Geochemical modeling of magmatic gas scrubbing

The EQ3/6 software package, version 7.2 was successfully used to model scrubbing of magmatic gas by pure water at 0.1 MPa, in the liquid and liquid-plus-gas regions. Some post-calculations were necessary to account for gas separation effects. In these post-calculations, redox potential was considered to be fixed by precipitation of crystalline α-sulfur, a ubiquitous and precocious process. As geochemical modeling is constrained by conservation of enthalpy upon water-gas mixing, the enthalpies of the gas species of interest were reviewed, adopting as reference state the liquid phase at the triple point. Our results confirm that significant emissions of highly acidic gas species (SO2(g), HCl(g), and HF(g)) are prevented by scrubbing, until dry conditions are established, at least locally. Nevertheless important outgassing of HCl(g) can take place from acid, HCl-rich brines. Moreover, these findings support the rule of thumb which is generally used to distinguish SO2-, HCl-, and HF-bearing magmatic gases from SO2-, HCl-, and HF-free hydrothermal gases. Mailing address: Prof. Luigi Marini, Laboratorio di Geochimica, Dipartimento per lo Studio del Territorio e delle sue Risorse (DipTeRis), Università degli Studi di Genova, Corso Europa 26, 16132 Genova, Italy; e-mail: lmarini@dipteris.unige.it


Introduction
Gases are released from magmas both during eruptions (syn-eruptive or active degassing) and in quiescent periods that either may preceed an eruption (pre-eruptive degassing) or may not (passive degassing).Studies of pre-eruptive degassing are of uppermost interest since we hope that we are able to interpret the changes in the composition and/or fluxes of volatiles as signals of the evolution of the magmatic system towards an eruptive episode.However, this is not a sim-ple exercise due to the common occurrence of interfering processes, mainly magmatic gas scrubbing by waters and interaction of these aqueous solutions with rocks.The effects of water-rock interaction are enhanced by the high reactivity of most magmatic gases and are similar to what takes place in a Giggenbach's flask during fumarolic gas sampling.In particular, CO 2(g) , H 2 S (g) , HCl (g) , and HF (g) are readily neutralised whereas the behavior of SO2 is unique, in that it may be disproportionate to SO4
All these processes can be satisfactorily modeled by means of reaction path modeling, a powerful geochemical tool originally proposed by Helgeson (1968), who worked out the thermodynamic relationships needed to evaluate the irreversible gas-water-rock exchanges.The first computer code for reaction path modeling, PATHI, was developed by Helgeson and coworkers, who presented several applications, including hydrothermal rock alteration and ore deposition, in a series of ground breaking scientific papers (e.g., Helgeson et al., 1969).
The theoretical evolution of water composition and the changes in secondary mineral assemblages during progressive neutralization of an initially acidic aqueous solution originated by absorption of magmatic gases in meteoric water was modeled by Reed (1997), who recognised the appearance/disappearance of a series of intermediate pH buffers until the final, thermodynamically stable mineral paragenesis attains equilibrium with the aqueous solution.Although this theoretical model is in line with our present understanding of most hydrothermal systems with close magmatic association (Giggenbach, 1997), it cannot be applied throughout.For instance, this model fails to explain the origin of the deep, acidic Na-Cl-SO 4 waters of the Miravalles geothermal system (Costa Rica), which are probably generated through interaction of deep, neutral Na-Cl waters with rocks depleted in alkali and alkali-earth metals and enriched in SiO2, Al2O3, and sulfate minerals, representing old solfataras buried by recent volcanics, as suggested by Bob Fournier in an informal communication about 10 years ago.Reaction path modeling supports the Fournier's hypothesis (Marini et al., 2003a).
Gas-water-rock interactions in crater lake waters were modeled by Marini et al. (2003b) to investigate the reasons for the bimodal pH distribution in crater lake waters, which exhibit an acidic mode at pH 0.5-1.5 and a near neutral mode at pH 6-6.5 (Varekamp et al., 2000).The neutralization of acid SO 4 -Cl waters with andesite at both low temperature and high temperature, followed by cooling below 100°C, rarely produce aqueous solutions with pH in the 3.5-5 range, in line with available analytical data.
Reaction path modeling has recently been used to model magmatic gas scrubbing taking place through mixing of magmatic gas with airsaturated water, reaction of a magmatic gas/airsaturated water mixture with rock, and other processes in an excellent paper by Symonds et al. (2001).It turns out that scrubbing by water prevents significant SO 2(g) and most HCl (g) emissions until dry pathways are established, apart from moderate HCl (g) degassing from acidic hydrothermal waters (pH < 0.5).Degassing of SO 2(g) from long-resident boiling hydrothermal systems is also prevented by magmatic gas scrubbing.
This paper investigates the irreversible gaswater mass exchanges taking place during addition of magmatic gas to pure water at 0.1 MPa, chiefly by means of the EQ3/6 software package, version 7.2 (Wolery, 1992;Wolery and Daveler, 1992).The purpose of this exercise is to test the EQ3/6 software package on a rather complex problem, which is similar to case 1 of Symonds et al. (2001).They modeled the addition of magmatic gas to air-saturated water instead of pure water as we intend to do.Hence, our results are not fully comparable with those of Symonds et al. (2001), as we neglect dissolved atmospheric gases (i.e., O2(aq) and N2(aq)) and, consequently, the derived N species (mainly NH4 + (aq)) in our computations.It must be stressed that the relatively high concentrations of NH4 + (aq) predicted by Symonds et al. (2001) are based on the assumption of instantaneous equilibration, which is rarely attained between N species in abiotic systems.On the other hand, the absence of dissolved O 2 in our model determines more reducing conditions, with respect to the model of Symonds et al. (2001), in the initial steps of gas-water interactions (ξ <0.4 mol) and, consequently, some differences in the concentrations of S species.

