First-principles Molecular Dynamics Study Of The Structure And Equation Of State Of MgSiO₃Melt At High Pressure And High Temperature

Previous experimental and theoretical studies as well as geophysical observationshave provided evidence for the possible presence of silicate melts at the top of thetransition zone and core-mantle boundary. A detailed knowledge of structural andequation of state （EoS） of silicate liquids at high pressure and temperature conditionsis crucial for better understanding mantle dynamics and thermal evolution of the Earth.Due to the difficuly of experiment and modeling of silicate liquid, the structural andthermodynamic properties of molten silicates at relevant pressure conditions remainedlargely unknown.The microstructure and and the equation of state of liquid MgSiO₃at0¹44GPa,2000⁶000K have been carried out by first-principles molecular dynamicssimulations. The simulated results and disscussion are as follows:1. Based on the first-principles molecular dynamics simulations and paircorrelation function （PCF） analyses, we investigated the microscopic structure ofMgSiO₃melt under high pressure and temperature.（1） The first peak positions of paircorrelation functions of O-Si, O-Mg and O-O pairs were1.635,1.970and2.695repectivly, under about zero pressure and2000K, the calculated bond lengths have agood agreement with experimental values1.620and2.120.（2） With pressure andtemperature increasing, the structure of MgSiO₃melt has been significantly changed.In particular, as pressure increasing, the structure becomes much denser; thecalculated average bond length between two atoms decreases with increasingtemperature.（3） The calculated bond lengths of O-Si, O-Mg and O-O are1.610,1.835and2.300under133GPa,4000K conditions.（4） The average Si-O coordinationnumbers increase from4to6and the ratio of bridging oxygen numbers increase from31.3%to72.9%, from the surface atmospheric to the core-mantle boundary.2. Based on the simulation pressure, temperature and volume results, wecalculated the equation of state of MgSiO₃melt by fitting the third-orderBirch-Murnaghan equation of state.（1） At0K, the density of MgSiO₃amorphousincreases monotonously with pressure increasing; and it appears a rapid growth within0²0GPa, while, the growth is relatively slow when the pressure is higher than20 GPa.（2） Under high temperature and high pressure, the density increases withincreasing pressure and decreases with increasing temperature. The strength of theinfluences of temperature on the density of MgSiO₃melt is also controled by pressure,wchich reached maximum when the pressure is nearly10GPa.（3） In this study, thegrowth rate of temperature relative pressure （ΔT/ΔP） decreases with increasingdensity in the MgSiO₃melt, and the growth rate descends by about75%when thedensity is greater than2.71g/cm3. That possibly implys the temperature’s effects onthermodynamic behavior of MgSiO₃melt is higher in shallow lithosphere with lowerdensity. With pressure appraching to the deep mantle, temperature’s effects decreaseand pressure’s effects become the dominant factor.（4） The calculated bulk modulusparameter is K0=21.62GPa and K0=21.99GPa at0GPa and2000K, byrespectively fitting the third-order Birch-Murnaghan and Vinet equation of state.In summary, the first principles molecular dynamics simulation of liquid MgSiO₃enriched the theoretical research of mantle melt, and provied new research results forphysical and theromdynamic properties of the melt composition in the deep Earth.