Lead isotopic composition of the potassic rocks from Roccamonfina (South Italy)

Pb isotope data are presented for the potassic rocks from Roccamonfina and some other Campanian volcanoes. Pb isotope variations for Roccamonfina fall within the previously found range of values for the Italian potassic volcanism and form similar shallow regression lines in the Pb-Pb diagrams. Their interpretation as two-component mixing lines is well supported by other geochemical evidence. The relation of the mixing processes deduced for Roccamonfina and for the wider regional volcanism is discussed. The enriched and anomalous mantle under Italy is proposed to be a result of various degrees of metasomatism of a range of “Atlantic island” type mantle compositions by an LIL-element-enriched mantle “fluid”. The mantel “fluid” may either derive its isotope and trace element characteristics from an old LIL-element-enriched mantle source or from continental crust which has to some extend retained its geochemical identity in the upper mantle.     

Variations in the chemical and isotopic composition of fluids discharged from the Taupo Volcanic Zone, New Zealand

Contents of H₂O, CO₂ and Cl in well discharges from six explored geothermal systems of the Taupo Volcanic Zone, New Zealand, point to the existence of two distinct source fluids. The fluid present in discharges from systems along the eastern boundary is characterised by high CO₂ contents, 1.6 ± 0.5 , at mole ratios of 3.9 ± 1.5. High (0.06) and (12) weight ratios in these waters suggest that all four constituents are derived from associated andesitic rock. Geothermal discharges in the western parts of the TVZ, dominated by rhyolitic magmatism, are characterised by low CO₂ contents, 0.12 ± 0.05 , and low (0.14 ± 0.1) ratios. Again, relative Cl, B, Li and Cs contents agree with those of this potential source rock. High and ratios in the east are typical of fluids affected by the addition of volatiles released from subducted marine sediments. For the western systems, these ratios resemble more closely those expected for mantle-derived volatiles. The isotopic compositions of all deep waters point to the presence of variable amounts of a magmatic component, some 14 ± 5% in the eastern and 6 ± 2% in the western systems. The observed variations are explained in terms of interaction of volatiles released from the subducted sediments with material of the mantle wedge to form a volatile-charged, high-alumina basalt. Its convective rise, in a direction opposite to that of the down-going slab, leads to high enrichment in volatiles of the magmas generated beneath the eastern parts of the TVZ and increases their ability to intrude the continental crust. Further fractional crystallisation and assimilation leads to the formation of volatile-rich andesitic melts, partly extruded to form the volcanoes of the andesitic arc, partly intruded to act as source rocks for the high-gas geothermal systems. Batches of high-alumina basalt, depleted in subducted volatiles, travel farther west to pond beneath a zone of crustal extension. Following extensive fractionation, highly siliceous melts, carrying predominantly mantle-type volatiles, rise beneath the western part of the TVZ to supply both heat and volatiles to the geothermal systems there.     

Chemical and isotopic signatures of the basement rocks from the Campi Flegrei geothermal field (Naples, southern Italy): inferences about the origin and evolution of its hydrothermal fluids

The Campi Flegrei (Naples, Campanian Plain, southern Italy) geothermal system is hosted by Quaternary volcanic rocks erupted before, during and after the formation of the caldera that represents one of the major structural features in the Neapolitan area. The volcanic products rest on a Mesozoic carbonate basement, cropping out north, east and south of the area. Chemical (major, minor and trace elements) and stable isotope (C, H, O) analyses were conducted on drill-core samples recovered from geothermal wells MF-1, MF-5, SV-1 and SV-3, at depths of ˜ 1100 to 2900 m. The study was complemented by petrographic and SEM examination of thin sections. The water which feeds the system is both marine and meteoric in origin. Mineral zonation typical of a high-temperature geothermal system exists in all the geothermal wells; measured temperatures in wells are as high as ˜ 400 °C. The chemical composition of the waters suggests the existence of two reservoirs: a shallow reservoir (depth < 2000 m) fed by seawater that boiled at 320 °C and became progressively diluted by steam-heated local meteoric water during its ascent; and a deeper reservoir (depth > 2000 m) of hypersaline water. The drill-cores are mainly hydrothermally altered volcanics of trachy-latitic affinity, but some altered pelites and limestones are also present. Published Na, Mg and K concentrations of selected geothermal waters indicate that the hydrothermal fluids are in equilibrium with their host rocks, with respect to K-feldspar, albite, sericite and chlorite. The measured δ¹⁸O_(SMOW) values of rocks range from +4.3 to + 16.5%. The measured δD_(SMOW) values range from − 79 to − 46%. The calculated isotopic composition of the fluids at equilibrium with the samples vary from + 1 to + 8.3%. δ¹⁸O and from − 52 to + 1%. δD. The estimated isotopic composition of the waters at equilibrium with the studied samples confirmed the existence of two distinct fluid types circulating in the geothermal system. The shallower has a marine water signature, while the deeper water has a signature consistent both with magmatic and meteoric origins. In the latter case, the recharge of this aquifer likely occurs at the outcrop of the Mesozoic Limestones surrounding the Campanian Plain; after infiltration, the water percolates through evaporitic layers, becoming hypersaline and D-depleted.     

