Rare-earth element (REE) behaviour in the alteration facies of the active magmatic–hydrothermal system of Vulcano (Aeolian Islands,Italy)

This work reports a study on the rare-earth element (REE) behaviour in the high-sulfidation-type superficial alteration and in the phyllic and propylitic alteration in the subsurface of the island of Vulcano. The results of this study demonstrate notable differences in the REE behaviour in the different alteration facies. In silicic alteration, all REE are equally strongly depleted; in advanced argillic alteration, LREE are quite immobile whereas HREE are strongly depleted; in intermediate argillic, phyllic and propylitic alteration, REE patterns are almost unchanged compared with their fresh rock equivalents. These features indicate that the behaviour of REE in hydrothermal alteration facies at La Fossa is mainly controlled by pH, availability of complexing ions in the fluid (mainly SO₄²⁻ and, in the silicic facies, F⁻) and the presence of secondary minerals able to host REE in their structures. The origin of the acid fluids is related to the hypogenic introduction of prevalently magmatic acid gases produced by the degassing of the shallow magmatic system of La Fossa volcano. In the active high-temperature fumarolic field of La Fossa, REE behaviour illustrates the transition from a dry vapor system, corresponding to the high-temperature fumaroles, to a wet condensing system in the surrounding area. The results of this research, carried out on an active high-sulfidation system, provide a useful tool for the study of high-sulfidation epithermal ore deposits and particularly help to understand the alteration processes during the main stage of hypogene wall rock alteration.     

Lead isotope composition of the sublimates from the fumaroles of Vulcano (Aeolian Islands,Italy): inferences on the deep fluid circulation

The fumarolic fluids of Vulcano (Aeolian Islands, Italy) consist of a mixture of both deep and shallow components. The final products, the fumarolic gases and the sublimates associated with them, provide information on the complex interactions that occur at depth. As radiogenic isotopes do not undergo fractionation after they are incorporated in a fumarolic gas, they can be used directly to characterize the components that mixed. Lead isotopes are particularly suitable, because seawater, which plays an important part in the formation of the fumarolic fluids of Vulcano, contains only negligible amounts of it (10^-12 g/g). Therefore, the lead present in the fumarolic gases (and sublimates) is derived from the magmatic component and a water-rock interaction process. The lead isotope compositions of the lead sulfosalt sublimates collected from the Fossa Crater of Vulcano in 1924, and between 1989 and 1993, are given. The lead isotope ratios of most of the samples are the same within the range of analytical error, regardless of their collection date. The only samples that display slight variations are those collected in 1993. On the whole, the compositional trend of the lead isotopes of the sublimates coincides with that of the latitic-rhyolitic activity of Fossa and differs substantially from that of the pre-Fossa trachy-basaltic activity. The lead composition of the sublimates is very different from that of the Calabrian basement rocks. The data presented here show that the magma presently degassing at Vulcano has the same lead isotopic composition as the products of the recent activity of Fossa, whereas the fumarolic fluid circulation of Vulcano has not involved basement rocks similar to the Calabrian metasediments.     

Fluids in early stage hydrothermal alteration of high-sulfidation epithermal systems: A view from the Vulcano active hydrothermal system (Aeolian Island,Italy)

High-sulfidation (HS) epithermal systems have elements in common with passively degassing volcanoes associated with high T, acid fumarole fields or acid crater lakes. They are considered to form in two stages, the first of which involves advanced argillic alteration resulting from intense, strongly acidic fluid–rock interaction. The La Fossa hydrothermal system (Vulcano Island) represents a classic example of such an active HS system and can be considered as a modern analogue of this early stage of alteration, resulting in a core of intense silicic (90–95% pure SiO₂) alteration surrounded by alunitic alteration zones.     

Evolution of Br/Cl ratios in fumarolic salammoniac from Vulcano (Aeolian Islands,Italy)

 Bromine/chlorine ratios in salammoniac samples, collected from 1983 to 1994 at the Fossa crater fumaroles of Vulcano (Aeolian Islands), were determined. The data obtained in this work have been compared to those obtained by Coradossi et al. (1985) in order to carry out a systematic study of the Br/Cl ratio variations since 1977. Br/Cl ratios range from 0.04 to 0.29. The Br/Cl ratio variations observed have been interpreted as the result of different contributions among deep and shallow components feeding the fumarolic fluids of Vulcano. The Br/Cl increases, which occurred from 1977 to 1979, in 1983–1984 and in 1992–1993, have been connected to an increased input of magmatic gases. Received: 10 November 1995 / Accepted: 12 July 1996     

Genesis of chlorine and sulphur in fumarolic emissions at Vulcano Island (Italy): assessment of pH and redox conditions in the hydrothermal system

Chlorine- and sulphur-bearing compounds in fumarole discharges of the La Fossa crater at Vulcano Island (Italy) can be modelled by a mixing process between magmatic gases and vapour from a boiling hydrothermal system. This allows estimating the compounds in both endmembers. Magma degassing cannot explain the time variation of sulphur and HCl concentrations in the deep endmember, which are more probably linked to reactions of solid phases at depth, before mixing with the hydrothermal vapours. Based on the P–T conditions and speciation of the boiling hydrothermal system below La Fossa, the HCl and S_tot contents in the hydrothermal vapours were used to compute the redox conditions and pH of the aqueous solution. The results suggest that the haematite–magnetite buffer controls the hydrothermal fO₂ values, while the pH has increased since the end of the 1970s. The main processes affecting pH values may be linked to Na–Ca exchanges between evolved seawater, feeding the boiling hydrothermal system, and local rocks. While Na is removed from water, calcium enters the solution, undergoes hydrolysis and produces HCl, lowering the pH of the water. The increasing water–rock ratio within the hydrothermal system lowers the Ca availability, so the aqueous solution becomes less acidic. Seawater flowing towards the boiling hydrothermal brine dissolves a large quantity of pyrite along its path. In the boiling hydrothermal system, dissolved sulphur precipitates as pyrite and anhydrite, and becomes partitioned in vapour phase as H₂S and SO₂. These results are in agreement with the paragenesis of hydrothermal alteration minerals recovered in drilled wells at Vulcano and are also in agreement with the isotopic composition of sulphur emitted by the crater fumaroles.     

Role of magma mixing during the recent activity of La Fossa di vulcano (Aeolian Islands, Italy)

Relatively homogeneous trachytes have been erupted for approximately 3800 years at la Fossa di Vulcano. From the Punte Nere eruptive cycle up to the Palizzi cycle the products varied little, while after the Palizzi cycle (1600 + 1000 a B.P.) to the latest eruption, 1888–1890 AD, a spectrum of compositions, with rhyolite dominating, characterized the erupted products.A stratigraphic sequence, starting with the Palizzi lava flow, has been studied, focussing the attention on lavas and volcanic bombs, to define the role that magma mixing processes have played in the recent history of La Fossa di Vulcano. Textural and chemical analyses of whole rocks, glass, groundmass, and mineral phases indicate that only the breadcrust bombs, erupted during the 1888–1890, show evidence of mixing between trachytic and rhyolitic end-members. Interestingly, in the deposits of the same eruption, trachytic bombs also occur.The lava flows erupted before 1888–1890 display general features suggesting that they entrained crystals and lava fragments during magma ascent. During the 1888–1890 eruption the trachytic bombs were erupted before the breadcrust bombs, which have a more evolved and hybrid composition. These characteristics, together with the change of the nature of the products after the Palizzi cycle, require a complex volcanological model for the recent history of la Fossa di Vulcano.     

Signs of potential renewal of eruptive activity at La Fossa (Vulcano,Aeolian Islands)

