Orlando Vaselli

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.

Petrology and geochemistry of late Tertiary/Quaternary mafic alkaline volcanism in Romania

Abstract Alkaline volcanic activity occurred in the Persani Mountains (eastern Transylvanian Basin) and Banat (eastern Pannonian Basin) regions of Romania between 2.5 Ma and 0.7 Ma. This volcanism followed an extended period of subduction-related mostly andesitic and dacitic magmatism in the Eastern Carpathian arc. The Persani Mts. alkaline activity coincided with the last phase of subduction-related activity. Several lava flows and pyroclastic deposits in the Persani Mts. carry peridotitic mantle xenoliths and amphibole megacrysts. Major- and trace-element geochemistry indicates that the alkaline magmas are primitive, silica-undersaturated alkali basalts and trachybasalts (7.8–12.3 wt.% MgO; 119–207 ppm Ni; 210–488 ppm Cr) which are LREE-enriched. Mantle-normalised trace-element diagrams reveal an overall similarity to continental intraplate alkali basalts, but when compared with a global average of ocean island basalts (OIB), the Banat lavas are similar to average OIB, whereas the Persani Mts. basalts have higher Rb, Ba, K and Pb and lower Nb, Zr and Ti. These features slightly resemble those of subduction-related magmas, particularly those of a basaltic andesite related to the nearby older arc magmas. With 87 Sr 86 Sr varying from 0.7035-0.7045 and 143 Nd 144 Nd from 0.51273-0.51289, the Romania basalts are indistinguishable from those of the western Pannonian basin (Hungary and Austria) and Neogene alkali basalts throughout Europe. Amphibole megacrysts have similar isotopic signatures, and their REE patterns indicate derivation by crystallisation from a mafic alkaline magma. The age-corrected Sr and Nd isotopic compositions of a calc-alkaline basaltic andesite related to the preceeding period of subduction also lies within the field of the younger alkaline magmas. Pb isotopic ratios of the Romanian alkali basalts do not lie on the NHRL, but overlap the field of Tertiary alkali basalts from the western Pannonian basin, and have unusually high 207 Pb 204 Pb at a given 206 Pb 204 Pb . Thus it is probable that, although the Romanian alkali basalts have a strong asthenospheric (i.e. OIB-type mantle source) component, their Pb isotopic characteristics were derived from mantle which was affected by the earlier subduction.

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.

Chemical composition of fumarolic gases and spring discharges from El Chichòn volcano, Mexico: causes and implications of the changes detected over the period 1998–2000

Abstract Since the March–April 1982 eruption of El Chichon volcano, intense hydrothermal activity has characterised the 1-km-wide summit crater. This mainly consists of mud and boiling pools, fumaroles, which are mainly located in the northwestern bank of the crater lake. During the period 1998–2000, hot springs and fumaroles discharging inside the crater and from the southeastern outer flank (Agua Caliente) were collected for chemical analyses. The observed chemical fluctuations suggest that the physico-chemical boundary conditions regulating the thermodynamic equilibria of the deep rock/fluid interactions have changed with time. The chemical composition of the lake water, characterised in the period 1983–1997 by high Na + , Cl − , Ca 2+ and SO 4 2− contents, experienced a dramatic change in 1998–1999, turning from a Na + –Cl − - to a Ca 2+ –SO 4 2− -rich composition. In June 2000, a relatively sharp increase in Na + and Cl − contents was observed. At the same time, SO 2 /H 2 S ratios and H 2 and CO contents in most gas discharges increased with respect to the previous two years of observations, suggesting either a new input of deep-seated fluids or local variations of the more surficial hydrothermal system. Migration of gas manifestations, enhanced number of emission spots and variations in both gas discharge flux and outlet temperatures of the main fluid manifestations were also recorded. The magmatic-hydrothermal system of El Chichon is probably related to interaction processes between a deep magmatic source and a surficial cold aquifer; an important role may also be played by the interaction of the deep fluids with the volcanic rocks and the sedimentary (limestone and evaporites) basement. The chemical and physical changes recorded in 1998–2000 were possibly due to variations in the permeability of the conduit system feeding the fluid discharges at surface, as testified by the migration of gas and water emanations. Two different scenarios can be put forward for the volcanic evolution of El Chichon: (1) build-up of an infra-crater dome that may imply a future eruption in terms of tens to hundreds of years; (2) minor phreatic–phreatomagmatic events whose prediction and timing is more difficult to constrain. This suggests that, unlike the diminished volcanic activity at El Chichon after the 1982 paroxistic event, the volcano-hydrothermal fluid discharges need to be more constantly monitored with regular and more frequent geochemical sampling and, at the same time, a permanent network of seismic stations should be installed.

