Abstract
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 a-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.
Highlights
Gases are released from magmas both during eruptions and in quiescent periods that either may preceed an eruption or may not
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
This paper investigates the irreversible gaswater mass exchanges taking place during addition of magmatic gas to pure water at 0.1 MPa, by means of the EQ3/6 software package, version 7.2 (Wolery, 1992; Wolery and Daveler, 1992)
Summary
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). 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. 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. The absence of dissolved O2 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
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