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Exploring Thermodynamic Properties of Earth’s Core-Forming Materials

Periodic Reporting for period 4 - Earth core (Exploring Thermodynamic Properties of Earth’s Core-Forming Materials)

Reporting period: 2019-12-01 to 2021-09-30

What is the problem/issue being addressed?
It is known that the Earth’s core is less dense than pure iron by about 7%, which is due to the presence of a light element(s) such as Si, S, C, O, and H. The goal of this project is to construct a thermodynamic model of the Earth’s central core. A particular focus is on the identification of the light element because the inclusion of these elements in iron liquid depends on the pressure, temperature, and chemical environment and hence provides us invaluable information about the origin and evolution of the solid Earth. We are examining phase relations and density of phases in Fe-light element systems by conducting high-pressure and -temperature experiments and employing thermodynamic calculations based on the experimental data.

Why is it important for society?
One of the important implications of the project is that the behaviour of the light element in the core should be linked to the origin of the magnetic field through geodynamo and hence the habitability of a planet. The convecting liquid iron core generates magnetic fields, which are protecting life on the surface from the harmful solar wind. The liquid convection is driven mostly by the preferential partitioning of the light element into the liquid outer core over the solid inner core. Therefore, for a planet, just being at an appropriate distance from the central star which stabilises liquid H2O is not the sufficient condition to be habitable; presence of light elements in the core is likely another key condition. This project will bring society a better understanding of Earth’s stable magnetic fields which have protected life since its birth.

What are the overall objectives?
The key research questions that the project is trying to address are:
1. What are the thermodynamic properties of iron and iron-light elements alloy under core pressure and temperature conditions?
2. What are the light elements dissolved in the Earth’s core?
3. How was/is the origin, current state, and evolution of the Earth’s core?

The conclusions of the action
We have established a thermodynamic model including liquids for each Fe-light element system, which can be applicable to Earth’s core pressure-temperature conditions. Under core conditions, Fe-Si liquids can be treated as ideal solutions as well as Fe-O. In contrast, Fe-S and Fe-C liquids remain nonideal to the shallow and bottom parts of the outer core respectively, which is contrary to theoretical predictions. None of the studied binary systems alone cannot explain the seismic properties of the core, while Fe-6wt%Si-1wt%O gives the density and P-wave velocity of liquids close to those of the outer core. Although the density of the crystallising phase from the Fe-Si-O liquid is not perfectly consistent with that of the inner core, we conclude that the Earth’s core should be Si-rich, which implies that core formation should have occurred under reducing conditions.
We also determine phase relations of Fe-Ni-Si alloy and found that the addition of Ni and Si simultaneously results in unexpected phase relations Fe, which enlarges the fcc stability field. This suggests that future studies should simultaneously include Ni and light elements for more realistic core systems.
Overview of the results and their exploitation and dissemination
We conducted high-pressure (P) and -temperature (T) experiments on Fe-alloys in diamond anvil cells (DAC). Based on experimental data, we constructed thermodynamic models for binary systems. As indicated below, we published these experimental and thermodynamic data in refereed journals. On publication of the results of the Fe-Ni-Si system, press release was made through the websites of the University of Edinburgh and Deutsches Elektronen-Synchrotron (DESY).

Work performed
1. We first set up diamond anvil cell (DAC) laboratory at the University of Edinburgh.
2. Fe-Si: We conducted high-P-T experiments with in-situ X-ray diffraction (XRD) in internally resistive-heated DAC on Fe-Si alloy and placed constraints on the P-T location of the boundary between the face-centred cubic (fcc) and hexagonal close-packed (hcp) phases. Published as Komabayashi et al. (2019, American Mineralogist).
Based on the experimental data, we constructed a thermodynamic model for the system Fe-Si which includes thermal equations of state (EoS) and mixing properties for Fe-Si liquids. We calculated the density and longitudinal wave (P-wave) velocity of the liquids and density of solid hcp phase, which were then compared with seismological observations. Published as Komabayashi (2020, Physics and Chemistry of Minerals).
3. Fe-S: We conducted high-P-T experiments with in-situ XRD in laser-heated DAC on solid Fe3S and established its thermal EoS. Published as Thompson et al. (2020, Earth and Planetary Science Letters).
We also conducted melting experiments on Fe3S in internally resistive- and laser-heated DAC. Then we constructed a thermodynamic model for melting of the system Fe-S. In prep as Thompson et al..
4. Fe-C: We conducted high-P-T experiments with in-situ XRD in laser-heated DAC on solid Fe3C and established its thermal EoS. (McGuire et al. 2021, American Mineralogist).
We also conducted melting experiments on Fe3C. Then we constructed a thermodynamic model for melting of the system Fe-C. In prep as McGuire et al..
5. Fe-Ni-Si: We conducted high-P-T experiments with in-situ XRD in internally resistive-heated DAC on Fe-Ni-Si alloy to 200 GPa and 3900 K and placed constraints on the P-T location of the fcc-hcp transition boundaries. The stability of the fcc phase is more enhanced which causes a depression of melting point of the hcp phase at the inner core depths. We demonstrated that the phase relations of the ternary system cannot be inferred from the binary systems. Published as Komabayashi et al. (2019, Earth and Planetary Science Letters). Press release was made through the University of Edinburgh and DESY.
6. Fe-Si-O: From our thermodynamic models for Fe-FeO and Fe-Si, we constructed a model for physical properties of Fe-Si-O liquids. We calculated the seismologically observable parameters and compared them with the observations. Published as Komabayashi (2020, Physics and Chemistry of Minerals).
7. Fe-Si-S: We conducted high-P-T experiments with in-situ XRD in laser-heated DAC on Fe-Si-S alloy and collected the unit-cell volume data of the hcp phase. In prep as Komabayashi et al.
8. Other systems
Fe-N: Compression and decompression experiments on fcc γʹ-Fe4N at room temperature were conducted in DAC with in-situ XRD and its room temperature EoS was constructed. Published as Breton (2019, American Mineralogist).
KCl: KCl is often used as a pressure medium and pressure marker in a DAC study. We evaluated its thermal EoS based on experimental data up to 230 GPa at 300 K and to 61 GPa at 1500–2600 K. Published as Tateno et al. (2019, American Mineralogist).
1. We demonstrated that under core conditions, Fe-Si liquids behave as ideal solutions, based on which we assessed melting/crystalising temperatures of Fe phases (e.g. hcp) which were not measured by experiment before. We found that addition of Si to the Fe phases hardly affects the melting temperatures under high pressures. Published as Komabayashi (2020).

2. We demonstrated that the phase relations of Ni-bearing Fe-light element system cannot simply be inferred from its end-member binary systems. Published as Komabayashi et al. (2019).
diamond anvil