Setting up the model
The model consists in the progressive addition of hot magmatic gas into 1000 g of pure water initially at 25°C.Pressure is maintained at 0.1 MPa throughout the process.The magmatic gas is represented by sample 79-2 from Merapi Volcano, Indonesia (Le Guern et al., 1982), with a discharge temperature of 810°C and an equilibrium temperature of 915°C (Symonds et al., 1994).This is the same magmatic gas considered by Symonds et al. (2001) and its chemical composition is given in table I.
The overall process under investigation determines first the gradual heating of the aqueous solution from 25°to 100°C, second its gradual isothermal evaporation at 100°C, and third gas-gas mixing at increasing temperature.The first and second stages of the process, taking place in the pure liquid and liquid-plus-gas regions, can be modeled by means of the EQ3/6 software package, provided that some suitable pre-computations and post-calculations are carried out (see below).Upon total evaporation of the aqueous solution, the subsequent mixing of the gas phase thus produced with magmatic gas cannot be modeled by means of EQ3/6.However, simple mass-balance and equilibrium computations are sufficient to describe this gas-gas mixing process if the number of species considered is reasonably low.
Reaction path modeling in the pure liquid and liquid-plus-gas regions was performed in reaction path mode, through a purely stoichiometric approach, assuming immediate attainment of chemical equilibrium.The thermodynamic database COM was used.This is the most complete database available in the EQ3/6 software package and includes the thermodynamic properties of several solids, aqueous species, and gases, mostly derived from SUPCRT92 (Johnson et al., 1992).
In EQ3/6 runs, activity coefficients of ionic solutes were computed by means of the B-dot equation of Helgeson (1969).
Table I.Chemical composition (molar fractions) of the magmatic gas and of the gas produced through total vaporization of the liquid phase.Sample 79-2 from Merapi Volcano, Indonesia (Le Guern et al., 1982) is considered to be representative of the magmatic gas by Symonds et al. (2001) and in this study as well.

