Регистрация / Вход
Прислать материал

Chemical and Physical Properties and Composition of the Earth’s Core

Name
Ohtani
Surname
Eiji
Scientific organization
V.S. Sobolev Institute of Geology and Geophysics, Novosibirsk, Russia
Academic degree
Doctor of Science
Position
head of laboratory
Scientific discipline
Earth Sciences, Ecology & Environmental Management
Topic
Chemical and Physical Properties and Composition of the Earth’s Core
Abstract
We conducted high-pressure mineral physics studies of the Earth’s central regions. The amounts of light elements in the core are the key to clarify the formation of the Earth. We made high pressure experiments on the phase relations, compression, and sound velocity measurements. Our results on the equation of state and the sound velocity of solid and liquid iron and iron–nickel alloys strongly suggest that the core contains light elements to reduce both density and sound velocity.
Keywords
High Pressure, Core, phase relation, compression, sound velocity, partitioning
Summary

We conducted high-pressure mineral physics studies of the Earth’s central regions. The amounts of light elements in the core are the key to clarify the formation process of the Earth. We made high pressure experiments on the phase relations, compression, and sound velocity measurements of the iron-light element systems.

Sound velocity is the most accurate information in seismology and can provide important clues on the structure and composition of the core. In spite of its importance, the sound velocity data of the core materials at high pressure and temperature are still very limited due to technical difficulties. Recent seismological studies revealed that inner core shows seismological anisotropy together with internal structure of the inner core, such as inner-most inner core. There are debated matters also on the structure and composition of the inner core, i.e., inner core might be hexagonal close-packed iron (hcp) or body-centered cubic (bcc) structure.

We measured the compressional velocity of hcp-iron and the other alloys by the inelastic X-ray scattering (IXS) method using DAC at high pressure and temperature. Inelastic X-ray scattering spectra were taken at BL35XU, Spring-8. We made the IXS measurement to 174 GPa, the highest pressure at room temperature in this method [1]. Compressional velocity measurements at high temperature were made by using both external heating and laser heating [2]. We developed a portable laser heating system and successfully measured the IXS spectra to temperatures up to 3000 K at 163 GPa.

Measurements of sound velocities in metals at Mbar pressures, with specific focus on the compressional sound velocity of hcp-iron were reviewed [3]. A critical comparison of our results [1] and literature results, coherently analyzed using consistent metrology (pressure scale, equation of state), allows us to propose reference relations for the pressure and density dependence of the compressional velocity of hexagonal close-packed iron at ambient temperature. This provides a key base line upon which to add complexity, including high-temperature effects, pre-melting effects, effects of nickel and/or light element incorporation, necessary for an accurate comparison with seismic models, and ultimately to constrain Earth’s inner core composition. 

Hexagonal close-packed iron (hcp-Fe) is a main component of Earth’s inner core. The difference in density between hcp-Fe and the inner core in the Preliminary Reference Earth Model (PREM) shows a density deficit, which implies an existence of light elements in the core. We measured the compressional sound velocity (VP) of hcp-Fe up to 163 GPa and 3000 K using inelastic X-ray scattering combined with a laser-heated sample in a diamond anvil cell [4]. We propose a new high-temperature Birch’s law for hcp-Fe, which gives us the VP of pure hcp-Fe up to the core conditions. We find that Earth’s inner core has a 4 to 5% smaller density and a 4 to 10% smaller VP than hcp-Fe. Our results demonstrate that components other than Fe in Earth’s core are required to explain Earth’s core density and velocity deficits compared to hcp-Fe. Assuming that the temperature effects on iron alloys are the same as those on hcp-Fe, we narrow down light elements in the inner core in terms of the velocity deficit. Hydrogen is a good candidate; thus, Earth’s core may be a hidden hydrogen reservoir. Silicon and sulfur are also possible candidates and could show good agreement with PREM if we consider the presence of some melt in the inner core, anelasticity, and/or a premelting effect.

Inner core was crystallized from the outer core. The light elements contents of the inner and outer core must be controlled by the element partitioning between the liquid and solid iron alloy, i.e., outer and inner cores.  We call it the inner core fractionation. The phase relationships and the crystallization temperatures in the Fe-S-Si system were determined up to 60 GPa and the Fe-S system up to 120 GPa [5] using a laser-heated diamond anvil cell combined with X-ray diffraction technique. On the basis of X-ray diffraction patterns, we confirmed that hcp/fcc Fe-Si alloy and Fe3S were stable phases under subsolidus conditions in the Fe-S-Si system. Because of dissolution of silicon into iron, the boundary of fcc and hcp phase shifts towards higher pressure compared to that of pure iron. Both solidus and liquidus temperatures are significantly lower than the melting temperature of pure Fe and increases with pressure in this study. As compared with the slope of the present Fe-S-Si liquidus and temperature profiles of the core of planets, inner core crystallization of the Earth occurred at the bottom of the liquid outer core whereas the crystallization of the Martian core must begin at the core-mantle boundary of the Mars. 

We made an experimental study on solid–liquid partitioning in the Fe–S–Si, Fe–S–Si–O, and Fe–S–Si–O–Ni systems up to 148 GPa and demonstrated that the metallic liquid phase is relatively sulfur rich, whereas the coexisting hcp-Fe phase is silicon rich. Based on our partitioning data, the equation of state of solid and liquid iron alloys, and the seismologically observed density of the inner and outer cores, the total amount of light elements in the bulk core was constrained to be 7.4–9.9 wt. %.. The present results demonstrate that the present-day Earth has a sulfur-rich outer core and a sulfur depleted inner core, and the difference in light element contents creates the seismologically observed density contrast between the inner and outer cores.

The experimental results on partitioning of Si and S between the liquid and solid iron alloys and the compressional velocity data of hcp-Fe, Fe3S, and Fe0.83Ni0.09Si0.08 determined by the IXS method revealed that the PREM outer and inner cores can be accounted for by a pair of the sulfur-enriched outer core and sulfur-depleted and silicon-enriched inner core.

 

References:

[1] Ohtani E et al., Sound velocity of hexagonal close-packed iron up to core pressures. Geophysical Research Letters, 40, 5089-5094, 2013.

[2] Ohtani E et al., Sound velocity measurement by inelastic X-ray scattering at high pressure and temperature by resistive heating diamond anvil cell. Russian Geology and Geophysics, 56, 1-2, 190-195, DOI: 10.1016/j.rgg.2015.01.012, 2015.

[3] Antonangeli D and Ohtani E, Sound velocity of hcp-Fe at high pressure: experimental constraints, extrapolations and comparison with seismic models. Progress in Earth and Planetary Science 2:3, DOI 10.1186/s40645-015-0034-9, 2015.

[4] Sakamaki T, Ohtani E et al., Constraints on Earth’s inner core composition inferred from measurements of the sound velocity of hcp-iron in extreme conditions, Sci. Adv. 2, e1500802, 10.1126/sciadv.1500802, 2016.

[5] Kamada S, Ohtani E et al., Melting relationships in the Fe–Fe3S system up to the outer core conditions, Earth and Planetary Science Letters 359-360, 26–33, 2012.