Controls on magma permeability in the volcanic conduit during the climactic phase of the Kos Plateau Tuff eruption (Aegean Arc)

Abstract

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.

Permeability measurements of Campi Flegrei pyroclastic products: An example from the Campanian Ignimbrite and Monte Nuovo eruptions

“…1). In summary, these studies have illustrated that permeability is overall a function of vesicularity; yet abundant scattering in the permeabilityvs.-vesicularity data from the previous works implies that permeability depends also on other parameters of which the most important are vesicle size and throat size, vesicle shape and tortuosity (Wright et al, 2009;Degruyter et al, 2010a;Polacci et al, 2012).…”

“…5). We propose here that crystallinity of the KPT magma was sufficiently high to counter balance the viscosity effect and allow vesicle to grow, coalesce and form permeable vesicle pathways well visible in tube pumices (Bouvet de Maisonneuve et al, 2009;Degruyter et al, 2010a). This process of vesicle coalescence enhancement by high crystallinity has been modelled numerically (Huber et al, 2012) and it has been documented to occur in crystal-rich magma analogues (Belien et al, 2010); it is likely to play a fundamental role on the development of permeability in any crystal-rich magma (e.g., basaltic magmas such as Stromboli (Bai et al, 2011), or silicic mushes (Huber et al, 2012)).…”

“…CI and KPT samples). For this reason, we suggest that similarities in permeability data can be generated by a balance between melt viscosity and crystal content as these will have a strong control on vesicle size and throat size and tortuosity (Wright et al, 2009;Degruyter et al, 2010a;Polacci et al, 2012).…”

Heterogeneous vesiculation of 2011 El Hierro xeno-pumice revealed by X-ray computed microtomography

“…Because the diameters at which we observed the main modes are about one order of magnitude above the detectability limit of structures in the 3D images, and because of the automated sampling procedure applied, we note that our data are unlikely to reflect resolution or sampling artefacts (cf. Degruyter et al 2010;Giachetti et al 2011). …”

High-resolution 3D analyses of the shape and internal constituents of small volcanic ash particles: The contribution of SEM micro-computed tomography (SEM micro-CT)

“…Conventional X-ray micro-CT analyses have been carried out recurrently to investigate volcanic rocks (e.g. Proussevitch et al, 1998;Degruyter et al, 2010), but the limited spatial resolution to~6 μm in X-ray shadow projections of many systems (Jerram and Higgins, 2007) makes those instruments poorly suited for smaller objects such as volcanic ash particles (s. Fig. 4 in Ersoy et al, 2010).…”