Permeability measurements of natural and experimental volcanic materials with a simple permeameter: Toward an understanding of magmatic degassing processes

Permeability measurement of quenched volcanic porous materials is an important approach to understand permeability development and degassing of vesicular silicic magmas. In this study, we developed a gas permeameter to measure permeability of natural samples and experimental products. The permeameter has broad measurement ranges of pressure difference (101–105 Pa) and gas-flow rate (10− 9–10− 5 m3/s). These ranges enable us to measure viscous permeability in the range of 10− 17–10− 9 m2 for 1 centimeter-scale samples, using the Forchheimer equation, which includes the inertial effect of gas flow permeating through the samples. In addition, we improved the procedure for performing permeability measurements of mm-sized products of decompression experiments. Although a previous study reported the first permeability data for vesicular silicic glass products of decompression experiments, we found an overestimation in their permeability data due to problems in sample preparation, especially for very low permeability samples. Our improved measurements give lower permeability values than those of Takeuchi et al. (2005)(Takeuchi, S., Nakashima, S., Tomiya, A., Shinohara, H., 2005. Experimental constraints on the low gas permeability of vesicular magma during decompression. Geophys. Res. Lett., 32, L10312 doi:10.1029/2005GL022491). o evaluated the inertial effect on apparent permeability calculated using Darcyʹs law under the conditions of permeability measurement and magmatic degassing. The permeability measurements indicate that the apparent permeabilities of highly permeable samples (> 10− 10 m2 in viscous permeability) are affected considerably by the inertial effect under a high-pressure gradient (> 105 Pa/m) and reduced by more than 0.5 orders of magnitude from their viscous permeabilities. Also under magmatic conditions, the reduction of apparent permeability by more than 0.5–1.0 orders of magnitude may occur during the degassing of highly permeable magmas driven by a high-pressure gradient.