analysed with the UV laser ablation technique without any fractionation due to attainable high temperatures (Wiechert and Hoefs, 1995). The applied laser microprobe device is built up by an optical system for beam adjustment and sample observation, a vacuum system to purify the sample gas and to transform the oxygen gas into CO2 with heated diamonds, and a gas source mass spectrometer. The reproducibility for in situ measurements of 8180 is about +0.2%0. In order to prohibit recombination of the vapourized oxygen and metal atoms during laser ablation the sample chamber is filled with fluorine gas. Rocks consisting of a fine-grained or vitrious and feldspar rich matrix and of porous structures as well as carbonates and/or water containing minerals start to react with the fluorine before laser treatment, which leads to high blanks. To reduce the blank a special acid treatment has been developed prior to the introduction of the sample into the sample chamber. Despite this treatment some samples still generate a high blank. In these cases the blank can be reduced by the fluorination of the reactive part of the sample. One peridotitic xenolith and one crustal xenolith have been selected for this study and analysed in detail. Oxygen isotope ratios have been measured using an ArF-excimer laser ablation technique. Figure 1 shows a traverse of oxygen isotope analyses in a peridotitic xenolith, which mainly consists of olivines and minor amounts of orthopyroxenes and clinopyroxenes. Olivines are both finegrained (about 200 gm) as well as coarse-grained (about 1 mm). Oxygen isotope ratios of olivines

Upper crustal tomographic images of the Amiata‐Vulsini geothermal region, central Italy

We computed the three‐dimensional P wave velocity structure of the upper crust beneath the adjacent Amiata and Vulsini Quaternary volcanoes (central Italy), inverting local earthquake arrival times. We used 676 selected earthquakes out of more than 3000 (in the M range 1.5–4.0) located between 1977 and 1992 by two contiguous microseismic networks operated by Ente Nazionale Energia Elettrica to monitor three geothermal areas (namely, Latera, Torre Alfina, and Mount Amiata). The resolving capability of the data set has been verified both with a priori synthetic tests and an a posteriori analysis of the resolution matrix. We find a 40×25 × 7 (h) km3 crustal volume where we can resolve details as small as 2 to 4 km. Our analysis is favored by numerous geological and geophysical data collected for geothermal exploration. These independent data give us specific constraints for both choosing a starting model and interpreting tomographic results. We find high‐velocity anomalies in the upper 3 km of the crust beneath Latera, Torre Alfina, and Mount Amiata geothermal fields, related to uplifted limestone units. The lateral extension of limestone units is defined by the three‐dimensional velocity pattern. A negative anomaly clearly identifies the Radicofani Graben, a Plio‐Quaternary depression filled with low velocity clayey sediments, between 1 and 3 km depth. A sharp gradient between high velocities (∼6.9 km/s) in the northern region and lower velocities in the southern one is recovered at 7 km depth. This feature may indicate the transition between the Tuscan province uplifted metamorphic crust and thickened carbonate series of the Latium province.