Since the end of the last magmatic eruption (1890), activity of La Fossa (southern Tyrrhenian Sea, Italy) has consisted of fumarolic emissions of fluctuating intensity. Fluids are discharged principally at two fumarolic fields located in the northern rim of the active crater and at the beach sited at its northern foot. Increased thermal, seismic and geochemical activity has been recorded since 1978, when an earthquake of M=5.5 occurred in the region. This paper combines available geophysical and geochemical information in order to develop a tentative interpretation of two episodes of apparent unrest which occurred in 1985 and 1987–1988, enhancing the risk of renewal of the eruptive activity. The 1985 unrest consisted essentially of a sharp build up of the internal pressure in the shallow hydrothermal system, which was induced by the injection of hot gases of magmatic origin. The crater fumaroles displayed significant increases in CO₂ and other acid species, but their outlet temperature did not change. Conversely, the 1987–1988 episode was characterized by appreciable modifications at the crater fumaroles, with only secondary effects at the fumarole system of the beach. The sliding of part of the eastern flank of the La Fossa cone into the sea occurred on 20 April 1988, when the region was affected by crustal dilatation producing a seismic sequence of relatively high intensity. Both episodes of unrest were accompanied by increases of local microseismic activity, which affected the nothern sector of the island in 1985, and the southern one in 1988. Finally, a phase of appreciable areal contraction was detected in 1990, probably due to the effect of the cooling and crystallization of magma at relatively shallow depths, accompanying the increased thermal activity at the crater fumaroles. Regional tectonic stress seems to play an important role in the transition of the volcanic system from a phase of relative stability to a phase of apparent unrest, inducing the heating and the expansion of shallow hydrothermal fluids. Available information is insufficient to indicate whether or not the volcano is building towards the renewal of a magmatic eruption, and there is no evidence to hypothesize episodes of significant magma migration. The frequency of measurements of many parameters needs to be increased in order to learn more about the temporal relationships between geochemical and geophysical variations preceding and accompanying periods of increased thermal activity. This will probably be a valid tool for recognizing short-term precursors of a future eruption, reducing the risk of false alarms.     

Imaging and modelling the subsurface structure of volcanic calderas with high-resolution aeromagnetic data at Vulcano (Aeolian Islands,Italy)

In this paper, we present a magnetic model of the subsurface structure of Vulcano island based on high-resolution aeromagnetic data. Three profiles across the most intense magnetic anomalies over the Piano and Fossa calderas were selected for the magnetic modelling, which was constrained by structural and volcanological data, previous geophysical models, paleomagnetic data, and borehole stratigraphy obtained from two deep wells. The interpretation of the magnetic sources represents a significant contribution to the understanding of the Piano and Fossa calderas’ underlying structure, providing us with evidence of the lateral discontinuity between them at depth. We propose that the positive magnetic anomalies in the Piano caldera area are caused by: (a) the remnants of an early submarine volcano; (b) an outcropping dyke swarm related to the feeding system of the Primordial Vulcano phase (beneath Mt. Saraceno); and (c) the presence of a non-outcropping dyke system intruded along a NE–SW-oriented intra-caldera fault (beneath the eastern part of the Piano caldera). Offshore, to the west, the magnetic anomaly map suggests the presence of a submarine volcanic structure, not revealed by bathymetric data, which could represent the eruptive centre, the presence of which has been indirectly deduced from the outcrop of eastern-dipping lavas on the western seashore. Magnetic modelling of the Fossa caldera points to the presence of a highly magnetized cone-like body inside the Fossa cone, centred beneath the oldest crater rims. We interpret this body as a pile of tephritic lavas emplaced in an early phase of activity of the Fossa cone, suggesting that the volume of mafic lavas that erupted at the beginning of the construction of the Fossa edifice was more significant than has previously been deduced. Furthermore, the presence of a magnetized body inside the Fossa cone implies that high temperatures are contained in very limited spaces, do not affect its bulk inner structure, and are restricted to fumarolic conduits and vents. In addition, structures beneath the western and northern part of the Fossa caldera are revealed to have null or low magnetization, which can be ascribed to the presence of pyroclasts and hyaloclastites in this area as well as to a large volume of hydrothermally altered materials. This suggests that the hydrothermal system, with a very limited extension at present, affected a larger area in the past, especially beneath the western part of the caldera.     

Time-space variation of volcano-seismic events at La Fossa (Vulcano, Aeolian Islands, Italy): new insights into seismic sources in a hydrothermal system

We investigated the relationship between volcano-seismic events, recorded at La Fossa crater of Vulcano (Aeolian Islands, Italy) during 2004-2006, and the dynamics of the hydrothermal system. During the period of study, three episodes of increasing numbers of volcano-seismic events took place at the same time as geothermal and geochemical anomalies were observed. These geothermal and geochemical anomalies have been interpreted as resulting from an increasing deep magmatic component of the hydrothermal fluids. Three classes of seismic events (long period, high frequency and monochromatic events), characterised by different spectral content and various similarity of the waveforms, have been recognised. These events, clustered mainly below La Fossa crater area at depths of 0.5–1.1 km b.s.l., were space-distributed according to the classes. Based on their features, we can infer that such events at Vulcano are related to two different source mechanisms: (1) fracturing processes of rocks and (2) resonance of cracks (or conduits) filled with hydrothermal fluid. In the light of these source mechanisms, the increase in the number of events, at the same time as geochemical and geothermal anomalies were observed, was interpreted as the result of an increasing magmatic component of the hydrothermal fluids, implying an increase of their flux. Indeed, such variation caused an increase of both the pore pressure within the rocks of the volcanic system and the amount of ascending fluids. Increased pore pressures gave rise to fracturing processes, while the increased fluid flux favoured resonance and vibration processes in cracks and conduits. Finally, a gradual temporal variation of the waveform of the hybrid events (one of the subclasses of long period events) was observed, likely caused by heating and drying of the hydrothermal system.     

