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Les déséquilibres radioactifs 238U-230Th-226Ra : discussions sur les sources et processus responsables du volcanisme de la Cordillère des Andes et sur la déglaciation en Islande

Abstract : Thanks to precise analyses of uranium series radioactive disequilibria, permitted by improvements of mass spectrometry data acquisition and optimization of chemistry, two questions were tackled and answers about the periods of glaciation in Iceland and the petrogenesis of andean lavas were proposed.
Two methods were used to date lava flows from the Reykjanes peninsula and the island of Heimaey, in Iceland, directly linked to the periods of deglaciation or reheating around the last ice age. An original method based on the use of segregation veins present in the lava flows has been developed. Thanks to the formation of this particular geological object this methods permits to be freed from xenocrysts when using the internal isochron method in the 238U-230Th system. Using this method some constraints are brought both on the quaternary geology of the area of Reykjavik and on the age of the end of the Würm glaciation on Heimaey. In parallel, due to the homogeneity of the historical and Holocene lavas of the Reykjanes peninsula, regarding the (230Th/232Th) and Th/U ratios, it is possible to derive the (230Th/232Th) ratio in time to determine the age of lava flows installed during the ice age. Thus we confirmed that the magnetic excursion recorded in the Skálamælifell lava flow is most probably the same one as that of Laschamp-Olby and that at this time (approximately 48.000 years) the Reykjanes peninsula was entirely covered with a glacier. Thanks to this method the extent of the glacier was also constrained by dating of a picritic shield around 22.000 years, thus the peninsula has to be free of any ice cap despite the fact that glaciation is the most intense around this time.
Then andean arc magmas petrogenesis was constrained in time and space. Almost systematic excesses of 226Ra compared to 230Th in lavas of austral, southern and northern volcanic zones of the Andes (AVZ, SVZ and NVZ) constrain the ascending time from the source to the surface after their formation which is necessarily fast (less than 8.000 years to preserve these disequilibria). We show that eruptive activity and excesses of 238U (and to a lesser extent 226Ra) compared to 230Th are linked. This observation suggests that the most active volcanoes (Villarrica and Llaima for the SVZ) are those whose source is enriched in fluids, coming from the subducted plate and sediments which cover it, during partial melting of the mantle wedge and that radioactive disequilibria tell us about the geodynamic context on a regional scale. Finally a more precise study of particular volcanoes (Nevado de Longavi (SVZ) and Guagua Pichincha (NVZ)), suggests that uranium series disequilibria record the evolution of a volcano in time. Thus the complex history of Nevado de Longavi and the particular signals of this volcano can be translated in term of variation of partial melting degree of the source, fractional crystallization or crustal contamination. In the case of Guagua Pichincha volcano, the adakitic source of the volcanic activity could be discussed thanks to the study of 238U and 226Ra excesses compared to 230Th and a model of adakites formation by partial melting of an hydrated mantle wedge followed by fractional crystallization of garnet at high pressure will be proposed.
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Contributor : Jérôme Chmeleff <>
Submitted on : Monday, December 12, 2005 - 1:26:09 PM
Last modification on : Wednesday, November 20, 2019 - 3:06:07 AM
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  • HAL Id : tel-00011187, version 1


Jérôme Chmeleff. Les déséquilibres radioactifs 238U-230Th-226Ra : discussions sur les sources et processus responsables du volcanisme de la Cordillère des Andes et sur la déglaciation en Islande. Géochimie. Université Blaise Pascal - Clermont-Ferrand II, 2005. Français. ⟨NNT : 2005CLF21601⟩. ⟨tel-00011187⟩



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