The chemical composition of submarine gases collected at Kolumbo is presented in Table 1. These gases are dominated by CO₂, which is present at concentrations up to 99.1% (Figure 3A). The concentration of CH₄ ranges from 1052 to 5521 ppm, while C₂H₆ and C₃H₈ exhibit much narrow ranges of 95–128 and 14–20 ppm, respectively (values measured in the Giggenbach bottle; Giggenbach, 1975). He varies between 9 and 40 ppm, while H₂ and CO range from 170 to 716 ppm and from 2 to 7 ppm, respectively. The O₂ and N₂ contents varied depending on the degree of sample contamination by ambient air, with maximum values of 5.1 and 21%, respectively (Figure 3B, 4A). ²⁰Ne ranges between 0.082 and 4.2 ppm, while ⁴⁰Ar ranges between 69 and 834 ppm (Figure 4B). It should be noted that ⁴⁰Ar was not measured in the sample with the greatest air contamination (O₂ = 5.1%, N₂ = 21%, and ²⁰Ne = 4.2 ppm). In order to evaluate the extent of air or the presence of air-saturated water (ASW), O₂ and Ar are plotted versus ²⁰Ne in Figure 4A,B, respectively. All of the analyzed samples fell along the line representing air rather than ASW, indicating that this contamination is probably due to sampling or extraction procedures. In order to determine the gas composition before air contamination (G_f), data were corrected based on the O₂ content measured in each analysis as follows:

where Gs is the concentration of the gaseous species measured in the sample (e.g., He), Ga is the concentration of the same gaseous species in air (e.g., He = 5.24 ppm) and F is the fraction of air calculated from the O₂ concentration measured in the sample divided by the atmospheric O₂ concentration (20.947%). We point out that even assuming that all of the ²⁰Ne or ⁴⁰Ar measured in our samples is of atmospheric origin (as for O₂), the recalculated percentages of air contamination would still be comparable. The back-corrected chemical composition of Kolumbo gases is reported in Table 2.

Furthermore, the concentration of gaseous Hg(0) differed markedly (and significantly) among the seven investigated chimneys, from 61 to 1301 ng m^-3 (Table 3).

Table 1 reports the isotopic compositions of CO₂, CH₄, and Ar in the analyzed gases. The C-isotope composition (δ¹³C_CO2) varied between -0.04 and 1.15‰ V-PDB, whereas increasing ratios were positively correlated with the concentrations of He, H₂, CO, and CH₄. The CH₄-isotope composition varied over a narrow range for both C (δ¹³C = -18.8 to -17.6‰ V-PDB) and H (δD = -118.6 to -115.4‰ V-SMOW).

The ³He/⁴He and ⁴He/²⁰Ne ratios used in the present study were previously measured and discussed by Rizzo et al. (2016). Here we further measured the Ar-isotope composition, which was found to be close to the values typically encountered in atmospheric air (⁴⁰Ar/³⁶Ar = 295.5 and ³⁸Ar/³⁶Ar = 0.188; Ozima and Podosek, 1983). ⁴⁰Ar/³⁶Ar in hydrothermal gases varies between 296 and 316, while ³⁸Ar/³⁶Ar is between 0.187 and 0.189. However, ⁴⁰Ar was corrected for atmospheric contamination (⁴⁰Ar^∗) in the samples having ⁴⁰Ar/³⁶Ar > 308 as follows:

This correction is useful for obtaining accurate estimates of the ⁴He/⁴⁰Ar^∗ ratio, which in Kolumbo gases vary between 4.8 and 5.5. These ratios are within the ratio range typical of the mantle (⁴He/⁴⁰Ar = 1–5; e.g., Ozima and Podosek, 1983; Marty, 2012), confirming that Kolumbo gases preserve the features that they have inherited from the magma.

