Angelo Minissale

Geochemistry of Quaternary travertines in the region north of Rome (Italy): structural, hydrologic and paleoclimatic implications

In the Tyrrhenian region of central Italy, late Quaternary fossil travertines are widespread along two major regional structures: the Tiber Valley and the Ancona–Anzio line. The origin and transport of spring waters from which travertines precipitate are elucidated by chemical and isotopic studies of the travertines and associated thermal springs and gas vents. There are consistent differences in the geochemical and isotopic signatures of thermal spring waters, gas vents and present and fossil travertines between east and west of the Tiber Valley. West of the Tiber Valley, δ13C of CO2 discharged from gas vents and δ13C of fossil travertines are higher than those to the east. To the west the travertines have higher strontium contents, and gases emitted from vents have higher 3He/4He ratios and lower N2 contents, than to the east. Fossil travertines to the west have characteristics typical of thermogene (thermal spring) origin, whereas those to the east have meteogene (low-temperature) characteristics (including abundant plant casts and organic impurities). The regional geochemical differences in travertines and fluid compositions across the Tiber Valley are interpreted with a model of regional fluid flow. The regional Mesozoic limestone aquifer is recharged in the main axis of the Apennine chain, and the groundwater flows westward and is discharged at springs. The travertine-precipitating waters east of the Tiber Valley have shallower flow paths than those to the west. Because of the comparatively short fluid flow paths and low (normal) heat flow, the groundwaters to the east of the Tiber Valley are cold and have CO2 isotopic signatures, indicating a significant biogenic contribution acquired from soils in the recharge area and limited deeply derived CO2. In contrast, spring waters west of the Tiber Valley have been conductively heated during transit in these high heat-flow areas and have incorporated a comparatively large quantity of CO2 derived from decarbonation of limestone. The elevated strontium content of the thermal spring water west of the Tiber Valley is attributed to deep circulation and dissolution of a Triassic evaporite unit that is stratigraphically beneath the Mesozoic limestone. U-series age dates of fossil travertines indicate three main periods of travertine formation (ka): 220–240, 120–140 and 60–70. Based on the regional flow model correlating travertine deposition at thermal springs and precipitation in the recharge area, we suggest that pluvial activity was enhanced during these periods. Our study suggests that travertines preserve a valuable record of paleofluid composition and paleoprecipitation and are thus useful for reconstructing paleohydrology and paleoclimate.

Multiple source components in gas manifestations from north-central Italy

Abstract Gas manifestations in north-central Italy consist of CO 2 -rich gases with minor N 2 rich emissions and discharge either along with thermal springs or into cold and stagnant waters. ‘Cold’ gases are prevalently C02 -dominated (> 90%) while gases related to the thermal waters have variable composition: from CO 2 > 99.5% to N2 > 90%. The variable composition of ‘thermal’ gases is caused by differences in the thermal regime and lithology of the ascent paths, where there is mixing of gases from multiple sources, such as N 2 rich atmospheric and deep C0 2 -rich metamorphic end-members. Elevated concentrations of CH 4 and H 2 in these gases are generally related to the presence of active geothermal systems at shallow depth, such as the Larderello-Travale field in Tuscany. The δ 13 C values between coexisting CH 4 and CO 2 in all samples analyzed suggest that CH 4 originates abiogenically in 200–400°C hydrothermal systems. Far from geothermal areas, where the thermal gradient is lower or the water/gas ratio is high because of large inflow of meteoric waters, H 2 and CH 4 are usually lower. In some cases, they can be scrubbed or oxidized (especially H 2 ), while the residual rising gas becomes indirectly enriched in N 2 and C0 2 . Carbon dioxide is also enriched in some discharged gases because it is produced at shallow depth in lower temperature conditions ( 15 N values for N 2 to near + 7.0%o suggest that, for some gas samples that contain excess nitrogen (e.g. where N2/Ar >> 83), this component probably derives from ammonia-rich feldspars and micas within the Palaeozoic schist basement rocks. However, other samples show evidence of a shallow component of CH 4 and N 2 from Neogene basin sediments. The areal distribution of the 3 He/ 4 He ratio points to a general prevalence of atmospheric and crustal 4 He in the gas discharges in central Italy. A significant component of mantle 3 He is only found in the geothermal areas of Larderello where the large regional thermal anomaly suggests the presence of a deep magmatic body.

