A pyroclastic flow deposit showing a standard depositional sequence outcrops along the north-eastern cliff of Ustica Island (South Tyrrhenian Sea). Layer 1, at the base of this standard unit, consists of plane-parallel laminated and cross laminated red ash with accretionary lapilli and vesicles (interpretable as ground surge deposit). Layer 2 is a breccia, containing ash, lapilli and block size fragments up to 7 m in diameter, showing either no fragment organization or a coarse tail reverse grading (interpretable as the deposit of the pyroclastic flow proper). Layer 3 at the top of the flow unit is formed by plane-parallel laminated red ash (interpretable as ash cloud surge deposit). The accretionary lapilli of layer 1 have been classified. The following five characteristics have been noted: 1) ratio of the core diameter C to the longest total diameter D of lapillus, C/D > 0.7, 2) ratio of the longest diameter Di to the shortest diameter Ds of lapillus, Di/Ds < 1.5, 3) vesicles within <50% of the population; 4) dense texture, 5) maximum grain size of accreted ash > 500 , mm. These characteristics are consistent with the interpretation that layer 1 was deposited by a surge. Six oriented blocks were collected from layer 2 and then submitted to progressive thermal demagnetization. The paleomagnetic directions are closely grouped around a single component of magnetization, with observed blocking temperatures suggesting an emplacement temperature above 570° C. The vesicularity of the juvenile clasts, collected from layer 2, was studied. A vesicularity index of 33% and a vesicularity range of 65% indicate a fragmentation due to a magma/water interaction only, without significant vesiculation. X-ray fluorescence analyses show that the breccia blocks plot in the hawaiite field with an alkali sodic affinity.
During 1971 a little pit-crater appeared in a previous depression, on the south-eastern flank of Mt. Etna´s Central Cone. The embryonic SE Crater gave a scarce and discontinuous explosive and effusive activity until the formation of a cinder cone producing a significant lava flow, on 1984. A new cinder cone started to grow up, on 1988. It disrupted and buried the previous one in a few years, changing the morphology of Etna´s summit zone.
The results of a detailed field study of the products related to the final-stage collapse (166.000 years, Vepe caldera) of the most recent caldera-forming eruption affecting the Latera Volcanic Complex are presented. The eruptive sequence (Pitigliano Formation) is composed of basal pumice flows (unwelded sequence), rheomorphic welded airfall tuffs (welded sequence) which in turn pass gradually to lava-like deposits, and final lava flows that form low-angle exogenous domes. In the more distal outcrops polygenetic breccia layers are interbedded with the welded sequence, whose variations in physical properties, textural characteristics, and other field evidence suggest an important fallout component followed by rheomorphism. Furthermore, the occurrence of variations in the proportions of the different kinds of near-surface and deep-seated rocks of the polygenetic breccia layers supports the hypothesis of the progressive opening of several source vents localized around the NW rim of the Vepe caldera. Collapsing eruptive columns, followed by lava fountains respectively produced the lowermost unwelded sequence (pumice flows) and the welded sequence (rheomorphic airfall tuffs). The latter grades upwards and proximally into a lava-like deposits. Nevertheless, during the final stage of the eruption, lavas outpoured quietly from a caldera ring-fracture. The eruptive styles were regulated by the degree of magmatic fragmentation and the rapid subsidence of a "piston-like" block, which mostly occurred when the magma chamber had been nearly completely tapped and the residual melt of the deeper part of the magmatic reservoir was squeezed out, forming low-angle exogenous domes.