The chemical composition of gases corrected for air contamination as well as the δ¹³C_CO2 show a variability clearly modulated by a process of selective dissolution of gases in water driven by their different solubilities (Figure 5, 6). In order to determine the original composition of the intact gases, which is necessary for evaluating the origin of CO₂ and the pressure and temperature conditions of the hydrothermal system, we initially defined the boundary conditions for modeling. Due to the lack of direct measures of temperature in the fluids discharged from the Kolumbo chimneys, we assumed a homogeneous emission temperature of 220°C, as measured by Sigurdsson et al. (2006) (see section “Sampling and Analytical Techniques”). This assumption is reasonable because it is based on direct measurements made in the same part of the floor of Kolumbo submarine crater and from vents showing a sustained flux of gas bubbles, which indicates the concomitant release of high-temperature fluids (Sigurdsson et al., 2006). Regardless of the accuracy of this estimate, it should be remembered that any slight difference in temperature between vents would mostly influence the extent of the CO₂ dissolution in water rather than the path of fractionation of less-soluble species, especially when considering elemental ratios. Since gases are emitted on the floor of Kolumbo submarine crater, which is at around 500 m b.s.l., we assumed a constant pressure of 50 bar. Based on the assumption of a temperature of 220°C, the saturated vapor pressure would be ∼23 bar. This means that the partial pressure of CO₂ would be ∼27 bar within a bubble of fluids exsolved from the vents located at the hydrostatic pressure of 50 bar.

Considering that we are dealing with submarine emissions, it is reasonable to assume that hydrothermal water condenses in the seawater together with the highly soluble acidic gases (SO₂, HCl, and HBr). We cannot exclude that the CO₂ continued to dissolve in water inside the gas-tight bottles during the time that lapsed between sampling and gas extraction aboard the vessel, mostly because of cooling.

The residual gas phase that we studied comprised mainly CO₂ and secondarily N₂, H₂, CO and CH₄, as well as noble gases at low levels and trace gases. We ultimately assumed a geochemical system comprising CO₂, N₂, H₂, CO, CH₄, He, Ne and Ar, all of which dissolve in liquid water according to Henry’s law. After the condensation of water vapors and highly soluble acidic gases, a further progression of gas dissolution would induce the preferential dissolution of CO₂ in water and the consequent enrichment of less-soluble species such as N₂, H₂, CO, hydrocarbons and noble gases (Sander, 2015). Since our dry gas mixture comprised > 97% CO₂, we deduce that the extent of fractionation was low. Therefore, this process can be better evaluated using other components such as CH₄ versus He (Figure 5, 6) rather than the CO₂ concentration.

Following the thermodynamic approach proposed by Fernandez-Prini et al. (2003 and references therein) and based on our pressure and temperature boundary conditions, we calculated Henry’s constant k_H and the gas–liquid distribution constant K_D for each species included in our geochemical system. We simulated a condensation process under equilibrium conditions as expressed by the Rayleigh (1896) equation:

where Rv₀ is the initial ratio of the bulk composition (e.g., He/CO₂), Rv is the same instantaneous ratio in the residual gas phase (v), f is the fraction of the residual gas phase and α is the fractionation factor determined by the solubility ratio of the species under consideration (e.g., k_H-He/k_H-CO2). We also assumed open-system conditions considering the continuous removal of gas-saturated parcels of water.

The initial composition applied in our model is reported in Table 2 and in the captions of Figure 5–7. Similarly to gas levels, the isotopic composition of gaseous CO₂ (δ¹³C_CO2) changes as a result of its dissolution in water (Figure 7) and the isotopic fractionation between gaseous and dissolved inorganic CO₂ [CO₂(aq)]. CO₂(aq) is referred to as dissolved inorganic C (DIC) and is equal to the sum of the aqueous species H₂CO₃, HCO3−, and CO32−. The fractionation process is modeled using the Rayleigh equation as follows (Clark and Fritz, 1997):

where (δ¹³C_CO2)₀ is the initial CO₂-isotope composition, f is the fraction of the residual gas phase, and ε is the fractionation factor between DIC and gaseous CO₂ (CO_2(g)). This fractionation factor is obtained by summing up the fractionation factors of dissolved C species and CO_2(g) weighted for their molar fraction with respect DIC (Zhang et al., 1995; Allègre, 2008):

The molar fraction of each C species depends on the water temperature and pH. We calculated each fractionation factor at 220°C based on the approach of Zhang et al. (1995), while the molar fractions of H₂CO₃ and HCO3− were set to 0.96 and 0.04, respectively. We also assumed that the molar fraction of CO32− was 0 given that the pH of water on the floor of Kolumbo submarine crater is typically lower than 8 (Mandalakis et al., 2019), which means that the terms involving [CO32−] can be neglected in the above equation. Under these conditions, ε_DIC-CO2(g) was calculated to be -0.79. The molar fractions were also used in the following equation describing the dissociation of H₂CO₃ in order to estimate the pH of water in which CO₂ was dissolved:

The chemical and δ¹³C_CO2 variability of gases emitted from Kolumbo (Figure 7, 8) is well-modeled by a trend of CO₂ dissolution in water having H₂CO₃ and HCO3− molar fractions of 0.96 and 0.04, respectively, which at 220°C corresponds to pH ∼5. It is particularly interesting that this value is identical to that measured by Carey et al. (2013) in fluids emitted by a neighboring vent on the floor of Kolumbo submarine crater thereby further validating the boundary conditions selected for our model.

