HR: 0800h
AN: V41B-0602    [Abstracts]
TI: F and Cl Diffusion in Phonolitic Melts
AU: * Balcone, H
EM: balcone@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, CNRS - Equipe Geologie des systemes volcaniques, 4 pl. Jussieu, 46-00, 3ème étage, cc 89, Paris, 75005, France
AU: Baker, D
EM: donb@eps.mcgill.ca
AF: Earth and Planetary Sciences, McGill University, 3450 University Street, Rm 238, Montreal (Quebec), H3A2A7, Canada
AU: Villemant, B
EM: villema@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, CNRS - Equipe Geologie des systemes volcaniques, 4 pl. Jussieu, 46-00, 3ème étage, cc 89, Paris, 75005, France
AU: Boudon, G
EM: boudon@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, CNRS - Equipe Geologie des systemes volcaniques, 4 pl. Jussieu, 46-00, 3ème étage, cc 89, Paris, 75005, France
AB: Halogens discharged into the atmosphere from magma degassing are highly variable from one volcano to another. The impact of their degassing on Earth's climate and their health hazards will mainly depend on initial volatile contents, eruptive style and degassing kinetics. In this study, fluorine and chlorine diffusion were measured in two natural phonolitic melts, one from Vesuvius and the other from Laacher See, at 0.5 and 1.0 GPa, between 1250 and 1450 ° C at anhydrous conditions and with about 2 and 5wt% of dissolved water. The two different starting materials allow us to investigate the alkali effect, Na vs. K, on halogen diffusion. One composition was a K₂O-rich (~10wt%) phonolitic melt corresponding to the white pumice phase of the 79AD eruption of Vesuvius, and the other a Na₂O-rich (~10wt%) phonolitic melt corresponding to most differentiated melt of the Laacher See (12 000 BC). The diffusion-couple technique in a piston cylinder was used. Experiments were performed both with only one halogen diffusing and with a mixture of halogens (F, Cl) diffusing in order to evaluate the interactions between the halogens during diffusion. Diffusion coefficients for fluorine range between 4*10^-11 m²/s at 1250 ° C and 8*10^-11 m²/s at 1450 ° C for the Na-rich melt and between 1*10^-11 m²/s at 1250 ° C and 8*10^{- 11} m²/s at 1450 ° C for the K-rich melt at anhydrous conditions. Diffusion coefficients for chlorine range between 2*10^-12 m²/s at 1250 ° C and 1*10^-11 m²/s at 1450 ° C for the Na-rich melt and between 9*10^-11 m²/s at 1250 ° C C and 7*10^-11 m²/s at 1450 ° C for the K-rich melt at anhydrous conditions. Fluorine diffusivity is typically higher than chlorine in the Na-rich phonolitic melt by one order of magnitude, whereas in the K-rich phonolitic melt fluorine and chlorine diffusivity are similar. At low temperature fluorine diffusion is more rapid in the Na-rich phonolitic melt; conversely chlorine diffuses faster in the K-rich phonolitic melt. Compared to the results obtained for fluorine and chlorine in a basaltic melt and in a rhyolitic melt, chlorine diffusivity is similar in both rhyolitic and phonolitic melts, and significantly lower than in basaltic melt, contrary to F diffusion coefficients which are comparable and only weakly dependent on the melt composition. F and Cl diffusion depend upon the dominant alkali and always differ from one another in the same phonolitic melt composition. These results will help us to better understand the degassing or the lack of degassing of theses species during syn-eruptive melt decompression and vesiculation. The contrasting volatile diffusivities in magmatic melts during magma vesiculation may be a key controlling factor of the composition of the vapour phase (bubbles) produced.
DE: 1036 Magma chamber processes (3618)
DE: 8410 Geochemical modeling (1009, 3610)
DE: 8419 Volcano monitoring (7280)
DE: 8430 Volcanic gases
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting