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Melting of host rocks by a shallow sill intrusion

Oleg Melnik 1, Muriel Gerbault2, Anastasia Borisova2

  • Affiliations: 1Department of Earth Sciences, University Oxford, Oxford, UK 2Géosciences Environnement Toulouse, Observatoire Midi-Pyrénées, Toulouse, France

  • Presentation type: Poster

  • Presentation time: Tuesday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 192

  • Programme No: 3.2.15

  • Theme 3 > Session 2

Abstract

In several volcanic areas magma bodies have been accidentally penetrated during drilling operations aimed at  supercritical hydrothermal resources. The first such well was drilled in the Krafla geothermal field in 2008--2009. Drilling stopped at a depth of 2,096 meters, and cuttings of fresh rhyolitic glass were retrieved. In the Menengai caldera in Kenya, multiple wells have penetrated syenitic magma located 2 kilometers beneath the caldera floor. In both cases, there was an abrupt transition from solid rock to molten magma, accompanied by an extreme temperature gradient. We have developed a 2D model of two-layered convection (for cases where magma and host rocks have different bulk compositions), based on the Navier--Stokes equations for incompressible magma with temperature- and crystal-content-dependent densities and viscosities in the Boussinesq approximation. The energy equation includes the latent heat release associated with crystallization. Phase diagrams for the magma and host rocks are calculated using the MELTS software. Simulations are performed on multiprocessor clusters using the OpenFOAM package. Both rhyolitic and basaltic magmas can efficiently melt the roof of the sill, forming a convective layer of molten rock with an initially linearly increasing thickness. As the intruded magma cools, convection slows, and the propagation velocity of the melting front decreases. The high-temperature gradient in unmelted rock becomes more uniform once melting ceases. A simplified 1D model, based on effective thermal conductivity, is capable of reproducing the results of 2D simulations with sufficient accuracy. This model allows for parametric studies of magma-rock evolution with reduced computational demands.