The enthalpy of liquid water and gas species
Modeling is also based on the assumption of conservation of enthalpy upon water-gas mixing.The enthalpies tabulated in SUPCRT92 are apparent standard molal enthalpies of formation from the elements, as defined by Benson (1968), and cannot be used for these computations.In steam tables a different reference state is chosen, either liquid water at 0°C (Denbigh, 1971) or liquid water at the triple point, i.e., 273.16 K, 0.006113 MPa.The latter reference state, with S H2O(l), triple =0, G H2O(l), triple =0, was adopted by the 5th International Conference on the Properties of Steam (Helgeson and Kirkham, 1974).For our purpose, the same convention must be adopted not only for H 2 O, but also for all the gas species of interest.Therefore, for all gas species, we obtained the enthalpy value at the triple point consistent with this convention (see table II for details).The enthalpy of the ith gas species at any temperature T and any pressure P (0.1 MPa in our case) can be computed by means of the relation (e.g., Denbigh, 1971) Table II.Triple point temperature, enthalpy at the triple point, and coefficients of the empirical power series in the temperature, which were used in this work to fit the molar heat capacity of relevant gas species and liquid water (CP data from Chase, 1998).(2.1) where C P is the molar heat capacity, V is the molar volume, and α is the coefficient of thermal expansion of the i-th gas species.As usually done in the literature (e.g., Denbigh, 1971, and references therein), the heat capacity of gas species was expressed as empirical power series in the temperature (table II) fitting the C P data reported by Chase (1998).
Consequently, the first integral of eq. ( 2.1) was computed as follows: (2.3) whereas the second integral of eq.(2.1) was neglected assuming ideal gas behavior.In fact, for ideal gases V=RT/P, and (2.4) It follows that: 1−αT=0 and .
This assumption is reasonable in the case under consideration, due to the comparatively small pressure increments from P triple to 0.1 MPa pressure.
Computed enthalpies for H 2 O (l) , H 2 O (g) , H 2(g) , CO 2(g) , CO (g) , CH 4(g) , N 2(g) , and NH 3(g) are in excellent agreement with those reported by the NIST Chemistry WebBook (http://webbook.nist.gov/).Enthalpies of H 2 O (l) , H 2 O (g) , H 2(g) , CO 2(g) , CO (g) , CH 4(g) , N 2(g) , NH 3(g) , S 2(g) , SO 2(g) , H 2 S (g) , HCl (g) , and HF (g) at 0.1 MPa pressure and variable temperature are shown in fig. 1.This plot shows that, at magmatic temperatures (let us say > 800 K): a) the enthalpy of any gas is significantly higher than that of liquid water; and b) H 2 O (g) has the highest enthalpy and is generally followed by CH 4(g) , NH 3(g) , CO 2(g) , SO 2(g) , S 2(g) , and H 2 S (g) , whereas HCl (g) , HF (g) , CO (g) , N 2(g) , and H 2(g) have much lower enthalpy values.From a) it follows that the temperature of liquid water is expected to increase upon input of magmatic gases of any composition.However, considering that the main constituent of magmatic gases is by far H 2 O (g) , followed by CO 2(g) and SO 2(g) (e.g., Symonds et al., 1994), the enthalpy of magmatic gases is not expected to diverge substantially from that of pure steam.
where indices w, g, and m indicate the initial pure water, the magmatic gas, and the aqueous solution formed upon addition of n g moles of magmatic gas to 1000 g of pure water, MW w is the molecular weight of water, and X j is the molar fraction of the j-th gas species in the magmatic gas.Both the enthalpies of solute species and the enthalpy of mixing are neglected.The temperature of the aqueous solution was derived from the enthalpy computed by eq.(2.5), based on the first relationship in table II solved with respect to T, i.e. (2.6) Then, temperature was related to the reaction progress variable ξ, i.e., the moles of magmatic gas added to 1000 g of pure water, by means of a second-degree polynomial equation.This is (2.7) for the considered example and was used to constrain reaction path modeling by EQ6 in the temperature range 25-99°C.
It must be noted that, in the system under consideration, the aqueous solution attains saturation with respect to crystalline α-sulfur upon addition of 0.022 mol of magmatic gas.This fact has to be taken into account in post-calculations (see below).
For ξ<0.4 mol, the total fluid pressure, obtained by summing the partial pressures of gas species (mainly H 2 O, CO 2 , and H 2 S) given by EQ6, is lower than the external pressure of 0.1 MPa and results of EQ6 are fully representative.For ξ≥0.4 mol, total fluid pressure exceeds 0.1 MPa, indicating that the aqueous solution experiences gas exsolution (degassing) until total fluid pressure attains 0.1 MPa.Single-step degassing was modeled starting from the compositions of the aqueous phase computed by EQ6, referring to the following mass balance: (2.8) and assuming equilibrium gas-liquid partitioning of gas species, i.e.Oxygen fugacity was, therefore, computed based on the thermodynamic constant of reaction (2.11) and the activity of H 2 S in the degassed liquid phase, which is assumed to be equal to its molality.This is a reasonable assumption for neutral aqueous species.The composition of the degassed liquid was then recomputed by means of EQ3NR, introducing temperature, fO 2 , fCO 2 , fH 2S , and total Cl and F molalities, as input data, and forcing pH by the electroneutrality condition.The differences between the pH values thus computed and those of the initial undegassed liquid are in all cases except one less than 0.0004 pH units, probably due to rounding off effects.This virtual pH constancy upon gas loss is not surprising as the aqueous phase is a relatively dilute solution of strong acids (see below).

