Magma degassing during subglacial eruptions and its use to reconstruct palaeo-ice thicknesses

Tuffen, Hugh; Owen, Jacqueline; Denton, Joanna S.

doi:10.1016/j.earscirev.2010.01.001

Cited by 57 publications

(66 citation statements)

References 83 publications

(157 reference statements)

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“…We have shown that degassing occurs through fracturing in rhyolitic melts, supporting suggestions that repeated fracturing and healing (RFH) assists degassing, suppressing vesiculation and favoring obsidian formation (Gonnermann and Manga, 2005;Rust and Cashman, 2007;Rust et al, 2004;Tuffen et al 2010).…”

Section: Autobrecciation and Melt Degassing: Obsidian Formationmentioning

confidence: 60%

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

Cabrera

Weinberg

Wright

et al. 2010

Geology

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We present water content transects across a healed fault in pyroclastic obsidian from Lami pumice cone, Lipari, Italy, using synchrotron Fourier transform infrared spectroscopy.Results indicate that rhyolite melt degassed through the fault surface. Transects define a trough of low water content coincident with the fault trace, surrounded on either side by high water content plateaux. Plateaux indicate that obsidian on either side of the fault equilibrated at

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Section: Autobrecciation and Melt Degassing: Obsidian Formationmentioning

confidence: 60%

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

Cabrera

Weinberg

Wright

et al. 2010

Geology

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“…During subglacial eruptions, rapid quenching restricts the exsolution of moderately soluble volatile species such as H 2 O and S, resulting in subglacially erupted tephras retaining higher volatile contents than their subaerial counterparts (Moore and Calk 1991;Óladottir et al 2007;Tuffen et al 2010). The most evolved melt inclusions from the Hvítárvatn tephra layer contain 0.60 ± 0.04(1σ) wt% H 2 O and provide the best estimate of immediately pre-eruptive H 2 O content.…”

Section: Inclusion-hosted Bubblesmentioning

confidence: 99%

The evolution and storage of primitive melts in the Eastern Volcanic Zone of Iceland: the 10 ka Grímsvötn tephra series (i.e. the Saksunarvatn ash)

Neave

Maclennan

Thordarson

et al. 2015

Contrib Mineral Petrol

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evolved assemblage is close to being in equilibrium with the matrix glass, trace element disequilibrium between primitive and evolved assemblages indicates that they were derived from different distributions of mantle melt compositions. Juxtaposition of disequilibrium assemblages probably occurred during disaggregation of incompatible trace element-depleted mushes (mean La/Yb melt = 2.1) into aphyric and incompatible trace element-enriched liquids (La/Yb melt = 3.6) shortly before the growth of the evolved macrocryst assemblage. Post-entrapment modification of plagioclase-hosted melt inclusions has been minimal and high-Mg# inclusions record differentiation and mixing of compositionally variable mantle melts that are amongst the most primitive liquids known from the EVZ. Coupled highfield strength element (HFSE) depletion and incompatible trace element enrichment in a subset of primitive plagioclase-hosted melt inclusions can be accounted for by inclusion formation following plagioclase dissolution driven by interaction with plagioclase-undersaturated melts. Thermobarometric calculations indicate that final crystal-melt equilibration within the evolved assemblage occurred at ~1140 °C and 0.0-1.5 kbar. Considering the large volume of the erupted tephra and textural evidence for rapid crystallisation of the evolved assemblage, 0.0-1.5 kbar is considered unlikely to represent a pressure of long-term magma accumulation and storage. Multiple thermometers indicate that the primitive assemblage crystallised at high temperatures of 1240-1300 °C. Different barometers, however, return markedly different crystallisation depth estimates. Raw clinopyroxene-melt pressures of 5.5-7.5 kbar conflict with apparent melt inclusion entrapment pressures of 1.4 kbar. After applying a correction derived from published experimental data, clinopyroxene-melt equilibria return mid-crustal pressures of 4 ± 1.5 kbar, which are consistent with pressures estimated from the major element Abstract Major, trace and volatile elements were measured in a suite of primitive macrocrysts and melt inclusions from the thickest layer of the 10 ka Grímsvötn tephra series (i.e. Saksunarvatn ash) at Lake Hvítárvatn in central Iceland. In the absence of primitive tholeiitic eruptions (MgO > 7 wt%) within the Eastern Volcanic Zone (EVZ) of Iceland, these crystal and inclusion compositions provide an important insight into magmatic processes in this volcanically productive region. Matrix glass compositions show strong similarities with glass compositions from the AD 1783-1784 Laki eruption, confirming the affinity of the tephra series with the Grímsvötn volcanic system. content of primitive melt inclusions. Long-term storage of primitive magmas in the mid-crust implies that low CO 2 concentrations measured in primitive plagioclase-hosted inclusions (262-800 ppm) result from post-entrapment CO 2 loss during transport through the shallow crust. In order to reconstruct basaltic plumbing system geometries from petrological data with greater confidence, mineral-mel...

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“…Magmatic glasses typically contain both primary (magmatic) and secondary (meteoric) water (Newman et al, 1986;Tuffen et al, 2010;Denton et al, 2012), the latter recording hydration by external water, which diffused into the glass post eruption, plus water adsorbed during sample preparation (Giachetti et al, 2015). It has been assumed previously that, at the low water concentrations characteristic of obsidian (b 1 wt.%), water dissolves in the glass predominantly as hydroxyl groups OH − , as a result of equilibrium speciation (Dixon et al, 1995;Zhang, 1999).…”

Section: Volatile Content Measurementsmentioning

confidence: 99%

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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Magma degassing during subglacial eruptions and its use to reconstruct palaeo-ice thicknesses

Cited by 57 publications

References 83 publications

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

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

The evolution and storage of primitive melts in the Eastern Volcanic Zone of Iceland: the 10 ka Grímsvötn tephra series (i.e. the Saksunarvatn ash)

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

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