Periodic Reporting for period 1 - LECOR (Light elements in the core)
Période du rapport: 2022-09-01 au 2025-02-28
Since the discovery of the Earth’s internal structure and the existence of a dense metallic core about a century ago, the idea of iron being the dominant component of the core gained firm support confirmed by cosmochemical and geochemical observations, seismic data, the theory of geomagnetism, and high-pressure studies. However, although closely matching, the velocities of seismic waves traveling through the core are significantly slower than those in a pure iron-nickel alloy. The observed core density- and velocity- deficit suggests that around 3-7 wt% of the light element(s) should be present in the inner core in order to explain the observed mismatch. Moreover, the inner core is anisotropic, with the compressional waves traveling faster along the polar axis than in the equatorial plane. Thus, the candidate material should be also able to explain the observed anisotropic pattern.
Nonetheless, the nature of the light element(s) in the core remains unconstrained, with hydrogen, carbon, oxygen, silicon, and sulfur being the most plausible candidates. The laboratory measurements on the physical properties of some candidate materials at high pressures and room temperature are available in the literature, but data at simultaneous high pressures and temperatures as most relevant to the Earth's core are almost absent. Therefore, experimental determination of sound velocities, densities, and plastic properties of candidate iron alloys at relevant pressure-temperature conditions is critically needed to evaluate the composition, seismic signatures, and geodynamics of the remotest region of our planet.
In LECOR, we aim to identify the most plausible candidate element, extending state-of-the-art measurement techniques considerably. In particular, we will study the elasticity and plastic deformation mechanisms of candidate binary and ternary iron alloys and compounds in situ at extreme pressure-temperature conditions using a combination of state-of-the-art synchrotron X-ray techniques developed in our group. We will interpret this novel data within the most recent geophysical and geochemical models, to better determine the composition of the Earth’s core. Such would open fascinating avenues to refine theories about the formation of planets, in general.
Nonetheless, the nature of the light element(s) in the core remains unconstrained, with hydrogen, carbon, oxygen, silicon, and sulfur being the most plausible candidates. The laboratory measurements on the physical properties of some candidate materials at high pressures and room temperature are available in the literature, but data at simultaneous high pressures and temperatures as most relevant to the Earth's core are almost absent. Therefore, experimental determination of sound velocities, densities, and plastic properties of candidate iron alloys at relevant pressure-temperature conditions is critically needed to evaluate the composition, seismic signatures, and geodynamics of the remotest region of our planet.
In LECOR, we aim to identify the most plausible candidate element, extending state-of-the-art measurement techniques considerably. In particular, we will study the elasticity and plastic deformation mechanisms of candidate binary and ternary iron alloys and compounds in situ at extreme pressure-temperature conditions using a combination of state-of-the-art synchrotron X-ray techniques developed in our group. We will interpret this novel data within the most recent geophysical and geochemical models, to better determine the composition of the Earth’s core. Such would open fascinating avenues to refine theories about the formation of planets, in general.
Over the first two years, we have determined sound velocities, densities, and plastic behavior of various iron-bearing compounds and alloys of iron with hydrogen, carbon, and silicon at extreme pressure-temperature conditions.
In particular, we determined sound velocities and densities of Fe7C3 employing nuclear inelastic scattering and x-ray diffraction experiments in diamond anvil cells at pressures up to 175 GPa and 2300 K and extrapolated them to inner core conditions. We show that sound velocities of Fe7C3 match seismic velocities observed in the Earth’s core within the error. However, the densities of Fe7C3 at core conditions are 7% less dense compared to the core. Altogether this study demonstrates that carbon is likely an important component of the core but should be present in amounts less than the eutectic composition in the Fe-Fe7C3 system.
Another main achievement is the investigation of the effect of silicon and carbon on the deformation of the hexagonal-close packed iron alloy employing radial x-ray diffraction at high-pressure, high-temperature conditions. We revealed the low anisotropy of the sound velocity (~2 %) of the Fe-Si-C alloy that is compatible with the anisotropy observed in the outer shells of the Earth’s inner core. This finding provides an explanation for the heterogeneous depth-dependent elastic anisotropy in the inner core originating from the stratification of silicon and carbon within the inner core upon its crystallization. The research provides a necessary link connecting the experimentally determined physical properties of iron alloys with the remote seismic observations.
From the technical point of view, we have successfully commissioned the setup for the measurement of powdered samples at extreme conditions for determining the aggregate compressional sound velocities. The Inelastic X-ray Scattering instrument coupled with the experiments at extremely high pressure-temperature conditions did not exist in Europe before.
In particular, we determined sound velocities and densities of Fe7C3 employing nuclear inelastic scattering and x-ray diffraction experiments in diamond anvil cells at pressures up to 175 GPa and 2300 K and extrapolated them to inner core conditions. We show that sound velocities of Fe7C3 match seismic velocities observed in the Earth’s core within the error. However, the densities of Fe7C3 at core conditions are 7% less dense compared to the core. Altogether this study demonstrates that carbon is likely an important component of the core but should be present in amounts less than the eutectic composition in the Fe-Fe7C3 system.
Another main achievement is the investigation of the effect of silicon and carbon on the deformation of the hexagonal-close packed iron alloy employing radial x-ray diffraction at high-pressure, high-temperature conditions. We revealed the low anisotropy of the sound velocity (~2 %) of the Fe-Si-C alloy that is compatible with the anisotropy observed in the outer shells of the Earth’s inner core. This finding provides an explanation for the heterogeneous depth-dependent elastic anisotropy in the inner core originating from the stratification of silicon and carbon within the inner core upon its crystallization. The research provides a necessary link connecting the experimentally determined physical properties of iron alloys with the remote seismic observations.
From the technical point of view, we have successfully commissioned the setup for the measurement of powdered samples at extreme conditions for determining the aggregate compressional sound velocities. The Inelastic X-ray Scattering instrument coupled with the experiments at extremely high pressure-temperature conditions did not exist in Europe before.
The research related to the nuclear inelastic scattering investigation of Fe7C3 is a significant technological breakthrough beyond the state-of-the-art. For the first time, it was possible to perform nuclear inelastic scattering experiments to derive sound velocities of materials at pressures relevant to the Earth’s core and at high temperatures. Achieving this breakthrough relied on both major development in the high-pressure high-temperature sample environment by our group and the utilization of the most recent advances in nuclear resonance scattering techniques of the nuclear resonance scattering beamline of the ESRF.