Pre-eruptive volatile content, degassing paths and depressurisation explaining the transition in style at the subglacial rhyolitic eruption of Dalakvísl, South Iceland

Owen, Jacqueline; Tuffen, Hugh; McGarvie, David W.

doi:10.1016/j.jvolgeores.2013.03.021

Cited by 9 publications

(27 citation statements)

References 82 publications

(165 reference statements)

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“…The latter equation, when combined with a model of melt viscosity as a function of temperature and H2O concentration, can be used to approximate vesicle growth rates at a range of different water concentrations. We acknowledge that the residual water concentration will decrease during vesicle growth, due to diffusive transfer of H2O from melt to vapour, as examined in variably-vesiculated Icelandic obsidian (Owen et al, 2013). However, at the low confining pressure within an ejected bomb (1 atm), this decrease is negligible, even when high vesicularity has been reached.…”

Section: Relationship With Natural Bomb Textures At Chaiténmentioning

confidence: 98%

Post-fragmentation vesiculation timescales in hydrous rhyolitic bombs from Chaitén volcano

Browning

Tuffen

James

et al. 2020

Journal of South American Earth Sciences

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Highlights:• Suite of high temperature bubble growth experiments performed on a rhyolitic bomb from the 2008 eruption of Chaitén volcano • Hot-stage microscopy allowed the tracing of in-situ bubble growth at different temperatures • Experimentally derived growth rates placed in context of a cooling volcanic bomb to determine the amount of post-fragmentation vesiculation • Our model recreates textures observed in metre scale volcanic bombs ABSTRACT Bubble nucleation and growth dynamics exert a primary control on the explosivity of volcanic eruptions. Numerous theoretical and experimental studies aim to capture the complex process of melt vesiculation, whereas textural studies use vesicle populations to reconstruct magma behaviour. However, post-fragmentation vesiculation in rhyolitic bombs can create final quenched bubble (vesicle) textures that are not representative of the nature of fragmenting magma within the conduit. To examine bubble growth in hydrous rhyolitic bombs, we have used heated stage microscopy to directly observe vesiculation of a Chaitén rhyolite melt (with an initial dissolved water content of ~0.95 wt. %) at atmospheric pressure and magmatic temperatures upon reheating.Thin wafers of obsidian were held from five minutes up to two days in the heated stage at temperatures between 575 °C and 875 °C. We found that bubble growth rates, measured through changes in bubble diameter, increased with both temperature and bubble size. The average growth rate at the highest temperature of 875 °C is ~1.27 µm s -1 , which is substantially faster than the lowest detected growth rate of ~0.02 µm s -1 at 725 °C; below this temperature no growth was observed. Average growth rate Vr follows an exponential relationship with temperature, T and inferred melt viscosity h, where Vr = 5.57×10 -7 e 0.016 and Vr = 3270.26e -1.117h . Several stages of evolving bubble morphology were directly observed, including initial relaxation of deformed bubbles into spheres, extensive growth of spheres, and, at higher temperatures, close packing and foam formation. Bubble deformation due to bubble-bubble interaction and coalescence was observed in most experiments. We use our simple, experimentally-determined relationship between melt viscosity and bubble growth rates to model post-fragmentation vesicle growth in a cooling 1 m-diameter rhyolitic bomb. The results, which indicate negligible vesicle growth within 2-3 cm of the bomb surface, correspond well with the observed dense margin thickness of a Chaitén bomb of comparable dimensions. The experiments described can be used to effectively reconstruct the post-fragmentation vesiculation history of bombs through simple analytical expressions which provide a useful tool for aiding in the interpretation of pumiceous endmember textures in hydrous rhyolitic bombs.

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Section: Relationship With Natural Bomb Textures At Chaiténmentioning

confidence: 98%

Post-fragmentation vesiculation timescales in hydrous rhyolitic bombs from Chaitén volcano

Browning

Tuffen

James

et al. 2020

Journal of South American Earth Sciences

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“…This equates to approximately 65 m of ice, 60 m of water, 20 m of solid rock or 10 m of 50% vesicular basalt, assuming densities of 917, 1,000 and 2,770 kg m -3 for ice, water and K1918 basalt respectively, with errors of ~20%. However, natural samples often experience loading by a combination of materials (Tuffen et al, 2010;Owen et al, 2012;Owen et al, 2013b;Owen, 2016;Owen et al, in press). There is strong consistency between the EPMA data gathered at the University of Hawaii, that gathered at the University of Edinburgh and that already published for Katla ( Fig.…”

Section: = ℎmentioning

confidence: 99%

“…In silicic systems, permeable pathways in magma (whether connected bubbles or tuffisite veins) often serve to enhance magma degassing and outgassing, and may contribute to a transition towards more effusive activity (Jaupart and Allègre, 1991;Jaupart, 1998). Permeable outgassing is also thought to occur during subglacial volcanism and contribute towards transitions in eruptive behaviour (Owen et al, 2013a;Owen et al, 2013b;Owen, 2016). Whilst the K1918 samples show plenty of visual evidence for permeable pathways (Figs.…”

Section: Links With Eruptive Behaviourmentioning

confidence: 99%

“…where the volatiles remain trapped within the magma) (Anderson and Fink, 1989;Hoblitt and Harmon, 1993;Anderson et al, 1995;Martel et al, 1998;Villemant and Boudon, 1998;Villemant and Boudon, 1999;Villemant et al, 2003;Adams et al, 2006;Villemant et al, 2008;Humphreys et al, 2009;Owen et al, 2013a). Transitions to effusive volcanism are often characterized by a shift towards open system degassing behaviour, where volatiles are able to efficiently outgas from the magma (Jaupart and Allègre, 1991;Jaupart, 1998;Namiki and Manga, 2008;Owen et al, 2013b).…”

Section: Introductionmentioning

confidence: 99%

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Basalt, Unveiling Fluid-filled Fractures, Inducing Sediment Intra-void Transport, Ephemerally: Examples from Katla 1918

Owen

Shea

Tuffen

2019

Journal of Volcanology and Geothermal Research

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This article documents textures within Katla 1918 pyroclasts, where particle-filled fractures and bubbles have been observed. These features are analogous to tuffisite veins; particle-filled fractures which represent the preserved remains of transient degassing pathways in shallow conduits. Such fractures have long been considered restricted to high viscosity silicic melts. However, through BSE images and compositional maps, we have identified similar tuffisite-like features in crystal-poor basalt pyroclasts from the 1918 A.D. subglacial eruption of Katla, Iceland (K1918). Clast textures record transient mobility of juvenile/lithic particles, melt droplets and gas through magmatic fractures and connected vesicles. Key evidence includes (1) the presence of variably sintered fine-ash particles within variably healed fractures and vesicles (present in >80% of clasts analysed), (2) compositional maps that reveal the presence of foreign particles within preserved and healed permeable pathways, and (3) lower vesicularities immediately surrounding 'fracture' walls, suggestive of diffusive volatile loss into a permeable network. The 1918 A.D. eruption of Katla occurred under a thick glacier, however the ice was quickly breached, owing partly to the explosive nature of the eruption. We propose that the formation and preservation of these transient permeable networks have been facilitated by rapid decompression of a relatively volatile-rich magma, in a confined subglacial environment, with combined magmatic and phreatomagmatic fragmentation, followed by rapid quenching by meltwater. Tuffisite veins in rhyolite demonstrate repeated fracture-healing cycles, which drive incremental release of overpressured gas and help to defuse explosive eruptions. Interestingly, the permeable network at Katla failed to defuse the 1918 A.D. eruption, which involved a particularly violent subglacial eruptive phase. It is unclear whether this demonstrates an inability of mafic tuffisite-like features to efficiently degas magma (perhaps owing to the especially transient nature of permeable pathways in low viscosity magmas) or an ability to enhance fragmentation by providing infiltration pathways for external water. The latter scenario may explain the rapid melting of the overlying glacier as the large surface area-to-volume ratio of fractured magma would allow rapid heat transfer. Nevertheless, we document a previously undescribed texture in basaltic magmas. It is intriguing why it has not, to the best of our knowledge, been documented elsewhere. Have these permeable pathways been overlooked in the past (e.g. mistaken for bad sample preparation or not noticed without high magnification BSE images) and are in fact a widespread phenomenon in subglacial (and other?) basalts; or do our samples in fact represent a rarely preserved texture? Either way, they offer a new insight into the degassing and fragmentation of subglacial basalt.

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“…A ring of tuyas was emplaced during the eruption, confined by an ice mass >400 m thick (Tuffen et al, 2002;McGarvie et al, 2006). Explosive activity formed steepsided tephra piles before the effusive emplacement of intrusions and lava caps (Tuffen et al, 2002(Tuffen et al, , 2008Owen et al, 2013b).…”

Section: Introductionmentioning

confidence: 99%

Widespread tephra dispersal and ignimbrite emplacement from a subglacial volcano (Torfajökull, Iceland)

Moles

McGarvie

Stevenson

et al. 2019

Geology

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The tephra dispersal mechanisms of rhyolitic glaciovolcanic eruptions are little known, but can be investigated through the correlation of eruptive products across multiple depositional settings. Using geochemistry and geochronology, we correlate a regionally important Pleistocene tephra horizon-the rhyolitic component of North Atlantic Ash Zone II (II-RHY-1)-and the Thórsmörk Ignimbrite with rhyolitic tuyas at Torfajökull volcano, Iceland. The eruption breached an ice mass >400 m thick, leading to the widespread dispersal of II-RHY-1 across the North Atlantic and the Greenland ice sheet. Locally, pyroclastic density currents traveled across the ice surface, depositing the variably welded Thórsmörk Ignimbrite beyond the ice margin and ~30 km from source. The widely dispersed products of this eruption represent a valuable isochronous tie line between terrestrial, marine, and ice-core paleoenvironmental records. Using the tephra horizon, estimates of ice thickness and extent derived from the eruption deposits can be directly linked to the regional climate archive, which records the eruption at the onset of Greenland Stadial 15.2.

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Pre-eruptive volatile content, degassing paths and depressurisation explaining the transition in style at the subglacial rhyolitic eruption of Dalakvísl, South Iceland

Cited by 9 publications

References 82 publications

Post-fragmentation vesiculation timescales in hydrous rhyolitic bombs from Chaitén volcano

Post-fragmentation vesiculation timescales in hydrous rhyolitic bombs from Chaitén volcano

Basalt, Unveiling Fluid-filled Fractures, Inducing Sediment Intra-void Transport, Ephemerally: Examples from Katla 1918

Widespread tephra dispersal and ignimbrite emplacement from a subglacial volcano (Torfajökull, Iceland)

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