Luigi Marini

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.

CO2 degassing and energy release at Solfatara volcano, Campi Flegrei, Italy

In the present period of quiescence, the Solfatara volcano, 1 km far from Pozzuoli, releases 1500 t d−1 of hydrothermal CO2 through soil diffuse degassing from a relatively small area (0.5 km2). This amount of gas is comparable to that released by crater plume emissions of many active volcanoes. On the basis of the CO2/H2O ratio measured in high-temperature fumaroles inside the degassing area, we computed a total thermal energy flux of 1.19×1013 J d−1 (138 MW). Most of this energy is lost by shallow steam condensation and transferred to the atmosphere through the hot soil of the degassing area. The thermal energy released by diffuse degassing at Solfatara is by far the main way of energy release from the whole Campi Flegrei caldera. It is 1 order of magnitude higher than the conductive heat flux through the entire caldera, and, during the last 20 years, it was several times higher than the energy associated with seismic crises and ground deformation events. It is possible that changes in the energy flux from a magma body seated underneath Solfatara and/or argillification processes at relatively shallow depths determine pressurization events in the hydrothermal system and consequently ground deformation and shallow seismic swarms, as recorded during the recent episodes of volcanic unrest centered at Pozzuoli.

Hydrothermal gas equilibria: the H2O-H2-CO2-CO-CH4 system

Abstract The difficulty in measuring reservoir gas concentrations in geothermal systems often forces the use of gas ratios in a separated vapor phase to investigate reservoir conditions. Measured CO/CO 2 and H 2 /H 2 O ratios of fumarolic fluids and vapors from geothermal wells representative of twenty-two different hydrothermal systems are consistent with theoretical values obtained from either of two commonly used redox buffers, indicating that CO and H 2 attain chemical equilibrium in the hydrothermal reservoir. Use of different f O 2 -buffers has little effect on these functions. Many measured CH 4 /CO 2 ratios are, instead, inconsistent with theoretical values obtained with any redox buffer. Since CH 4 /CO 2 ratios are strongly affected by redox conditions in the gas equilibration zone, this disagreement between measured and theoretical values likely indicates that either no unique f O 2 -buffer is active in all the hydrothermal environments or that CH 4 is not in equilibrium with the other gases. The weight of CH 4 on the 3log(X CO /X CO 2 ) + log(X CO /X CH 4 ) function is relatively small. Therefore this function and the log(X CO /X CO 2 ) − log(X H 2 /X H 2 O ) function, both of which are independent upon redox conditions, were used. These functions gave reasonable estimates of the equilibrium temperature and either the fraction of separated steam or the fraction of condensed steam in each sample. From these data, the CO/CO 2 , H 2 /H 2 O, and H 2 /CO ratios in the hypothetical single saturated vapor phase were calculated and used to investigate f O 2 and f CO 2 distributions in the considered twenty-two hydrothermal systems. Recalculated f CO 2 values are generally consistent, within one-half log-unit, with the full equilibrium function of Giggenbach (1984), Giggenbach (1988) although production of thermometamorphic CO 2 might locally take place. It is evident that no unique f O 2 -buffer is active in all the hydrothermal environments. This fact imply that CH 4 could have attained chemical equilibrium with other gas species in the H 2 O-H 2 -CO 2 -CO-CH 4 system.

Geochemical evidence for the existence of high-temperature hydrothermal brines at Vesuvio volcano, Italy

Abstract A high-temperature hydrothermal system is present underneath the crater area of Vesuvio volcano. It is suggested that NaCl brines reside in the high-temperature reservoir and influence the chemical composition of the gases discharged by the fumaroles of the crater bottom (vents FC1, FC2, and FC5). These have typical hydrothermal compositions, with H2O and CO2 as major components, followed by H2, H2S, N2, CH4, and CO (in order of decreasing contents) and undetectable SO2, HCl, and HF. Fumarolic H2O is either meteoric water enriched in 18O through high-temperature water-rock oxygen isotope exchange or a mixture of meteoric and arc-type magmatic water. Fumarolic CO2 is mainly generated by decarbonation reactions of marine carbonates, but the addition of small amounts of magmatic CO2 is also possible. All investigated gas species (H2O, CO2, CO, CH4, H2, H2S, N2, and NH3) equilibrate, probably in a saturated vapor phase, at temperatures of 360 to 370°C for vent FC1 and 430 to 445°C for vents FC2 and FC5. These temperatures are confirmed by the H2-Ar geoindicator. The minimum salt content of the liquid phase coexisting with the vapor phase is ∼14.9 wt.% NaCl, whereas its maximum salinity corresponds to halite saturation (49.2–52.5 wt.% NaCl). These poorly constrained salinities of NaCl brines reflect in large uncertainties in total fluid pressures, which are estimated to be 260 to 480 bar for vents FC2 and FC5 and 130 to 220 bar for vent FC1. Pressurization in some parts of the hydrothermal system, and its subsequent discharge through hydrofracturing, could explain the relatively frequent seismic crises recorded in the Vesuvio area after the last eruption. An important heat source responsible for hydrothermal circulation is represented by the hot rocks of the eruptive conduits, which have been active from 1631 to 1944. Geochemical evidence suggests that no input of fresh magma at shallow depths took place after the end of the last eruptive period.

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.

Oxidation state of iron in silicate glasses and melts: a thermochemical model

The acid and base dissociation constants of FeO and Fe2O3 components in silicate melts are defined in terms of observed relationships between atomistic properties of dissolved oxides (nephelauxetic parameters, electronegativity, fractional ionic character of the bond) and polymerization constants in simple systems. These constants are obtained from the Toop–Samis model depicting the Gibbs free energy of mixing of binary MO–SiO2 melts, which is coupled with the amphoteric treatment of altervalent dissolved oxides. Model parameterization is carried out on the basis of the extended set of data concerning thermodynamic activity of FeO in melts buffered by equilibrium with pure iron metal and a gaseous phase and on the various measurements of bulk redox state of iron in chemically complex melts at various T, fO2 conditions. Dissociation constants are related to thermodynamic parameters of the main dissolved species (O2−, Fe2+, Fe3+, FeO2−) without any significant error progression. As an ancillary result, thermochemical calculations allow to quantify to some extent the systematic errors in the FeII/FeIII bulk redox ratio arising from the utilization of Mossbauer spectroscopy on quenched melts and glasses.

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.

Soil diffuse degassing and thermal energy fluxes from the Southern Lakki Plain, Nisyros (Greece)

Two diffuse soil CO2 flux surveys from the southern Lakki plain show that CO2 is mainly released from the hydrothermal explosion craters. The correspondence between high CO2 fluxes and elevated soil temperatures suggests that a flux of hot hydrothermal fluids ascends towards the surface. Steam mostly condenses near the surface and the heat given off is conductively transferred to the atmosphere through the soil, accompanied by a large CO2 flux. It was calculated, that 68 t d−1 of hydrothermal CO2 are released through the total surveyed area of ∼1.3 km². Admitting that a steam flux of 2200 t d−1 accompanies this CO2 flux, the thermal energy released through steam condensation amounts to 58 MW.