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| Titel |
Crystallinity-vesicularity interrelation in silicic pyroclasts - Neutron and X-Ray Computed Tomography constraints on magma permeability |
| VerfasserIn |
S. Wiesmaier, B. Scheu, K.-U. Hess, B. Schillinger, A. Flaws, D. B. Dingwell |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250066276
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| Zusammenfassung |
| The permeability of magma controls gas escape during magma ascent and thus may control
eruption behaviour, varying from quiet degassing to explosive fragmentation (Mueller et al.,
2008). Yet, the spatial distribution of connected vs. isolated vesicle structures in magma
remains poorly constrained. Additionally, the crystal distribution may influence magma
permeability: a) do fractures in crystals provide additional pathways to melt-based volatile
migration? and b) do low surface-tension crystal faces catalyse bubble nucleation and
growth?
In felsic pyroclasts, the size, shape and interconnectivity of vesicles and phenocrysts have
been quantified by 3D tomography. We applied high resolution neutron computed
tomography (NCT) at 20 μm and X-ray Computed Tomography (XCT) at 5–10 μm
resolution on large samples of 15–50 cm3 to investigate the 3D structure of vesicular (Φ =
0.45–0.72), silica-rich pyroclastic material from various explosive eruptions. Samples are of
the 2004 vulcanian and the 1783 plinian eruption of Asama (Japan), the 1997 eruption of
Soufrière Hills Volcano (Montserrat) and the June 1991 vulcanian event of Unzen
(Japan).
Volume reconstructions of the pore space and different crystal phases were calculated
with Tomoview, our custom-made software. The reconstructed volumes showed an
interrelation between vesicle and crystal distribution. Differential overlapping of crystal and
vesicle subvolumes trace the crystal outlines exceptionally well. Furthermore, Tomoview
detected connected pathways that frequently exploited inter-fracture space of fragmented
crystals. Crystal fragmentation thus appears to provide an additional mechanism for
generating pore space. The evolution of a permeable network may thus be affected by the
crystal content, which ultimately biases the eruptive behaviour of silicic magma. |
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