Torre Alfina Research Articles

Light hydrocarbons in gas-emissions from volcanic areas and geothermal fields.

Geochemical investigation of gas emissions at low temperatures in areas of Quaternary volcanism shows a systematic enrichment of aromatic hydrocarbons and alkenes in fumaroles from active volcanic areas (Vulcano and Phlegrean Fields, Italy; S. Miguel, Terceira and Graciosa, Azores; Chichon, Mexico; Uzon and Geyser Valley, Kamchatka) and geothermal systems (Larderello area, Italy) with respect to those pertaining to areas of recent but exhausted volcanism (Vulsini and Sabatini volcanic districts, Torre Alfina volcano, Italy). The general behaviour of hydrocarbons in the investigated gas-emissions, together with thermodynamic and experimental data (taken from the industrial treatment of petroleum) suggest possible abiogenic processes acting on degradation of alkane hydrocarbons when source temperatures exceed 150° and acid species together with high active-surface area clay minerals are available. Benzene shows very similar enrichments in emissions from Vulcano beach and Phlegrean Fields, Kamchatka and Azores, pointing out the possible occurrence of catalytic reactions at the source. Alkenes display significant relative increases in crater fumaroles from Chichon volcano. Moreover, lower carbon-number alkenes (propene and ethene) progressively appear with increasing alkenes/alkanes ratio, suggesting an increasing importance of thermal-cracking reactions. The general similarities in distribution patterns from different active volcanic environments allow us to reappraise the influence of the nature of sources and to emphasize the effects of abiogenic reactions. With reference to areas of active volcanism, the distribution of hydrocarbons can represent a useful parameter to evaluate the prevailing physico-chemical conditions and their time-space variabilities. Accordingly they can give a significant contribution to the geochemical monitoring of active volcanic areas.

Chemical relationship between discharging fluids in the Siena-Radicofani graben and the deep fluids produced by the geothermal fields of Mt Amiata, Torre Alfina and Latera (Central Italy)

The thermal springs discharging in the Siena-Radicofani basin and the deep fluids within the geothermal systems of Piancastagnaio (Mt Amiata), Torre Alfina and Latera (Vulsini Mts) have a common origin. The chemical composition and evolution towards the low enthalpy of the springs as compared to the high enthalpy of the geothermal fluids are affected by both the structural setting of the region and the deep hydraulic conditions. Recharge of both the shallow thermal aquifer and the deep geothermal systems takes place in the outcrop areas of Mesozoic carbonate rocks, which constitute the main potential geothermal reservoir in central Italy. The waters of meteoric origin are heated at depth, as a consequence of anomalous heat flow in the region; these waters acquire a CO 2-rich rising gas phase, equilibrate equilibrate with the reservoir rocks and, finally, assume their CaHCO 3SO 4 composition. If these waters discharge rapidly from the border fault systems of the Siena-Radicofani basin they maintain their original composition. If, instead, they emerge from the inner faults of the graben, their temperature and dissolved solids increase so that they become NaCl with a high content of NH 4, H 3BO 3 and Li +. The chemical evolve towards high enthalpy. In this case the deep fluids again acquire an NaCl composition similar to carbonate reservoir is at a structural high, buried by both allochthonous flyschoid series (“Ligurids”) and by Neogenic pelitic formations, a slower recharge occurs in the reservoir, and consequently the fluids evolve towards high enthalpy. In this case the deep fluids again acquire an NaCl composition similar to that of the springs emerging inside the graben, since part of the reservoir recharge also occurs through the leakage of saline fossil waters stored within the Neogenic sedimentary formations at the border of the geothermal systems.

Petrological significance of high-pressure ultramafic xenoliths from ultrapotassic rocks of Central Italy

