The Influence of Phenocrysts on Degassing in Crystal-Bearing Magmas With Rhyolitic Groundmass Melts

The porosity at which a magma becomes permeable (i.e., the percolation threshold; ϕ_c) is important for magma degassing; it is also poorly constrained in crystal-bearing systems. To address this, we conduct high pressure-temperature decompression experiments on water-saturated rhyolitic melts with variable crystal contents. We find that crystal-bearing run products become permeable at ~55-vol.% vesicularity (crystal free), a value that is similar to that found in decompression-crystallization experiments using basaltic andesite compositions. Our results provide insight into controls on the eruption styles of hydrous, crystal-bearing magmas in general and controls on pulsatory Vulcanian behavior, in particular.