Chemical and isotopic variations in fumarolic discharge and thermal waters at Vulcano Island (Aeolian Islands, Italy) during 1996: evidence of resumed volcanic activity

Gas samples from some fumaroles at ‘La Fossa' crater and Baia di Levante on Vulcano Island and from a diffuse soil gas emission were analysed during 1995–1996, along with water samples from thermal wells in the area of Vulcano Porto. During 1996, we observed a significant increase both in the gas/steam ratio and in the CO₂ concentration, as well as strong variations in δ¹³C_CO2, δD_H2O and δ¹⁸O_H2O of fumarolic gases. These variations are probably related to an increased inflow of deep fluids of magmatic origin. The temperatures of fumaroles did not show remarkable variations except for fumarole F11. In this case, temperature increased by about 80°C from February to August 1996. During the same period, remarkable variations in temperature, phreatic level and chemical and isotopic composition of water were also recorded in one of the geothermal wells in the Vulcano Porto area (Camping Sicilia; T60°C). The observed variations in this well are probably related to a pressure build-up, occurring at least in the surficial part of the system, because of increased gas flux and/or decreased permeability of the fumarolic degassing system. Chemical and isotopic composition of the water showed that during this evolutionary phase, the content of fumarolic condensate in this well was about 80 to 90%. Based on the observation of physical and chemical variables of the Camping Sicilia fluids, during this phase of activity, it is concluded that this area is affected by a phreatic eruption hazard if a volcanic episode with high energy discharge in a limited time span occurs. It follows that this well may be considered as a preferential point for volcanic activity monitoring, both in the case of normal routine surveillance and in the case of inaccessibility to the crater area.     

Characteristics,extent and origin of hydrothermal alteration at Mount Rainier Volcano,Cascades Arc,USA: Implications for debris-flow hazards and mineral deposits

Hydrothermal alteration at Mount Rainier waxed and waned over the 500,000-year episodic growth of the edifice. Hydrothermal minerals and their stable-isotope compositions in samples collected from outcrop and as clasts from Holocene debris-flow deposits identify three distinct hypogene argillic/advanced argillic hydrothermal environments: magmatic-hydrothermal, steam-heated, and magmatic steam (fumarolic), with minor superimposed supergene alteration. The 3.8 km³ Osceola Mudflow (5600 y BP) and coeval phreatomagmatic F tephra contain the highest temperature and most deeply formed hydrothermal minerals. Relatively deeply formed magmatic-hydrothermal alteration minerals and associations in clasts include quartz (residual silica), quartz–alunite, quartz–topaz, quartz–pyrophyllite, quartz–dickite/kaolinite, and quartz–illite (all with pyrite). Clasts of smectite–pyrite and steam-heated opal–alunite–kaolinite are also common in the Osceola Mudflow. In contrast, the Paradise lahar, formed by collapse of the summit or near-summit of the edifice at about the same time, contains only smectite–pyrite and near-surface steam-heated and fumarolic alteration minerals. Younger debris-flow deposits on the west side of the volcano (Round Pass and distal Electron Mudflows) contain only low-temperature smectite–pyrite assemblages, whereas the proximal Electron Mudflow and a < 100 y BP rock avalanche on Tahoma Glacier also contain magmatic-hydrothermal alteration minerals that are exposed in the avalanche headwall of Sunset Amphitheater, reflecting progressive incision into deeper near-conduit alteration products that formed at higher temperatures.     

