Roberto Cioni

Soil CO2 flux measurements in volcanic and geothermal areas

Abstract The accumulation chamber methodology allows one to obtain reliable values of the soil CO2 flux, ϕsoil CO2, in the range 0.2 to over 10 000 g m−2 d−1, as proven by both laboratory tests and field surveys in geothermal and volcanic areas. A strong negative correlation is observed between Δϕsoil CO2/Δt and ΔPatm/Δt. Maps of classes of log ϕsoil CO2 for the northern sector of Vulcano Island, Solfatara of Pozzuoli, Nea Kameni Islet and Yanbajain geothermal field evidence that active faults and fractures act as uprising channels of deep, CO2-rich geothermal or magmatic gases. The total diffuse CO2 output was evaluated for each surveyed area.

Origin of the fumarolic fluids of Vulcano island, Italy and implications for volcanic surveillance

Variations in δD and δ18O values with H2O contents and outlet temperatures indicate that the fumaroles of La Fossa crater have discharged mixtures of magmatic water and marine hydrothermal water, since 1979. The contribution of meteoric water was low in the period 1979–1982 and very low afterwards. The δ18O values of the marine-hydrothermal component of +5 to +7.2‰ are due to isotopic exchange with the 18O-rich silicates of the rocks under high-temperature and low-permeability conditions. The δ18O value of the magmatic end-member is generally +3.5 to +4.3‰, although values as high as +5.5 to +6.5‰ were reached in the summer of 1988, when magma degassing appears to have extended into the core of the magma body. The δD values of the end-member were close to -20‰, typical of andesitic waters. Both the isotopic values and chemical data strongly support a ‘dry’ model, consisting of a central magmatic gas column and a surrounding hydrothermal envelope, in which marine hydrothermal brines move along limited fracture zones to undergo total evaporation on approaching the conduits of magmatic fluids. The vents at the eastern and western boundaries of the fumarolic field are fed by fluids whose pressure is governed by the coexistence of vapor, liquid and halite, giving rise to a high risk of phreato magmatic explosions, should magma penetrate into these wet environments. Most La Fossa eruptions were triggered by an initial hydrothermal blast and continued with a series of phreatomagmatic explosions. The fluids discharged by the Forgia Vecchia fumaroles are mixed with meteoric water, which is largely evaporated, although subordinate loss of condensed steam may be responsible for scrubbing most of the acidic gas species. The temperatures and pressures, and the risk of a sudden pressure increase, are low. A boiling hydrothermal aquifer at 230° C is present underneath the Baia di Levante beach. This area has a minor risk of hydrothermal explosions.

Reactions governing the chemistry of crater fumaroles from Vulcano Island, Italy, and implications for volcanic surveillance

Abstract More than 200 chemical and isotope analyses of fumarolic fluids collected at the Fossa Grande crater, Vulcano Island, during the 1980s show that the main process controlling these fluids is mixing between the gas released by a magma body and the vapour produced through evaporation of brines of marine origin. Large variations in the relative contribution of these two sources have been observed during the last 10 a. The main species (H 2 O and CO 2 ), the inert gases (He and N 2 ), and the D content of steam are fixed by the mixing processes; they are therefore the best tracers the fraction of the deep magmatic component in the fumarolic fluids discharged at the surface. In contrast, the “fast” species (H 2 and CO) equilibrate at T,P values close to the outlet temperature and atmospheric pressure, and under redox conditions governed by the SO 2 H 2 S buffer, as indicated by thermodynamic calculations. Acid gases (HCl, HF, H 2 S and SO 2 ) are partly contributed by the magmatic component and partly produced by the reactions between hot rocks, steam and salts which take place in the “dry” zones surrounding the central magmatic gas column, as suggested by the good agreement between their analytical and theoretical contents.