The temperature and pressure conditions at which gaseous species equilibrate in the hydrothermal system beneath the floor of Kolumbo submarine crater can be estimated from concentration data of minor reactive species together with CO₂ and H₂O (Chiodini and Marini, 1998 and references therein). The most-important assumptions in this approach are that the gaseous species have attained chemical equilibrium in the hydrothermal system and are quenched during their ascent to the sea surface. We evaluated two stability diagrams based on the concentrations of H₂, CO, CH_4, and CO₂ in the dry gas phase (Figure 10), following the approach proposed by Chiodini and Cioni (1989) and Chiodini et al. (2001) and subsequently modified by Chiodini et al. (2006) for submarine gases at Panarea. Because H₂O represents the main component of hydrothermal fluids, we assumed that pure water coexisted with vapor, whereas the fugacity of water vapor (fH₂O) as a function of temperature (Giggenbach, 1987) can be expressed as follows:

where T is the temperature in kelvin. It is also important to consider the fugacity of O₂ (fO₂), since this controls the redox conditions at which hydrothermal systems are formed. Since the H₂O concentration of hydrothermal vapors in our samples was not available and we have no information on the mineral assemblage that could fix fO₂, we considered the redox buffer typically proposed for hydrothermal systems by D’Amore and Panichi (1980):

Complementarily, we considered the following reactions and temperature-dependent equilibrium constants based on the thermodynamic data reported by Stull et al. (1969) and Giggenbach (1980, 1987):

Finally, the following equations derived by Chiodini et al. (2006) were taken into account:

where P is the partial pressure. The plot of log(CO/CH₄) versus log(H₂/CH₄) in Figure 10A suggests that Kolumbo gases would have reached an equilibrium at 200–250°C and P_CO2 ∼ 50 bar. In comparison with the submarine hydrothermal system of Panarea and the high-temperature fumaroles of Vulcano (Aeolian Islands, Italy), Kolumbo gases fall close to or within the range for Panarea gases, confirming their hydrothermal nature. The same pressure and temperature conditions were derived by examining the plot of log(CO/CO₂) versus log(H₂/CH₄) (Figure 10B). Given the compositional changes that may arise in hydrothermal gases due to the partial dissolution of CO₂ in water (see section “Gas–Water Interaction of Magmatic Fluids”), we recalculated the pressure and temperature conditions by applying the pristine composition estimated for each gas vent. Based on the values of Henry’s law constant, CO₂ was found to differ considerably from H₂, CO and CH₄, with the former having a greater tendency to partition from the gas phase into the aqueous phase (Sander, 2015). Therefore, the correction for gas–water partitioning was deemed necessary only when considering ratios that involved CO₂ [e.g., log(CO/CO₂); Figure 10B]. By applying the pristine composition, we estimated a gas equilibrium temperature of ∼200°C and P_CO2 ∼ 10 bar. These values are slightly lower than the pressure and temperature conditions derived from the plot of log(CO/CH₄) versus log(H₂/CH₄), but they remain well within the typical values reported for hydrothermal gases. In general, the estimated temperature using CO/CH₄, CO/CO_2, and H₂/CH₄ was equal to or below the highest temperature measured on the floor of Kolumbo submarine crater vents by Sigurdsson et al. (2006) and Carey et al. (2013). This suggests that part or all of these gaseous species did not achieve equilibrium under the assumed redox conditions or that the magmatic activity has waned in recent years.

To investigate these two possibilities, we applied the approach proposed by Chiodini and Cioni (1989) and Chiodini et al. (2006), which is based on the reaction:

Since this reaction does not involve gaseous O₂, Chiodini et al. (2006) suggested that this approach provides equilibrium pressure and temperature estimates that are independent of the redox conditions. The resulting equations that allow pressure and temperature to be determined are

This specific approach revealed that Kolumbo gases equilibrated at 200–250°C and when P_CO2 was slightly below 50 bar (Figure 11A). The presence of CO₂ in one of the above equations prompted us to again consider the pristine composition of gases prior to the gas–water partitioning process. This yielded a slightly lower equilibrium temperature (189–216°C), whereas P_CO2 remained the same (Figure 11A). This temperature range is comparable to those estimated using CO/CH₄, CO/CO₂ and H₂/CH₄ ratios, suggesting that the assumed redox conditions are not responsible for the difference in the estimated equilibrium temperatures.

