Experimental Research Of The Candidate Material Of The Earth's Outer Core By Shock Wave

On the basis of geophysical observations, cosmochemical constraints, and high-pressure experimental data, the Earth's liquid outer core consists of mainly liquid iron alloyed with about10percent (by weight) of light elements. Although the concentrations of the light elements are small, they nevertheless affect the Earth's core:its rate of cooling, the growth of the inner core, the dynamics of core convection, the evolution of the geodynamo, and the chemical reactions happening at the core-mantle boundary. The light elements in the Earth's outer core may be Silicon (Si), Oxygen (O), Sulfur (S), Carbon (C), and Hydrogen (H), and at least two of them are present in the Earth's outer core. However, there are lots of controversy about the identity and abundance of the light elements, although they are of substantial geophysical and geochemical interest.In order to make a further constraint on the light elements in the Earth's outer core, supported by National Natural Science Foundation of China (Grant No.41074056) and the Fundamental Research Funds for the Central Universities(Grant No.20410042), and based on the lots of researching literatures and theoretical estimation, we choose Fe92.5O2.2S5.3(Fe/FeO/FeS mixture,92.5wt%Fe,2.2wt%O,5.3wt%S) as the object of study, and measure their equation of state, sound velocity and melting behavior at high pressure using two stage light gas gun. Combining the experimental data of Fe9oO8S2, we provide an integrate and self-consistent thermodynamical method to describe the density and bulk sound velocity of Fe-O-S system under the conditions of the Earth's outer core for constraining the abundance of O and S in the outer core. The main contents and the innovations in the dissertation as bellow:1. Using multi-anvil press, we explore the condition of temperature and pressure on which Fe2O3reacts with Fe to produce FeO. Under the pressure about2GPa and temperature about800℃, the mixture powder of pure Fe2O3,FeS and Fe is compressed to a big compact sample. The X-ray shows that Fe3+in the sample has been reduced to Fe2+completely. The abundance of Fe, O and S are92.5wt%,2.2wt%and5.3wt%, respectively. The density of the sample is about6.880g/cm3, which is consistent well with the theoretical value based on additive law.2. Using two stage light gas gun, the equation of state of Fe92.5O2.2S5.3were mea-sured in the pressure range from73GPa to233GPa applying electrical pins and revers-impact method. The Hugoniot parameter is Co=3.71O(±0.12)km/s and λ=1.610(±0.04). Based on the Hugoniot parameter of Fe, FeO and FeS we also calculate the equation of state of Fe92.5O2.2S5.3according to the additive law, which are in good agreement with the experiment data. It indicates that there is no chemical reaction between Fe, FeO and FeS under shock wave.3. We also obtained longitudinal and bulk sound velocity along Hugoniot using reverse impact method and optical analysis technique in the pressure region93to208GPa with DISAR appliance. Compared our experimental results with those of Fe90O8S2and Fe, it is found that O and S can increase the bulk sound velocity of Fe, and the effect of O is much stronger than S. Therefore it is firmly confirmed the possibility of O and S present in the Earth's outer core.4. Based on the experimental data of sound velocity along Hugoniot, it can be found that Fe92.5O2.2S5.3is melted completely at164GPa, namely equilibrium melt. According to the law of conversation of energy, the equilibrium melting temperature is about3500K. at164GPa. Taking this point as a reference, the extrapolated melting temperature is about5000K at330GPa, which is the pressure at the inner/outer core boundary of the Earth. Compared the melting temperature of Fe92.5O2.2S5.3with that of Fe, Fe-O, Fe-S and other Fe-O-S system, we found that O and S has different effect on the melting temperature of Fe at high pressure. More S is in the Fe-O-S system, is there more melting depression of Fe. Therefore, when we consider the light elements' effect on the melting temperature of iron, we not only have to pay attention to their abundance, but also have to pay attention to the identity.5. We provide a self-consistent, integrate method based on thermodynamic theory to determine the density and bulk sound velocity of Fe-O-S system under any pressure and temperature. The calculated bulk sound velocity of Fe, Fe90O8S2and Fe92.5O2.2S5.3are also consistent well with the experimental data. It indicates this method is reasonable and related thermodynamic parameters are accurate. Applying this method, the density and bulk sound velocity of various Fe-O-S systems are calculated and compared with those of Earth's outer core. In the errors range, both the density and bulk sound velocity of Fe92.5O2.2S5.3match with the PREM model basically, but the best fit is Fe90O0.5Ss9.5. It indicates the abundance of O in the Earth's outer core could be no more than2.5wt%.Based on geochemical constraints, there is2-3wt%S in the outer core. Recent continuous accretion models constrained from siderophile-element-partitioning data, point towards more reduced conditions during early Earth accretion, leading to a core with low oxygen and high silicon contents. Our study provides an independent constraint on the oxygen content in the core, placing less than2.5wt%oxygen in the liquid outer core with an optimal value of0.5wt%. In terms of oxygen content in the core, our result is in a good agreement with a recent core composition model (8wt%Si,2wt%S and,0.5wt%O) based on core-mantle element-partitioning data.