Thermodynamic And Elastic Properties Of Orthoenstatite And High-pressure Clinoenstatite At High Temperatures And High Pressures

The compositions and structures of the Earth's interior are of great significance to understand the origins,evolutions and dynamics of the Earth.The wave velocities structure of Earth's interior can be determined by seismologic observation and analysis.Meanwhile,the elastic properties of minerals under Earth's conditions can be measured or calculated by the method of mineral physics.By comparing the results of seismologic observation with the elastic data of minerals,one can constrain the composition and structure of the Earth's interior.At present,the major research methods of mineral physics are experimental measurement and calculation.In recent years,with the rapid development of computer software and hardware technology,some achievements have been made in elasticity of minerals by the first-principles calculation based on density functional theory.The upper mantle is an important part of the Earth.In the pyrolite model,the upper mantle is constitute of olivine,garnet,orthopyroxene and clinopyroxene.The elasticity of the upper mantle's major minerals under high pressures and high temperatures conditions is important for us to understand the seismologic structure of upper mantle.The experiments and first-principles calculations have investigated the elastic moduli and wave velocities of olivine,garnet and clinopyroxene at high pressures and high temperatures.But there are no experimental or calculated elastic data of orthopyroxene under mantle conditions.Many previous seismic studies report anomalously low VP/VS zones in mantle wedge at the Earth's depth of 50-120 km.These low VP/VS ratios may be related to the enrichment of orthopyroxene.To get a better understanding of the compositions and structures in these zones and the upper mantle,we calculated the thermodynamic and elastic properties of orthoenstatite(MgSiO3),the Mg end-member orthopyroxene under high pressures and high temperatures conditions using first principle calculations based on density function theory with local density approximation.The calculated equations of state,thermodynamic properties,elastic constants,elastic moduli and wave velocities of orthoenstatite agree well with the experimental results.Bulk and shear moduli increase nonlinearly with pressure at mantle temperatures,but the shear modulus and VS show very weak pressure dependence in comparison with VP.Compared to other major minerals in the upper mantle,orthoenstatite has the lowest compressional velocities(VP),shear velocities(VS),and VP/VS ratio down to the depth of approximately 300 km.The enrichment of opx in the upper mantle can cause the unusually low VP/VS observed in the mantle wedge.In addition to the global discontinuities,such as 410 km discontinuity and 660 km discontinuity in the mantle,there are some local discontinuities.The local X-discontinuities in the upper mantle have been observed in a number of seismic studies in the depth range of 250-330 km.The X-discontinuities are often attributed to phase transitions of minerals in the upper mantle.For example,the coesite-stishovite phase transition and the transition of orthoenstatite to high-pressure clinoenstatite.In order to investigate the formation of X-discontinuities,we calculated equation of state,elastic moduli,and wave velocities of high-pressure clinoenstatite under the pressure and temperature conditions of Earth's upper mantle using first-principles calculation.Bulk moduli and shear moduli show nonlinear relationship with pressure in the range of 0-35 GPa at the mantle temperatures.The shear modulus and S wave velocity exhibit weak pressure dependence compared with bulk modulus and P wave velocity.The phase transition of orthopyroxene to high-pressure clinoenstatite,occurring at a depth around 300 km,will cause an increase of-3%-4%,5%and 2.8%for P wave velocity,S wave velocity and density,respectively.The local enrichment of orthopyroxene(20%-40%)and subsequent phase transition can produce a significant impedance contrast(1.5%-3%),which may explain the local X-discontinuity at the depth range of 250-300 km.