Characterization of juvenile pyroclasts from the Kos Plateau Tuff (Aegean Arc): insights into the eruptive dynamics of a large rhyolitic eruption

Article Article scientifique Article Characterization of juvenile pyroclasts from the Kos Plateau Tuff (Aegean Arc): insights into the eruptive dynamics of a large rhyolitic eruption eng 2009 author Bouvet De Maisonneuve Caroline 552167 author Bachmann Olivier 58577 author Burgisser Alain 89047 643-658 10.1007/s00445-008-0250-x 0258-8900 Silicic pumices formed during explosive volcanic eruptions are faithful recorders of the state of the magma in the conduit, close to or at the fragmentation level. We have characterized four types of pumices from the non-welded rhyolitic Kos Plateau Tuff, which erupted 161,000 years ago in the East Aegean Arc, Greece. The dominant type of pumice (>90 vol.%) shows highly elongated tubular vesicles. These tube pumices occur throughout the eruption. Less common pumice types include: (1) “frothy” pumice (highly porous with large, sub-rounded vesicles), which form 5–10 vol.% of the coarsest pyroclastic flow deposits, (2) dominantly “microvesicular” and systematically crystal-poor pumices, which are found in early erupted, fine-grained pyroclastic flow units, and are characterized by many small (<50 μm in diameter) vesicles and few mm-sized, irregular voids, (3) grey or banded pumices, indicating the interaction between the rhyolite and a more mafic magma, which are found throughout the eruption sequence and display highly irregular bubble shapes. Except for the grey-banded pumices, all three other types are compositionally identical and were generated synchronously as they are found in the same pyroclastic units. They, therefore, record different conditions in the volcanic conduit leading to variable bubble nucleation, growth and coalescence. A total of 74 pumice samples have been characterized using thin section observation, SEM imagery, porosimetry, and permeametry. We show that the four pumice types have distinct total and connected porosity, tortuosity and permeability. Grey-banded pumices show large variations in petrophysical characteristics as a response to mingling of two different magmas. The microvesicular, crystal-poor, pumices have a bimodal bubble size distribution, interpreted as reflecting an early heterogeneous bubble nucleation event followed by homogeneous bubble nucleation close to fragmentation. Finally, the significant differences in porosity, tortuosity and permeability in compositionally identical tube and frothy pumices are the result of variable shear rates in different parts of the conduit. Differential shear rates may be the result of either: (1) pure shear, inducing a vertical progression from frothy to tube and implying a relatively thick fragmentation zone to produce both types of pumices at the same time or (2) localized simple shear, inducing strongly tubular vesicles along the wall and near-spherical bubbles in the centre of the conduit and not necessarily requiring a thick fragmentation zone. Dewey 550 Open Access - Licence nationale Springer 29 Section des sciences de la Terre et de l'Environnement 15 false Bulletin of Volcanology 71 6 a527c16c-fef7-4994-a0a8-1e40fa4426eb Article (Published version) Public BouvetDeMaisonneuve2008_Article_CharacterizationOfJuvenilePyro.pdf 852387 application/pdf openAccess Faculté des sciences / Section des sciences de la Terre et de l'Environnement

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Characterization of juvenile pyroclasts from the Kos Plateau Tuff (Aegean Arc): insights into the eruptive dynamics of a large rhyolitic eruption	Bulletin of Volcanology			2009	384	312

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Burgisser, Alain