Experimental constraints on the low gas permeability of vesicular magma during decompression

Efficiency of degassing through connected bubble network in magma has been evaluated based on gas permeability of magma. In order to investigate permeability variation with vesicularity of magma vesiculating during decompression, we performed a series of decompression experiments and permeability measurements of quenched experimental products for Mt. Usu dacite. The decompression experiments were performed by internally heated pressure vessel with rapid-quenching device at the Geological Survey of Japan, AIST. Samples were held for 135 minutes at 900 _E#8249;C and 150 MPa. Then, they were isothermally decompressed to a final pressure of 50, 35, 20, 15 or 10 MPa during 10-30 seconds. After the decompression, they were held for 1 hour at each final pressure, and then quenched. The permeabilities were calculated by using Darcy_fs law for compressible gas and results of steady-state gas-flow measurement. The vesicularities were measured by image analysis of back-scattered electron images of the experimental products. The permeabilities of the experimental products are less than 10-15.5 m2 at vesicularity less than 45 vol %, and increase by about 2 orders of magnitude with the vesicularity increase from 45 to 80 vol %. The permeability-vesicularity relationship of the experimental products is quite different from those obtained for natural eruptive materials by previous measurements, which reported a dramatic increase in permeability from 10-15.5 to 10-13 m2 with vesicularity increase from 2 to 40 vol %. The quenched experimental products can be considered as snapshots in vesiculating magma during decompression. Therefore, the permeability variation in vesiculating magma should be close to that obtained from the experiments than that obtained from natural eruptive materials, because the latter has experienced complex deformation in the late stages of eruption. Textural analysis for the experimental products shows that the gas permeability of vesicular magma developed in the bubble coalescence and shape relaxation processes. Bubble number density of the experimental products decreases with increasing of vesicularity. The decrease of bubble number density suggests that bubble coalescence occurred in the timescale of the experiments. Some of the bubbles in the experimental products have a spherical shape, suggesting that the relaxation of bubble shape also occurs after the coalescence. Because the bubble coalescence and shape-relaxation are time-dependent processes, the gas permeability may have time-dependence due to bubble coalescence and shape-relaxation.