Temporal evolution of a three component system: the island of Lipari (Aeolian Arc,southern Italy)

The volcanic products from Lipari define an evolutionary trend with a high gradient of K-enrichment, similar to the calc-alkaline to potassic volcanism of other islands in the Aeolian arc. Stratigraphic reconstruction of the island based on field and geochronological data indicate that the volcanic activity can be subdivided in two stages. The first stage, from 223 to 42 ka, consists of six eruptive cycles and is characterized by basalts and basalt-andesites showing progressive increase in both SiO₂ and K₂O contents with time. The second stage consists of four cycles erupted since 42 ka and is marked by an apparent rejuvenation of the geochemical system with the appearance of the first rhyolitic products. Fractional crystallization, assimilation and mixing models suggest that the geochemistry of Lipari volcanism evolved with time by a complex interplay between two mantle-derived components, one sub-alkaline and the other alkaline, in addition to crustal melts and/or crustally-derived materials. A petrogenetic model in which fractional crystallization was subordinate to mixing best fits the geochemical data and petrographic observations of macro- and microscopic features. Melts from the crustal and mantle end-members are almost always present in the system but the relative proportions appear to vary with time. The sub-alkaline mantle component (source of Tyrrhenian tholeiites) is an important contributor to the early evolution of the volcanism in Lipari; input from the alkaline mantle component (source of the Roman Comagmatic Province) increases with time, and the crustal component becomes dominant in the later activity. The preferred petrogenetic model for the temporal evolution of the volcanic system in Lipari involves melting initially caused by an increase in the thermal input related to the opening of the Tyrrhenian Sea and/or to subduction processes. The quick rise of the isotherms and almost contemporaneous melting of source materials with different compositions favored complex mixing during ascent of the melts.     

The effect of variations in rainfall on the chemical composition of vulcano fumaroles (Italy)

Analytical data for samples collected over a period of 17 months at the crater fumaroles of Vulcano are given. Fluids are both condensed and absorbed in KOH solutions, thus providing a complete analysis of acid and condensable species. Variations in H₂O, CO₂, SO₂, H₂S, HCl, B, F, Br, NH₄ concentrations are taken into account. Rainwater represents an important modifying facor of the basic chemical composition of the investigated discharges, which do not seem to have undergone any other significant change during the above mentioned span of time. While acting essentially as a diluting factor for CO₂ and SO₂, the inflow of meteoric water can introduce substantial modifications in concentrations of other constituents. As a consequence, any correct comparison of fumaroles compositions either for different systems or for different samplings at the same system should take into consideration the influence of meteorological conditions.     

Genetic significance of chemical,isotopic, and petrographic features of some peralkaline salic rocks from the island of Pantelleria

An anorthoclase phenocryst separate from a pantellerite welded tuff and a slightly peralkaline nonhydrated sodatrachyte glass both have⁸⁷Sr/⁸⁶Sr ratios of0.7030 ± 0.0002. The sodatrachyte glass, withSiO₂ = 63,Al₂O₃ = 15.5,and CaO= 1.4wt%and Sr= 53ppm, is interpreted as an intermediate member in the differentiation sequence through which pantellerite melts were derived from primary mafic magmas. The very low⁸⁷Sr/⁸⁶Sr ratios, in conjunction with K/Rb ratios of 500 to 700 found for the sodatrachytes, show that the primary mafic magmas were derived from mantle material which had been depleted in incompatible elements by a much earlier episode of magma generation.     

