Browsing by Subject "Thermoelasticity"
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Item A Computational Study of the Earth's Lower Mantle Thermal Structure and Composition(2018-05) Valencia-Cardona, JuanAn enhanced determination of the internal dynamic processes that take place in our planet's interior could be achieved if its composition and thermal structure are well resolved. However, its extreme pressure and temperature conditions make this task a grand challenge in the geophysical sciences. Alternate methods of study are required to overcome such physical constraints. In this work, we use the thermoelastic properties of various minerals, computed from ab initio calculations, to study the composition and thermal structure of the Earth's lower mantle. The mineral phases of this region of the Earth are: (Fe, Al)-bearing MgSiO3 bridgmanite, (Fe, Al)-bearing MgSiO3 post-perovskite, (Mg, Fe)O ferropericlase, and CaSiO3 perovskite. The thermoelastic properties of different lower mantle aggregates are computed by varying the molar concentration of these mineral phases, whose seismic velocities and densities are compared to one-dimensional seismic models, for validation, along their self-consistent temperature gradient. We particularly focus on the effect of a pressure-induced reordering of the electronic structure of Fe in ferropericlase, known as spin-crossover, on the temperature profile of the lower mantle. The anomalous behavior caused by spin-crossover has not been observed in one-dimensional seismic studies. Thus, we present a novel way to achieve this by using a common seismic observable known as the Bullen's parameter. Finally, we study the seismic and thermodynamic signatures of a major phase transition, bridgmanite to post-perovskite, which are of vital importance to shed light on the enigmatic behavior of the deep lower mantle, otherwise known as the D" region.Item Thermoelastic properties of iron- and aluminum-bearing bridgmanite at high pressures and temperatures(2016-01) Shukla, GauravThermoelastic properties of the Earth’s forming minerals play an important role in deciphering the tomographic images of seismic observations. In spite of the considerable progress in the experimental measurements of the elastic properties of minerals at high pressures and temperatures, the available data is still quite limited to constrain the composition and thermal structure of the Earth’s interior. The first-principles atomistic calculations have often complimented the experimental measurements in the study of minerals under high pressure and temperature conditions. In this work, we present the first-principles investigation of the effect of iron (Fe) and aluminum (Al) on the thermoelastic properties of MgSiO3 perovksite (also known as bridgmanite), the most abundant mineral of the Earth’s lower mantle. First, we investigate the pressure induced iron state changes in Fe-bearing MgSiO3 and MgGeO3 perovskite (a low-pressure analog of MgSiO3 ) within the local density (LDA+U) and the generalized gradient approximation augmented by the Hubbard-type correction (GGA+U). We showed that the iron state transitions occur at particular average Fe-O bond-length irrespective of mineral composition (MgSiO3 or MgGeO3 ) or the exchange and correlation functional used in the calculations (LDA+U or GGA+U). We further study the effect of disorder, iron concentration, and temperature on the spin crossover in Fe3+ -bearing bridgmanite using LDA+U calculations. Thermal effects have been addressed within the quasiharmonic approximation using density functional perturbation theory (DFPT). Then, we calculate the aggregate elastic moduli (bulk and shear modulus) and acoustic velocities for the Fe- and Al-bearing bridgmanite to investigate the effect of iron state changes and its possible consequences to the lower mantle composition.