The Agua de Pau volcano, located on the centre of the S. Miguel island, represents one of the most dangerous active volcano of the Azores archipelago. Its volcanic history appears to be characterized by recurrent plinian events which follow long periods of quiescence. The last one, dated at 4550 years ago, produced a widespread pumice fall deposit (Fogo A) followed by minor pyroclastic flows and surges. The large number of exposures and their well preservation give the opportunity of modeling future similar events, which can not be ruled out according to the cyclic pattern of the volcanic activity. The modeling was focussed on the pyroclastic flow hazard. It started with the recognition on the field of flow termini, whose height above sea level together with the distance from the eruptive vent were used to reconstruct the slope and the height of the energy line. The obtained results have pointed out the occurrence of potentially dangerous areas on the northern and southern side of the volcano, well in coincidence with the main populated villages. On the northern side possible pyroclastic flows can reach the southern peripheries of the main towns at velocities of the order of 15 m/s, while on the southern side the pyroclastic flows can travel troughout villages at velocities of the order of 65 ÷ 70 m/s. On the basis of the areal distribution of the associated ash-cloud surge deposits areas potentially exposed to seriously dangerous volcanic events were extended well outside the hazardous areas for pyroclastic flows.
The amount of S0 2 recently emitted by Vulcano cannot be satisfactorily explained by the degassing of magma volumes linked with the recent activity of La Fossa and Vulcanello. A systematic study of the sulfur content of melt inclusions in phenocrysts from samples, representative of the volcanic series, suggests that the sulfur may partially come from primary basalt, analogous to that which erupted 50.000 years ago at La Sommata, at the beginning of the collapse that resulted in the formation of La Fossa caldera. This magma, still active at depths, is similar, due to its richness in volatile elements (H20 = 2.88%; S = 2800-1800 ppm; Cl = 4000 - 2500 ppm; F = 640 ppm), to Mount Etna´s primary magmas; the high volatile elements content is a characteristic directly inherited from the mantle of this Southern Italian regon.
A detailed mapping (scale 1:5000) was carried out on the northern wall of Valle del Bove (VDB), a deep depression located in the eastern side of Mount Etna. The products of several volcanic centres that preceded the actual volcano are exposed along the valley walls. Stratigraphic criteria suggested by Pasquare´ et al. (1991; 1992) were used during this study. The oldest volcanics are exposed in the eastern side of the wall and represent the remains of at least three small eruptive centres. The outcrops do not show the contact with the substratum, but they allow a partial morphological reconstruction of these small centres, gathered in Rocca Capria lithosome. The volcanics of Rocca Musarra, dipping towards NE, probably represent the remains of an old eruptive centre situated in the southern part of VDB perhaps overlying Rocca Capra lithosome. The products belonging to Concazze Synthem, whose main eruptive centre is the Ellittico, are deposited with angular unconformity on the older centres. These products crop out in the central and western parts of the northen wall. It is a volcanic succession that deepens from west to east. At the bottom, it is mainly composed of lava, whereas it is mostly pyroclastic at the top. The activity of the Ellittico ended with a large caldera collapse. The caldera edges are exposed at Pizzi Deneri and Punta Lucia. A subhorizontal lava succession represents the products that filled the depression and began the construction of the present volcano. The recent and present day products are distributed inside and outside the Valle del Bove and emerge from NE-SW oriented eruptive fissures, following the main tectonic lineaments of this area. The succession of volcanics found between the Ellittico caldera and the present topographic surface, is identified as Il Piano Synthem (see Calvari et al., 1994). Concazze Synthem and Il Piano Synthem make up the Mongibello Supersynthem. Several volcano-stratigraphic aspects of the VDB northern wall have been re-examined in order to reconstruct a new and more detailed framework of both the stratigraphy of the eruptive centres and the spatial relationships between them.
