The interface between magma and Earth’s atmosphere and its influence on the gas composition of volcanic plumes

Magmatic gases that reach Earth’s surface are among the scarce sources of information on the planet’s interior. Their composition is dominated by H₂O, CO₂, and sulfur species and largely differs from that of today’s atmosphere of the Earth, particularly, by the amounts of oxygen. When magmatic gases are emitted directly to the atmosphere (e.g. at lava lakes), the process is further characterised by huge temperature gradients (hundreds to more than a thousand K per metre). The rapid cooling and fast mixing with atmospheric oxygen defines an early phase in the lifetime of a volcanic plume, which can crucially influence the plume’s later composition. Few attempts have been made to include the often extreme dynamics of this early plume phase into the scope of volcanic gas studies.

Naturally, magmatic degassing processes are difficult to study and thus bound to large uncertainties in crucial parameters, such as gas temperature, gas composition, and mixing. Further, heterogeneous processes involving ash and aerosols might have a strong impact on the processes.

We developed a model to study the C-H-O-S gas phase reaction kinetics of the first seconds of a volcanic gas emission. The entire cooling process of the volcanic gases is covered and studied considering its dynamics and regarding large ranges of mixing scenarios, gas compositions and emission temperatures.

We find that many major processes are far from (the often assumed) thermal equilibrium (TE). Particularly, large amounts of HO_X (OH + HO₂), exceeding TE by orders of magnitude, form at high temperature as soon as sufficient O₂ entered the plume. High OH levels lead to rapid oxidation of emitted species, such as CO, H₂, and SO₂. Strikingly, CO levels can be both, reduced and enhanced by high temperature processing, depending on the assumed initial conditions. Moreover, we observe that the enthalpy change associated with the chemical conversions can lead to a significant net heating of the plume.

Overall, and despite of the simplifications made, our model indicates a major influence of the dynamics within the interface between magma and the atmosphere (i.e. the early volcanic plume). The composition of gas samples that interacted only a tenth of a second with the atmosphere might substantially differ from the magmatic gas composition. This would lead to enhanced uncertainties in the quantification of magmatic parameters (such as temperature and redox state), when derived from measurements of gas ratios in the volcanic plume.