Mercury emissions and stable isotopic compositions at Vulcano Island (Italy)

Sampling and analyses methods for determining the stable isotopic compositions of Hg in an active volcanic system were tested and optimized at the volcanic complex of Vulcano (Aeolian Islands, Italy). Condensed gaseous fumarole Hg_(fum)^T, plume gaseous elemental Hg_(g)⁰ and plume particulate Hg_(p)^II were obtained at fumaroles F0, F5, F11, and FA. The average total Hg emissions, based on Hg^T/SO₂ in condensed fumarolic gases and plumes, range from 2.5 to 10.1 kg y^? 1, in agreement with published values [Ferrara, R., Mazzolai, B., Lanzillotta, E., Nucaro, E., Pirrone, N., 2000. Volcanoes as emission sources of atmospheric mercury in the Mediterranean Basin. Sci. Total Environ. 259(1–3), 115–121; Aiuppa, A., Bagnato, E., Witt, M.L.I., Mather, T.A., Parello, F., Pyle, D.M., Martin, R.S., 2007. Real-time simultaneous detection of volcanic Hg and SO₂ at La Fossa Crater, Vulcano (Aeolian Islands, Sicily). Geophys. Res. Lett. 34(L21307).]. Plume Hg_(p)^II increases with distance from the fumarole vent, at the expense of Hg_(g)⁰ and indicates significant in-plume oxidation and condensation of fumarole Hg_(fum)^T.Relative to the NIST SRM 3133 Hg standard, the stable isotopic compositions of Hg are δ²⁰²Hg_(fum)^T = ? 0.74‰ ± 0.18 (2SD, n = 4) for condensed gaseous fumarole Hg_(fum)^T, δ²⁰²Hg_(g)⁰ = ? 1.74‰ ± 0.36 (2SD, n = 1) for plume gaseous elemental Hg_(g)⁰ at the F0 fumarole, and δ²⁰²Hg_(p)^II = ? 0.11‰ ± 0.18 (2SD, n = 4) for plume particulate Hg_(p)^II. The enrichment of Hg_(p)^II in the heavy isotopes and Hg_(g)⁰ in the light isotopes relative to the total condensed fumarolic Hg_(fum)^T gas complements the speciation data and demonstrates a gas-particle fractionation occurring after the gas expulsion in ambient T° atmosphere. A first order Rayleigh equilibrium condensation isotope fractionation model yields a fractionation factor α_cond-gas of 1.00135 ± 0.00058.     

Simulation of syn-eruptive floods in the circumvesuvian plain (southern Italy)

During explosive eruptions the deposition of fine-grained volcanic ash fallout reduces soil permeability, favouring runoff of meteoric water and thus increasing the occurrence of catastrophic floods. A fully dynamic, two-dimensional model was used to simulate flooding scenarios in the Vesuvian area following an explosive volcanic eruption. The highest risk occurs in the catchment area of the Acerra-Nola Plain N and NE of Vesuvius. This plain has a population of 70,000 living in low-lying areas. This catchment area is vulnerable to ash fall because it lies downwind of the dominant synoptic circulation and it lacks a natural outflow toward the sea. Our numerical simulations predict dangerous scenarios, even in quiescent periods, during extreme rain events (return periods of 200 years have been considered), and a significant increase in the extent of the flooded areas due to renewed volcanic activity. Based on these simulations a hazard zonation has been proposed. Editorial responsibility: A Woods     

Chemical and isotopic assessment in volcanic thermal waters: Cases of Ischia (Italy) and São Miguel (Azores,Portugal)

Isotopic fractionation of ¹⁰B/¹¹B provides a sound tool for identifying hydrogeochemical processes in complex areas, owing to its ability to discriminate between various scenarios. In addition, the occurrence of boron as a minor element in areas of active volcanism allows its use in comparison with concentrations of other conservative or non‐conservative ions. This allows the detection of water mixtures of diverse origin and temperature, deep or shallow, including fresh water, seawater and even brines. This tool was applied in studies of the volcanic islands of Ischia and São Miguel, across widely differing geographical and climatic contexts. Five groups of waters have been identified in Ischia Island: marine, transition, hot carbonated, cold carbonated and fresh waters. For São Miguel Island the identified groups are cold carbonic, hot carbonic, boiling and acidic boiling waters. Copyright © 2008 John Wiley & Sons, Ltd.     