Mineralogical and geochemical data on mafic volcanics of shoshonite suite from the islands of Procida-Vivara and Ventotene are presented and discussed. The investigated rocks include: i) lithic lava clasts and juvenile vesiculated fragments from hydromagmatic tuffs at Procida-Vivara; ii) samples from lava flows, and lithic lava clasts from a pyroclastic unit at Ventotene. Composition ranges from shoshonite basalt to shoshonite, and latite. Petrographic observations and mineral chemical data on samples from Procida-Vivara show some evidence of phase disequilibrium, as testified by normal and reverse zoning in clinopyroxene and plagioclase phenocrysts, coexistence of two different clinopyroxenes (a Mg-rich and an Fe-rich diopside), and, in the most evolved samples, of alkali-feldspar, magnesian olivine and calcic plagioclase. Sr-isotope composition is always more radiogenic in separated clinopyroxenes and plagioclases, relative to both matrix and whole rock. As regards the rocks from Ventotene, petrographic and mineralogical data indicate equilibrium between phenocrysts and groundmass. The restricted chemical variations within each group of samples are generally explained in terms of fractional crystallization of the observed mineral phases. However, the juvenile fragments from the Solchiaro tuff Procida, as well as the lavas from Ventotene, show strong enrichments in LILE and LREE, and positive correlations between 87Sr/86Sr and degree of chemical evolution, which strongly suggest that open-system processes were operating during evolution. Petrographic and geochemical data suggest interaction between distinct batches of magmas with different degree of chemical evolution and Sr-isotope composition. Furthermore, quantitative models show that a liquid akin to the most primitive lithic lava clasts (shoshonite basalt) might represent a parent magma for more evolved shoshonite suite magmas only if open-system evolution processes were involved. Finally, the most primitive among the investigated rocks have shown that a marked geochemical and Sr-isotope difference existed between the source reactions of the magmas from Procida-Vivara and Ventotene districts.
Sixteen samples, taken from different structural sectors of Mount Etna volcano, have been dated to study its evolution in time. The data were obtained through a potassium argon technique specially developped for dating young volcanic rocks, down to an age of zero, with typical accuracies ranging between 103 and 104 years. Control was obtained by analysing samples from modern flows, by comparing the dating with available stratigraphic evidence, and by checking it with previous radiometrical dating. The results allow the eruptive history of the volcano to be reconstructed. Two periods of tholeiitic to subalkaline activity were distinguished: one at around 520,000 ± 40,000 years B.P. whose products, partly submarine, outcrop in the south-eastern sector of the volcano, and the other between 330,000 and 270,000 years B.P., located in the south-western sector. The oldest alkaline products date to 168,000 ± 8,000 years B.P. This raises the question of the continuity in the volcanic activity and suggests a possible gap between the subalkaline and the alkaline series. The same problem also arises in the subsequent history. Indeed, it appears to be divided into three phases separated by quiescent periods: 168,000 to 100,000 years B.P. (lavas belonging to the Ancient Alkali Centers), around 80,000 to 60,000 years B.P. (the Trifoglietto Unit) and younger than 35,000 years (the Mongibello Unit). The volcanological evolution and the nature of the erupted products reflect the alternation of extensional and comprensional stress fields, which depend upon the geodynamical activity in the area at the time.
A new calorimetric method for measuring the output of thermal convective energy emitted by submerged fumaroles is herewith described. This method, which has been applied to the Baia di Levante (Vulcano) manifestations, together with the measurements of the gaseous output of the same manifestations, has made it possible to estimate the outputs of thermal convective energy, water vapour and CO2 in the entire area. The energy and vapour emission rates have been ascertained to be one order of magnitude lower than those of the crater and are comparable with those measured at the crater fumaroles in 1983. The values of the gas/vapour ratio obtained from the output ratios and from geobarometric equilibria are comparable, suggesting that an equilibrium had been reached in the system feeding the fumaroles of Baia di Levante at the time the measurements were effected. The method presented here can supply important information, vital for the assessment of the risk of a phreatic explosion in the area and therefore completes the data obtained from the estimations of the pressure of fluids at depth.
