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Modélisation thermodynamique et Spectroscopies IRTF, RMN des pyroxènes : des xénolites à la rhéologie mantellique

Abstract : Over the last decades, peridotite xenoliths brought up by kimberlites and basalts have largely and critically contributed to the understanding of the physical and chemical state of the Earth’s upper-mantle, a prerequisite for direct modelling of deep deformations such as those at work in post-glacial rebound. Such an approach first requires quantification of the physical conditions (P, T, σ) in control of mineral chemistry and rock textures. An original protocol for thermodynamic inversion of experimental data on mutually-equilibrated pyroxenes, was first developed to obtain a reliable geothermobarometer. Moreover, FTIR spectroscopic studies of pyroxenes show that these nominally anhydrous minerals (NAMs) actually contain dissolved water, from 38 to 450 ppm for cpx’s and from 19 to 184 ppm for opx’s. According to its disruptive action onto the crystalline network, such water directly affects the physical and chemical properties of the Earth’s mantle (viscosity, phase diagrams, electrical conductivity). Dissolution of this water component is correlated to fO2, P and T, but only indirectly to mineral composition. Last, 1H, 27Al and 29Si MAS NMR analyses on the same pyroxenes (as iron-bearing minerals), as well as on kaolinite+magnetite mixes, have authenticated NMR spectra for natural pyroxenes, but have also yielded constraining data on site occupancies, specially for Al. These complementary tools applied here with some success, open new prospects for the understanding and modelling of deep processes.
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Karine Begaudeau. Modélisation thermodynamique et Spectroscopies IRTF, RMN des pyroxènes : des xénolites à la rhéologie mantellique. Sciences de la Terre. Université de La Rochelle, 2011. Français. ⟨NNT : 2011LAROS335⟩. ⟨tel-00696775⟩

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