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Abstract:
Eruptive dynamics of silicic volcanoes are highly controlled by the possibility of gases escaping from the magmatic column. A recent study (Collombet et al., 2021) shows that outgassing probably occurs deeper in the magmatic column (several kilometers) than previously thought. This early outgassing is linked to bubble channeling which occurs when crystal content reaches 40 to 70 vol% (Parmigiani et al., 2017). This particular crystal content should allow the magma to become permeable at lower gas content than initially thought.
We propose to model the evolution of bubble coalescence and channelization in synthetic magmas using decompression experiments in internally-heated autoclave. In these experiments, we propose to test the influence of crystal size and crystal content on the permeability in a gas-poor mixture.
Preliminary results suggest the presence of permeability at 10% bulk porosity for a bulk crystal content of 50 vol%. It confirms that one important parameter for the formation of permeability in gas-poor magma is the number density of bubbles, controlled by decompression rate (Toramaru, 1995). Based on our preliminary results and assumptions on the future samples, the role and process of nucleation are then to be considered seriously in the problematic of channelization and further in magmatic degassing.
The parametrized behavior of bubble coalescence and permeability at low gas volume fraction and high crystal fraction will then be integrated into large-scale numerical model to predict the evolution of degassing in magmatic columns.
