Simulates On Melting, Elastic And Thermodynamic Properties Of Major Material Of The Lower Mantle

The behavior of Earth materials at high pressure is central to our understanding of the structure, dynamics, and origin of the Earth. Over the range of conditions that exist within the Earth's mantle, the physical properties of condensed matter depend more strongly on pressure than on other factors such as temperature. The high pressure physical properties of Earth materials are difficult to obtain directly through laboratory experiments. However, computer simulations have been increasingly popular in exploring various properties of the Earth's materials at the geophysically relevant conditions. In this paper, the physical properties of the main composition (MgSiO₃ perovskite and post-perovskite, MgO and CaSiO₃ perovskite) of the Earth's lower mantle at high pressures have systemically study using state-of-the-art computer simulation techniques. The main work contains the following sections:1. The melting curve of MgSiO₃ perovskite and MgO are simulated by using the constant temperature and pressure molecular dynamics method combined with effective pair potentials. Understanding the melting behavior of Earth materials at the pressure and temperature conditions of the Earth's lower mantle is crucial to deciphering the early history and differentiation of the Earth and to determining if partial melting of the mantle is responsible for the seismologically observed ultra-low velocity zone at the core-mantle boundary. In this work, Firstly, the reliability of the present potential model has been verified. Secondly, the pressure-volume equations of state of MgSiO₃ perovskite and MgO were predicted at higher temperatures and higher pressures. Melting is arguably one of the least well understood processes in condensed matter physics, so a rigorous study of the minerals melting is prohibited either by technical problems or by the present state of the theory. This makes the problem of finding melting temperature of the minerals really challenging. A possible solution is to calculate the temperature of overheating (thermal instability temperature) and then estimate the degree of overheating. It is found that the degree of superheating of MgSiO₃ perovskite and MgO are 42ï¼…and 30ï¼…, respectively. According to this value, the melting curve of MgSiO₃ perovskite and MgO were determined.2. High-pressure elasticity of MgSiO₃ perovskite polymorph, MgO and CaSiO₃ perovskite. Experimental studies in understanding high-pressure behavior of elastic properties of relevant phases are still lacking. First-principles calculations provide the ideal complement to the laboratory approach because they require no input from experiment; that is, there are no free parameters in the theory. Such calculations have true predictive power and can supply critical information including that which is difficult to measure experimentally. High-pressure elasticity of the relevant minerals contains the following sections: 1) The observed B1 phase of MgO was found to be stable up to 397 GPa, precluding the B1-B2 phase transition within the lower mantle. MgO was found to be highly anisotropic in its elastic properties, with the magnitude of the anisotropy first decreasing between 0 and 20 GPa and then increasing from 20 to 150 GPa. We found the high pressure reversal of the sign of elastic anisotropy in MgO and the prediction that MgO has a large elastic anisotropy in the lowermost mantle. The Cauchy condition was found to be strongly violated in MgO and CaSiO₃ perovskite, reflecting the importance of noncentral many-body forces. 2) The bulk modulus of CaSiO₃ perovskite is similar to that of MgSiO₃ perovskite; however, its shear modulus is much higher at pressures corresponding to the lower mantle. This suggests that CaSiO₃ perovskite can no longer be considered as an invisible component in modelling the composition of the lower mantle, and even small amounts of the mineral may affect significantly the seismic properties, particularly shear wave velocity, of the generally accepted Mg-rich silicate perovskite dominated composition of this region. Moreover, CaSiO₃ perovskite exhibits strong anisotropy at pressures corresponding to the transition zone and the top of the lower mantle. 3)Comparison between the volumes of the MgSiO₃ perovskite phase to the post-perovskite phase at the same pressure-temperature conditions indicates that the post-perovskite phase is always denser than the perovskite. According to the usual condition of equal enthalpies, it is shown that the transition from the perovskite phase to the post-perovskite phase occurs at the pressure of 108 GPa. It is found that the MgSiO₃ post-perovskite phase has similar bulk modulus and larger shear modulus than perovskite at relevant pressures. This phase is remarkably anisotropic. Comparisons with seismological observation show that post-perovskite may be the most abundant mineral in the D" region.3. The thermodynamic properties of CaSiO₃ perovskite are systemically predicted using the quasi-harmonic Debye model for the first time at high pressure and high temperature. It can be seen that the thermal expansion coefficient increases with T³ at low temperatures and gradually approaches a linear increase at high temperatures, and then the increasing trend becomes gentler. The effects of the pressure on the thermal expansion coefficient are very small at low temperatures; the effects are increasingly obvious as the temperature increases. As pressure increases, the thermal expansion coefficient decreases rapidly and the effects of temperature become less and less pronounced, resulting in linear high temperature behaviour. The thermal expansion coefficient and heat capacity are shown to converge to a nearly constant value at high pressures and temperatures. Unlike the thermal expansion coefficient and heat capacity, the entropy is nearly insensitive to pressure.