Report

Teaser, summary, work performed and final results

Periodic Reporting for period 3 - Earth core (Exploring Thermodynamic Properties of Earth’s Core-Forming Materials)

Teaser

• 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...

Summary

• 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?
The data obtained in this project provides us basic and critical information about the origin and evolution of the solid Earth. One of the important implications is that the nature of the light element in the core should be related to the origin of the magnetic field through geodynamo and hence the habitability of a planet. The convecting liquid iron core generates the magnetic field, which is protecting life on the surface from the harmful solar wind. The convection of the liquid outer core 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 field which has protected life since its birth.

• What are the overall objectives?
The key research questions that my 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?

Work performed

Objective 1 -Setting up a laboratory-
I first set up my laboratory in two separate rooms at the School of GeoSciences of the University of Edinburgh. The discussion about the refurbishment started early in 2015 and all the engineering work was finished in November 2015. In parallel, I started procurement of equipment for my laboratory. I installed major instruments which were Ion Slicer (JEOL) and Laser-Raman spectrometer (HORIBA). I also built an optic system (Princeton Instruments; Laser 2000) to measure the temperature in a resistive heating system in the diamond anvil cell (DAC).
Objective 2 -Fe-Si(-Ni)-
The phase transition between a face-centred cubic (fcc) and hexagonal close-packed (hcp) structure in Fe-Si alloys were examined in an internally resistive heated diamond anvil cell under high-pressure (P) and -temperature (T) conditions to 71 GPa and 2000 K by in-situ synchrotron X-ray diffraction. Starting from the precisely constrained phase loop in Fe-4wt%Si, we have constructed a thermodynamic model for the phase transition. It is well known that the mixing properties of Fe and hypothetical Si for the Fe phases are negatively non-ideal at 1 bar but we found that they become ideal with increasing P and T. The entropy change upon melting of each end-member, ∆SFe and ∆SSi, is fairly large and therefore the melting phase loop of the Si-bearing Fe phases would be wide unless the melting temperatures of the end-members (TFe, TSi) are close. The compositions of coexisting liquid and solid have been reported to be close (<1 wt%Si) at pressures greater than 50 GPa, which implies that TFe and TSi are close and as a consequence, the melting two-phase loop should be nearly temperature independent. The liquidus temperature of Fe alloy is therefore, not changed by the presence of Si at the inner core-outer core boundary, which implies that Si does not significantly affect the thermal structure of Earth’s core. The results were submitted to a journal.
The same transition in the system Fe-Si-Ni was also investigated with the same technique and the data analysis is in progress.
Objective 3 -Fe-S-
In-situ high-pressure and -temperature X-ray diffraction experiments were conducted on the system Fe-S in a laser-heated DAC. Our focus is on melting relations of Fe3S and equation of state of solid Fe3S which is the most iron rich sulfide compound. Because solid Fe3S is only stable under high pressure, the phase needs to be synthesized in the DAC. We first tried two starting materials: a mixture of powder material (Fe+FeS) and Fe3S which was presynthesised in a multianvil apparatus. However neither of them successfully forms monomineralic Fe3S in the DAC under high pressure. As such, we found that the synthesis of solid Fe3S in the DAC is extremely difficult. We finally found a solution with a custom-made alloy material which is homogeneous in composition at the nm scale. The new starting material made it possible to synthesise solid Fe3S in the DAC. We collected data on the sample to 100 GPa, 3000 K. The X-ray diffraction data analysis is in progress and recovered samples will be seen in a SEM.

Final results

I used my own cutting edge high-pressure and –temperature experimental technique, an internally-heated diamond anvil cell in our in-situ works. This technique shows much smaller uncertainties in the pressure and temperature than the conventional laser heating system and therefore, we were able to place tight constraints on the phase relations of iron alloys systems. Our results made it possible to deduce thermodynamic data of the systems studied.