Fumaroles in ice caves on the summit of Mount Rainier—preliminary stable isotope, gas, and geochemical studies

The edifice of Mount Rainier, an active stratovolcano, has episodically collapsed leading to major debris flows. The largest debris flows are related to argillically altered rock which leave areas of the edifice prone to failure. The argillic alteration results from the neutralization of acidic magmatic gases that condense in a meteoric water hydrothermal system fed by the melting of a thick mantle of glacial ice. Two craters atop a 2000-year-old cone on the summit of the volcano contain the world's largest volcanic ice-cave system. In the spring of 1997 two active fumaroles (T=62°C) in the caves were sampled for stable isotopic, gas, and geochemical studies.Stable isotope data on fumarole condensates show significant excess deuterium with calculated δD and δ¹⁸O values (−234 and −33.2‰, respectively) for the vapor that are consistent with an origin as secondary steam from a shallow water table which has been heated by underlying magmatic–hydrothermal steam. Between 1982 and 1997, δD of the fumarole vapor may have decreased by 30‰.The compositions of fumarole gases vary in time and space but typically consist of air components slightly modified by their solubilities in water and additions of CO₂ and CH₄. The elevated CO₂ contents (δ¹³C_CO2=−11.8±0.7‰), with spikes of over 10,000 ppm, require the episodic addition of magmatic components into the underlying hydrothermal system. Although only traces of H₂S were detected in the fumaroles, most notably in a sample which had an air δ¹³C_CO2 signature (−8.8‰), incrustations around a dormant vent containing small amounts of acid sulfate minerals (natroalunite, minamiite, and woodhouseite) indicate higher H₂S (or possibly SO₂) concentrations in past fumarolic gases.Condensate samples from fumaroles are very dilute, slightly acidic, and enriched in elements observed in the much higher temperature fumaroles at Mount St. Helens (K and Na up to the ppm level; metals such as Al, Pb, Zn Fe and Mn up to the ppb level and volatiles such as Cl, S, and F up to the ppb level).The data indicate that the hydrothermal system in the edifice at Mount Rainier consists of meteoric water reservoirs, which receive gas and steam from an underlying magmatic system. At present the magmatic system is largely flooded by the meteoric water system. However, magmatic components have episodically vented at the surface as witnessed by the mineralogy of incrustations around inactive vents and gas compositions in the active fumaroles. The composition of fumarole gases during magmatic degassing is distinct and, if sustained, could be lethal. The extent to which hydrothermal alteration is currently occurring at depth, and its possible influence on future edifice collapse, may be determined with the aid of on site analyses of fumarole gases and seismic monitoring in the ice caves.     

Guagua pichincha volcano, Ecuador: fluid geochemistry in volcanic surveillance

The densely populated metropolitan area of Quito is located on the slopes of the active Guagua Pichincha volcano at only 10 km from the crater. Recently, the Italian Ministry of Foreign Affairs sponsored a project for the mitigation of volcanic hazard in this area. The geochemical study carried out as part of this project was aimed at constructing a geochemical model of the zone for use in volcanic surveillance.According to this geochemical model, a hydrothermal aquifer (T = 200–240°C), fed both by meteoric waters and by fluids released by a magma body, lies at shallow levels beneath Guagua Pichincha crater. The crater fumaroles are essentially fed by steam boiled off from the hydrothermal aquifer. The high flow rate fumaroles located in the dome area show significant SO₂ contents, which suggest a relatively high contribution of magmatic fluids in the zone of the aquifer feeding them. The absence of SO₂ in the fumarolic discharges near the southern crater wall indicates instead that the magmatic fluids dissolve entirely into the aquifer here. The hot springs located at the western end of the crater represent the lateral discharge of the hydrothermal aquifer.On the basis of this model, it is likely that an increment in the flux of both the magmatic fluids and the heat from a magma body produces an increase, albeit small, of the pressure-temperature conditions of the hydrothermal system and consequent changes in flow rate and fluid chemistry of the fumarolic vents. In particular, total sulphur and possibly hydrochloric acid may increase in all the vents and sulphur dioxide may appear in other fumarolic discharges. The varying thermodynamic conditions in the hydrothermal aquifer can be evaluated on the basis of the equilibria among carbon species and hydrogen. Only minor delayed changes are expected in the physical-chemical characteristics of the springs located at the western end of the crater.     

