Melt fracturing and healing: A mechanism for degassing and origin of silicic obsidian

Cabrera, Agustín; Weinberg, Roberto Ferrez; Wright, Heather; Zlotnik, Sergio; Fernand, Raymond Alexander

doi:10.1130/g31355.1

Cited by 63 publications

(51 citation statements)

References 27 publications

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“…Therefore, in order to account for gas removal of an entire magma body by shear fracturing, there would have to be multiple cycles of fracturing and healing occurring throughout magma ascent (Tuffen & Dingwell, 2005;Cabrera et al, n.d.). The theory of continuous, cyclical outgassing is further supported by seismic evidence from other volcanoes, whereby earthquake repose times match calculated obsidian fracture healing times (Tuffen et al, 2003;Cabrera et al, 2010;Yoshimura & Nakamura, 2010). Outgassing is also likely to occur via collapse of bubble networks, however (Okumura et al, 2008;Okumura et al, 2009;Okumura et al, 2012), which may be the dominant mechanism at greater depths in the conduit i.e., several thousand metres (Okumura et al, 2009).…”

Section: Model For Obsidian Formationmentioning

confidence: 60%

“…The stage 2 band shows brittle fracture of a weak, bubble-rich plane, when viscous shear of unrelaxed (high viscosity, high local strain-rate) magma leads to local accumulation of shear stress, sufficient to induce brittle deformation and fracture the magma (Wright & Weinberg, 2009). If connectivity to lower pressures is achieved, the sudden stress release and pressure drop on fracture development induces rapid outgassing of both the volatiles inside the bubbles and the melt immediately surrounding the fracture, similar to the process shown by the work of (Cabrera et al, 2010) and explored in more detail by . Gas loss instigates welding of magma fragments to form a bubble-free, dense, higher viscosity, 'healed' obsidian band (stage 3 in Fig.…”

Section: Effect Of Shear On Obsidian Degassing and Outgassingmentioning

confidence: 71%

“…Early work by (Eichelberger et al, 1986) suggested that silicic magma rises as a highly permeable foam which, upon extrusion, loses gas via development of bubble connectivity, enabling outgassing to the conduit walls (Jaupart & Allegre, 1991). Another body of work describes shear-induced fracturing of magma, leading to permeable pathways through which volatiles can escape (Stasiuk et al, 1996;Gonnerman & Manga, 2003;Okumura et al, 2009;Cabrera et al, 2010;Castro et al, 2012). There is widespread evidence for this non-explosive magma fragmentation, in the form of brittle fracture and healing.…”

Section: Effusive Silicic Volcanismmentioning

confidence: 99%

“…Local increases in melt H 2 O can result from shear-induced bubble collapse and resorption (Yoshimura & Nakamura, 2008;Watkins et al, 2012), and growth of spherulites (Sparks, 1997;Von Aulock et al, 2013 bubble exsolution and outgassing via bubble networks or fractures (Cabrera et al, 2010;Castro et al, 2012), both of which may conversely lead to a local increase in meteoric water content (e.g., Giachetti & Gonnerman, 2013;Von Aulock et al, 2013).…”

Section: Role Of Heterogeneities On Structural Evolution Of An Obsidimentioning

confidence: 99%

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

Shields

Mader

Caricchi

et al. 2016

Journal of Volcanology and Geothermal Research

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Obsidian flow emplacement is a complex and understudied aspect of silicic volcanism. Of particular importance is the question of how highly viscous magma can lose sufficient gas in order to erupt effusively as a lava flow. Using an array of methods we study the extreme textural heterogeneity of the Rocche Rosse obsidian flow in Lipari, a 2 km long, 100 m thick,~800 year old lava flow, with respect to outgassing and emplacement mechanisms. 2D and 3D vesicle analyses and density measurements are used to classify the lava into four textural types: 'glassy' obsidian (b15% vesicles), 'pumiceous' lava (N 40% vesicles), high aspect ratio, 'shear banded' lava (20-40% vesicles) and low aspect ratio, 'frothy' obsidian with 30-60% vesicles. Textural heterogeneity is observed on all scales (m to μm) and occurs as the result of strongly localised strain. Magnetic fabric, described by oblate and prolate susceptibility ellipsoids, records high and variable degrees of shearing throughout the flow. Total water contents are derived using both thermogravimetry and infrared spectroscopy to quantify primary (magmatic) and secondary (meteoric) water. Glass water contents are between 0.08-0.25 wt.%. Water analysis also reveals an increase in water content from glassy obsidian bands towards 'frothy' bands of 0.06-0.08 wt.%, reflecting preferential vesiculation of higher water bands and an extreme sensitivity of obsidian degassing to water content. We present an outgassing model that reconciles textural, volatile and magnetic data to indicate that obsidian is generated from multiple shear-induced outgassing cycles, whereby vesicular magma outgasses and densifies through bubble collapse and fracture healing to form obsidian, which then re-vesiculates to produce 'dry' vesicular magma. Repetition of this cycle throughout magma ascent results in the low water contents of the Rocche Rosse lavas and the final stage in the degassing cycle determines final lava porosity. Heterogeneities in lava rheology (vesicularity, water content, microlite content, viscosity) play a vital role in the structural evolution of an obsidian flow and overprint flow-scale morphology. Post-emplacement hydration also depends heavily on local strain, whereby connectivity of vesicles as a result of shear deformation governs sample rehydration by meteoric water, a process previously correlated to lava vesicularity alone.

