High‐temperature mixtures of magmatic and atmospheric gases

Abstract

[1] Recent measurements of BrO, NO x , and near-source sulfate in volcanic plumes suggest that volcanic vents might not simply act as point sources of emissions into the troposphere, but may also act as hightemperature reaction sites where mixtures of magmatic and ambient atmospheric gases may combine, giving new and previously unexpected reaction products. The detection of such species demands that a more complex model be developed for the interaction of volcanoes and atmospheres. We show that general thermodynamic models can be applied successfully to volcanic gas equilibria by comparing the results from HSC Chemistry with those from two volcanic gas equilibrium models (Solvgas and Gasmix). Using a thermodynamic model optimized for volcanic gas chemistry (C-O-S-H-F-Cl-Br-I-N-Ar speciation), we show that the volume ratio of atmospheric gas to magmatic gas in a high-temperature mixture is an important parameter of the volcanic plume chemistry, and our results suggest that even small amounts of air (a few % for an H 2 O-rich magmatic gas) in the high-temperature mixture are sufficient to yield elevated levels of reactive nitrogen, halogen (Cl, Br, and I), and sulfur species within the volcanic plume. Further modifications of the plume chemistry may also occur due to low-temperature reactions, and chemical schemes for the modification of halogen (Cl, Br, I), nitrogen, and sulfur chemistry are suggested, within the constraints imposed by recent measurements.

Reactive halogen chemistry in volcanic plumes

“…The fact that high-temperature volcanic gases are equilibrium systems at pressure, temperature, oxygen fugacity (P-T-fO 2 ) conditions of the melt has long been recognized [Gerlach and Nordlie, 1975]. In line with recent modeling efforts satisfactorily using the same HSC software [Gerlach, 2004;Martin et al, 2006], we assume the gases to be in equilibrium also in the case of high-temperature airvolcanic gas mixtures, particularly when reaction kinetics proceed faster than air dilution itself which we assume to be the case for T ! 600°C.…”

“…Nevertheless, it has been shown by thermodynamic models that significant amounts of atomic halogen species (i.e., Cl, Br) can be produced by high-temperature oxidative dissociation in a volcanic gas-air mixture [Gerlach, 2004;Aiuppa et al, 2005;Martin et al, 2006], particularly above the so-called compositional discontinuity, at which drastic changes in the speciation of gases occur [Gerlach and Nordlie, 1975]. The subsequent dilution of the volcanic gas-air mixture with ambient air leads to the entrainment of O 3 and HO x at the plume edges promoting the onset of autocatalytic radical reactions, including the oxidation of atomic species (XO, X = I, Br, Cl), in particular bromine:…”

“…More recently, Martin et al [2006] presented more detailed equilibrium calculations for volcanic gas-air mixtures.…”

Rapid oxidation of mercury (Hg) at volcanic vents: Insights from high temperature thermodynamic models of Mt Etna's emissions

“…Thermodynamic models have been used in a number of recent studies to model the compositional changes occurring as hot magmatic gases mix and react with cold atmospheric gases at volcanic vents (e.g., Gerlach, 2004;Martin et al, 2006;Bagnato et al, 2007;Bobrowski et al, 2007;Martin et al, 2009b). The assumption in these models is that magmatic and atmospheric gases equilibrate instantaneously at the vent until a temperature (typically N500°C) is reached where the composition becomes frozen (i.e., the quenching temperature).…”

“…While there remain uncertainties about the validity of the equilibrium assumption (e.g., Aiuppa et al, 2007;Martin et al, 2009b), thermodynamic models enable parametric dependences to be investigated in the absence of chemical kinetic schemes. The model used here runs within commercial software (HSC Chemistry v5.1) and is based upon the high-T thermodynamic model of Martin et al (2006), which includes 110 gas-phase species of C, O, S, H, Cl, F, Br, I and N. To the existing model we incorporate Hg, HgH, HgO, HgS, HgF, HgCl, HgBr, HgI, HgF 2 , HgCl 2 , HgBr 2 , HgI 2 , HgSO 4 , HgSO 4 .HgO, HgSO 4 .2HgO and Hg 2 SO 4 . All compounds are included as both gas and condensed phases except for HgH (gas phase only) and the Hg sulphates (condensed phases only).…”