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Minéralogie physique de phases silicatées alumino-calciques du manteau terrestre. Implications géodynamiques

Abstract : Mineral physics could provide answers to many questions we asked about mineral phases present in the Earth's mantle, their characteristics, their crystal structure, their phase transitions. In the second part of the twentieth century, high pressure and high temperature experiments could give essential informations about materials from the deep Earth: these data could then be combined to those obtained by seismology measurements, geochemistry analyses, experimental and theoretical geodynamics, for a better understanding of the deep parts of our planet. Many former studies revealed that silicate phases bearing calcium and/or aluminium could display very interesting characteristics and properties, with important geodynamics implications. The combination of calcium and aluminium is know to be very useful for mineral phases: indeed, calcium is able to be substituted by atoms which display large cations, while aluminium when replacing silicon atoms could allow the eventual charge compensation required by the substitution of calcium. Moreover, there is an increasing amount of data which reveal the existence of many new (Ca,Al)-rich silicate phases at (P,T) conditions of the Earth's mantle: these phase are found to display very original structure and properties. In this thesis manuscript, we report the main results obtained about the aluminous calcium perovskite, Al-CaSiO3, which is one of the three main mineral phases present in the lower mantle. We show that this phase is able to incorporate huge amount of natural actinides uranium and thorium which provide the main part of the heat produced in our planet, by radioactive decay. Then the Al-rich Ca-perovskite bearing U and Th could be the thermal engine of the Earth's lower mantle. These results obtained by mineral physics experiments and methodology are presented with the objective to better constrain the recent geodynamics models. Here, we propose that the (U,Th)-Al-CaSiO3 perovskite alone is able to provide the entire bottom heating of the big domes observed in the cross sections of the mantle obtained by seismic tomography. The possible relation between our results from mineral physics and the volume of "hot" materials present at the bottom of the mantle, is also discussed. The second silicate phase bearing Ca and Al presented in this thesis is the new high pressure phase named CAS phase of composition CaAl4Si2O11. After many experimental studies performed at high pressure on basaltic crust assemblage, it is now commonly accepted that the CAS phase is one of the main mineral phases present in the oceanic crust (Mid-Ocean Ridge Basalt, MORB) subducted to the lowermost lower mantle. The CAS phase is shown to be one of the last solid residual phases (with Ca-perovskite) when the oceanic crust is partially molten, as expected when this crust reaches the D'' region. Here, we show that the CAS phase bears an isosymmetrical transition where some silicon atoms adopt a coordination 5, in the trigonal bipyramidal site (2 face-sharing tetraedra). The implications of such intermediate coordination (between coordinations 4 and 6) is discussed in terms of diffusion processes, diffusion creep deformation, viscosity: it appears that the formation of SiO5 groups strongly favours the deformation properties of these materials, and then enhances their transport properties. It is clear that the coordination of silicon atoms could have a strong direct effect on the dynamic processes occurring in the deep mantle. With the two studies presented in this thesis, we see that experimental mineral physics can provide essential data for models in geodynamics, thermal behaviour and in seismology. Seismic waves give informations about the structure of the deep Earth and the density profile, while experimental geodynamics reproduce the rheological behaviour of the mantle with appropriate fluids and a bottom heating: it is then important to provide complementary data about the Earth materials. The study of the CAS phase shows that the macroscopic properties of the mantle could find their origin in the microscopic structure of the Earth's mineral phases.
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Laurent Gautron. Minéralogie physique de phases silicatées alumino-calciques du manteau terrestre. Implications géodynamiques. Sciences de la Terre. Université de Marne la Vallée, 2008. ⟨tel-00743660⟩

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