New geological insights and structural control on fluid circulation in La Fossa cone (Vulcano,Aeolian Islands,Italy)

Electric resistivity tomography (ERT), self-potential (SP), soil CO₂ flux, and temperature are used to study the inner structure of La Fossa cone (Vulcano, Aeolian Islands). Nine profiles were performed across the cone with a measurement spacing of 20 m. The crater rims of La Fossa cone are underlined by sharp horizontal resistivity contrasts. SP, CO₂ flux, and temperature anomalies underline these boundaries which we interpret as structural limits associated to preferential circulation of fluids. The Pietre Cotte crater and Gran Cratere crater enclose the main hydrothermal system, identified at the centre of the edifice on the base of low electrical resistivity values (<20 Ω m) and strong CO₂ degassing, SP, and temperature anomalies. In the periphery, the hydrothermal activity is also visible along structural boundaries such as the Punte Nere, Forgia Vecchia, and Palizzi crater rims and at the base of the cone, on the southern side of the edifice, along a fault attributed to the NW main tectonic trend of the island. Inside the Punte Nere crater, the ERT sections show an electrical resistive body that we interpret as an intrusion or a dome. This magmatic body is reconstructed in 3D using the available ERT profiles. Its shape and position, with respect to the Pietre Cotte crater fault, allows replacing this structure in the chronology of the development of the volcano. It corresponds to a late phase of activity of the Punte Nere edifice. Considering the position of the SP, soil CO₂ flux, and temperature maxima and the repartition of conductive zones related to hydrothermal circulation with respect to the main structural features, La Fossa cone could be considered as a relevant example of the strong influence of pre-existing structures on hydrothermal fluid circulation at the scale of a volcanic edifice.     

A new scenario for the last magmatic eruption of La Soufrière of Guadeloupe (Lesser Antilles) in 1530 A.D. Evidence from stratigraphy radiocarbon dating and magmatic evolution of erupted products

La Soufrière of Guadeloupe is a dangerous volcano characterized over the last decade by moderate seismic and fumarolic unrest. In the last 15,000 years it has experienced phreatic and magmatic eruptions and unusually numerous flank collapse events sometimes associated with a magmatic eruption. We propose a new age of 1530 A.D. and a new eruptive scenario for the last magmatic eruption on the basis of a novel statistical analysis of radiocarbon age dates, and new field and geochemical data. This eruption is the only magmatic eruption likely to have occurred in Guadeloupe during the last 1400 years. The eruption mainly involved an andesitic magma which, in the first phase of the eruption, partially mixed with a slightly more differentiated magma stored in a small and shallow magma chamber. Ascent of magma to the surface generated a partial collapse of the hydrothermally altered edifice that increased the magma discharge and led to a sub-plinian phase with scoria fallout and column-collapse pyroclastic flows followed by near-vent pyroclastic scoria fountains. The eruption ended with growth of a lava dome. Our revised interpretation of the last magmatic eruption of La Soufrière constitutes the most likely key to a future magmatic eruption scenario for this volcano which displays strong evidence of unrest since 1992.     

Organic and inorganic geochemistry of low temperature gas discharges at the Baia di Levante beach, Vulcano Island, Italy

Discharge from subaereal and submarine gas vents of the Baia di Levante beach gases from the Vulcano Island were sampled for major and trace gas components in May and November 1995.Chemical compositions and equilibrium calculations suggest three different groups of CO₂-rich gas emissions depending on their distance from the La Fossa crater: (1) gas vents close to the Faraglione area are characterised by high H₂S contents, high calculated equilibrium temperatures based on inorganic species and relatively high proportion of alkene compounds; (2) gas vents close to Vulcanello are characterised by low calculated equilibrium temperatures and low amounts of alkenes; and (3) Pontile sample has the highest equilibrium CO₂ pressure (up to 68 bars) which may account for the observed absence of benzene. The relative large variability of H₂S in the Baia di Levante beach gas discharge may be attributed to either different interactions between iron sulphides and weakly acid waters or catalytic effect of elemental sulphur on the de-hydrogenation of cyclo-hexane. Thermodynamic calculations suggest that the main inorganic species and CH₄ may have re-equilibrated at relatively shallow depth (10–200 m b.s.l. and 30–600 m b.s.l. for a lithostatic and hydrostatic pressure, respectively). The slow kinetics of reactions in the C_nH_2n/C_nH_2n+2 systems, with respect to that of CH₄–CO–CO₂, may explain the observed propene/propane ratios, which can only be reached at reaction temperatures of 300–350°C. This low speed of reactions can also explain the observed disequilibrium of C1–C4 alkanes.     

