Equation Of State Of Fe-S System

On the basis of seismology, geophysical observations, cosmochemical constraints, and high-pressure experimental data, it is found that the Earth’s liquid outer core consists of90wt.%iron alloyed with about10wt.%of light elements. Although the concentrations of the light elements are small, they nevertheless affect the dynamics and thermodynamic characteristics of the Earth’s core. 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 there are two kinds of light elements present in the Earth’s outer core. Among these elements, most researchers considered that the amounts of O, S and Si are more than other light elements, so O, S and Si are considered as the main light elements. However, according to the theory and experiments, O and Si can not co-exist in the Earth’s outer core. Because the Earth’s outer core with lots of O is formed at oxidation conditions, while the Earth’s outer with lots of S is formed at relative reduction conditions. Therefore, the materials of the Earth’s outer core may consist of Fe-O-S or Fe-Si-S ternary system. Recently, the shock wave experiment conducted by Huang et al (2011) revealed that the Earth’s outer core is depleted in oxygen, and the maximum content is2.5.wt%with an optical value of0.5wt.%, their work also suggested that the Fe-S-Si ternary system is the most possible component of the Earth’s outer core (Evidence for an oxygen-depleted liquid outer core of the Earth, Nature,2011,479).In order to make a further constraint on the light elements in the Earth’s outer core, it is necessary to make clear the effect of S or Si on the density and sound velocity of Fe at the conditions of Earth’s outer core. Based on a lot of researching literatures and theoretical estimation, we choose Fe88.24Sn11.76(Fe/FeS mixture,88.24wt.%Fe,11.76wt.%S) as the object of study. Using two stage light gas gun, we measured their equation of state along Hugoniot to find the effect of S on the dentity of Fe at high pressure and temperature. And these experimental results also provide necessary parameters to measure the sound velocity. The main contents and the innovations in the dissertation as bellow:1. Using the multi-anvil press, we explore the best conditions to synthesize the compact Fe-S sample with large volume. It’s found that the best condition is about2GPa and800℃. The average bulk immersion density is about6.50g/cm3, which is close to the theoretical density6.48g/cm3based on the density of the Fe and FeS. SEM/EDS instrument shows that FeS uniformly distributes in the sample, and there is no pore among Fe and FeS powder. The abundance of S is about11.76wt.%.2. Using two stage light gas gun, the equation of state of Fe88.24S11.76were measured in the pressure range from about90GPa to200GPa applying electrical pins method. The Hugoniot parameter is C0=3.468(±0.142) km/s, λ=1.618(±0.054) Based on the Hugoniot parameter of Fe and FeS, we also calculate the equation of state of Fe88.24S11.76according to the additive law, which are in good agreement with the experimental data. It indicates that the additive law is effective and there is no obvious chemical reaction between Fe and FeS under shock wave.3. Using thermodynamical method, we calculate the density and bulk sound velocity of Fe88.24S11.76under outer core condition based on the experimental Hugoniot parameter, and compare the results with the PREM, It is found that density of Fe-11.76wt.%S are lower than that of the Earth’s outer core, while the bulk sound velocity is higher than that of Earth’s outer core under the same conditions. It indicates the abundance of S in the Earth’s outer core should be less than11.76wt.%.4. To satisfy the density of the outer core (limited by PREM) as a starting point, we calculate the density and sound velocity of Fe90S10. The calculated results indicate that10wt.%S can match the density limit, but its sound velocity is lower than the data of PREM. These results suggest that:even without considering the geochemical abundance of the S element, the outer core can not be composed of Fe-S system.Fe-S-Si is the most possible component of the Earth’s outer core. Combined with the geochemical limits on the S content (2-3wt.%), the amount of Si content in the Earth’s outer core is considerable. The following work is to measure the sound velocity of the Fe-S system, as well as Fe-Si system (reasonable Si content should be about10wt.%) along Hugoniot, and make full preparations for the research of Fe-S-Si system at high pressure and temperature.