Isobaric-isothermal evaporation at 100°C
During this second stage of the overall process, temperature is kept constant at 100°C by H 2 O (l) -H 2 O (g) coexistence.Initially, progressive addition of magmatic gas to water was modeled by means of EQ6 neglecting the effects of gas separation.Knowing the enthalpy of the aqueous solution (computed by eq.(2.5)), the fraction of separated steam, y=n H2O(g) /(n H2O(g) + +n H2O(l) ), was then calculated assuming isoenthalpic boiling, i.e.
(2.12) and equilibrium gas-liquid partitioning of CO 2 (eq.(2.9)).Subscripts o, l, and g refer to the unboiled aqueous solution, the separated (boiled) liquid phase and the separated gas phase.Since H 2 O and CO 2 represent more than 98% (on molar basis) of the gas species present in the separated gas phase, other gas species were neglected in eq.(2.12).Obtained y values were used to distribute the moles of each chemical species computed by EQ6 between the separated liquid and gas phases by means of simple mass balances (equivalent to eq. (2.8) with y instead of f), assuming equilibrium partitioning for gas species (eq.(2.9)).Again, partition coefficients were computed from the gas solubility data reported in the COM thermodynamic database of EQ3/6 by means of eq.(2.10) and f O2 was calculated referring to reaction (2.11).Then the composition of the boiled liquid was recomputed by means of EQ3NR as for the first stage of the process.In this case, the differences between computed pH values and those of the unboiled aqueous solution range from 0.005 to 0.09 pH units, since the change in pH upon boiling cannot be properly accounted for by a simple mass balance like eq. (2.8).

