Bruno Capaccioni

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.

Light hydrocarbons in hydrothermal and magmatic fumaroles: hints of catalytic and thermal reactions

Volcanic gaseous mixtures emitted from active volcanoes frequently show variable amounts of saturated (alkanes), unsaturated (alkenes) and aromatic volatile hydrocarbons. Three major patterns of distributions can be recognized, apparently related to the chemical-physical environment of formation of the gas exhalations: alkane-rich, low-temperature gas emissions from recently active volcanic areas; aromatic-rich hydrothermal manifestations; and alkene-rich, ‘magmatic’ fumaroles on active volcanoes. Thermodynamic data, together with theoretical and practical findings from the petroleum industry, point to two main types of reactions occurring in these volcanic environments: cracking and reforming. Cracking processes, mainly caused by thermal effects, occur when hydrocarbon-bearing hydrothermal fluids enter and mix with a hot and dry, rapidly rising magmatic gas phase. The most probable products are light alkenes with carbon numbers decreasing with increasing reaction temperatures. The presence of aromatic species in hydrothermal fluids can be linked to reforming processes, catalysed by several possible agents, such as smectites and zeolites, generally present in the hydrothermally altered volcanic terranes, and facilitated by long residence times in a hydrothermal envelope.

Monitoring of active but quiescent volcanoes using light hydrocarbon distribution in volcanic gases: the results of 4 years of discontinuous monitoring in the Campi Flegrei (Italy)

During the period from January 1991 to January 1994, the 10-yr geochemical monitoring in the Campi Flegrei (Southern Italy), usually performed using the inorganic gas fraction, was implemented by discontinuous monitoring of the hydrocarbon gas fraction. The measured compositions have revealed the presence of significant amounts of at least 11 hydrocarbon species, related to alkane, alkene, and aromatic classes. Thermodynamic calculations showed the possible existence of partially independent equilibria within the C1/C2/C3 and C1/C2/C3/C4 alkane systems, as well as among alkene/alkane pairs at redox conditions controlled by the geothermal (FeO/Fe1,5O) redox buffer system, at temperatures between 200 and 400°C and PH2O controlled by the presence of a saline brine. Unlike the constancy of the measured and calculated ‘inorganic’ temperature, equilibrium temperature based on organic species has displayed fluctuations and a significant increase starting from the second half of 1993. According to their apparent correlation with other independent observations, such as the measured increases of vapor fraction, H2S/CO2 and H2/N2 ratios on the monitored fumarole in addition to a general increase of dissolved CO2 in the Campi Flegrei groundwaters, the observed variations of the organic parameters were interpreted as the result of a general increase in acid-bearing gas flux throughout the re-equilibration zone.

1980–1990: Ten years of geochemical investigation at Phlegrean Fields (Italy)

Abstract A systematic geochemical investigation on the thermal manifestations of the volcanic area of Phlegrean Fields started in 1980, with the aim of obtaining useful information about the changes possibly occurring at the same time of vertical ground movements, termed as “bradyseismic”, which frequently interested this area. The attention was initially given to the fumaroles and the boiling pools of Solfatara and Pisciarelli, then subsequently extended to thermal springs, dug wells, and crater lakes. Analytical data pertaining to a period preceding the last crisis of 1982–1984 were then available, as well as other data collected during the previous crisis of 1970; a comprehensive picture of the variations observed before, during and after the acceleration of vertical movement has been drawn. According to the results obtained, the intrusion of a magmatic body as the trigger of the observed ground deformation is considered unrealistic. The whole area appears now in a quiet stage, and no further perturbation is expected without previous significant tectonic activity; chemical precursors appear suitable for an early detection of changes occurring at depth, to forecast possible consequences to be expected in the surface environment.

Intense magmatic degassing through the lake of Copahue volcano, 2013-2014

Here we report on the first assessment of volatile fluxes from the hyperacid crater lake hosted within the summit crater of Copahue, a very active volcano on the Argentina-Chile border. Our observations were performed using a variety of in situ and remote sensing techniques during field campaigns in March 2013, when the crater hosted an active fumarole field, and in March 2014, when an acidic volcanic lake covered the fumarole field. In the latter campaign, we found that 566 to 1373 t d−1 of SO2 were being emitted from the lake in a plume that appeared largely invisible. This, combined with our derived bulk plume composition, was converted into flux of other volcanic species (H2O ~ 10989 t d−1, CO2 ~ 638 t d−1, HCl ~ 66 t d−1, H2 ~ 3.3 t d−1, and HBr ~ 0.05 t d−1). These levels of degassing, comparable to those seen at many open-vent degassing arc volcanoes, were surprisingly high for a volcano hosting a crater lake. Copahues unusual degassing regime was also confirmed by the chemical composition of the plume that, although issuing from a hot (65°C) lake, preserves a close-to-magmatic signature. EQ3/6 models of gas-water-rock interaction in the lake were able to match observed compositions and demonstrated that magmatic gases emitted to the atmosphere were virtually unaffected by scrubbing of soluble (S and Cl) species. Finally, the derived large H2O flux (10,988 t d−1) suggested a mechanism in which magmatic gas stripping drove enhanced lake water evaporation, a process likely common to many degassing volcanic lakes worldwide.

