How H2O transport and trapping affect the dynamics of magmatic systems from melt generation to eruption triggering

H₂O and other volatiles play a major role in the dynamics of magmatic systems from magma source to volcanic eruption. At the source, H₂O facilitates melting and within the crust, H₂O fluxing of hot rocks favours the formation and stabilisation of magma mush. H₂O dissolved in melt lowers its viscosity and density and thus helps melt extraction and transport. H₂O exsolution during magma transport causes crystallisation and stalling of the magma, while further exsolution in a crystallizing magma chamber increases pressure on the chamber walls and facilitates chamber failure and eruption. During the magma final journey towards the surface, formation and expansion of bubbles fragment the magma, increasing the explosivity of the volcanic eruption.

We have developed numerical simulations that model flux melting in the deep crust, and other simulations that model H₂O exsolution in a growing and crystallising magma chamber in the upper crust. A parameter that is crucial in both processes is the permeability of the magma, mush, and solid rocks through which H₂O is moving.

Our flux melting models show that due to complex interactions between the effect of H₂O and heat, the amount of melt produced by flux melting depends on how fast H₂O is transferred relative to heat. By absorbing latent heat, H₂O flux melting decreases temperatures and promotes heat transfer in the melting areas. The extent of the melting region depends on the competition between the transfer of H₂O, which induces melting and cooling, and the transfer of heat that smooths temperature anomalies. The amount of crustal melting also depends on permeabilities contrast. The presence of an impermeable layer above a more permeable layer increases the production of melt by trapping H₂O in the permeable layer.

The ease of transfer of exsolved H₂O within a crystallising magma body depends on crystal fractions. Our models of magma chambers growth and solidification show that preferential transfer of H₂O through the mush that surrounds a magma chamber results in the formation of H₂O layers at the top the mush and in the chamber’s roof. If the permeability of the country rocks is low, the H₂O layers grow, their buoyancy increases, and the pressure eventually exceeds the yield strength of the country rock and causes fracturing. Depending on how well the H₂O layers connect to the liquid magma in the chamber below, fracturing of the country rock can result either in the release of H₂O only and possibly causes a bradyseismic crisis, or in the release of H₂O, mush, and magma, and triggers a crystal-rich eruption.

We know from volcano degassing and from petrological data that magmas are rich in H₂O. However, our knowledge of how this H₂O is transferred from magmas and through the crust is limited. This knowledge is important though as our simple numerical models indicate that the modalities of H₂O transport and accumulation strongly affect the dynamics of a magmatic system.