Late Quaternary deformation on the island on Pantelleria: New constraints for the recent tectonic evolution of the Sicily Channel Rift (southern Italy)

Structural observations carried out on the volcanic Island of Pantelleria show that the tectonic setting is dominated by NNE trending normal faults and by NW-striking right-lateral strike-slip faults with normal component of motion controlled by a ≈N 100°E oriented extension. This mode of deformation also controls the development of the eruptive fissures, dykes and eruptive centres along NNE–SSW belts that may thus represent the surface response to crustal cracking with associated magma intrusions. Magmatic intrusions are also responsible for the impressive vertical deformations that affect during the Late Quaternary the south-eastern segment of the island and producing a large dome within the Pantelleria caldera complex. The results of the structural analysis carried out on the Island of Pantelleria also improves the general knowledge on the Late Quaternary tectonics of the entire Sicily Channel. ESE–WNW directed extension, responsible for both the tectonic and volcano-tectonic features of the Pantelleria Island, also characterizes, at a greater scale, the entire channel as shown by available geodetic and seismological data. This mode of extension reactivates the older NW–SE trending fault segments bounding the tectonic troughs of the Channel as right-lateral strike-slip faults and produces new NNE trending pure extensional features (normal faulting and cracking) that preferentially develop at the tip of the major strike-slip fault zones. We thus relate the Late Quaternary volcanism of the Pelagian Block magmatism to dilatational strain on the NNE-striking extensional features that develop on the pre-existing stretched area and propagate throughout the entire continental crust linking the already up-welled mantle with the surface.     

Stratigraphy and significance of Brown Tuffs on the Aeolian Islands (southern Italy)

Brown Tuffs (BT) are volcaniclastic ash deposits prominently represented in the stratigraphic profiles of all the Aeolian Islands (and Capo Milazzo on the northern coast of Sicily). Detailed stratigraphy and tephrochronology together with available radiometric ages suggest that they were emplaced over a long time interval spanning from the end of the last interglacial period (ca. 80 ka BP) up to 4–5 ka BP (age of the overlying Punte Nere pyroclastic products on Vulcano). The most complete BT succession is documented on Lipari where 14 distinct and successive units are subdivided by the interbedding of widespread tephra layers, local volcanic products, paleosols and epiclastic deposits and the occurrence of local erosive surfaces. Inter-island occurrence of Ischia-Tephra (a widely known tephra layer in the Aeolian archipelago dated at 56 ka BP) and Monte Guardia pyroclastics from Lipari (dated at 22–20 ka BP) subdivides the BT succession in Upper (UBT), Intermediate (IBT) and Lower BT units (LBT), which can be correlated at regional level: the LBT was emplaced between 80 and 56 ka BP, the IBT between 56 and 22 ka BP and the UBT between 20 and 4–5 ka BP. On the basis of stratigraphy, similarity in lithology and textural features, morphology of glass fragments, composition and consistency of thickness and grain-size variations, UBT units correlate with Piano Grotte dei Rossi tuffs on Vulcano island. They were generated by pulsating hydromagmatic explosive activity giving rise to pyroclastic density currents spreading laterally from a source located inside the La Fossa caldera on Vulcano island. Composition is in agreement with this hypothesis since UBT compositional features match those of Vulcano magmas erupted in that period. The effect of co-ignimbrite ash clouds (or associated fallout processes from sustained eruptive columns) is seen to explain the presence of UBT in areas further away from the suggested source (e.g. Salina and Lipari islands and Capo Milazzo). The origin of UBT exposed on Panarea island is still a matter of debate, due to contrasting compositional data. Due to large uniformity of lithological, textural and componentry characters with respect to the UBT, the lower portions of the BT succession (LBT-IBT) are considered to be the result of recurrent, large scale hydromagmatic eruptions of similar type. Moreover, for the IBT units, the correlation with Monte Molineddo 3 pyroclastics of Vulcano island (on the basis of lithological, compositional and stratigraphic matching) again suggests source(s) related to the Vulcano plumbing system and located within the La Fossa Caldera.     

Late quaternary tephra layers along the Cilento coastline (southern Italy)

The results of stratigraphic and tephrostratigraphic analyses of several pyroclastic levels, collected along the coastal sector of the Cilento region (southern Italy), are presented. Some of these levels are here described for the fist time, others, already known in literature, were reconsidered in order to better understand their stratigraphic position and to point out the possible volcanic sources.     

The Campania-Lucania (southern Italy) earthquake of 23 November 1980

We present the main seismological results of our study of the Campania-Lucania earthquake of 23 November 1980. A complete set of far field and local data has been analysed. From long-period body waves data we determine the fault plane solution (φ₁ = 140°,δ₁ = 60°,φ₂ = 75°,δ₂ = 54°), a depth of 15 km and calculate a seismic moment of 6 × 10²⁵ dyne cm and a source duration of 6 s. From data of a local network deployed immediately after the event we determine aftershock locations: they are aligned in a direction NW-SE that fit extremely well with the focal solution determined above. We can choose as fault plane the plane striking 140° and dipping at 60° and the event is a normal event with a large component of left-lateral strike slip. The source area evaluated from this aftershock distribution 14 km × 40 km is quite suitable for an earthquake of a seismic moment of 6 × 10²⁵ dyne cm.     