Computer-assisted image analysis on clast shape fabric from the Orvieto-Bagnoregio ignimbrite (Vulsini District, central Italy): implications on the emplacement mechanisms

Abstract Computer-assisted Image Analysis can be succesfully used to derive quantitative information about grain-size distribution, particle shape and fabric on both consolidated and unconsolidated solid aggregates. We have developed a new analytical method that provides a series of quantitative textural parameters from whatever particulate deposits by combining commercial image acquisition system with devoted C-software. After exhaustive tests of the method, we applied it to a widespread Quaternary ignimbrite formation in central Italy (the Orvieto-Bagnoregio formation). The results suggest some new aspects of emplacement mechanisms of ignimbrites: (1) elongated particles shows variable degrees of flow-related preferred orientation both on horizontal and vertical planes; (2) vertical variations of flow-related preferred particle orientations follow a “zig-zag” pattern that we interpret to result from deposition by progressive aggradation during the passage of a particulate flow. The filling up of paleovalleys by means of progressive aggradation proceeds flank to flank due to alternating flow directions, induced by the already deposited material; (3) the occurrence of vertically spaced peaks on the strength of clast orientation suggests the existence of discrete depositional units deposited by aggradation from an unsteady but persisting flow. Strong fabrics are inferred to result from the high amount of shear stress imparted to particles at the depositional boundary layer, which at the same time can be responsible for the development of the basal inversely graded layer. Image analysis can also provide useful indications of paleoflow directions, paleotopographic details and ignimbrite source areas.

Plinian fall deposits from Vulsini Volcanic District (Central Italy)

Stratigraphic investigations of the Vulsini Volcanic District indicate the existence of five prominent plinian fall deposits, deposited over a period of about 0.34 Ma. The oldest deposit (‘Basal pumices’) crops out mainly in the peripheral areas and is one of the largest plinian events in the Vulsini District, with a volume of about 9 km3 and a calculated column height exceeding 30 km. Subcircular patterns of isopachs and isopleths around the Bolsena lake indicate emplacement in wind-free conditions and suggest a source vent location in the northernmost sector of the present Bolsena lake, where it is possible that an older central volcanic structure existed. The four younger plinian fall deposits are related to Bolsena activity. The oldest of these, named the ‘Ponticello’ and ‘Orvieto-Bagnoregio’ pumices, are mainly distributed on the north-eastern sector of the Vulsini District. Their volumes are an order of magnitude lower than that of the basal unit, and the estimated column heights do not exceed 20 km. The younger ‘Ospedaletto’ pumice deposit has a NE-SW dispersal axis, whereas the youngest ‘Casetta’ pumice deposit is found only in the north-eastern sector of the district. Their estimated volumes are respectively 1.2 and 0.1 km3, whereas the inferred vent areas appear to be located slightly to the east of the Ponticello and Orvieto-Bagnoregio pumice deposit source areas. The chronology of the Ospedaletto and Casetta pumices indicates that the final plinian activity from the Bolsena complex is contemporaneous with the Latera activity. The decreasing volumes from the oldest to the youngest units, together with the progressive shifting northeastwards of the source vents, may be related to the volcano-tectonic subsidence of the Bolsena area. The source vents for the post-Basal pumices events are mainly clustered just north of Bolsena village where the greatest displacement during subsidence occurred. Accordingly, source vent shifting appears to be related to the progressive opening of normal faults from the inner to the outer margin of the Bolsena depression.

HCl degassing from extremely acidic crater lakes: preliminary results from experimental determinations and implications for geochemical monitoring

Abstract Crater lakes are monitored to detect volcanic unrest starting from the assumption that they behave as condensers for magmatic gases. A further assumption is that acidic gases such as HCl are conservative once dissolved in water. This is not true for extremely acidic crater lakes, whose H+ activity is high enough to induce Cl− hydrolysis and consequently HCl degassing. This study presents the results of experimental determinations at 40–45°C demonstrating that HCl degassing from acidic water depends on pH and Cl− concentration. HCl degassing starts at pH values c. 0.05–0.1 with a rate of 5–10 mg min−1 l−1, increasing up to c. 70 mg min−1 l−1 at pH<−0.2. This implies that the rate of HCl removal from a crater lake with a volume of 104–105 m3 and a seawater-like Cl− concentration ranges from 5 to 50 t h−1. The estimated HCl/H2O ratio in the separated vapour phase (0.01–0.2) is coherent with HCl/H2O ratios of fumaroles. Our experiments imply that: (i) the presence of very acidic gas species in fumaroles can be associated with a liquid-dominated feeding system, and (ii) dissolved in extremely acidic crater lakes, Cl− behaves as a non-conservative component.

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.

Sampling and analysis of light hydrocarbons in volcanic gases

SummaryVolcanic gas emissions are strictly related to a specific stage of volcanic activity. Complete knowledge of their chemical composition can be extremely useful for efficient geochemical surveillance of active volcanism sites. A new, effective and simple method for the evaluation of light hydrocarbons in volcanic gases is described. A rapid sampling procedure and GC-FID analysis are employed. Data from different volcanic areas are shown and a their possible correlation is also discussed.