Magma system maturation and ground deformation
In collaboration with J. Biggs from University of Bristol, we explored a new conceptual model to explain the pattern of ground deformation in volcanic areas. According to this model, series of magma pockets are aligned along faults. As the magmatic system matures, the rock between the pockets heats up and soften. In a first stage, when rocks are still cold, the deformation induced by magma pressurisation is localised above the magma pockets. In a second stage, the rocks between magma pockets are hot and becomes viscous and deformations broaden. In a final stage, magma pockets connect, form a large magma chamber, and the deformation covers the whole magmatic area.
We used numerical simulation to calculate that after 10s of thousand years the effect of rock softening affects the ground deformation. The calculations also showed that only magma pockets that are less than 1 km apart do fully converge. Thus, connexion of magma chambers either involves chambers that are separated by a relatively thin screen of country rock or mechanical rupture of the country rock plays a role. These results are published in the Philosophical Transactions of the Royal Society (doi.org/10.1098/rsta.2018.0005).
Volatiles release
To model the release of water from a growing and solidifying magma chamber, we used data from petrological experiments to produce an algorithm that calculates temperatures, and fractions of liquid, crystals, and water. We implemented in the simulation the transport of the exsolved water. In part of the magmatic system where crystals are in contact and form a solid network, bubbles are rapidly transported through channels, whereas, where the magma is poor in crystals, the bubbles are suspended in the liquid and their velocity is so low that they can be considered immobile. Bubbles ascending through channels are arrested when they encounter liquid magma or solid rock. During solidification, water concentrates at certain levels with both the concentration value and the concentration depth depending on the magma emplacement rate. The most interesting result is the formation of layers of gas in the crystal mush and solidified magma that surrounds the active magma chamber. The pattern of those gas layers depends on the composition of the magma and on its initial water content. A magma that crystallises over a narrow range of temperatures is less able to concentrate gas than a magma that crystallises over a wide range of temperatures. These results have been submitted for publication to Lithos and have been part of a keynote presentation at the Hutton symposia that took place in Nanjin, China, in 2019. We also calculated the flux of water released by the magma chamber. The data indicate that the volume added by gas exsolution does not suffice to cause an overpressure that would rupture the wall of the chamber. However, if gas-rich layers are connected, their buoyancy might cause chamber failure and trigger an eruption. Another paper that explores the role of volatiles in the failure of magma chambers is in preparation.
Enclave deformation
In the context of the collaboration with the volcanology team of my host institute, enclaves deformation was modeled. Granitic magma commonly contains enclave of mafic magmas. Those enclaves are often deformed. The intensity of the deformation was believed to be linked to the contrast in viscosity between the granitic and the mafic magma. Our study that uses computational fluid dynamics with discrete element modelling suggests that it is not the case. According to the simulations, the deformation is linked to the formation of force chains between the crystals and is independent of viscosity. It is an important result because enclave deformation is used to infer the cooling rate of magma, hence the dynamics of magma chambers. These results have been published in Journal of Volcanology and Geothermal Research (doi.org/10.1016/j.jvolgeores.2020.106790)