A gravity and magnetic study of the volcanic island of Ischia, Naples (Italy)

Gravity and magnetic data for the volcanic island of Ischia, Naples, Italy, have been analyzed and interpreted in light of recent geological and volcanological data to define a model of the island's shallow and deep structures. From the interpretation of the gravity data it appears that the shallow structures consist of pyroclastic material (p=2.0 g/cm³). Within these pyroclastics there are domes and lava flows of higher density and eruptive centres filled with lower density material. The basement is a “horst” with the shallowest depth at about 1.0 km, south of the centre of the island, if we assign a density contrast of 0.5 g/cm³ relative to the above pyroclastics.Interpretation of magnetic data measured at 725 stations showed that the basement derived from the gravity interpretation is magnetized. Moreover, this basement is less magnetized on the western side of Ischia which may be caused by the anomalous thermal state of the area, as indicated by surface fumaroles, hot springs etc. and temperature measurements in deep drillings.     

The Pb isotopic composition of Jurassic cherts overlying ophiolites in the North Apennines,Italy

In the Mesozoic eugeosynclinal sequences of the Italian North Apennines, ophiolitic rocks are commonly stratigraphically overlain by thick bedded chert sequences of Late Jurassic age. The isotopic composition of Pb contained in upper basalts, lower ferruginous (and in places manganiferous) cherts, middle cherts and upper cherts indicates that simple mixing occurred in the sediments between basalt-derived Pb, and more radiogenic Pb provided by fine-grained terrigenous detritus, radiolarian silica and possibly seawater.The most ferruginous cherts are those nearest the ophiolitic basement, which is interpreted as ocean crust formed early in the opening of the Apennine ocean. Hematite content, considered to represent original Fe hydroxy-oxide chemical precipitate in the sediment, decreases upwards in a general way to very low values in the upper cherts. The most ferruginous cherts also contain the highest volcanogenic Pb component and total Pb, whereas higher cherts contain mainly terrestrially-derived Pb. These relations indicate a volcanic-hydrothermal origin for a significant portion of the Pb in the basal to middle parts of the chert formation, and suggest a similar origin for the hematitic Fe. The vertical distribution of hematite and volcanogenic Pb suggests that some initial Fe precipitates, carrying such Pb, had a widespread lateral dispersal after formation at an active ridge.     

Earthquake focal mechanisms and stress inversion in the Irpinia Region (southern Italy)

The goal of this study was to estimate the stress field acting in the Irpinia Region, an area of southern Italy that has been struck in the past by destructive earthquakes and that is now characterized by low to moderate seismicity. The dataset are records of 2,352 aftershocks following the last strong event: the 23 November 1980 earthquake (M 6.9). The earthquakes were recorded at seven seismic stations, on average, and have been located using a three-dimensional (3D) P-wave velocity model and a probabilistic, non-linear, global search technique. The use of a 3D velocity model yielded a more stable estimation of take-off angles, a crucial parameter for focal mechanism computation. The earthquake focal mechanisms were computed from the P-wave first-motion polarity data using the FPFIT algorithm. Fault plane solutions show mostly normal component faulting (pure normal fault and normal fault with a strike-slip component). Only some fault plane solutions show strike-slip and reverse faulting. The stress field is estimated using the method proposed by Michael (J Geophys Res 92:357–368, 1987a) by inverting selected focal mechanisms, and the results show that the Irpinia Region is subjected to a NE–SW extension with horizontal σ ₃ (plunge 0°, trend 230°) and subvertical σ ₁ (plunge 80°, trend 320°), in agreement with the results derived from other stress indicators.     

Hot springs of Rhodesia: Their noble gases,isotopic and chemical composition

Four of Rhodesia's hottest spring complexes (54–100°C) were studied. Tritium contents were very low, compared to adjacent rivers, indicating that the samples studied were indigenous, deep-seated water which had undergone negligible intermixing with surface water.The noble-gas measurements revealed: (1) the waters are meteoric; (2) the noble gases were kept in closed-system conditions in the ground; (3) paleotemperatures are 26–31°C; (4) the boiling Binga springs lost part of their noble gases.Independently, measurements of stable isotopes indicate the meteoric origin of the springs. The chemical composition of the waters clearly reflects their origin from two groups of rocks — one from Karroo sediments and one from crystalline rocks.