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The permeability of transient porous networks in magmas in-situ

^^ Anthony Lamur^^ , Philip Benson, Jackie E. Kendrick, Fabian B. Wadsworth, Yan Lavallée

Abstract

It is well known that the ease with which volatiles can escape magmas dictates the subsequent eruptive style. The volatile escape rate is controlled by both how fast effective pressure (=magmastatic pressure -- pore pressure) can build up and how permeable the porous network is. While a huge number of permeability measurements have been performed on cold lavas from volcanoes all over the world, in which the porous network is frozen-in, static, and 'fossilized', very few studies have tried to systematically understand the permeability of porous networks in high-temperature magma itself, which by their molten nature are transient and dynamic. Here we use a suite of sintered two-phase (melt and pore) samples, of varying starting porosity, in a novel high-temperature, low-pressure triaxial cell explicitly designed to replicate shallow magmatic conditions. The samples are first brought up to a temperature above their glass transition temperature, allowing for the porous network to evolve as further sintering progresses under 1) quasi-hydrostatic conditions and 2) constant differential stress. During these experiments, the permeability is assessed by keeping a constant pore pressure differential across the sample, and monitoring flow rate. We show that the permeability measured in these dynamic magmatic conditions reflects the architecture of the transient porous network that itself reflects the stress state of the magmatic system. As such, we posit that existing permeability relationships, which require an immutable porous model, likely fail to encompass the full range and transient nature of porous networks in magmas.