Stability And Thermal Equation Of State Of (Mg, Fe) SiO₃-Perovskite At Lower Mantle Conditions

Shock compression has become an important method to advance our understanding of the deep Earth's interior, and has played an important role in the research of solid Earth science. In this thesis, the experimental techniques and analysis methods in shock wave Physics and Thermodynamics are used to study the phase stability and Equation of State of (Mg, Fe)SiO3 perovskite, which has become generally accepted as the dominant phase of the lower mantle. This is the dominant problem to constraint the mineral composition of the Earth's lower mantle.The main work and achievements of this thesis are as followings: (1)A new shock recovery apparatus was designed and the sample was recovered successfully with shock velocity up to 6.0km/s, which provided us a important way to investigate the phase stability of (Mg, Fe)SiO3. This is new promotion in shock wave experimental techniques.(2) Five shots of shock recovery experiments with the initial sample of (Mg0.92, Fe0.08)SiO3 enstatite were conducted at pressure range from 60 to 110GPa (the corresponding temperature is estimated as 2400~5000K). The recovered sample were analysesed by X-ray diffraction (XRD) and infrared absorption spectra (IR). The results indicate that the main phase of recovered samples is not perovskite structure, but single-chain structure enstatite. Especially, both XRD and IR observation shows no evidence for the existence of oxides SiO2 and (Mg0.92, Fe0.08)O in recovered samples. Therefore, there is no possibility for the chemical decomposition reaction of (Mg0.92, Fe0.08)SiO3 to oxides SiO2 and (Mg0.92, Fe0.08)O under shock compression from 60 to 110GPa.(3) The Gibbs energy and molar volume of all phases in the reaction MgSiO3(Pv) = MgO(Pe) + SiO2(St) are calculated using latest experimental thermodynamic parameters under lower mantle conditions (1000-3500K and 30-150GPa). When pressure higher than 30 GPa, the differences of Gibbs energy and molar volume of the above reaction are all positive. The results suggest that there is no possibility of the chemicaldecomposition reaction of (Mg0.92, Fe0.08)SiO3 to oxides assemblages. The parameters of the possible new phase of SiO2 have no influence on the conclusion.(4) In order to discuss the phase stability of (Mg, Fe)SiO3-perovskite at lower mantle conditions, the previous Hugoniot, sound velocity experimental results and now shock recovery experiments were analyzesed together. All the above shock wave experiments have proved no possibility of the chemical decomposition of (Mg, Fe)SiO3up to 140GPa. However, there is a small scattering in the pressure-compressional sound velocity curve between 60 and 90GPa, and also some small changes in XRD and IR spectra between recovered samples and original samples, which may owe to certain structure change of perovskite.(5) The thermal equation of state of (Mg, Fe)SiO3-perovskite was obtained from previous Hugoniot experiments: 0 =1.84(2), q=1.69(3), Kos=260.1(9) GPa, and K=4.18(4), with p=4.19g/cm.(6) The static experimental data can be reproduced by the above thermal equation of state of (Mg, Fe)SiO3-perovskite, which shows a good agreement between shock wave data and static data. This also shows the high reliability of our thermal Equation of State in wide pressure and temperature range.(7) (Mg0.92, Fe0.08)SiO3-perovskite matches the density profile of PREM of the lower mantle well. Therefore, we prefer a perovskite-dominant lower mantle model.