Abstract
<strong class="journal-contentHeaderColor">Abstract.</strong> The permeability of magma in volcanic conduits controls the fluid flow and pore pressure development that regulates gas emissions and the style of volcanic eruptions. The architecture of the permeable porous structure is subject to changes as magma deforms and outgasses during ascent. Here, we present a high-resolution study of the permeability distribution across two conduit shear zones (marginal and central) developed in the dacitic spine that extruded towards the closing stages of the 1991â1995 eruption at Unzen volcano, Japan. The marginal shear zone is approximately 3.2âm wide and exhibits a 2âm wide, moderate shear zone with porosity and permeability similar to the conduit core, transitioning into a <span class="inline-formula">â¼</span>â1âm wide, highly sheared region with relatively low porosity and permeability, as well as an outer 20âcm wide cataclastic fault zone. The low-porosity, highly sheared rock further exhibits an anisotropic permeability network, with slightly higher permeability along the shear plane (parallel to the conduit margin), and is locally overprinted by oblique dilational Riedel fractures. The central shear zone is defined by a 3âm long by <span class="inline-formula">â¼</span>â9âcm wide fracture ending bluntly and bordered by a 15â40âcm wide damage zone with permeability enhanced by <span class="inline-formula">â¼</span>â3 orders of magnitude; directional permeability and resultant anisotropy could not be measured from this exposure. We interpret the permeability and porosity of the marginal shear zone to reflect the evolution of compactional (i.e. ductile) shear during ascent up to the point of rupture, which was estimated by Umakoshi et al. (2008) at <span class="inline-formula">â¼</span>â500âm depth. At this point the compactional shear zone would have been locally overprinted by brittle rupture, promoting the development of a shear fault and dilational Riedel fractures during repeating phases of increased magma ascent rate, enhancing anisotropic permeability that channels fluid flow into and along the conduit margin. In contrast, we interpret the central shear zone as a shallow, late-stage dilational structure, which partially tore the core of the spine, leaving a slight permanent displacement. We explore constraints from monitored seismicity and stick-slip behaviour to evaluate the rheological controls, which accompanied the shift from compactional toward dilational shear as magma approached the surface, and discuss their importance in controlling the permeability development of magma evolving from overall ductile to increasingly brittle behaviour during ascent and eruption.
Highlights
Outgassing pathways and volcanic eruptionsThe style and timing of activity exhibited during a volcanic eruption are strongly influenced by the presence and mobility of volatiles in magma (Sparks, 1997; Woods and Koyaguchi, 1994) and surrounding conduit wallrock (Jaupart and Allègre, 1991)
Volatiles are exsolved into gas bubbles (Navon et al, 1998; Sparks, 2003) as their solubility decreases with decompression (Liu et al, 2005), crystallisation (Tait et al, 1989), and heat generated by crystallisation (Blundy et al, 2006) and shear (Lavallée et al, 2015)
The study of evolving monitored signals and eruptive products at Unzen depicts a wide range of outgassing pathways, which evolve during the course of magma ascent and lava dome eruptions
Summary
The style and timing of activity exhibited during a volcanic eruption are strongly influenced by the presence and mobility of volatiles in magma (Sparks, 1997; Woods and Koyaguchi, 1994) and surrounding conduit wallrock (Jaupart and Allègre, 1991). Close examination of the architecture of shallow dissected conduits and structures in ventproximal silicic lava exposes complex shearing histories that would impact the permeable porous network of erupting magma These structures reveal porosity contrasts through the lavas, and strain localisation near the conduit margins is commonly identified via the presence of flow bands and variably porous shear zones with a spectrum of configurations (Gaunt et al, 2014; Kendrick et al, 2012; Kennedy and Russell, 2012; Pallister et al, 2013a; Smith et al, 2001; Stasiuk et al, 1996; Tuffen and Dingwell, 2005); features that are preserved to differing extents in crystal-poor and crystal-rich magmas (Calder et al, 2015; Lavallée and Kendrick, 2021). The study of evolving monitored signals and eruptive products at Unzen depicts a wide range of outgassing pathways, which evolve during the course of magma ascent and lava dome eruptions
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