Mardi 4 Octobre à 10h00 dans l’amphi Recherche (pôle physique) : Michael Heap (EOST Strasbourg)
Title: Mechanisms of porosity and permeability loss in conduit wall rock: From localised cataclastic pore collapse to distributed viscous flow
Explosive silicic volcanism is driven by gas overpressure in systems that are inefficient at outgassing. The conduit margin zone—defined as the wall rock and the magma close to the conduit margin—is thought to be the annulus through which magmas outgas. Therefore, a decrease of porosity and permeability for the material within this zone could impede such outgassing and lead to pressure build-up and explosive behaviour. This study employs high-pressure (effective pressure of 40 MPa), high-temperature (up to 800 °C) triaxial deformation experiments using porous andesite to elucidate the mechanisms of porosity and permeability loss in the region of the conduit margin zone that precludes macroscopic brittle failure (depth = ~1 km). It is shown that the mechanism facilitating compaction in volcanic rocks is localised cataclastic pore collapse at all temperatures below the glass transition of the amorphous groundmass glass T_g. In this regime, porosity is only reduced within the bands of crushed pores; the host rock porosity remains unchanged. Although these features may disrupt outgassing of the nearby magma-filled conduit, it is unclear whether they will form a coherent low-permeability barrier. A simple model of heat transfer from a hot magma-filled conduit in to the country rock suggests that hours to days (the frequency of Vulcanian explosions at many active stratovolcanoes) are sufficient to heat ~0.01-1 m of wall rock adjacent to the conduit to temperatures above T_g, upon which the micromechanism driving compaction switches from localised pore collapse to distributed viscous flow. This change in deformation mechanism is accompanied by a substantial reduction in strength and a substantial decrease in porosity, the result of widespread viscous pore flattening and closure. A low-porosity layer that envelops the conduit will severely inhibit outgassing and could allow pore pressure to build to that preparatory for the next Vulcanian explosion, during which the compacted layer is fractured and the process can start again. This study therefore highlights that small changes in the temperature can result in a change in deformation micromechanism that drastically alters the mechanical and hydraulic properties of the wall rock adjacent to the conduit, with implications for pore pressure augmentation leading to explosive behaviour.