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Etudes texturales en 2D et 3D des produits des explosions vulcaniennes du volcan Soufrière Hills, 1997 (Montserrat)

Abstract : In 1997, Soufrière Hills Volcano experienced two periods of repetitive Vulcanian explosions, implying 88 explosions. Each of these explosions emptied the first 0.5-2 km of the conduit. The aim of this PhD was to constrain and to quantify the vesiculation processes that occurred during each of these explosions, by a textural analysis of the natural products. This study involved measurements by 2D image-analysis, 3D analysis by X-Ray computed microtomography, a detailed textural study of a series of breadcrust bombs, and chemical measurements of in particular matrix-glass water contents (using Raman microspectrometry) and chlorine contents. Textural analysis in 2D leaded us to recognize several vesicle size populations. The smallest ones (smaller than few tens of microns) are syn-explosive, whereas most of the largest ones (> several hundreds of μm) in pumices are pre-explosive. Nearly 15% of the largest vesicles in pumices, and all these large vesicles in the breadcrust bombs, formed by fracturation of amphibole phenocrysts. A population of intermediate vesicles, formed by the coalescence of the small ones, is visible in pumices. Syn-explosive nucleation took place heterogeneously on Fe-Ti oxides and vesicle growth occurred mainly by decompression. Coalescence affected all vesicles larger than few tens of microns. X-ray computed microtomography allowed us to work with a 3D vision of the system. For the first time, vesicle and crystal size distributions were performed down to 2 μm, by combining several data sets. The observations made, and the parameters obtained, are consistent with those performed using 2D image-analysis. Moreover, 3D allowed us to constrain the coalescence process that occurred between neighbouring vesicles whatever their sizes. Breadcrust bombs textural analysis showed that these samples underwent a vesiculation process that was intermediate between that of the dense samples from the degassed plug, and the pumices. Post-fragmentation vesiculation of the breadcrust bomb interior is the major mechanism leading to the fracturation of their dense rind, giving the breadcrusted aspect. Chemical analyses showed a strong relationship between the matrix-glass water contents of the sample and its texture. Pumices can retain huge amount of water (until ~2.8 wt%) whereas dense samples and breadcrust bombs are more degassed. Measured chlorine contents are constant in all samples and could be explained by low distribution coefficient (~5) between the vapour phase and the melt. Between two successive explosions (~10h), degassing occurred in partially open-system. We demonstrated that syn-explosive vesiculation was important during each Vulcanian explosion of the Soufrière Hills Volcano in 1997 and that the development of the sample texture was controlled by the time interval between decompression and the onset of surface cooling. Syn-explosive decompression rates estimated using vesicle number densities (>0.3-6.5 MPa s-1) are consistent with those obtained by numerical modelling.
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https://tel.archives-ouvertes.fr/tel-00463901
Contributor : Thomas Giachetti <>
Submitted on : Monday, March 15, 2010 - 2:28:56 PM
Last modification on : Wednesday, March 4, 2020 - 12:22:19 PM
Long-term archiving on: : Friday, June 18, 2010 - 9:22:03 PM

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  • HAL Id : tel-00463901, version 1

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Thomas Giachetti. Etudes texturales en 2D et 3D des produits des explosions vulcaniennes du volcan Soufrière Hills, 1997 (Montserrat). Sciences de la Terre. Université Blaise Pascal - Clermont-Ferrand II, 2010. Français. ⟨tel-00463901⟩

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