Geochemistry of water and gas discharges from the Mt. Amiata silicic complex and surrounding areas (central Italy)

Abstract The Mt. Amiata volcano in central Italy is intimately related to the post-orogenic magmatic activity which started in Pliocene times. Major, trace elements, and isotopic composition of thermal and cold spring waters and gas manifestations indicate the occurrence of three main reservoir of the thermal and cold waters in the Mt. Amiata region. The deepest one is located in an extensive carbonate reservoir buried by thick sequences of low-permeability allochthonous and neo-autochthonous formations. Thermal spring waters discharging from this aquifer have a neutral Ca-SO4 composition due to the presence of anhydrite layers at the base of the carbonate series and, possibly, to absorption of deep-derived H2S with subsequent oxidation to SO42− in a system where pH is buffered by the calcite–anhydrite pair ( Marini and Chiodini, 1994 ). Isotopic signature of these springs and N2-rich composition of associated gas phases suggest a clear local meteoric origin of the feeding waters, and atmospheric O2 may be responsible for the oxidation of H2S. The two shallower aquifers have different chemical features. One is Ca-HCO3 in composition and located in several sedimentary formations above the Mesozoic carbonates. The other one has a Na-Cl composition and is hosted in marine sediments filling many post-orogenic NW–SE-trending basins. Strontium, Ba, F, and Br contents have been used to group waters associated with each aquifer. Although circulating to some extent in the same carbonate reservoir, the deep geothermal fluids at Latera and Mt. Amiata and thermal springs discharging from their outcropping areas have different composition: Na-Cl and Ca-SO4 type, respectively. Considering the high permeability of the reservoir rock, the meteoric origin of thermal springs and the two different composition of the thermal waters, self-sealed barriers must be present at the boundaries of the geothermal systems. The complex hydrology of the reservoir rocks greatly affects the reliability of geothermometers in liquid phase, which understimate the real temperatures of the discovered geothermal fields. More reliable temperatures are envisaged by using gas composition-based geothermometers. Bulk composition of the 67 gas samples studied seems to be the result of a continuous mixing between a N2-rich component of meteoric origin related to the Ca-SO4 aquifer and a deep CO2-rich component rising largely along the boundaries of the geothermal systems. Nitrogen-rich gas samples have nearly atmospheric N2/Ar (=83) and 15 N / 14 N (δ=0‰) ratios whereas CO2-rich samples show anomalously high δ15N values (up to +6.13 ‰), likely related to N2 from metamorphic schists lying below the carbonate formations. On the basis of average 13 C / 12 C isotopic ratio (δ13C around 0‰), CO2 seems to originate mainly from thermometamorphic reactions in the carbonate reservoir and/or in carbonate layers embedded in the underlying metamorphic basement. Distribution of 3 He / 4 He isotopic ratios indicates a radiogenic origin of helium in a tectonic environment that, in spite of the presence of many post-orogenic basins and mantle-derived magmatics, can presently be considered in a compressive phase.

Origin and evolution of ‘intracratonic’ thermal fluids from central-western peninsular India

