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| Titel |
238U-230Th disequilibrium in fossil hydrothermally altered material: a tool for dating volcano flank-collapse events. |
| VerfasserIn |
A. Salaun, B. Villemant, M. Gérard, J. C. Komorowski, P. Louvat, G. Manhès, A. Michel, J. Moureau |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250030555
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| Zusammenfassung |
| Flank instability has been recognized as a very common and often recurrent process in the
evolution of volcanoes. Prolonged and extensive hydrothermal system activity is a major
cause of volcano instability in promoting conditions of mechanical weakness of the
edifice that have led to numerous partial edifice-collapses with emplacement of
debris avalanche deposits (DAD). The purpose of this study is to evaluate potential
applications of U-series disequilibrium analysis of altered material collected in
active hydrothermal systems and in DADs that have sampled different regions of
the paleo-hydrothermal systems developed prior to collapse on La Soufrière of
Guadeloupe and Soufriere Hills (Montserrat) volcanoes. Significant redistribution of
chemical elements occurs during hydrothermal alteration with a large fractionation
between elements of the U-decay series. Indeed, samples of active hydrothermal
systems show large variations in U/Th ratio at generally constant (230Th/232Th). This
chemical fractionation between U and Th offers the opportunity to date hydrothermal
alteration and to constrain the age of the active hydrothermal system involved in edifice
collapses. The evolution of trace element compositions (REE, U, Th) are documented
throughout the successive stages of andesite alteration and discussed in terms of relative
element mobility and associated mineralogical assemblages. A progressive HREE
depletion occurs together with significant U enrichment at low alteration degrees,
followed by a final U depletion. Glass alteration in smectite is accompanied by large U
depletions relative to Th. On the contrary, U enrichment is associated with silica
polymorphs. All these processes are discussed in terms of relative U behaviours
during leaching, incorporation and adsorption processes. Andesitic products that
have been affected by hydrothermal alteration are complex assemblages of glass,
phenocrysts, xenocrysts with complex magmatic histories, as a well as country
rock lithologies and hydrothermal minerals. The detailed analytical procedure that
we have developed allows to precisely correlate U-Th mobility and 238U-230Th
disequilibrium to specific geochemical processes regardless of the initial material
heterogeneity. We can therefore date the formation of smectite, a typical product of glass
alteration. This work has important implications for improving hazard assessment from
potentially unstable andesitic volcanoes hosting an active hydrothermal system. |
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