Controls on magma permeability in the volcanic conduit during the climactic phase of the Kos Plateau Tuff eruption (Aegean Arc)
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X-ray computed microtomography (µCT) was applied to pumices from the largest Quaternary explosive eruption of the active South Aegean Arc (the Kos Plateau Tuff; KPT) in order to better understand magma permeability within volcanic conduits. Two different types of pumices (one with highly elongated bubbles, tube pumice; and the other with near spherical bubbles, frothy pumice) produced synchronously and with identical chemical composition were selected for µCT imaging to obtain porosity, tortuosity, bubble size and throat size distributions. Tortuosity drops on average from 2.2 in frothy pumice to 1.5 in tube pumice. Bubble size and throat size distributions provide estimates for mean bubble size (~93–98 μm) and mean throat size (~23–29 μm). Using a modified Kozeny-Carman equation, variations in porosity, tortuosity, and throat size observed in KPT pumices explain the spread found in laboratory measurements of the Darcian permeability. Measured difference in inertial permeability between tube and frothy pumices can also be partly explained by the same variables but require an additional parameter related to the internal roughness of the porous medium (friction factor f 0 ). Constitutive equations for both types of permeability allow the quantification of laminar and turbulent gas escape during ascent of rhyolitic magma in volcanic conduits.
KeywordsMicrotomography Pumice Permeability Tortuosity Outgassing Volcanic conduit
This project was supported by the Swiss NSF grant #200021-111709/1 to WD and OB. WD greatly acknowledges the UGCT team at the University of Ghent (B. Masschaele, V. Cnudde, J. Vlassenbroeck, M. Dierick and L. Van Hoorebeke) and kindly thanks B. Lindquist, R. Ketcham and A. Proussevitch for the use of their respective softwares and quick response to questions. We are grateful for constructive comments from two anonymous reviewers.
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