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| Titel |
Integration of porosity, connectivity and permeability measurements to determine syn-eruptive degassing processes during a sub-plinian basaltic eruption |
| VerfasserIn |
Simone Jordan, Lucia Gurioli, Matthieu Colombier, Jean-Luc Le Pennec, Olivier Roche |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250101239
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| Publikation (Nr.) |
 EGU/EGU2015-345.pdf |
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| Zusammenfassung |
| Degassing of the volatile phases is considered to have a major control on the eruption
dynamics, particularly in controlling shifts between explosive and extrusive eruption styles.
The sub-plinian eruption of the basaltic monogenetic La Vache and Lassolas cone complex in
the Chaîne des Puys, France, about 8600 years ago, was an unusual large event that raises the
question of the processes that controlled the explosivity of non-differentiated magma and the
evolution of this sub-plinian event. This study combines the results of density, porosity,
connectivity and permeability measurements of juvenile clasts to determine the state of
vesiculation and the presence of open degassing pathways within the melt prior to
fragmentation. The volume of connected vesicles is measured using a Helium-Pycnometer,
while permeability measurements are conducted using a permeameter recently
built at the Laboratoire Magma et Volcans, following Takeuchi et al. (2008). The
permeameter has broad measurement ranges of pressure difference (101-105 Pa) and
gas-flow rate (10-9-10-5 m3/s). These ranges enable us to measure viscous (Darcian)
permeability in the range of 10-17-10-9 m2 for 1 centimetre-scale samples (such as scoria
clasts) using the Forchheimer equation (Rust and Cashman, 2004) that accounts for
inertial effects caused by non-laminar flow at high gas flow rates. This technique
is a relatively new approach to determine the permeability of quenched samples.
The integration of porosity and connectivity measurements provides information
about the percentage of connected and isolated vesicles, with the connected vesicles
forming potential degassing pathways. Our results show that the permeability and the
vesicularity of the La Vache and Lassolas pyroclasts correlate very well, defining a
trend that is also shown by the permeability data derived from the literature for the
Cascades (Saar and Manga, 1999) and the Ambrym volcano (Polacci et al., 2012). The
connectivity data of the samples, however, show that the vesicles in each clast are fully
connected to each other, while the permeability of the clasts varies by two orders
(10-11-10-13m2). Textural analysis of the different vesicle networks will give better insights
into the cause of this discrepancy. These interesting results indicate the importance
of both connectivity and permeability measurements and textural analysis for the
evaluation of the presence of a vesicle network and the potential for degassing prior to
fragmentation.
References:
Polacci, M., Baker, D.R., La Rue, A., Mancini, L., Allard, P., 2012. Degassing behaviour
of vesiculated basaltic magmas: an example from Ambrym volcano, Vanuatu Arc. J.
Volcanol. Geotherm. Res., 233-234, 55-64.
Rust, A.C., Cashman, K.V., 2004. Permeability of vesicular silicic magma: inertial and
hysteresis effects. Earth Planet. Sci. Lett., 228, 93-107.
Saar, M.O., Manga, M., 1999. Permeability-porosity relationship in vesicular basalts.
Geophs. Res. Lett., 26, 111-114.
Takeuchi, S., Nakashima, S., Tomiya, A., 2008. Permeability measurements of
natural and experimental volcanic materials with a simple permeameter: Toward an
understanding of magmatic degassing processes. J. Volcanol. Geotherm. Res., 177, 329-339. |
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