Isobaric gas-gas mixing at 100-915°C
For this stage of the process, first we computed the specific enthalpy of the gas mixture Hgm, referring to water only, i.e., by the equation (2.13) in which subscript T S is the temperature (100°C) at the considered pressure (0.1 MPa), n g,ev indicates the moles of magmatic gas consumed to evaporate the aqueous solution completely, and n g are the moles of magmatic gas added to the system after complete vaporization of the liquid.The first term of the numerator refers to the enthalpy of the steam produced through total vaporization of the aqueous solution and the first term of the denominator identifies the corresponding moles of steam.
The second term of the numerator represents the enthalpy of the magmatic steam added to the system after complete vaporization of the liquid and the second term of the denominator identifies the corresponding moles of magmatic steam.The temperature of the gas mixture was derived from the enthalpy computed by eq.
(2.13), based on the H H2O -T relationship (table II) solved with respect to T, i.e. (2.14) The molar fraction of magmatic steam in the gas mixture, x s , was readily computed as follows: (2.15) and used to calculate the concentrations of major gas species (H2O(g), CO2(g), SO2(g), H2S(g), HCl(g), and HF(g)) in the gas mixture by means of simple mass balances, based on the compositions of the magmatic gas (table I) and of the gas produced through total vaporization of the aqueous solution.The concentrations of minor gas species (H2(g), CO(g), S2(g)) in the gas mixture were then calculated referring to the following reactions: (3.1) and those constrained by the H 2 S-SO 2 magmatic redox buffer (Giggenbach, 1987) (3.2) The latter buffer is closely approached upon dry-up of the system, when the SO 2(g) concentration in the gas phase equals that of H 2 S (g) (see below).
The evolution of the R H parameter against ξ and temperature is shown in figs.3b and 4b, respectively.This parameter is defined as RH= (Giggenbach, 1987) and, therefore, it is directly comparable with the analytical data of fumaroles.

Sulfur precipitation
As anticipated, the aqueous solution becomes saturated with respect to crystalline α-sulfur upon addition of 0.022 mol of magmatic gas.In the log (moles of S) -logξ plot (fig.5), the moles of S precipitated experience a very sharp initial in- (2.16) (2.17) (2.18) whose thermodynamic constants were obtained by means of SUPCRT92 (Johnson et al., 1992).

Addition of magmatic gas to pure water at 0.1 MPa: modeling results
Apart from minor differences arising from the choice of modeling magmatic gas scrubbing by pure water rather than by air-saturated water, our results are very similar to those of Symonds et al. (2001).

Temperature
The plot of temperature versus the reaction progress variable ξ (fig.2) shows a comparatively smooth increase from the initial temperature of liquid water, 25°C, to the saturation temperature at 0.1 MPa pressure (100°C).However, the transition from the single liquid phase region to the biphase liquid-plus-gas region occurs at ξ of 0.4 mol (see above), corresponding to a temperature of 32.4°C only.The temperature of 100°C is attained through addition of 4.29 mol of magmatic gas and it is kept constant up to ξ of 79.3 moles by coexistence of H 2 O (l) and H 2 O (g) .Upon dry-up of the system, temperature immediately experiences a very sharp increase.Then it flattens for temperatures approaching that of the magmatic gas, 915°C.

Redox potential
The diagram of log fO 2 versus ξ (fig.3a) is similar in shape to fig. 2, due to the marked temperature dependence of log fO 2 .As shown by fig.4a, log fO 2 values are initially intermediate between those fixed by the FeO-FeO1.5hydrothermal redox buffer (Giggenbach, 1987, temperature crease, but for ξ >0.14 they follow a linear trend which is parallel to that of the moles of S added to the system.This means that for ξ >0.14 mol the ratio between the amount of S removed from the system and the amount of S added to the system is constant.Since this ratio is 0.81, it can be concluded that most of the S added to the system is precipitated as crystalline α-sulfur, as already observed by Symonds et al. (2001).