Irreversible water–rock mass transfer accompanying the generation of the neutral, Mg–HCO3 and high-pH, Ca–OH spring waters of the Genova province, Italy

In a recent survey of the spring waters of the Genova province, many neutral Mg–HCO3 waters and some high-pH, Ca–OH waters were found in association with serpentinites. All the springs are of meteoric origin as indicated by the stable isotopes of water and dissolved N2 and Ar. Interaction of these meteoric waters with serpentinites determines a progressive evolution in the chemistry of the aqueous phase from an immature Mg-rich, SO4–Cl facies of low salinity to an intermediate Mg–HCO3 facies (pH 7.0–8.5, PCO210−3.5–10−2.5 bar, Eh 150–250 mV), and to a mature Ca–OH facies (pH 10–12, PCO2 10−9.4−10−10.6 bar, Eh-390 to-516 mV). The irreversible water–rock mass transfer leading to these chemical changes in the aqueous phase was simulated through reaction path modeling, assuming bulk dissolution of a local serpentinite, and the precipitation of gibbsite, goethite, calcite, hydromagnesite, kaolinite, a montmorillonite solid mixture, a saponite solid mixture, sepiolite, and serpentine. The simulation was carried out in two steps, under open-system and closed-system conditions with respect to CO2, respectively. The calculated concentrations agree with analytical data, indicating that the computed water-rock mass transfer is a realistic simulation of the natural process. Moreover, the simulation elucidates the role of calcite precipitation during closed-system serpentinite dissolution in depleting the aqueous solution of C species, allowing the concurrent increment in Ca and the acquisition of a Ca–OH composition. Calcium–OH waters, due to their high pH, tend to absorb CO2, precipitating calcite. Therefore, these waters might be used to sequester anthropogenic CO2, locally preventing environmental impact to the atmosphere.

Hydrothermal eruptions of Nisyros (Dodecanese, Greece). Past events and present hazard

Abstract The detailed analysis of the craters of hydrothermal eruptions and related products present on Nisyros Island demonstrates the ephemerality of these morphological forms. In other words, the mere recognizable existence of the craters and associated deposits implies recency of hydrothermal activity. The minimum temperature required to cause the explosive phenomenon and, possibly, the depth of the reservoir (which can be evaluated on the basis of the correlation between the diameter of the crater and the depth of explosion as proposed by Fytikas and Marinelli, 1976) are therefore closely representative of the current hydrothermal circulation. Both field evidence and historical records indicate that all the deposits of hydrothermal eruption recognized on Nisyros Island were emplaced as debris flows. Almost all the ballistic ejecta were entrained in these debris flows and either redeposited far from their landing sites or involved in later crater collapse and erosion. This emplacing mechanism implies that the original products were characterized by a water content higher than about 5% by weight. Steam-driven hydrothermal eruptions, one of which took place in 1871, originated deposits of limited dispersion, as no sign of these erodible products can be found in the field today. Surface geology and fluid geochemistry, together with subsurface information (e.g., primary and hydrothermal lithologies, distribution of temperature with depth, physical-chemical characteristics of deep water-bearing zones) indicate that two distinct hydrothermal aquifers are present underneath the southeastern part of the caldera floor. Both aquifers were probably involved in the most important historically documented hydrothermal eruptions, which occurred in 1873. At that time, violent earthquakes fractured the brittle aquiclude separating the two aquifers and caused a sudden transfer of fluids from the deep to the shallow aquifer, thus triggering the hydrothermal eruptions. Hydrothermal eruptions will probably occur in future, and this hazard must be taken into serious consideration. The southern half of Lakki plain, where all past eruptions took place and active fumaroles are concentrated is the zone at highest risk. At present, gas geochemistry represents an effective tool to detect changes in the P,T conditions of the shallow aquifer, and particularly the phenomena of pressure build-up that may lead to a hydrothermal eruption.

Gas geobarometry for hydrothermal systems and its application to some Italian geothermal areas

Abstract By the use of the CO2, CH4, H2 and CO contents of geothermal gas discharges, the CO2 partial pressure in the gas equilibration zone can be calculated. Assuming that the partial pressures of water at any temperature are fixed by the presence of liquid water, then the following function, which is nearly independent of temperature, linking CO2 partial pressure to the ratio X H 2 / X C O is obtained: log ⁡ P C O 2 = 3.573 − 46 / T ( K ) − log ⁡ X H 2 / X C O Assuming a hydrostatic model, geothermal gradients for five high enthalpy and two medium to low enthalpy Italian geothermal areas have been estimated. High enthalpy systems can be clearly distinguished from those of low to medium enthalpy. Finally, accurate determinations of the H2/CO ratio in fluids discharged by selected fumaroles in volcanic areas can provide a method of monitoring pressure conditions in geothermal aquifers. This fact is particularly useful when the risk of a phreatic explosion in a volcanic area is to be assessed.

