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Experimental Evidence of Primary Permeability at Very Low Gas Content in Crystal‐Rich Silicic Magma
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Experimental Evidence of Primary Permeability at Very Low Gas Content in Crystal‐Rich Silicic Magma
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Experimental Evidence of Primary Permeability at Very Low Gas Content in Crystal‐Rich Silicic Magma
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Journal Article
Experimental Evidence of Primary Permeability at Very Low Gas Content in Crystal‐Rich Silicic Magma
2024
Overview
Eruptive dynamics is influenced by gas escape from the ascending magma. Gas pathways form in the magma via bubble coalescence, leading to gas channeling. Magmatic crystals play a key role in gas channel formation. This work constrains experimentally decompression‐induced coalescence in high‐crystallinity silicic magmas without external deformation, focusing on low gas content and bimodal crystal size (microlites and phenocrysts). All percolating samples have permeabilities of 10−14 m2 at bulk porosities of 7–10 vol% and bulk crystallinities up to 75 vol%. Our results demonstrate the possibility of coalescence‐related outgassing at high pressure (120–350 MPa) and without external strain, which corresponds to magma stagnating deep in a volcanic conduit. Channeling at such low gas content implies that bimodal crystallinity favors effusive over explosive volcanic behavior. It may also be the missing physical mechanism explaining gas transfer across magmatic systems despite high melt viscosity and low or absent magma extrusion. Plain Language Summary The way volcanoes erupt is mainly controlled by the ability of gases to escape from the magma (outgassing). An efficient way to outgas is to connect bubbles together (coalescence) up to the point where channels form in the magma (channeling) in which gas can circulate toward the surface. We investigate experimentally how coalescence and channeling happen in immobile, crystal‐rich viscous magma in conditions (pressure and temperature) similar to those found in deep volcanic conduit, where only a small amount of gas (<10%) is present. Our experiments demonstrate that bubble connections are possible thanks to a large amount of crystals on which bubbles can lean, deform, and join each other. That gas can escape from magma at depth could favor effusive eruption over volcanic explosions and even bring new insights on degassing in immobile magma. Key Points Gas permeability in silicic magma is possible with gas volumes below 10% thanks to high crystallinity Outgassing at low gas content can happen without external deformation nor fracturation of the magma Volcanic outgassing in stagnant magmas is explained by deep conduit bubble channeling that may be a key mechanism in eruptive transition