Chemistry of the liquid phase
The pH of the aqueous solution decreases almost linearly with logξ, up to ξ of ∼40 mol (fig.6), whereas the decrease in pH becomes dramatic above this threshold, when the liquid phase is essentially a very concentrated aqueous solution of hydrogen chloride.In particular, the «last drop» of liquid (for ξ of 79.3 moles during the progressive addition of hot (915°C) magmatic gas into 1000 g of pure water initially at 25°C.Pressure was kept at 0.1 MPa throughout the simulation.Fig. 4a,b.Plot of a) log fO 2 and b) RH versus temperature showing the expected changes in these variables during the progressive addition of hot (915°C) magmatic gas into 1000 g of pure water, initially at 25°C (line labelled simulation).Pressure was kept at 0.1 MPa throughout the simulation.Also shown are the FeO-FeO1.5hydrothermal redox buffer and the H2S-SO2, magmatic redox buffer of Giggenbach (1987).The latter was computed for the same fH 2O of the computer simulation.and y of 0.993) is dominated by HCl (aq) , with a molality of 20.2, followed by H + and Cl − , which are present with equal molalities of 7.96.Since activity coefficients of ionic solutes were computed by means of the B-dot equation of Helgeson (1969), which is expected to be accurate only in dilute aqueous solutions (i.e., for ionic strength lower than 1 molal; Wolery, 1992), these values are highly uncertain.
On the one hand, the concentrations of dissolved CO 2 and H 2 S (which enter preferentially the gas phase, with B i at 100°C of 1405 and 29.8, respectively) increase with ξ in the single liquid phase but experience a marked decrease with ξ upon transition from the single liquid region to the biphase liquid-plus-gas region (figs.7 and 8).The corresponding dissociation products, i.e., HCO 3 − and HS − ions, show variations similar to those of CO 2 and H 2 S. On the other hand, the concentrations of dissolved HCl and HF (which preferentially remain into the liquid phase, with B i at 100°C of 0.00121 and 0.00221, respectively) increase continuously with ξ, both in the liquid region and in the liquid-plus-gas region (fig.whose pK values at 100°C are −0.62 and 3.85, respectively.These pK values correspond to the iso-activity pH's, where the activity of HX (aq) equals that of X − .Since pH decreases regularly with ξ, the iso-activity condition is attained much earlier for HF than for HCl.The distribution of HF and F − is further complicated by HF dimerization (3.5) and acid dissociation of the dimer (3.6) The evolution of dissolved S species (fig.8) is rather complicated due to the occurrence of several association/dissociation and redox reactions (Symonds et al., 2001).Dissolved hydrogen sulfide is the dominant aqueous S species for ξ ≤ 3 moles, whereas HSO 4 − ion prevails above this threshold, indicating an increase in f O2 .Concurrently, the concentration of aqueous SO 2 increases, although it remains a subordinate S species throughout the process.Similar to what was observed for pH and Cl species, also some S species (e.g., HSO 4 − and SO 4 2− ) experience dramatic changes approaching dry-up of the system.

Chemistry of the gas phase
In the biphase liquid-plus-gas region, the weight percentage of gases in the system considered increases almost linearly with ξ in the log-log plot of fig. 10, attaining 100% (dry-up of the system) for ξ of 79.3 moles.The gas phase is initially dominated by CO2(g), but H2O(g) becomes the prevailing gas species for ξ ≥ 4 moles (fig.11).Hydrogen sulfide is the third major gas species through most of the biphase liquid-plus-gas region.Approaching dry-up of the system, HF (g) first and HCl (g) and SO 2(g) afterwards appear in the gas phase and their concentrations increase progressively with ξ.In particular, HCl (g) experiences a large increase soon before dry-up and becomes the third major gas species, due to the high concentration of HCl (aq) in the «last drop» of liquid.
In the single gas region, H 2 O (g) and CO 2(g) are the two main constituents, initially followed by HCl (g) , whose molar fraction decreases with ξ.On the contrary, the concentrations of SO 2(g) , H 2(g) , S 2(g) , and CO (g) increase by several orders of magnitude, and H 2(g) becomes more important than HCl (g) for ξ >1600 mol.Gas phases relatively rich in the magmatic component are characterised by similar concentrations of SO 2(g) , H 2 S (g) , H 2(g) , and HCl (g) , whereas CO (g) , S 2(g) , and HF (g) are present in lower amounts.
As already recalled, the similarity in H 2 S (g) and SO 2(g) molar fractions explains why redox conditions are consistent with the H 2 S-SO 2 magmatic redox buffer of Giggenbach (1987).

