Giovanni Chiodini

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.

Rate of diffuse carbon dioxide Earth degassing estimated from carbon balance of regional aquifers: The case of central Apennine, Italy

Central Italy is characterized by an anomalous flux of deeply derived CO2. In the western Tyrrhenian sector of central Italy, CO2 degassing occurs mainly from focused emissions (vents and strong diffuse degassing) and thermal springs, whereas in the eastern Apennine area, deep CO2 is dissolved in “cold” groundwaters of regional aquifers hosted by Mesozoic carbonate-evaporite formations. Influx of deep CO2 into 12 carbonate aquifers (12,500 km2) of the central Apennine is computed through a carbon mass balance that couples aquifer geochemistry with isotopic and hydrogeological data. Mass balance calculations estimate that 6.5×1010 mol yr−1 of inorganic carbon are dissolved in the studied aquifers. Approximately 23% of this amount derives from biological sources active during the infiltration of the recharge waters, 36% comes from carbonate dissolution, while 41% is representative of deep carbon sources characterized by a common isotopic signature (δ13C ≅ −3‰). The calculated deep CO2 influx rate ranges from 105 to 107 mol yr−1 km−2, increasing regionally from east to west in the study area.

Quantification of deep CO2 fluxes from Central Italy. Examples of carbon balance for regional aquifers and of soil diffuse degassing

Abstract In Central Italy non-volcanic CO 2 is discharged by focused degassing (strong diffuse emission and vents) and by high-CO 2 groundwater. 3 He / 4 He data and the carbon isotopic composition of CO 2 are compatible with derivation from mantle degassing and/or metamorphic decarbonation. The gases produced at depth accumulate in permeable reservoirs composed of Mesozoic carbonates. When total pressure (roughly corresponding to p CO 2 ) of the reservoir fluid exceeds hydrostatic pressure, a free gas phase forms gas reservoirs within the permeable host rocks from which gases may escape toward the surface. This process generates both the focused vents and the CO 2 -rich springs which characterise the study area. The storage and expulsion of CO 2 is controlled by fractures and faults and/or structural highs of permeable carbonate formations. Influx of deep CO 2 into the overlying groundwater yields a widespread elevated p CO 2 anomaly in the Tyrrhenian Central Italy aquifers. These aquifers release CO 2 to the atmosphere when groundwater is discharged at the surface from springs. The groundwater degassing flux is estimated from the carbon balance of regional aquifers computed by coupling aquifer geochemistry with isotopic and hydrogeological data. The resulting production rate of deep CO 2 ranges from 4×10 5 to 9×10 6 mol y −1 km −2 . In concert with the regional geologic setting, the deep CO 2 production rate increases westward. In the aquifers with anomalously high p CO 2 , the average CO 2 influx rate of the anomalous areas is several times higher than the value derived by Kerrick et al. [Kerrick, D.M., McKibben, M.A., Seward, T.M., Caldeira, K., 1995. Convective hydrothermal CO 2 emission from high heat flow regions. Chem. Geol., 121 (1995) 285–293.] as baseline for CO 2 emission from areas of high heat flow. The flux of CO 2 lost to the atmosphere from water emitted from springs is of the same order of magnitude as the influx of deep CO 2 into the aquifer.

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.

Dynamics of carbon dioxide emission at Mammoth Mountain, California

Mammoth Mountain, a dormant volcano in the eastern Sierra Nevada, California, has been passively degassing large quantities of cold magmatic CO2 since 1990 following a 6-month-long earthquake swarm associated with a shallow magmatic intrusion in 1989. A search for any link between gas discharge and volcanic hazard at this popular recreation area led us to initiate a detailed study of the degassing process in 1997. Our continuous monitoring results elucidate some of the physical controls that influence dynamics in flank CO2 degassing at this volcano. High coherence between variations in CO2 efflux and variations in atmospheric pressure and wind speed imply that meteorological parameters account for much, if not all of the variability in CO2 efflux rates. Our results help explain differences among previously published estimates of CO2 efflux at Mammoth Mountain and indicate that the long-term (annual) CO2 degassing rate has in fact remained constant since ∼1997. Discounting the possibility of large meteorologically driven temporal variations in gas efflux at other volcanoes may result in spurious interpretations of transients that do not reflect actual geologic processes.

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.

Early signals of new volcanic unrest at Campi Flegrei caldera? Insights from geochemical data and physical simulations

For the first time a physical model, constrained by monitoring data, is used to derive a quantitative estimate of the evolution in time of magmatic gases that enter a hydrothermal system of an active volcano. The site is Campi Flegrei (west of Naples, in Italy), a caldera that had a large ground inflation in 1982–1984 followed by 20 yr of subsidence. More recently the behavior of the system has changed: the magmatic component of fumaroles has increased, swarms of earthquakes are more frequent, and the ground has started a general uplifting trend, indicating that the hydrothermal system undergoes repeated injections of magmatic fluid. Physical simulations of the process show that total injected fluid masses are the same order of magnitude as those emitted during small to medium size volcanic eruptions, and their cumulative curve highlights a current period of increasing activity. Gas emission studies coupled with physical modeling can be extremely effective in predicting magmatic evolution and eruptive activity at volcanoes.