Fluid geochemistry of Nisyros island, Dodecanese, Greece

Abstract Two distinct hydrothermal aquifers are present beneath the Lakki plain: (a) a deep aquifer, characterized by temperatures higher than 290°C and chloride-rich fluids: and (b) a shallow aquifer, with a chloride content close to that of sea water and temperatures in the range 170–255°C. The vapor phase boiled from the shallow aquifer feeds the fumarolic vents in the southern half of the Lakki plain and its liquid phase flows northeast and southwest, contributing to the thermal springs located along the northern and southern coasts of the island. This conceptual hydrogeochemical model of Nisyros Island can be used to reconstruct past hydrothermal eruptions and to estimate the risk of such events happening again in the future.

Geochemical and seismological investigations at Vulcano (Aeolian Islands) during 1978–1989

Both geochemical and geophysical evidence indicates that the activity of the fumarolic system of the Fossa crater can be divided into two periods. From 1978 to 1983, such activity has been mainly controlled by two competing processes which affect the permeability of the fumarolic system at deep levels: (1) rock fracturing induced by faulting and (2) mineral deposition-alteration. The latter, which causes a slow decrease of the permeability at deep levels through clogging of fractures and voids, prevailed at the end of this period, determining a fluid pressure increment in the deep parts of the system. From 1984 to 1989, fluid pressure at deep levels remained persistently high, triggering local microseismic swarms. Furthermore, PT conditions much higher than before were attained in the zones where hydrothermal fluids seep into the conduits from lateral aquifers, owing to an uprising of the isotherms, a deepening of these seeping zones, or both.

Chemical geothermometry and geobarometry in hydrothermal aqueous solutions: A theoretical investigation based on a mineral-solution equilibrium model

Abstract The theoretical compositions of an aqueous solution in equilibrium with a mineral assemblage made up of low-albite, K-feldspar, either a Ca-Al-silicate or calcite, clinochlore, muscovite, quartz, anhydrite, and fluorite, under varying T-PCO2-mcl conditions of geothermal interest, indicate that 1. 1) the total SO4 content as well as the Na K , K 2 Mg , and SO 4 F 2 ratios are potential geothermometers; 2. 2) the total HCO3 content as well as the K 2 Ca , Ca Mg , HCO 3 F , and (HCO 3 ) 2 SO 4 ratios are potential PCO2 indicators; 3. 3) the Na, K, Ca, and Mg total contents as well as the Na 2 Mg and Na 2 Ca ratios are mainly controlled by the total ionic salinity and are therefore hardly suitable as geoindicators. A preliminary test of the equations involving total HCO3 content as well as K 2 Ca and HCO 3 F ratios as PCO2 indicators have provided satisfactory results.

Lake Bogoria hot springs (Kenya): geochemical features and geothermal implications

Abstract Many boiling springs and fumaroles are present along the shores of Lake Bogoria, which is a closed-basin alkaline saline lake typical of African Rifts. Two different geothermal waters, both of Na-HCO3 type have been recognized. The first, discharged by the boiling springs located along the western shores of the lake, comes from a shallow steam-heated thermal aquifer. Its temperature is close to 100°C, as indicated by chalcedony solubility, while the chloride content of these waters is slightly higher than 200 mg L−1. The second, recognizable in the southernmost boiling springs, is representative of a deeper and hotter geothermal reservoir. The occurrence of mixing and boiling processes complicates the interpretation of geochemical data. Nevertheless, a chloride content of about 660 mg L−1, an equilibrium temperature close to 170°C and a high carbon dioxide partial pressure, at least 10–20 bar, have been estimated for the deep geothermal reservoir.