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Section: Model For Obsidian Formationmentioning

confidence: 60%

Section: Effect Of Shear On Obsidian Degassing and Outgassingmentioning

confidence: 71%

Section: Effusive Silicic Volcanismmentioning

confidence: 99%

Section: Role Of Heterogeneities On Structural Evolution Of An Obsidimentioning

confidence: 99%

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

Shields

Mader

Caricchi

et al. 2016

Journal of Volcanology and Geothermal Research

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“…Thermal conductivity (k) (W/(m • C)) 1.48 1.5 Lesher and Spera, 2015 Heat capacity at constant pressure (C p ) (J/kg • C) 1,230 1,604 Turcotte and Schubert, 2002;Lesher and Spera, 2015 Initial temperature (T) ( • C) 900 Höskuldsson and Sparks, 1997 Density (ρ) (kg/m 3 ) 2,700 2,300 Höskuldsson and Sparks, 1997 Geothermal gradient ( • C/km) 100 Flóvenz and Saemundsson, 1993 Frontiers in Earth Science | www.frontiersin.org degassing and/or porosity reduction in the Sandfell rhyolite were primarily facilitated by the fracture bands forming, which increased the permeability in the magma and led to rapid crystallization due to undercooling (Hammer and Rutherford, 2002;Cabrera et al, 2011;Castro et al, 2012;Pistone et al, 2013;Shields et al, 2014;Heap and Kennedy, 2016;Kushnir et al, 2017). Several studies have shown that strain localization and fracturing induces local decompression, which causes volatiles to degas/escape along the strain planes (Okumura et al, 2009(Okumura et al, , 2010Caricchi et al, 2011;Castro et al, 2012;Kushnir et al, 2017).…”

Section: Host Rock Liquid Referencesmentioning

confidence: 99%

Syn-Emplacement Fracturing in the Sandfell Laccolith, Eastern Iceland—Implications for Rhyolite Intrusion Growth and Volcanic Hazards

et al. 2018

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Felsic magma commonly pools within shallow mushroom-shaped magmatic intrusions, so-called laccoliths or cryptodomes, which can cause both explosive eruptions and collapse of the volcanic edifice. Deformation during laccolith emplacement is primarily considered to occur in the host rock. However, shallowly emplaced laccoliths (cryptodomes) show extensive internal deformation. While deformation of magma in volcanic conduits is an important process for regulating eruptive behavior, the effects of magma deformation on intrusion emplacement remain largely unexplored. In this study, we investigate the emplacement of the 0.57 km 3 rhyolitic Sandfell laccolith, Iceland, which formed at a depth of 500 m in a single intrusive event. By combining field measurements, 3D modeling, anisotropy of magnetic susceptibility (AMS), microstructural analysis, and FEM modeling we examine deformation in the magma to constrain its influence on intrusion emplacement. Concentric flow bands and S-C fabrics reveal contact-parallel magma flow during the initial stages of laccolith inflation. The magma flow fabric is overprinted by strain-localization bands (SLBs) and more than one third of the volume of the Sandfell laccolith displays concentric intensely fractured layers. A dominantly oblate magmatic fabric in the fractured areas and conjugate geometry of SLBs, and fractures in the fracture layers demonstrate that the magma was deformed by intrusive stresses. This implies that a large volume of magma became viscously stalled and was unable to flow during intrusion. Fine-grained groundmass and vesicle-poor rock adjacent to the fracture layers point to that the interaction between the SLBs and the flow bands at sub-solidus state caused the brittle-failure and triggered decompression degassing and crystallization, which led to rapid viscosity increase in the magma. The extent of syn-emplacement fracturing in the Sandfell laccolith further shows that strain-induced degassing limited the amount of eruptible magma by essentially solidifying the rim of the magma body. Our observations indicate that syn-emplacement changes in rheology, and the associated fracturing of intruding magma not only occur in volcanic conduits, but also play a major role in the emplacement of viscous magma intrusions in the upper kilometer of the crust.

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Volcanic sintering: Timescales of viscous densification and strength recovery

Vasseur

Wadsworth

Lavallée

et al. 2013

Geophysical Research Letters

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[1] Sintering and densification are ubiquitous processes influencing the emplacement of both effusive and explosive products of volcanic eruptions. Here we sinter ash-size fragments of a synthetic National Institute of Standards and Technology viscosity standard glass at temperatures at which the resultant melt has a viscosity of ∼108–109 Pa.s at 1bar to assess sintering dynamics under near-surface volcanic conditions. We track the strength recovery via uniaxial compressive tests. We observe that volcanic ash sintering is dominantly time dependent, temperature dependent, and grain size dependent and may thus be interpreted to be controlled by melt viscosity and surface tension. Sintering evolves from particle agglutination to viscous pore collapse and is accompanied by a reduction in connected porosity and an increase in isolated pores. Sintering and densification result in a nonlinear increase in strength. Micromechanical modeling shows that the pore-emanated crack model explains the strength of porous lava as a function of pore fraction and size.

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Melt fracturing and healing: A mechanism for degassing and origin of silicic obsidian

Cited by 63 publications

References 27 publications

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

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

Syn-Emplacement Fracturing in the Sandfell Laccolith, Eastern Iceland—Implications for Rhyolite Intrusion Growth and Volcanic Hazards

Volcanic sintering: Timescales of viscous densification and strength recovery

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