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Numerical modelling of cooling magmatic bodies. Application to Krafla volcano.

Gabriel Girela Arjona , Deepak Garg, Antonella Longo, Paolo Papale

Abstract

We aim to simulate the dynamics of shallow magmatic bodies while cooling. In order to do so we use, and further develop, GALES (Garg and Papale, Frontiers in Earth Sciences 2022), a numerical simulation software that solves the 4D dynamics of multi-component fluids. Krafla case serves as a perfect example: in 2009, the IDDP-1 drilling inside the Krafla caldera got stuck at 2.1 km, retrieving quenched glass cuttings. I had drilled into an undetected rhyolitic magma body had been drilled. This body stood without apparent signs of crystallisation at the rooftop, opposing the most common belief that magmatic bodies at shallow depths should present a mushy region adjacent to the body's walls. We perform 2D numerical simulations of the thermo-fluid dynamics of a magma of the characteristics of the one encountered under Krafla, assuming thermodynamic equilibrium in a sill-like, disk-shaped body situated at 2.1 km depth. The properties density, heat capacities, single-phase and multiphase non-Newtonian viscosity, thermal conductivity, and compressibility, are locally computed as a function of pressure, temperature, phase distribution and composition. Early results show a strong influence of the volatile phase on the bulk properties, especially density. This translates in the development of small, rising plumes from the bottom, composed of crystals and exsolved gas. Results allow a first evaluation of the conditions under which a crystal mush can form and be stable close to the roof and margins of a shallow magmatic intrusion, and an estimation of the lifespan of a cooling magmatic body of these characteristics.