Crystal controls on permeability development and degassing in basaltic andesite magma

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 similar to 1-3 m s(-1). We measured the gas flux of the quenched samples using a bench-top permeameter and calculated the Darcian (k(1)) and inertial (k(2)) permeabilities using the Forchheimer equation. The experimental samples developed permeability at a critical vesicularity (Phi(c)) 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 Phi(c) > 63 vol% permeability threshold determined for crystal-free basaltic andesite melts. The percolation threshold decrease is observed when the microlites comprise >=similar to 20 vol% and can be explained by the onset of yield strength, which occurs when the crystals form a loosely packed, touching framework.