Strain‐induced outgassing of three‐phase magmas during simple shear

Abstract

A major factor determining the explosivity of silicic eruptions is the removal of volatiles from magma through permeability‐controlled outgassing. We studied the microstructural development of permeability during deformation of highly viscous magma by performing simple shear experiments on bubble (0.12–0.36 volume fraction) and crystal‐bearing (0–0.42 volume fraction) silicate melts. Experiments were performed under torsion, at high temperature and pressure (723–873 K and 150–200 MPa) in a Paterson deformation apparatus at bulk shear strains between 0 and 10. The experimental setup allows for gas escape if bubble connectivity is reached on the sample periphery. Three‐dimensional imaging and analysis of deformed bubbles was performed using X‐ray tomography. The development of localized deformation in all samples, enhanced by crystal content, leads to brittle fracture at bulk strains > 2 and sample‐wide fracturing in samples deformed to strains >5. A decrease in both bubble fraction and dissolved volatile content with increasing strain, along with strain‐hardening rheological behavior, suggests significant shear‐induced outgassing through the fracture networks, applicable to shallow conduit degassing in magmas containing crystal fractions of 0–0.42. This study contributes to our understanding of highly viscous magma outgassing and processes governing the effusive‐explosive transition.

Unravelling textural heterogeneity in obsidian: Shear-induced outgassing in the Rocche Rosse flow

“…We also see evidence for outgassing via shear-induced fracture and healing of the melt, also observed during shearing of analogue silicic magma samples (Okumura et al, 2010;Shields et al, 2014) and in natural samples (e.g., (Stasiuk et al, 1996;Tuffen et al, 2003;Castro et al, 2012;Cabrera et al, n.d.). Fig.…”

“…These veins of welded fragmental material describe shear fracture and healing of silicic magma (Tuffen et al, 2003) and may result in dense bubble-free obsidian bands (Castro et al, 2005a;Tuffen & Dingwell, 2005;Gonnerman & Manga, 2005;Cabrera et al, n.d.). (Shields et al, 2014) experimentally show the effect of shear on outgassing of viscous magmas, highlighting significant localisation of shear strain during deformation. Some authors postulate that localised shear-induced fracture alone is not an effective large-scale outgassing mechanism and that a combination of permeability via bubble connectivity and fracture development is required to fully outgas ascending magma Okumura et al, 2013;Sano et al, 2015) and result in volatile-poor obsidian lava.…”

“…Vesicles were segmented from the glass matrix by defining a greyscale threshold range for each sample, using an identical method to that described in (Shields et al, 2014). Image resolution was downsampled by a factor of 2 to enable processing with Blob3D.…”

“…Detailed degassing studies on tuffisite-bearing obsidian from Chaitén showed slightly higher water loss between 0.2 and 0.3 wt.% towards tuffisite vein margins . And shear experiments on silicic magma analogues by (Shields et al, 2014) showed that, in samples with multiple fractures, magmatic water decreased by 0.8 to 1.4 wt.% during deformation. This last water loss estimate is high, because experimental samples underwent rapid fracture healing due to high confining pressures and a lack of pore fluid pressure, which greatly enhances repeated fracture formation and hence water loss (Okumura et al, 2010), whereas fractures in natural obsidian would take much longer to heal (Yoshimura & Nakamura, 2010).…”

Rheological controls on the eruption potential and style of an andesite volcano: A case study from Mt. Ruapehu, New Zealand

“…Few studies exist of the more complex -and realistic -situation of three-phase suspen-3 sions (e.g., Shields et al, 2014). These include the rheological study of synthetic high-viscosity magmas containing both crystals and bubbles (e.g., Pistone et al, 2012Pistone et al, , 2013.…”