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Comportement des éléments traces au cours des processus de dégazage. Etude des volcans Piton de la Fournaise (Réunion) et Lascar (Chili)

Abstract : This study is aimed at better understanding the behavior of trace elements – and notably those of light elements such as Li and B – during magma degassing processes. For this purpose, we used a geochemical approach based on the analysis of fresh lavas and volcanic aerosols from Piton de la Fournaise (Réunion) and Lascar (Chile) volcanoes, respectively. Firstly, this thesis work focused on the role of gas transfers in triggering major eruptions and the time scales involved. Trace element analyses of recent lavas (1998-2008) of Piton de la Fournaise reveal anomalous abundances of volatile elements (e.g., Li, Cu, B, Tl, Bi, Cd) a few days prior to the April 2007 summit collapse. The kinetic (diffusive) fractionation of elements accounts for the observed anomalies. The short time-scales required to fractionate Li from Cd diffusively (minutes to hours) and Cd from Bi (few hours to two days) support the idea that the magmas underwent rapid pressure variations a few days before the summit collapse.Secondly, this study concentrated on the quiescent degassing activity of Lascar volcano. Both major gaseous species and trace element enrichment in gas and aerosols collected in the sustained plume over the period 2009 to 2012 suggest the involvement of two main degassing sources with contrasted geochemical signatures: a deep magmatic reservoir and a shallow hydrothermal system. Contributions from these two dominant sources vary with time in response to changes in volcanic activity. This temporal evolution has been shown not only by well-known tracers such as SO2 and HCl, but also by a trace element both volatile and highly fluid-mobile such as B.To determine the bulk boron concentration of our samples, we have developed a robust low-blank method based on isotope dilution ICP-MS. Applied to lavas of Piton de la Fournaise, this technique allowed us to quantify the amount of B lost during magma degassing (10-30%) and its dependency on degassing conditions. Applied to aerosols of Lascar, it enabled us to show that B volatility is enhanced during hydrothermal processes (gas-water, gas-rock interactions). Finally, applied to fertile peridotite xenoliths, it led us to establish new constraints on the behavior of B during mantle processes and estimate a primitive mantle B content of 0.26 ± 0.04 ppm.
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Contributor : Gabrielle Menard Connect in order to contact the contributor
Submitted on : Friday, November 21, 2014 - 9:36:52 AM
Last modification on : Tuesday, October 19, 2021 - 7:01:36 PM
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  • HAL Id : tel-01076248, version 2


Gabrielle Menard. Comportement des éléments traces au cours des processus de dégazage. Etude des volcans Piton de la Fournaise (Réunion) et Lascar (Chili). Sciences de la Terre. Université Blaise Pascal - Clermont-Ferrand II, 2014. Français. ⟨NNT : 2014CLF22460⟩. ⟨tel-01076248v2⟩



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