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

Abstract

AbstractThe porosity at which a magma becomes permeable (i.e., the percolation threshold; ϕc) 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.