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Propriétés élastiques des minéraux hydratés : applications à l'anisotropie sismique dans les zones de subduction

Abstract : The flow of material and the seismic signature of subduction zones are highly dependent on the elastic propertiesof two major types of hydrated minerals: serpentines (antigorite, lizardite and chrysotile) produced by thehydration of mantle minerals and glaucophane, a marker of blueschists and eclogites, which are metamorphicfacies characteristic of subduction zones. Detection of these phases is sometime difficult. It is important to knowtheir elastic properties in order to better understand the seismic images. The goal of this work is to measure andcalculate the elastic properties of hydrated minerals by Brillouin spectroscopy (for antigorite and glaucophane)and ab initio calculations (for lizardite). The precise knowledge of such properties allows linking them to theseismicity in the subduction zones including areas of low velocities detected by various seismic methods. Ourmeasurements were performed at room conditions for antigorite and glaucophane and at high-pressure forantigorite in a diamond anvil cell. Elastic measurements were then coupled with measurements of latticepreferredorientations by Electron Back-Scattered Diffraction. This allowed quantifying the high anisotropy(AVP= 37% and AVS= 50%) in the whole rock and the low seismic velocities of P and S waves. We were able tolink it to the shear wave splitting observed by seismology in the Ryukyu arc (Japan). Seismic observations showthat serpentine is present in areas of low seismicity; it appears to be not only an essential mineral of thesubduction zones but also a “lubricant” allowing sliding layers to slip over each other without leading toearthquakes. For glaucophane, blueschists exhibit a higher anisotropy than glaucophane eclogites, but theserocks are difficult to detect at higher depth, because of their high velocities comparable to those of thesurrounding mantle. We performed ab initio calculations for a similar serpentine, the lizardite, for which weestablished the elastic constants at various pressures and in the presence of iron or not. We highlighted ananomaly around 5 GPa for lizardite and 7 GPa for antigorite, which was later confirmed by Brillouin and Ramanspectroscopies at high pressure.
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  • HAL Id : tel-00660769, version 1

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Lucile Bezacier. Propriétés élastiques des minéraux hydratés : applications à l'anisotropie sismique dans les zones de subduction. Sciences de la Terre. Ecole normale supérieure de lyon - ENS LYON, 2011. Français. ⟨NNT : 2011ENSL0620⟩. ⟨tel-00660769⟩

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