Two suites of ultramafic xenoliths have been found in ultrapotassic lavas from the 0.9 Ma old Torre Alfina volcano sited at the northern border of the Vulsinian district (Central Italy). One group of Xenoliths consists of spinel-bearing lherzolites, harzburgites, minor wherlites and dunites with a maximum size of 3–4 cm. Some samples contain discrete laths of phlogopite. A second class consists of phlogopite-rich, glass-bearing peridotites. The first suite displays textural characteristics such as triple points, deformed olivine with well developed kink banding and porphyroclastic textures indicating equilibration at high pressure. Pressure estimates give values in the range 1.3–2.5 GPa, corresponding to mantle depths in the area, where the present-day Moho is about 25 km deep. Equilibration temperatures have been estimated in the range between 950–1000°C. The chemical composition of some phases, such as the very high Fo contents of olivines (up to Fo94 in harzburgites), Mg content of orthopyroxenes and CrCr+Al ratios of clinopyroxenes and spinels, suggest that these xenoliths represent peridotites which suffered different degrees of partial melting before being incorporated into the Torre Alfina magma. On the other hand, the occurrence of phlogopite speaks for metasomatic events. The phlogopite-rich, glass-bearing xenoliths consist of phlogopite, olivine, clinopyroxene, rare orthopyroxene and glass. Apatite is the most common accessory. Olivine is present in both euhedral and strained crystals. A few relics of protogranular textures are also observed. Textural and chemical evidence suggests that these xenoliths represent mica-rich peridotites which have undergone phlogopite breakdown during rapid rise to the surface with the development of a K-rich liquid which reacted with mafic phases producing a rapid growth of olivine and, to a lower extent, pyroxene. Originally, these xenoliths may have represented intensively metasomatized upper mantle. However, a cumulitic origin from previous potassic magmatic events cannot be excluded. The host lavas have compositions intermediate between high-silica lamproite and Roman-type ultrapotassic rock. They have high abundances of incompatible elements and radiogenic Sr, coupled with high Mg content, MgOCaO, Ni and Cr. These features support a genesis in a residual upper mantle which has suffered partial melting with the extraction of basaltic liquids, followed by metasomatic events which caused an enrichment in incompatible elements and radiogenic Sr. The presence of mantle-derived ultramafic xenoliths in the torre Alfina lavas testifies for a rapid uprise of the magma which reached the surface without suffering fractional crystallization and significant interaction with the upper crust. Accordingly, the Torre Alfina lavas represent an unique example of primitive potassic liquid in Central Italy.

Geothermal prospecting by geochemical methods on natural gas and water discharges in the vulsini mts volcanic district (central Italy)

The Latera and Torre Alfina geothermal fields were discovered in the Vulsini Mts district (central Italy) in the 70s. The fluid produced by the two geothermal systems is a high p CO 2 (around 7 MPa) sodium chloride solution (T.D.S. is 9200 ppm at Latera and 7800 at Torre Alfina), with high SiO 2 and H 3BO 3 contents. The fluid temperature taken at well bottom is about 155°C at Torre Alfina, whereas at Latera it ranges from 200 to over 350°C. In spite of these temperatures, recorded in producing wells, previous geochemical prospectings using geothermometers in natural thermal manifestations had predicted temperatures no higher than 140°C in all the Vulsini district. This contrasting feature between real temperatures and those evaluated during prospecting is caused by the fast circulation of large amounts of meteoric waters in the aquifer located in the shallow parts of the carbonate reservoir formations, and by the short interaction between the latter and the deep geothermal fluids. In the present study a new geochemical survey on thermal and cold springs, stream samples, as well as natural gas emissions has been carried out. A critical review of the main geothermometers, some considerations about the hydraulic behavior of the reservoir formations, and the cross comparison between NH 4 +/B ratio, p CO 2 and SiO 2 content in both cold and thermal waters, have led to the conclusion that in the Vulsini Mts there are no shallow anomalous areas apart from those already discovered at Latera and Torre Alfina. The present method could be successfully applied in other geothermal systems, where the potential reservoir is represented by carbonate formations.

CO 2 distribution in the atmosphere and noise survey after blow-out in alfina 1 well, Northern Latium, Italy

A water-dominated geothermal reservoir, with a gas cap (mainly CO 2) on the reservoir roof, was individualed in Torre Alfina zone, northern Latium, about 30 km south-east of Monte Amiata. The first exploratory well (Alfina 1), drilled in 1973, blew-out spontaneously so that no production casing could be inserted. After well shut-in several surface gas manifestations appeared which were considered of some risk to the environment. This paper describes the methods used to determine the degree of pollution and the total area involved. The study of the gas dispersion in the atmosphere, the noise resulting from production, and the meteorological conditions in the area, was used in drawing up a disposal plant project; this plant will be assembled on future wells as a safety measure where large quantities of fluids are involved.