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1. Thermal conductivity of Fe-Si alloys and thermal stratification in Earth’s core

   https://doi.org/10.1073/pnas.2119001119  

   Zhang, Youjun; Luo, Kai; Hou, Mingqiang; Driscoll, Peter; Salke, Nilesh P.; Minár, Ján; Prakapenka, Vitali B.; Greenberg, Eran; Hemley, Russell J.; Cohen, R. E.; et al (December 2021, Proceedings of the National Academy of Sciences)

   Light elements in Earth’s core play a key role in driving convection and influencing geodynamics, both of which are crucial to the geodynamo. However, the thermal transport properties of iron alloys at high-pressure and -temperature conditions remain uncertain. Here we investigate the transport properties of solid hexagonal close-packed and liquid Fe-Si alloys with 4.3 and 9.0 wt % Si at high pressure and temperature using laser-heated diamond anvil cell experiments and first-principles molecular dynamics and dynamical mean field theory calculations. In contrast to the case of Fe, Si impurity scattering gradually dominates the total scattering in Fe-Si alloys with increasing Si concentration, leading to temperature independence of the resistivity and less electron–electron contribution to the conductivity in Fe-9Si. Our results show a thermal conductivity of ∼100 to 110 W⋅m −1 ⋅K −1 for liquid Fe-9Si near the topmost outer core. If Earth’s core consists of a large amount of silicon (e.g., > 4.3 wt %) with such a high thermal conductivity, a subadiabatic heat flow across the core–mantle boundary is likely, leaving a 400- to 500-km-deep thermally stratified layer below the core–mantle boundary, and challenges proposed thermal convection in Fe-Si liquid outer core. 

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2. A hydrogen-enriched layer in the topmost outer core sourced from deeply subducted water

   https://doi.org/10.1038/s41561-023-01324-x  

   Kim, Taehyun; O’Rourke, Joseph G; Lee, Jeongmin; Chariton, Stella; Prakapenka, Vitali; Husband, Rachel J; Giordano, Nico; Liermann, Hanns-Peter; Shim, Sang-Heon; Lee, Yongjae (December 2023, Nature Geoscience)

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   The Earth’s core–mantle boundary presents a dramatic change in materials, from silicate to metal. While little is known about chemical interactions between them, a thin layer with a lower velocity has been proposed at the topmost outer core (Eʹ layer) that is difficult to explain with a change in concentration of a single light element. Here we perform high-temperature and -pressure laser-heated diamond-anvil cell experiments and report the formation of SiO2 and FeHx from a reaction between water from hydrous minerals and Fe–Si alloys at the pressure–temperature conditions relevant to the Earth’s core–mantle boundary. We suggest that, if water has been delivered to the core–mantle boundary by subduction, this reaction could enable exchange of hydrogen and silicon between the mantle and the core. The resulting H-rich, Si-deficient layer formed at the topmost core would have a lower density, stabilizing chemical stratification at the top of the core, and a lower velocity. We suggest that such chemical exchange between the core and mantle over gigayears of deep transport of water may have contributed to the formation of the putative Eʹ layer. 

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3. Nitrogen Content in the Earth’s Outer Core

   https://doi.org/https://doi.org/10.1029/ 2018GL080555  

   Bajgain, Suraj K.; Mookherjee, Mainak; Dasgupta, Rajdeep; Ghosh, Dipta B.; Karki, Bijaya B. (January 2019, Geophysical research letters)

   Using first principles molecular dynamic simulations, we explore the effects of nitrogen (N) on the density and sound velocity of liquid iron and evaluate its potential as a light element in the Earth’s outer core. Our results suggest that Fe-N melt cannot simultaneously explain the density and seismic velocity of the Earth’s outer core. Although ~2.0 wt.% N can explain the bulk sound velocity of the outer core, such N content only lowers the density of liquid Fe by ~3%. Matching both the velocity and density by the other light elements limits the N in the core to ≪2.0 wt.%. Our finding suggests that nitrogen is a minor to trace element in the Earth’s core and is consistent with the geochemical mass balance with terrestrial abundance of N and alloy-silicate partitioning data, which suggest that there cannot be significant N in the core. 

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4. Regional stratification at the top of Earth's core due to core–mantle boundary heat flux variations

   https://doi.org/https://doi.org/10.1038/s41561-019-0381-z  

   Mound, Jon; Davies, Chris; Rost, Sebastian; Aurnou, Jonathan M (June 2019, Nature geoscience)

   Earth’s magnetic field is generated by turbulent motion in its fluid outer core. Although the bulk of the outer core is vigorously convecting and well mixed, some seismic, geomagnetic and geodynamic evidence suggests that a global stably stratified layer exists at the top of Earth’s core. Such a layer would strongly influence thermal, chemical and momentum exchange across the core–mantle boundary and thus have important implications for the dynamics and evolution of the core. Here we argue that the relevant scenario is not global stratification, but rather regional stratification arising solely from the lateral variations in heat flux at the core–mantle boundary. Using our extensive suite of numerical simulations of the dynamics of the fluid core with het- erogeneous core–mantle boundary heat flux, we predict that thermal regional inversion layers extend hundreds of kilometres into the core under anomalously hot regions of the lowermost mantle. Although the majority of the outermost core remains actively convecting, sufficiently large and strong regional inversion layers produce a one-dimensional temperature profile that mimics a globally stratified layer below the core–mantle boundary—an apparent thermal stratification despite the average heat flux across the core–mantle boundary being strongly superadiabatic. 

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5. High‐Pressure Deformation of Iron–Nickel–Silicon Alloys and Implications for Earth’s Inner Core

   https://doi.org/10.1029/2020JB021077  

   Brennan, Matthew C.; Fischer, Rebecca A.; Couper, Samantha; Miyagi, Lowell; Antonangeli, Daniele; Morard, Guillaume (March 2021, Journal of Geophysical Research: Solid Earth)

   Abstract Earth’s inner core exhibits strong seismic anisotropy, often attributed to the alignment of hexagonal close‐packed iron (hcp‐Fe) alloy crystallites with the Earth’s poles. How this alignment developed depends on material properties of the alloy and is important to our understanding of the core’s crystallization history and active geodynamical forcing. Previous studies suggested that hcp‐Fe is weak under deep Earth conditions but did not investigate the effects of the lighter elements known to be part of the inner core alloy. Here, we present results from radial X‐ray diffraction experiments in a diamond anvil cell that constrain the strength and deformation properties of iron‐nickel‐silicon (Fe–Ni–Si) alloys up to 60 GPa. We also show the results of laser heating to 1650 K to evaluate the effect of temperature. Observed alloy textures suggest different relative activities of the various hcp deformation mechanisms compared to pure Fe, but these textures could still account for the theorized polar alignment. Fe–Ni–Si alloys are mechanically stronger than Fe and Fe–Ni; extrapolated to inner core conditions, Si‐bearing alloys may be more than an order of magnitude stronger. This enhanced strength proportionally reduces the effectivity of dislocation creep as a deformation mechanism, which may suggest that texture developed during crystallization rather than as the result of postsolidification plastic flow. 

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