The chemical and isotopic composition of several thermal springs and associated gas phases in a large sector of central-western peninsular India has been investigated. Such springs have meteoric isotopic signature and emerge, after very well developed convective circulation at depth, along important tectonic structures such as the Son–Narmada–Tapti rift zone and the West Coast Fault. Chemical components in both gas and liquid phases and geothermometric estimations suggest that such springs are not related to the presence of any active hydrothermal systems at shallow depth in any of the studied areas. The hottest convective water emerges at Tattapani at near boiling point for water at atmospheric pressure (>90°C) in association with an N2-rich gas phase of clear meteoric signature. Since such fluids do not carry any corrosive components, they could be conveniently exploited for industrial purposes, such as drying processes. From a tectonic point of view, the presence of thermal emergences scattered in a wide area along geologically well defined structures, which also generate frequent moderate earthquakes, suggests that such structures are active. Although the isotopic composition of thermal springs points to a meteoric origin, their feeding aquifers are not topographically driven as in most active Alpine orogenic belts. The relative high quantity of total helium in the associated gas phase suggests also that they are really deep, old, long circulating waters. We propose for such waters the term ‘intracratonic thermal waters’ since the isotopic signature of He in the gas phase does not show any release of primordial 3He in any of the areas of spring emergence. Based on the quite low 3He/4He ratio in the gas phase we suggest also that, in spite of its morphological shape, the Narmada–Son–Tapti rift zone cutting the Indian subcontinent in two is more related to paleo-suture rather than to a mid-continental rift system.

Thermal springs in Italy: their relation to recent tectonics

Abstract The large scale circulation of thermal waters in Italy is reviewed from both a chemical and an isotopic point of view. Two main areas are recognized: one in the Alpine region and the other in the central-southern Apennine region. The Alps are an area of very high hydraulic heads and low thermal gradients, where hot, deep-seated waters of meteoric origin rapidly reach the surface along active boundary fault systems. The Apennines, in contrast, are characterized by lower hydraulic heads and higher thermal gradients. All thermal springs in Italy (including those in active volcanic areas) originate essentially from meteoric waters, and most follow long pathways in Mesozoic carbonate-anhydritic aquifers before emerging, as indicated by relations among present hydrothermal activity in central-southern Italy, deep crustal structures and PCO2 content in thermal waters.

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)

Abstract 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 3 BO 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.

A geochemical traverse across the Eastern Carpathians (Romania): constraints on the origin and evolution of the mineral water and gas discharges

Abstract The inner sector of the Eastern Carpathians displays a large number of Na–HCO 3 , CO 2 -rich, meteoric-originated cold springs (soda springs) and bore wells, as well as dry mofettes. They border the southern part of the Pliocene–Quaternary Calimani–Gurghiu–Harghita (CGH) calc-alkaline volcanic chain. Both volcanic rocks and CO 2 -rich emissions are situated between the eastern part of the Transylvanian Basin and the main east Carpathian Range, where active compression tectonics caused diapiric intrusions of Miocene halite deposits and associated saline, CO 2 -rich waters along active faults. The regional patterns of the distribution of CO 2 in spring waters (as calculated p CO 2 ) and the distribution pattern of the 3 He/ 4 He ratio in the free gas phases (up to 4.5 R m / R a ) show their maximum values in coincidence with both the maximum heat-flow measurements and the more recent volcanic edifices. Moving towards the eastern external foredeep areas, where oil fields and associated brines are present, natural gas emissions become CH 4 -dominated. Such a change in the composition of gas emissions at surface is also recorded by the 3 He/ 4 He ratios that, in this area, assume ‘typical’ crustal values ( R m / R a =0.02). In spite of the fact that thermal springs are rare in the Harghita volcanic area and that equilibrium temperature estimates based on geothermometric techniques on gas and liquid phases at surface do not suggest the presence of shallow active hydrothermal systems, a large circulation of fluids (gases) is likely triggered by the presence of mantle magmas stored inside the crust. If total 3 He comes from the mantle or from the degassing of magmas stored in the crust, CO 2 might be associated to both volcanic degassing and thermometamorphism of recently subducted limestones.

