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Theses

Propriétés élastiques des olivines riches en fer

Abstract : Except for the fayalite end-member, elastic properties of iron-rich olivine (Fe,Mg)2SiO4 have never been fully studied despite their aknowledged natural existence on Earth and Mars. The purpose of this thesis is to investigate on the evolution of elastic properties with regard to the iron content of olivines. Fine-grained polycrystalline aggregates of four different compositions (Fa44, Fa63, Fa81 and Fa100) were sintered by Spark Plasma Sintering (SPS) from nano-powder olivines synthesised under controlled atmosphere. These samples were then studied at pressures up to 6,4 GPa using the DIA press at X17B2 beamline of the late National Synchrotron Light Source at Brookhaven National Laboratory. Unit cell of the samples and NaCl pressure calibrant were determined using X-ray diffraction and used to calculate the isothermal bulk modulus (KT0) by fitting to a Birch-Murnaghan equation of state. Despite our best efforts to apply a hydrostatic pressure on the sample, a high level of deviatoric stress was observed in every experiment. Correction for the deviatoric stress on the d-spacing data was applied using the 10 detectors setup of the beamline and the calculations from Singh (1993) but is not sufficient to calculate a precise pressure derivative K'. These corrected data combined with literature allow us to propose a two level evolution of KT0, with constant values of ~128 GPa for the Mg-rich olivines and ~135 GPa for the Fe-rich olivines, connected by a quick raise between Fa50 and Fa70. In addition, we conducted ultrasonic interferometric measurements combined with X-ray imaging on the Fa81 and Fa100 samples. The adiabatic bulk modulus (KS0) and shear modulus (G) were calculated from compression and shear wave velocities. When converted into KT0, KS0 values are significantly lower than the X-ray diffraction ones. On the opposite, G values are in good agreement with the linear evolution with iron content of olivines calculated by Liu et al. (2005).
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Nicolas Tercé. Propriétés élastiques des olivines riches en fer. Géophysique [physics.geo-ph]. Université Paul Sabatier - Toulouse III, 2016. Français. ⟨NNT : 2016TOU30397⟩. ⟨tel-02306986⟩

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