The findings of previous investigations of crater fumaroles at White Island (Giggenbach, 1987) and Vulcano (Chiodini et al., 1993, 1995) suggest that the two most-reactive gaseous species in hydrothermal systems are H₂ and CO, while CH₄ is one of the least reactive (Giggenbach, 1991; Taran and Giggenbach, 2003). To evaluate if CH₄ measured in Kolumbo gases attained full equilibrium, we considered the following formation reaction and relative equilibrium constant:

In the gas phase, log(K_CO2+H2) = -2248 / T + 2.485

In a single saturated liquid phase, as assumed in our case, gas equilibrium contents are computed using the vapor–liquid distribution coefficient (B) (Giggenbach, 1980; D’Amore and Truesdell, 1988; Chiodini and Marini, 1998):

Therefore, the above reported reaction for the liquid phase becomes

An equilibrium temperature of 250–300°C and log(K_CO2+H2) – log(B_CO/B_CO2) + log(B_H2) ≈ 0.0 ± 0.1 implies log(H₂/H₂O) ≈ log(CO/CO₂). Thus, the log(CO/CO₂) values measured in our samples and corrected for CO₂ dissolution can be used to roughly estimate the equilibrium temperature in liquid water, which varies in the narrow range of 263–276°C (Figure 11B). These values are higher than those previously calculated including CH₄, and thus we deduce that the latter species was not in equilibrium with the other gases and led to underestimations of the equilibrium temperature.

Based on the relationships proposed by Chiodini and Cioni (1989), we can finally calculate P_CO2 ∼ 30 bar and P_H2O ∼ 36 bar, which sum to a total pressure of ∼66 bar. Considering that Kolumbo gases are emitted at 500 m b.s.l., which corresponds to a pressure of ∼50 bar, the hydrothermal system must be located at a pressure of ∼116 bar (∼1000 m b.s.l.).

Based on this study and other previous findings (Sigurdsson et al., 2006; Carey et al., 2013; Kilias et al., 2013; Rizzo et al., 2016), in Figure 12 we propose a physical and geochemical model for the Kolumbo magmatic-hydrothermal system. We made some assumptions in order to simplify the system: (1) the permeability of the system was considered uniform, isotropic and sufficiently high; (2) the shallow magma chamber located 5–7 km beneath Kolumbo (Dimitriadis et al., 2010; Konstantinou and Yeh, 2012; Cantner et al., 2014) represents the source of magmatic gases feeding the hydrothermal system, (3) magmatic degassing does not or only weakly modifies these magmatic gases, and (4) seawater infiltrates from the sea bottom and favors condensation of magmatic gases both within and above the hydrothermal system.

At Kolumbo, magmatic gases (H₂O, CO₂, S-bearing species, halogens and noble gases) having ³He/⁴He ∼ 7 Ra (Rizzo et al., 2016) and δ¹³C_CO2 ∼-0.4‰ ascend from the magma chamber and feed conduits up to the hydrothermal system (Figure 12). During the cooling of magmatic gases, H₂O and more-acidic species (S and halogens) condensate to form a hydrothermal system at ∼270°C in which water at pH ≤ 5 probably circulates. We cannot exclude that the hydrothermal system has multiple levels; however, we are able to reconstruct the upper level that feeds the bottom vents of the crater. Hydrothermal waters are probably fed by seawater infiltrating from the sea bottom and any brine formed from the adsorption of acidic gases in groundwater by rock dissolution.

The hydrothermal system has a total pressure of ∼66 bar, corresponding to ∼1,160 m b.s.l. and 650 m below the crater bottom, if the hydrostatic pressure is assumed at depth. Hydrothermal gases within the hydrothermal system undergo gas–water interactions along the fractures feeding the floor of Kolumbo submarine crater and from the shallow vents that favor the removal of most of the acidic gases (S and halogens) and the partial dissolution of CO₂ (see section “Gas–Water Interaction of Magmatic Fluids”). This produces an excess of N₂, noble gases and reactive gases via migration paths that permit their rapid ascent under advective degassing. At the crater bottom, several vents discharge fluids at temperatures up to 220°C, variable gas fluxes and pH down to ∼5 (Sigurdsson et al., 2006; Carey et al., 2013). Acoustic and visual imaging of the ascending bubbles suggests that CO₂ is being dissolved into seawater within ∼10 m above the crater floor (Carey et al., 2013).