Redox Evolution of a Degassing Magma Rising to the Surface and its Effects on Eruptive Dynamics.
Résumé
Volatiles carried by magmas, either dissolved or exsolved, exert a fundamental role on a variety of geological phenomena such as magma dynamics or the composition of Earth's atmosphere. In particular, the redox state of volcanic gases emanating at the Earth surface is widely believed to mirror that of their magma source, and thought to have exerted a first order control on the secular evolution of atmospheric oxygen. Little attention has been paid, however, on chemical aspects in numerical or laboratory modeling studies of magma decompression, emphasis being put on physical aspects. Here, we use a coupled chemical-physical model of conduit flow that explicitly takes in account the chemical equilibrium and solubilities of the six species composing the H-O-S system. We show that the redox state evolution of an ascending magma is strongly dependent on the initial redox state and on the amount of excess gas in the reservoir. Reduced magmas with little excess gas in the reservoir show an increase of redox state during ascent whereas those with at least 5 wt% gas tend to better preserve the redox signature of their source, in agreement with petrologic evidence. Oxidized magmas show a dramatic decrease of redox state during ascent, the amount of excess gas playing only a small role in the process. Excess gas controls the redox evolution close to the NNO buffer, a small amount of gas oxidizing the magma and a large amount of gas reducing it. At shallow depths, such as in dome forming eruptions, we show that the redox state has a direct effect on the porosity evolution with depth and thus on eruption dynamics. Reduced to moderately oxidized magmas display a pressure threshold beyond which porosity evolution is strongly accelerated as pressure decreases. We thus conclude that a complete description of the physics of volcanic eruptions requires explicit consideration of chemical aspects and that the redox state of erupted magmas is not necessarily a good proxy of the redox state of the gases they emit. Both findings may require re-evaluation of current models aimed at quantifying the role of magmatic volatiles on geological processes.