Chemical evolution of thermal waters and changes in the hydrothermal system of Papandayan volcano (West Java,Indonesia) after the November 2002 eruption

Papandayan is a stratovolcano situated in West Java, Indonesia. Since the last magmatic eruption in 1772, only few hydrothermal explosions have occurred. An explosive eruption occurred in November 2002 and ejected ash and altered rocks. The altered rocks show that an advanced argillic alteration took place in the hydrothermal system by interaction between acid fluids and rocks. Four zones of alteration have been defined and are limited in extension and shape along faults or across permeable structures at different levels beneath the active crater of the volcano.     

Genesis and evolution of the fumaroles of vulcano (Aeolian Islands,Italy): a geochemical model

A geochemical model explaining the presence of fumaroles having different gas composition and temperature at the top of the crater and along the northeastern coast of Vulcano island is proposed. A pressurized biphase (liquid-vapor) reservoir at the depth of about 2 km is hypothesized. Energy and mass balance sheets controlP-T conditions in the system.P-T must vary along a boiling curve of brine as liquid is present. The CO₂ content in the steam is governed by the thermodynamic properties of the fluids in the H₂-NaCl-CO₂ system. On the assumption that oxygen fugacity in the system is between the HM-FMQ oxygen buffers, observed SO₂/H₂S, CO₂/CO, CO/CH₄ ratios in the fumarolic gases at the Fossa crater appear in equilibrium with a temperature higher than that observed, such as may exist at depth. The more reduced gas phases present on the sea-side may result from re-equilibrium processes in shallower aquifers. The suggested model would help in monitoring changes in volcanic activity by analyzing fumarolic gases.     

Pyroclastic density currents resulting from the interaction of basaltic magma with hydrothermally altered rock: an example from the 2006 summit eruptions of Mount Etna,Italy

After 16 months of quiescence, Mount Etna began to erupt again in mid-July 2006. The activity was concentrated at and around the Southeast Crater (SEC), one of the four craters on the summit of Etna, and eruptive activity continued intermittently for 5 months. During this period, numerous vents displayed a wide range of eruptive styles at different times. Virtually all explosive activities took place at vents at the summit of the SEC and on its flanks. Eruptive episodes, which lasted from 1 day to 2 weeks, became shorter and more violent with time. Volcanic activity at these vents was often accompanied by dramatic mass-wasting processes such as collapse of parts of the cone, highly unusual flowage processes involving both old rocks and fresh magmatic material, and magma–water interaction. The most dramatic events took place on 16 November, when numerous rockfalls and pyroclastic density currents (PDCs) were generated during the opening of a large fracture on the SE flank of the SEC cone. The largest PDCs were clearly triggered explosively, and there is evidence that much of the energy was generated during the interaction of intruding magma with wet rocks on the cone’s flanks. The most mobile PDCs traveled up to 1 km from their source. This previously unknown process on Etna may not be unique on this volcano and is likely to have taken place on other volcanoes. It represents a newly recognized hazard to those who visit and work in the vicinity of the summit of Etna.     

Varicolored and vesiculated tuffs from La Fossa volcano,Vulcano Island (Aeolian Archipelago,Italy): evidence of syndepositional alteration processes

Chemical and mineralogical data for samples collected from a surge sequence from La Fossa cone (Vulcano Island, Italy) show a wide variety of alteration states between adjoining beds, the macroscopic features of which are expressed by various degrees of reddening. The effects of the alteration processes on pyroclastic rocks are as follows: hydration and oxidation of each component of the pyroclastic rocks to varying degrees; formation of authigenic smectite; precipitation of a large variety of soluble salts; and corrosion on the surface of glass fragments (pitting). Dry surge beds, emplaced from a two-phase, dry steam + solid, suspension do not show significant alteration. By contrast, wet surge deposits, suggesting an emplacement from free water-bearing turbulent flows, show an increasing degree of alteration, passing from grey to red coloured beds. The strict relationship between the present alteration state and the depositional unit rules out any post-depositional processes. The occurrence of alteration in wet surge beds and the lack of alteration in dry beds shows that the main controlling agent was water condensed from the eruptive cloud and suggests a syn-depositional character to the alteration itself. These observed differences can be ascribed to the different chemical reactivities of the water, probably related to the amounts of acidic species carried by the eruptive cloud and/or by the efficiency of their capture during the condensation of the water vapour.