The Interface Between Magma and Earth's Atmosphere

Abstract

Volcanic gases crucially influence Earth's atmosphere, on long time scales by gradually altering the atmospheric redox state (Gaillard et al., 2011;Kasting, 1993), as well as on short time scales when large amounts are emitted instantaneously during large eruptions (Robock, 2000). Volcanic gas measurements reveal information about Earth's interior, and the chemistry within the cooled and diluted volcanic plume. In that context, the interface between the magmatic gases and the atmosphere and its influence on the gas composition requires consideration. Interpretations of fumarole emissions, particularly those which have considerably cooled at the point of sampling, include a quantitative treatment of gas-rock and gas-fluid interactions. The analysis encompasses both, thermodynamic equilibrium (TE) relationships and the quantification of relative rates of equilibration (see e.g., Giggenbach, 1987Giggenbach, , 1996Henley and Fischer, 2021). In contrast to that, the interaction between magmatic gas and atmospheric air in high-temperature emission processes, for example, at open vents, is difficult to access experimentally and therefore remains poorly studied. Such high-temperature volcanic gases are commonly solely interpreted on the basis of TE relations. While this is likely to be valid for gas bubbles within the magma body, we show that-in contrast to usual assumptions (e.g., Gerlach and Nordlie, 1975;Martin et al., 2006;Moussallam et al., 2019)-TE is not suited to describe magmatic gases at the direct interface between magma and atmospheric air and afterward. TE requires the Gibbs free energy to be minimized prior to macroscopic changes of the system. By modeling the chemical kinetics and turbulent atmospheric mixing in the magma-atmosphere interface, we show that major chemical conversions, cooling, and mixing usually take place on comparable time scales. This leads to complex interactions of these processes ultimately determining the gas composition of the plume within the first fractions of a second after emission into the atmosphere. Then, the gas composition greatly deviates from both, the magmatic gas TE and the TE of the diluted plume and its interpretation requires considering chemical kinetics (i.e., reaction rates) and the dynamics of the system.

Quantifying BrO and SO2 distributions in volcanic plumes—Recent advances in imaging Fabry-Pérot interferometer correlation spectroscopy

“…Since SO 2 can serve as a quasiconservative tracer for dilution this ratio can give important insights in plume chemistry (von Glasow et al, 2009) but can be also related to volcanic activity (e.g., Warnach et al, 2019). Imaging measurements of this ratio can help to understand the complex chemical conversion mechanism within early plume stages, which is so far not well understood (see, e.g., Kuhn et al, 2022).…”

High-spectral-resolution Fabry-Pérot interferometers overcome fundamental limitations of present volcanic gas remote sensing techniques

“…While tropospheric OH levels are in the sub-ppt-range, simulations of the high-temperature emission of magmatic gases into the atmosphere find ppm-levels of OH within the hot and early plume (e.g., Gerlach, 2004;Roberts et al, 2019). Such high amounts of OH are very likely to substantially influence the magmatic gas composition, particularly the redox state it represents, within split-seconds after their emission (Kuhn, 2022;Kuhn et al, 2022), taking into question the results based on common thermodynamic equilibrium assumptions prevailing in volcanic gas geochemistry (see, e.g., Symonds et al, 1994;Moussallam et al, 2019). The development of kinetic emission models paired with measurements of volcanic OH (and other short-lived radicals) is crucial for understanding the chemical evolution of volcanic gas plumes in the atmosphere.…”

“…OH is only expected in gas emissions with high temperature (e.g., Roberts et al, 2019;Kuhn, 2022;Kuhn et al, 2022). Nyiragongo -and the neighboring Nyamulagira-volcano are 10.3389/feart.2023.1039093 located in the Western Branch of the East African Rift in the Democratic Republic of Congo.…”

“…On the other hand, these dynamic changes can lead to periods (e.g., on the order of months to years), in which the lava lake and active lava flows are relatively easily accessible (e.g., Pouclet and Bram, 2021). These are rare opportunities for sampling the hot gas emissions of lava lakes (e.g., Le Guern, 1987) and to examine the still poorly understood high-temperature interface between magma and the atmosphere, which is likely to significantly influence all open-vent volcanic gas emissions (Kuhn et al, 2022). Section 2.3 describes measurements with the introduced prototype performed at an active lava flow within the crater of Nyiragongo volcano in February 2020.…”