Shock Equation Of State And Sound Velocity Of Fe/Fe-9 Wt% O Under High Temperature And High Pressure

Natural disasters on surface ralated to the structure,composition and state of movement of the Earth interior are inextricable.Knowing more about the composition of the Earth's internal matter and motion rule will help human strengthen the ability to resist these disasters.According to geophysical,geochemical,cosmological,and metallurgical studies,it is found that the Earth's core principally consists of iron and alloyed with approximately 4-10 wt%lighter element including silicon(Si),sulfur(S),oxygen(O),carbon(C)and hydrogen(H).What kinds of light elements and how much in Earth core are still unresolved.Particularly oxygen,it is directly related to the circumstance in the initial Earth accretion process.In order to constrain oxygen in the core,the shock equation of state and sound velocities of pure Fe and Fe-9 wt%O(Fe-9O)were studied using dynamic compression technique.The thermodynamic model was used to estimate the Fe-9O density,longitudinal and bulk sound velocity under the Earth core conditions.Our research can provide an important reference for constraining oxygen in the Earth core.The main research contents and innovative conclusions of this study are as follows.First of all,the effects of temperature on sound velocity were investigated.Sound velocitis of pure iron were measured by the reverse impact and optical analysis technology.The stress ranged from 65.4 GPa to 235.1 GPa.First-order derivative of particle velocity versus time was used to determine the elastic-plastic transition point during unloading.It is more accurate to gain the bulk sound velocity.Our results show that the longitudinal sound velocity of pure iron along Hugoniot has a sufficient linear relationship with density,which is closed to the static experimental results at room temperature.Therefore,the conclusion that the linear relationship between the longitudinal sound velocity and density at high temperature is still valid.That is,Birch's law is still working at high temperature.But the effects of temperature on bulk velocity and shear velocity can not be ignored.By studying the influence of temperature on sound velocity under high pressure,it provides an important experimental data for constraining the light element in the Earth core.Second,physical properties of Fe-9O were investigated under high temperature and pressure.Large-scale bulk Fe-9O samples were successfully synthesized by cubic anvil high-pressure apparatus.Fe-9O were characterized by various physical and chemical analyses.The results show that the prepared Fe-9O sample has uniform O element distribution and the mass of O element is 9.0(5)wt%.The chemical reaction completeled fully,and no Fe³⁺exists in the sample.The sample density is 6.908(16)g/cm³,close to the theoretical density.A new mixture model for predicting the adiabat is proposed based on the previous models.The prediction is better than the traditional models.The Hugoniot adiabatic is discontinuous at 88.7 GPa,which may be caused by the Fe_0.98O metallization.The Hugoniot parameters before and after the phase transition are:C_0,L=3.81(13)km/s,?_L=1.81(8);C_0,H=3.71(6)km/s,?_H=1.75(2).The high-pressure melting curves of Fe,Fe-9O were calculated according to Lindemann melting law,and extrapolated to the inner and outer core boundary(ICB).It is found that the melting temperature of pure iron at the boundary is about6200(280)K,which is 400 K higher than that of Fe-9O.Last but not least,oxygen in the Earth core was constrained.The density and sound velocity of Fe-9O under core conditions were calculated according to the thermodynamic model,then compared with the Preliminary Reference Earth Model(PREM).The results show that the density and sound velocity of the Fe-O matter are difficult to meet the dual requirements under the Earth core conditions.Therefore,it is concluded that there are no large amount of oxygen existing in the outer core,and the accretion process of the original Earth is done under reducing conditions.