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

Abstract

Permeability development in magmas controls gas escape and, as a consequence, modulates eruptive activity. To date, there are few experimental controls on bubble growth and permeability development, particularly in low viscosity melts. To address this knowledge gap, we have run controlled decompression experiments on crystal-free rhyolite (76 wt. % SiO 2 ), rhyodacite (70 wt. % SiO 2 ), K-phonolite (55 wt. % SiO 2 ) and basaltic andesite (54 wt. % SiO 2 ) melts. This suite of experiments allows us to examine controls on the critical porosity at which vesiculating melts become permeable. As starting materials we used both fine powders and solid slabs of pumice, obsidian and annealed starting materials with viscosities of ~10 2 to ~10 6 Pa s. We saturated the experiments with water at 900˚ (rhyolite, rhyodacite, and phonolite) and 1025˚C (basaltic andesite) at 150 MPa for 2-72 hours and decompressed samples isothermally to final pressures of 125 to 10 MPa at rates of 0.25-4.11 MPa/s. Sample porosity was calculated from reflected light images of polished charges and permeability was measured using a bench-top gas permeameter and application of the Forchheimer equation to estimate both viscous (k 1 ) and inertial (k 2 ) permeabilities. Degassing conditions were assessed by measuring dissolved water contents using micro-Fourier-Transform Infrared (-FTIR) techniques.All experiment charges are impermeable below a critical porosity ( c ) that varies among melt compositions. For experiments decompressed at 0.25 MPa s -1 , we find the percolation threshold for rhyolite is 68.3 ± 2.2 vol.%; for rhyodacite is 77.3 ± 3.8 vol.%; and for K-phonolite is 75.6 ± 1.9 vol.%. Rhyolite decompressed at 3-4 MPa s -1 has a percolation threshold of 74 ± 1.8 vol.%. These results are similar to previous experiments on silicic melts and to high permeability thresholds inferred for silicic pumice. All basaltic andesite melts decompressed at 0.25 MPa s -1 , in contrast, have permeabilities below the detection limit (~10 -15 m 2 ), and a maximum porosity of 63 vol.%. Additionally, although the measured porosities of basaltic andesite experiments are ~10-35 vol. % lower than calculated equilibrium porosities, -FTIR analyses confirm the basaltic andesite melts remained in equilibrium during degassing. We show that the low porosities and permeabilities are a consequence of short melt relaxation timescales during syn-and post-decompression degassing. Our results suggest that basaltic andesite melts reached  c > 63 vol. % and subsequently degassed; loss of internal bubble pressure caused the bubbles to shrink and their connecting apertures to seal before quench, closing the connected pathways between bubbles. Our results challenge the hypothesis that low viscosity melts have a permeability threshold of ~30 vol. %, and instead support the high permeability thresholds observed in analogue experiments on low viscosity materials. Importantly, however, these low viscosity melts are unable to maintain high porosities once the percolation threshold is ...

Universal Relationship Between Viscous and Inertial Permeability of Geologic Porous Media

“…To further establish whether equation (2) suitably describes real geologic porous media representing different processes inducing pore geometry variation, we focused on the results from groups of previous laboratory experiments where geologic samples were deformed through compression (Dan et al, 2016;Gostick et al, 2006;Jones, 1987;Ma et al, 2015;Ma et al, 2016;Schrauf & Evans, 1986;Zhou et al, 2015), torsion (Okumura et al, 2009), shearing (Javadi et al, 2014;Wang, 2017), or decompression (Lindoo et al, 2016;Takeuchi et al, 2009), and from modeling experiments where pore networks with ducts of different shapes were systematically varied (Veyskarami et al, 2016), like we did with our CFD simulations discussed below. The complete details on the data set including the type of geologic porous media, the geometry-varying processes represented, and the fitted parameters are provided in Table S4 in the supporting information.…”

Permeability During Magma Expansion and Compaction

“…The red curve represents the fit to Glass Mountain Plinian data (equation ) and the blue curves are based on equation . The arrows labeled ϕ cr indicate the maximum value of ϕ t for which permeabilities were below the detection limit (∼10 −17 m 2 for Takeuchi et al, and <10 −15 m 2 for Lindoo, ). (b) Connectivity, ϕ c / ϕ t , as a function of total porosity.…”

Outgassing from Open and Closed Magma Foams

“…Fragmentation can open pathways for outgassing that might subsequently heal (Tuffen et al, 2003;Cabrera et al, 2010). Previous work has focussed on the growth of bubbles in homogeneous foams and the resultant connectivity (Pistone et al, 2015;Lindoo et al, 2016), or on the disruption of foams during shearing (Okumura et al, 2009).…”

“…The development of porosity is central to the volcanic eruptions as it controls the buoyancy of magma (Gonnermann and Manga, 2007), its rheology (Llewellin et al, 2002) and the pore pressure (Martel et al, 2000) that influence eruptive behavior. During vesiculation, bubbles may coalesce and create a variably permeable network that regulates outgassing (Lindoo et al, 2016), which if efficient may result in foam collapse. If outgassing (i.e., the removal of volatiles from the system) is prevented by a low permeability network or barrier (Yoshimura and Nakamura, 2008), pressure may build up and the strain rate at the bubble walls may exceed the inverse of the relaxation time of the melt phase, in which case magma fragments, potentially triggering an explosive eruption (Dingwell, 1996).…”