Abstract

Understanding degassing of mafic magmas is important for modeling eruptions and examining controls on eruption style. We conducted high-pressure–high-temperature isothermal decompression experiments to investigate the effects of decompression-induced crystallization on permeability development and magma degassing. Experiments were performed on hydrous basaltic andesite (54 wt% SiO2) decompression rates equivalent to magma ascent velocities of ∼1–3 m s–1. We measured the gas flux of the quenched samples using a bench-top permeameter and calculated the Darcian (k1) and inertial (k2) permeabilities using the Forchheimer equation. The experimental samples developed permeability at a critical vesicularity (Φc) of 56.4 ± 2.7 vol% (at 0.125 MPa s–1) and 50.76 ± 5.6 vol% (at 0.083 MPa s–1), considerably lower than the Φc > 63 vol% permeability threshold determined for crystal-free basaltic andesite melts. The percolation threshold decrease is observed when the microlites comprise ≥∼20 vol% and can be explained by the onset of yield strength, which occurs when the crystals form a loosely packed, touching framework.

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