The carbonate-evaporite geothermal systems of Tuscany and Latium are flushed by a remarkable flux of CO2 of deep provenance. Available d13 C data suggest that the CO2 discharging at the surface in the region is originated by mixing of a mantle component and a thermometamorphic component. The variations of PCO2, presumably caused by changes in the CO2 flux of deep provenance, are likely quicker than temperature variations. Pco2. can therefore play a role more important than temperature as cause of deposition or dissolution of calcite and anhydrite. The results of an equilibrium model between aqueous solution (with a chloride content > 0.03 mol/kg) and the mineral paragenesis made up of anhydrite, calcite, fluorite, quartz, albite, adularia chlorite and illite indicate that, for PCO2 values lower than a certain threshold (that depends upon chloride content and temperature) the system can be sealed by precipitation of calcite caused by a PCO2 increase. For PCO2 values higher than that threshold, instead, the system can be sealed by precipitation of anhydrite determined by a PCO2 decrease. The deposition of anhydrite or calcite can completely seal the fractures acting as circulation paths for the hydrothermal solutions. Thus the natural flow of CO2 is stopped. This can be restored either by tectonic movements or hydraulic-pneumatic fracturing. In the absence of these phenomena, the deposition of anhydrite or calcite can cause the "death"of the geothermal reservoir. This phenomenology implies that an active geothermal system can be present, at depth, below a surface area of high CO2 flux only. It must be stressed that there is a good correspondence between surface areas of high CO2 flux (outlined on the basis of Pco, distribution in shallow waters) and the areal extension of geothermal reservoirs of high enthalpy (Monte Amiata and Latera), medium enthalpy (Torre Alfina) and low enthalpy (Viterbo).
Unusual pyroxenes have been identified in crustal xenoliths commonly enclosed in recent lavas from Mount Etna (Italy). The crystals exhibit specific habits, occurrence places and composition ranges reflecting the pnmary flyschoid nature and the alteration degree of the xenolith. They occur as (1) tangled up and acicular munerals of Ti, Al-poor and Si-rich clinopyroxenes and form typical reaction rims in the siliceous xenoliths, (2) Ti, Al-nch and Si-poor augites -diopsides probably inherited from the enclosing lava in the peraluminous xenoliths, and (3) grains of Ca-nch clinopyroxenes, i.e diopsides hedenbergites, fassaites or subsilicic aluminian ferrian diopsides or hedenbergites and constitute concentric texturai zonings and beddings in the carbonate xenoliths. They supply a cntenon for discriminating the siliceous, peraluminous and carbonate xenolith families. Reaction and zoning phenomena make these minerals tracers of elemental exchanges between xenolith and host lava. Pyroxene crystallization is associated with elemental inputs such as Fe, Mg, Ti, Ca or Si, and especially Sc and Co. The xenoliths testify to shallow interactions between hawaiitic magmas and a crustal basement. They constitute an indirect approach to interaction phenomena occurnng at wall rocks in the feeding dikes.
Among the ultrapotassic rocks of SE Spain, the outcrop of Zeneta (province of Murcia) underwent impressive hydrothermal transformations which mostly involved mafic phases (olivine, ± orthopyroxene, ± clinopyroxene). Some samples contain Mg-saponite, minor amount of a smectite with moderate Al and Fe and of silica phases as secondary minerals. The largest part of the samples exhibits heterogeneous distribution of secondary minerals. The secondary phases may be calcite, Al- and/or Fe-rich smectites, hisingerite, silica phases and rare Mn-hydroxides and celadonite In places, the silica phases (opale A, opale CT and minor chalcedony) may be very abundant; they substitute previous secondary minerals, fill voids, and may include rare crystals of goethite. In two cases, it was possible to evaluate the mobility of the components during the hydrothermal transformations: SiO2, Fe2O 3tot., MgO and Ni were stronglymobile (i.e. they changed their mass during the transformation), whereas TiO2, Al2O3, Na2O, P2O5, Co, Cr, Zr, NbPb, U, La, Nd, Sm, Eu were not significantly mobile. The stability of goethite is in agreement with temperature lower than 80-100° C at least during the strong silification stage.