Conclusions
The EQ3/6 software package, version 7.2, can be effectively used to compute the irreversible gas-water mass exchanges taking place during addition of magmatic gas to pure water at 0.1 MPa, in the liquid and liquid-plus-gas regions, similar to what was done by Symonds et al. (2001) by means of the software code CHILLER.However some post-calculations are needed to take in due account the effects of gas separation.In this specific case, these post-calculations were simplified by concurrent precipitation of crystalline α-sulfur, which fixes the redox potential.Post-calculations might be not as simple as in this case if precipitation of elemental sulfur does not take place.Luckily, this process is ubiquitous and precocious, considering that it begins upon addition of 0.45 g of magmatic gas only, at least in the case considered.
Since reaction path modeling is largely based on the assumption of conservation of enthalpy upon water-gas mixing, we reviewed the enthalpies of the gas species of interest, adopting as reference state the liquid phase at the triple point.Used data are presented (table II) for comparison with other possible test cases.
Our results of geochemical modeling are very similar to those obtained by Symonds et al. (2001), apart from minor differences due to our choice of modeling magmatic gas scrubbing by pure water rather than by air-saturated water, as done by Symonds et al. (2001).
As already shown by Symonds et al. (2001), results of reaction path modeling carried out in this investigation indicate that scrubbing by water prevents significant emissions of strongly acidic gas species (SO 2(g) , HCl (g) , and HF (g) ) until dry conditions are established, at least locally.However substantial HCl (g) degassing can occur from acidic brines rich in HCl (aq) .These results are also consistent with the rule of thumb which is generally used to distinguish SO 2 -, HCl-, and HF-bearing magmatic gases from SO 2 -, HCl-, and HF-free hydrothermal gases (Giggenbach, 1980(Giggenbach, , 1987;;Chiodini et al., 1993).
Finally, we would like to emphasize once more the power of reaction path modeling, which is also highly recommended for the correct interpretation of isotope data, especially multivalent elements, e.g., sulfur (e.g., Gambardella, 2001;Marini et al., 2002).

Fig. 2 .
Fig. 2. Changes in temperature during the progressive addition of hot (915°C) magmatic gas into 1000 g of pure water initially at 25°C.Pressure was kept at 0.1 MPa throughout the simulation.

Fig
Fig. 3a,b.Variations in a) oxygen fugacity and b)during the progressive addition of hot (915°C) magmatic gas into 1000 g of pure water initially at 25°C.Pressure was kept at 0.1 MPa throughout the simulation.

Fig. 5 .
Fig. 5. Moles of sulfur both added to the system and precipitated as crystalline α-sulfur during the progressive addition of hot (915°C) magmatic gas into 1000 g of pure water initially at 25°C.Pressure was kept at 0.1 MPa throughout the simulation.

Fig. 6 .
Fig. 6.Changes in the pH of the aqueous solution during the progressive addition of hot (915°C) magmatic gas into 1000 g of pure water initially at 25°C.Pressure was kept at 0.1 MPa throughout the simulation.

Fig. 7 .Fig. 8 .
Fig. 7. Changes in dissolved carbonate species during the progressive addition of hot (915°C) magmatic gas into 1000 g of pure water initially at 25°C.Pressure was kept at 0.1 MPa throughout the simulation.

Fig. 9 .
Fig. 9. Changes in dissolved chloride and fluoride species during the progressive addition of hot (915°C) magmatic gas into 1000 g of pure water initially at 25°C.Pressure was kept at 0.1 MPa throughout the simulation.

Fig. 10 .
Fig.10.Weight percentage of gas in the system during the progressive addition of hot (915°C) magmatic gas into 1000 g of pure water initially at 25°C.Pressure was kept at 0.1 MPa throughout the simulation.

Fig. 12 .
Fig. 12.Variations in the molar fractions of H2(g), HCl(g), and HF(g) during the progressive addition of hot (915°C) magmatic gas into 1000 g of pure water initially at 25°C.Pressure was kept at 0.1 MPa throughout the simulation.