4:25 PM - 4:40 PM
[SIT18-09] Dynamics of the inner core derived from the sound velocity of iron-light element compounds at high pressure and temperature
Keywords:Inner core, Sound velocity, Density, B2-FeNiSi alloy, hcp-FeNi alloy, Inelastic x-ray scattering
The phase relation of the Fe-Ni-Si system exhibits the coexistence of Si-rich Fe-Ni-Si alloy with the B2 structure and hexagonal close-packed (hcp) Fe-Ni alloy containing negligible amounts of Si under the core conditions (1). We showed experimentally that the relation of the compressional wave velocity vp and the density ρ (Birch’s law) of B2-Fe0.68Ni0.08Si0.26 has a weak or negligible temperature dependence. We extrapolated the ρ, vp, and shear wave velocity (vs) of these alloys to the inner core conditions for comparison with the Preliminary Reference Earth Model (PREM).
Assuming an inner core temperature of 6000 K., we estimated the composition of the inner core both at the inner core boundary (ICB) and the center of the core (COE) which can explain the ρ, vp, and vs of the PREM inner core. The inner core can be explained by a three-phase mixture of hcp-Fe0.95Ni0.05, B2-Fe0.68Ni0.06Si0.26, and Fe3S. We observed that the inner core composition at COE is sulfur-rich and silicon-deleted compared to that at ICB. The increase in sulfur content with depth in the inner core may have important implications. Considering the phase and melting relations of the iron-light element systems, iron-sulfide is a low melting temperature component that may be solidified in later stages of inner core crystallization. Therefore, we can suggest that the vp-ρ relation of the PREM inner core may provide evidence for a compositionally stratified inner core, formed by convective overturning (e.g., 2) or lopsided growth of the inner core (e.g., 3), with the sulfur-rich low-temperature component in the deep inner core. Enrichment of other light elements, such as oxygen, carbon, and hydrogen, in the deep inner core cannot account for the differences in density and sound velocity from the shallow inner core, based on Birch's law of these compounds. The difference in volume fractions of hcp-Fe–Ni, B2-Fe–Ni–Si, and Fe3S phases between the shallow and deep inner core could contribute to the distinct anisotropy observed in seismology (4).
References
(1) Ikuta, D. et al. (2021) Commun. Earth Environ. 2, 225.
(2) Cottaar S. and Buffett, B. (2012) Phys. Earth Planet. Inter. 198-199, 67-78
(3) Monnereau, M. et al. (2010) Science 328, 1014-1017 (2010)
(4) Wang, T. and Song, X. (2018) Phys. Earth Planet. Inter. 276, 247-257.
Assuming an inner core temperature of 6000 K., we estimated the composition of the inner core both at the inner core boundary (ICB) and the center of the core (COE) which can explain the ρ, vp, and vs of the PREM inner core. The inner core can be explained by a three-phase mixture of hcp-Fe0.95Ni0.05, B2-Fe0.68Ni0.06Si0.26, and Fe3S. We observed that the inner core composition at COE is sulfur-rich and silicon-deleted compared to that at ICB. The increase in sulfur content with depth in the inner core may have important implications. Considering the phase and melting relations of the iron-light element systems, iron-sulfide is a low melting temperature component that may be solidified in later stages of inner core crystallization. Therefore, we can suggest that the vp-ρ relation of the PREM inner core may provide evidence for a compositionally stratified inner core, formed by convective overturning (e.g., 2) or lopsided growth of the inner core (e.g., 3), with the sulfur-rich low-temperature component in the deep inner core. Enrichment of other light elements, such as oxygen, carbon, and hydrogen, in the deep inner core cannot account for the differences in density and sound velocity from the shallow inner core, based on Birch's law of these compounds. The difference in volume fractions of hcp-Fe–Ni, B2-Fe–Ni–Si, and Fe3S phases between the shallow and deep inner core could contribute to the distinct anisotropy observed in seismology (4).
References
(1) Ikuta, D. et al. (2021) Commun. Earth Environ. 2, 225.
(2) Cottaar S. and Buffett, B. (2012) Phys. Earth Planet. Inter. 198-199, 67-78
(3) Monnereau, M. et al. (2010) Science 328, 1014-1017 (2010)
(4) Wang, T. and Song, X. (2018) Phys. Earth Planet. Inter. 276, 247-257.