HR: 0800h
AN: V41B-0600    [Abstracts]
TI: Non-volatile vs Volatile Behaviours of Halogens During the 79 AD Plinian Eruption of Vesuvius, Italy
AU: Boudon, G
EM: boudon@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, CNRS - Equipe Geologie des systemes volcaniques, 4 place Jussieu, 46-00 3eme etage, cc 89, Paris, 75005, France
AU: * Balcone, h
EM: balcone@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, CNRS - Equipe Geologie des systemes volcaniques, 4 place Jussieu, 46-00 3eme etage, cc 89, Paris, 75005, France
AU: villemant, b
EM: villema@ccr.jussieu.fr
AF: Institut de Physique du Globe de Paris, CNRS - Equipe Geologie des systemes volcaniques, 4 place Jussieu, 46-00 3eme etage, cc 89, Paris, 75005, France
AU: villemant, b
EM: villema@ccr.jussieu.fr
AF: Université P.& M. Curie, 4 place Jussieu, 46-00 3eme etage, cc 89, paris, 75005, France
AU: Michel, A
EM: michel@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, CNRS - Equipe Geologie des systemes volcaniques, 4 place Jussieu, 46-00 3eme etage, cc 89, Paris, 75005, France
AB: M^te Somma-Vesuvius is one of the best studied active volcanoes on Earth primarily because of the variety of eruptive style and the related risk assessment in a densely populated region (700 000 to 3 millions people including Napoli). Determination of the pre-eruptive conditions and degassing behaviour during eruption of volatile species is fundamental for establishing the dynamics of the different eruption types. F and Cl behaviours during the 79AD plinian eruption were investigated through a detailed electron microprobe study of melt inclusions and matrix glass of pumice clasts from a complete sequence of the pumice-fallout deposits. These results highlight two different pre-eruptive states of the erupted magmas during the plinian phase. The white pumice (WP) magma, first emitted, and the upper part of the grey pumice (GP) magma were saturated relative to subcritical fluids (a Cl-rich H₂O vapour phase and a brine) which buffered the Cl melt content at ~ 5300 ppm. Conversely, the major part of the GP magmas was not H₂O-saturated and thus the Cl melt content was only controlled by the magma differentiation degree. These results and Cl solubility data for K- phonolitic melts allow to estimate the WP magma chamber was at a depth of ~ 7.5km and had a low vertical extent (<500 m). The GP magma reservoir was located just below but its vertical extent cannot be constrained. H₂O and halogen degassing paths of silicic H₂O-rich magmas during explosive plinian may be modelled using the residual volatile content of the melt and the vesicularity of representative pumice clasts. During the plinian phase of the 79AD eruption H₂O is efficiently degassed, following different degassing modes, contrary to F and Cl which remain trapped in the melts. These new data demonstrate that the 79AD magma reservoir was only partially saturated in H₂O (and Cl) prior eruption and provide new insights into the F and Cl behaviours in phonolitic melts. In H₂O undersaturated phonolitic melts, both F and Cl behave as incompatible and non volatile elements. In H₂O- saturated conditions Cl significantly partitions into the fluid phase. However Cl low diffusivity in K₂0-rich phonolitic melts may limit its extraction during rapid H₂O degassing as in plinian eruptions. Keywords: Vesuvius; 79AD eruption; plinian eruption; halogen behaviour; magma degassing; magma differentiation.
DE: 8409 Atmospheric effects (0370)
DE: 8410 Geochemical modeling (1009, 3610)
DE: 8428 Explosive volcanism
DE: 8430 Volcanic gases
DE: 8488 Volcanic hazards and risks
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting