Equation Of State Of MgSiO₃ Perovskite At Lower Mantle Condition And Fe-S-C Melting Behaviour Research With Related Geophysics Implications

(Mg,Fe) SiO₃ perovskite is the most abundant mineral in the Lower mantle.Its high pressure physical properties,such as equation of state,phase stability,are very crucial for constraining the mineralogy composition and describing dynamics process (such like mantle convection,earthquake and volcano mechanism) of the Earth interior.In my work,the eqation of state,crystal structure and thermodynamic stability of (Mg,Fe)SiO₃ perovskite have been investigated at lower mantle high pressure and temperature condition by combination of shock wave loading,static compression and computer simulation.Our results have significant implications for modeling lower mantle mineral composition model accurately.Hence it will improve our further understanding about the interior of the Earth.This dissertation is also devoted to the study of melting behavior of Fe-C-S ternary system under high pressure.Based on the analysis of quenched sample texture,we demonstrate how the melting relations,subsolidus formations and element partition in this system change along with pressure,temperature and starting component.Our results have important implications for planetary differentiation and core composition stratification of planetary body.The main achievements in this study are as followings:(1) Large MgSiO₃-perovskite samples were successfully synthesized by using modified multi-anvil sample assembly.The recovered samples are confirmed to be MgSiO₃ perovskite by electron microscopy analysis and Raman spectrum.Successful syntheses provide essential starting sample for further shock loading experiments.(2) Shock wave data on the pre-synthesized perovskite samples up to 107 GPa yielded a linear relationship between the shock wave velocity U_s and particle velocity u_p described by U_s=6.47(±0.63)+1.56(±0.31) u_p.Fitting experiment data to the Rankin-Hugoniot equation,we obtained the Grüneisen parameterγ₀= 1.33 with q=1.The best fitted values for the adiabatic bulk modulus K_0S and its pressure derivative K_0S' are 254(±10) GPa and 3.9(±0.17),respectively,which are in general agreement with values derived from static compression data.By direct comparison with dynamic compression data using enstatite as starting material,we observed that MgSiO₃ enstatite completely transformed into perovskite phase above 90GPa shock pressure.Improvement of the precision in determining the Hugoniot relationship by additional shock wave data is needed to further constrain the thermoelastic properties of perovskite.However,this is the first demonstration that direct shock wave loading of the pre-synthesized perovskite samples can provide a new way to determining the thermal equation of state of silicate perovskite without the complication of phase transformations along the Hugoniot path,ultimately leading to a better constrained thermoelastic parameters for this important mantle mineral.(3) High pressure melting experiments for Fe-S-C ternary system have been conducted on a piston-cylinder and multi-anvil press by using starting material with different Fe/(C+S) ratio from 3.5 to 20GPa and up to 1700K.For Fe(90wt%)-S(5 wt%)-C(5 wt%),two immiscible C-rich and S-rich Fe-C-S liquids were observed at 3.5GPa,1873 K.At 5GPa,only one homogeneous liquid is quenched,which indicates the miscibility gap close between 3.5GPa and 5GPa.Pressure changes the subsolidus phase relations fundamentally.At 5GPa,Fe₃Ccrystallize first with temperature decrease and coexist with FeS.However,Fe₇C₃appear instead of Fe₃C between 10-20GPa,which means Fe₃C melt incongruently above 5GPa.Meanwhile,FeS melts incongruently into Fe₃S₂+liquid above 10GPa.In the contrast group of Fe(90w%)-C(w2%)-S(8w%),Fe₃C,FeS and Fe are the major crystallized phases under solidus up to 10GPa.Given probable Earth's core element composition of Fe-C-S,our results document no composition stratification is expected in the planetary core with P>5GPa.Based on the crystallization sequence a C-rich solid inner core and S-rich liquid outer core can be deduced for the Earth.(4) High-pressure behavior of orthorhombic MgSiO₃ perovskite crystal has been simulated using density functional theory and plane-wave pseudopotentials approach up to 120 GPa pressure at zero-temperature.The lattice constants and mass density of the MgSiO₃ crystal as functions of pressure were computed and the corresponding bulk modulus and bulk velocity were evaluated.Our theoretical results agree well with high-pressure experiments data.A thermodynamic method was introduced to correct the temperature effect on the 0 K first-principles results of bulk wave velocity,bulk modulus and mass density to lower mantle P/T range. Taking into account the temperature corrections,the corrected mass density,bulk modulus and bulk wave velocity of MgSiO₃-perovskite estimated from the first-principles results is 2%,4%,and 1% lower than Preliminary Reference Earth Model (PREM) profile,respectively,supporting MgSiO₃-perovskite primarily composed lower mantle model.(5) Relative stability of different phases for MgSiO₃ and (Mg_0.75,Fe_0.25)SiO₃ within 0-120GPa are investigated using first-principles method.For MgSiO₃,the computed equation of state for orthorhombic phase of Pbnm space group agrees well with experimental results.The relative stability reduces from observed Pbnm orthorhombic phase to intermediated tetragonal P4mbm phase,and then to hypothetical cubic Pm(?)m phase.For (Mg_0.75,Fe_0.25)SiO₃ ,same sequence of relative phase stability is observed.Thus,our work suggests the low-symmetric orthorhombic MgSiO₃ should be favored in the lower mantle condition.However,adding Fe into MgSiO₃ will make it less stable at the same depth.