Fluid mixing in carbonate aquifers near Rapolano (central Italy): chemical and isotopic constraints

Chemical (major and trace elements) and isotopic compositions (dD and d 18 O in waters and d 13 Ci n CO 2 and 3 He/ 4 He in gases) of natural thermal (11) and cold (39) fluids (spring waters and gases) discharging from a tectonic window of Mesozoic limestones in central Italy, have proved to be the result of mixing processes inside the limestone formations. The limestones provide a preferential route for subsurface fluid migration and they gather both descending cold, Ca-HCO3, B-depleted groundwaters and rising convective Ca-SO4(HCO3), CO2-saturated, B-rich thermal waters. Atmospherically-derived descending gas components (N2, Ne, He), dissolved in rainfall that infiltrates the limestone system mix with N2, Ne, He-depleted hot rising waters. Boron in the liquid phase and N2 and Ne in the gas phase are the most useful elements to trace the mixing process. The deeper gas samples recognised in the area are associated with the hotter waters emerging in the area. In spite of being depleted in Ne and He and light hydrocarbons they have the higher measured 3 He/ 4 He ratios, suggesting a contribution of mantle 3 He to the gas phase. This contrasts with deep circulation in the crust which would lead to increased concentration of 4 He in the deeper gases. Paradoxically, there is more relative concentration of 4 He in the more air-contaminated gas samples than in the deeper gas samples. A similar paradox exists when the d 13 Co f CO 2 in the deeper gas samples is considered. The shallower air-contaminated gas samples, although they should be affected by the addition of soil- 13 C depleted organic C, have d 13 Ci n CO 2 more positive than the deeper gas samples recognized. Since any deep hydrothermal source of CO2 should generate CO2 with more positive values of d 13 C than those measured at surface, a multiple (single) calcite precipitation process from hydrothermal solutions, with C isotopic fractionation along the rising path inside the Mesozoic limestone formations, is proposed. # 2002 Elsevier Science Ltd. All rights reserved.

Hydrogeochemistry of the Campania region in southern Italy

Abstract A geochemical study of thermal springs, cold springs, stream waters and natural gas emissions has been carried out in the Campania region of southern Italy. This region hosts four Quaternary volcanic areas, and thermal springs and gas emissions occur in three of them. Most thermal springs discharge Na-Cl composition waters of connate origin derived from post-orogenic volcanic and sedimentary formations. Although high-enthalpy systems are present in two of the four volcanic areas, there appear to be no magmatic contributions to the thermal springs. Solute geothermometers are unreliable as spring waters are strongly affected by mixing with “shallow” brines before discharging. Thermal springs and gas emissions also occur in non-volcanic areas, where an extensive carbonate unit acts as a regional aquifer for cold, low-salinity, bicarbonate waters. Thermal features in these areas occur in fractured zones associated with active faults. Their compositions are determined only by the type of rock encountered by solutions before surface discharge. As in other areas of north-central Italy, the widespread occurrence of hot and cold CO2-rich springs, and gas emissions in both volcanic and non-volcanic zones, suggests a deep origin for the CO2.

Chemical composition of thermal springs, cold springs, streams, and gas vents in the Mt. Amiata geothermal region (Tuscany, Italy)

Abstract A geochemical study of thermal and cold springs, stream waters and gas emissions has been carried out in the Mt. Amiata geothermal region. The cold springs and stream waters do not seem to have received significant contribution from hot deep fluids. On the contrary, the thermal springs present complex and not clearly quantifiable interactions with the hot fluids of the main geothermal reservoir. The liquid-dominated systems in the Mt. Amiata area, like most of the high-enthalpy geothermal fields in the world, are characterized by saline, NaCl fluids. The nature of the reservoir rock (carbonatic and anhydritic), and its widespread occurrence in central Italy, favor a regional circulation of “Ca-sulfate” thermal waters, which discharge from its outcrop areas. Waters of this kind, which have been considered recharge waters of the known geothermal fields, dilute, disperse and react with the deep geothermal fluids in the Mt. Amiata area, preventing the use of the main chemical geothermometers for prospecting purposes. The temperatures obtained from the chemical geothermometers vary widely and are generally cooler than temperatures measured in producing wells. Other thermal anomalies in central Italy, apart from those already known, might be masked by the above-mentioned circulation. A better knowledge of deep-fluid chemistry could contribute to the calibration of specific geothermometers for waters from reservoirs in carbonatic rocks.