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| Titel |
Degassing dynamics at Mount Etna inferred from radioactive disequilibria (210Pb-210Bi-210Po) in volcanic gases |
| VerfasserIn |
Luca Terray, Pierre-Jean Gauthier, Giuseppe Salerno, Alessandro La Spina, Salvatore Giammanco, Pasquale Sellitto, Pierre Briole |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250130160
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| Publikation (Nr.) |
 EGU/EGU2016-10373.pdf |
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| Zusammenfassung |
| Volcanic gases are significantly enriched in the last short-half-life radionuclides of the 238U
series, namely the so-called Radon daughters 210Pb, 210Bi and 210Po. Because of their
contrasted volatilities, these isotopes are strongly fractionated upon degassing, which gives
rise to significant radioactive disequilibria between them in the gas phase. These disequilibria
carry precious information on shallow degassing processes beneath active volcanoes: they
remarkably constrain the magma residence time in the degassing reservoir and the duration of
gas extraction from magma to surface. On Mount Etna (Sicily), where the study of these
disequilibria was initiated thirty years ago (Lambert et al., EPSL, 1985-86), no
measurement of 210Pb, 210Bi and 210Po in the gases has been performed for the last twenty
years.
Here we present new 210Pb-210Bi-210Po radioactive disequilibria measurements in
volcanic plume gases of Mount Etna. Samples were collected in the bulk diluted plume at
kilometric distance from the summit area during the May 2015 eruption, then in more
concentrated plumes arising from each summit crater of Etna during quiescent degassing in
July 2015. We found values of (210Bi/210Pb) = 7.0 ± 0.3 and (210Po/210Pb) = 80 ± 6 during
both periods. These results suggest that 210Pb, 210Bi and 210Po are not significantly
fractionated during the transport of the plume from the crater rim to close-downslope sites
(<1 km).
None of the previous degassing models (Lambert et al., EPSL, 1985-86 ; Gauthier et al.,
JVGR, 2000) satisfactorily explain measured activity ratios. We propose here a new
degassing model based on the previous conceptualization designed for basaltic open-conduit
volcanoes, like Stromboli. This model considers extreme Radon enrichments in volcanic
gases as a source of 210Pb atoms produced by radioactive decay of 222Rn within gas bubbles
travelling to surface. We constrain a magma residence time of 470 ± 170 days and an
extraction time of the gases of 4.9 ± 0.8 days. Along with SO2 fluxes, we also derive a
volume of the degassing reservoir of 0.2-0.6 km3 in good agreement with previous estimates.
Results gathered from these campaigns have intriguing implication for potential routine
survey of the plume radioactivity, as part of the monitoring network of active volcanoes. |
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