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Magma fragmentation and juvenile pyroclast shapes, with a focus on lava fountains / Fragmentation magmatique et forme des pyroclastes juvéniles, avec un focus sur les fontaines de lave.

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Comida, Pier Paolo (2021). Magma fragmentation and juvenile pyroclast shapes, with a focus on lava fountains / Fragmentation magmatique et forme des pyroclastes juvéniles, avec un focus sur les fontaines de lave. Thèse. Québec, Doctorat en sciences de la terre, Université du Québec, Institut national de la recherche scientifique, 288 p.

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Résumé

Explosive fragmentation of ultramafic and mafic magmas can be magmatic or phreatomagmatic, and each mode can result in different eruption styles and form different juvenile pyroclasts. Strombolian pulses and lava fountains are a common example of magmatic fragmentation in basalts, driven by magma decompression and vesiculation. When magma meets external water, rapid magma cooling and the violent conversion of water into steam leads to phreatomagmatic eruptions, such as those forming maar-diatremes. Beyond the eruption style, fragmentation mechanisms can be either hydrodynamic or brittle. The former consists in the ductile breakup of magma into smaller liquid drops and filaments, like the achneliths observed from lava fountain deposits. Brittle fragmentation is caused by very high stresses, resulting in juvenile pyroclasts with specific external shapes, internal textures and surface features. Linking juvenile pyroclast characteristics and primary magma fragmentation requires a standardized methodology of analysis. In this thesis, magmatic and phreatomagmatic pyroclasts from the 1977 eruption of Ukinrek volcano (Alaska) were used to select the best size fraction (coarse ash from 0.71 to 0.5 mm) for the analysis of juvenile pyroclasts in comparative studies of magma fragmentation. Moreover, it was found that SEM image acquisition for this fraction should be performed for at least 50 juvenile grains, each with a pixel density of at least 20 000 pixels per particle. The software PASTA was developed to facilitate image processing and measurement of morphometric and textural parameters. The chosen coarse ash size fraction allows representative quantification of 2D vesicularity and crystallinity in cross-sections without significant post fragmentation changes. A very fine ash fraction (88–63 µm), on the other hand, is ideal for morphometric measurements and crucial for the study of surface characteristics, but special care must be taken during sample preparation and juvenile grains have to be confirmed using a dual mounting technique. Hydrodynamic magma fragmentation was investigated through laboratory experiments involving remelted volcanic rocks, at the Physikalisch Vulkanologisches Labor (PVL) in Germany. About 200–250 g of magma at 1200°C was generated within 1 hour, using an induction furnace. Magma fragmentation was triggered through the injection of compressed argon at 3 MPa or 10 MPa, and recorded by high-speed cameras between 2000 and 5000 frames per second. The three volcanic materials used in the experiments are an olivine-melilite (Hohenstoffeln, ~ 38% SiO₂), an alkali basalt (Billstein, ~ 45% SiO₂), and a basaltic-trachyandesite (Sommata, ~ 54% SiO₂). All magmas produced glassy, bubble- and crystal-poor, fluidal juvenile pyroclasts. The experimental pyroclasts were compared to natural juvenile pyroclasts from the 1959 eruption of Kīlauea Iki (~ 49% SiO₂) in Hawaii, which consist of glassy fluid particles, scoria and golden pumice. Juvenile pyroclasts from experiments and lava fountains were analyzed using the steps in the standardized methodology. Hydrodynamic fragmentation of magmas in the experiments is similar to the breakup of industrial liquid jets. Different breakup regimes are controlled by the balance of magma viscosity and surface tension, the ejection velocity and external aerodynamic effects. On one hand, ultramafic magmas of low viscosity ejected at high velocity (Hohenstoffeln 10 MPa) form a spray (regime IV), dominated by surface tension and aerodynamic forces. In contrast, intermediate magmas expelled at lower velocities (Sommata 3 MPa) are barely fragmented at the crucible exit (regime II), as higher viscosity causes stretching into multiple filaments before fragmentation. Similar regimes are observed in natural lava fountains. A modified Ohnesorge diagram (Reynolds number versus Weber number) has the potential to distinguish different breakup regimes, and future work is advised to improve this diagram. Hydrodynamic magma fragmentation in the experiments generated a panoply of fluidal juvenile pyroclasts. For coarse ash-sized particles, ultramafic magma ejected at the lowest velocity (Hohenstoffeln 3 MPa) mainly formed spheres (~ 50%), with high values of axial ratio and solidity (> 0.7 and > 0.9 respectively). Efficient hydrodynamic fragmentation was dominated by surface tension and rapid contraction (< 1 ms). In contrast, the intermediate magma ejected at high velocity (Sommata 10 MPa) primarily formed Pele’s hairs, with lower values of axial ratio and solidity (0.4 and 0.7, respectively). Final fluidal morphologies result from the competition between viscosity and surface tension, ultimately preserved by the fast cooling process. In ultramafic magmas, stretching is minor and relaxation times are short, resulting in compact morphologies. For intermediate magmas, the higher viscosity and stretching prevents contraction, allowing elongated shapes to be maintained. These fluidal particles are morphologically similar to achneliths generated during the 1959 eruption of Kīlauea Iki.

Type de document: Thèse Thèse
Directeur de mémoire/thèse: Ross, Pierre-Simon
Mots-clés libres: primary magma fragmentation; juvenile pyroclasts; fluidal; lava fountains; experiments; morphology; internal texture; standardized methodology
Centre: Centre Eau Terre Environnement
Date de dépôt: 04 mars 2022 16:40
Dernière modification: 04 mars 2022 16:40
URI: https://espace.inrs.ca/id/eprint/12480

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