An experimental study of permeability development as a function of crystal-free melt viscosity

The efficiency of volatile exsolution and degassing from an ascending magma influences eruption dynamics. We performed single-step decompression experiments using externally heated TZM alloy cold seal pressure vessels to investigate porosity-permeability relationships as a function of crystal-free melt viscosity. The experiments employed natural, finely powdered rhyolite (76 % SiO₂), rhyodacite (70 % SiO₂), K-phonolite (55 % SiO₂), and basaltic andesite (54 % SiO₂) starting compositions, with estimated viscosities varying between ~10⁶-10³ Pa s. We first held the experiments at water-saturated conditions of 900 (rhyolite, rhyodacite, and phonolite) and 1025°C (basaltic andesite) and 150 MPa for 2-72 hours. We decompressed the experiments isothermally to final pressures of 125 to 25 MPa at 1-5 MPa/s and quenched after holding for 0.25-60 minutes at the final pressure. After removing the run products from the capsules, we measured porosity using image processing methods and permeability using a gas permeameter. We employed the Forchheimer equation to estimate Darcian (viscous) and inertial permeabilities of each experiment. All experiments are impermeable below a critical porosity that appears to vary between the different melt compositions. For rhyolite samples, the permeability increases from 10^-14 to 10^-13 m² at 63 - 80 vol. %, below which samples are impermeable. Rhyodacite is impermeable until the experiments reach ~67 vol. % porosity, at which the samples have viscous permeability of 10^-12.3 m2; higher than the rhyolite at similar porosity. K-rich phonolite reaches viscous permeability of 10^-14.5 m² at 55 vol. % porosity. Basaltic andesite samples remain impermeable to pressures as low as 50 MPa. Our preliminary results indicate that the critical porosity at which magmas become permeable and degas during magma ascent may decrease as a function of decreasing melt viscosity. However, further experiments on the phonolite and basaltic andesite compositions are needed to verify this trend.