Significance of silicate melt pockets in upper mantle xenoliths from the Bakony–Balaton Highland Volcanic Field, Western Hungary

Abstract Silicate melt pockets with or without carbonate occur in 10% of upper mantle xenoliths from the alkali basalts of the Bakony–Balaton Highland Volcanic Field (BBHVF), Western Hungary. Based on the estimated bulk composition of the melt pockets, both the carbonate-free and the carbonate-bearing ones are considered to be the result of the reaction between primary mantle clinopyroxene and/or amphibole and external CaO, Al2O3, alkali-rich and MgO-poor fluids/melts, as metasomatic agents, migrating in the upper mantle. The metasomatic melt that produced the carbonate-bearing melt pockets was extremely rich in volatiles, whereas metasomatic melt that contributed to the formation of the carbonate-free melt pockets was particularly rich in silica and relatively poor in volatiles. These metasomatizing melts could have originated from the melting of the previously metasomatized upper mantle due to Middle Miocene mantle diapirism.

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.

Concentration, distribution, and translocation of mercury and methylmercury in mine-waste, sediment, soil, water, and fish collected near the Abbadia San Salvatore mercury mine, Monte Amiata district, Italy

The distribution and translocation of mercury (Hg) was studied in the Paglia River ecosystem, located downstream from the inactive Abbadia San Salvatore mine (ASSM). The ASSM is part of the Monte Amiata Hg district, Southern Tuscany, Italy, which was one of the worlds largest Hg districts. Concentrations of Hg and methyl-Hg were determined in mine-waste calcine (retorted ore), sediment, water, soil, and freshwater fish collected from the ASSM and the downstream Paglia River. Concentrations of Hg in calcine samples ranged from 25 to 1500 μg/g, all of which exceeded the industrial soil contamination level for Hg of 5 μg/g used in Italy. Stream and lake sediment samples collected downstream from the ASSM ranged in Hg concentration from 0.26 to 15 μg/g, of which more than 50% exceeded the probable effect concentration for Hg of 1.06 μg/g, the concentration above which harmful effects are likely to be observed in sediment-dwelling organisms. Stream and lake sediment methyl-Hg concentrations showed a significant correlation with TOC indicating considerable methylation and potential bioavailability of Hg. Stream water contained Hg as high as 1400 ng/L, but only one water sample exceeded the 1000 ng/L drinking water Hg standard used in Italy. Concentrations of Hg were elevated in freshwater fish muscle samples and ranged from 0.16 to 1.2 μg/g (wet weight), averaged 0.84 μg/g, and 96% of these exceeded the 0.3 μg/g (methyl-Hg, wet weight) USEPA fish muscle standard recommended to protect human health. Analysis of fish muscle for methyl-Hg confirmed that >90% of the Hg in these fish is methyl-Hg. Such highly elevated Hg concentrations in fish indicated active methylation, significant bioavailability, and uptake of Hg by fish in the Paglia River ecosystem. Methyl-Hg is highly toxic and the high Hg concentrations in these fish represent a potential pathway of Hg to the human food chain.

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.