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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. 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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3. 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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4. Iron Isotope Systematics of the Skaergaard Intrusion and Implications for its Liquid Line of Descent

   https://doi.org/10.1093/petrology/egad053  

   Lesher, Charles E. ; Brown, Eric L. ; Barfod, Gry H. ; Glessner, Justin ; Stausberg, Niklas ; Thy, Peter ; Tegner, Christian ; Salmonsen, Lars Peter ; Nielsen, Troels F. D. ( July 2023 , Journal of Petrology)

   The Skaergaard intrusion is one of the most thoroughly studied layered mafic intrusions on Earth and an exceptional example of (near) closed-system magmatic differentiation. We report new Fe isotope data for whole rocks, and magnetite and ilmenite separates through the layered series (LS) and upper border series (UBS) of the intrusion. δ56Fe values for gabbroic rocks range from 0.033‰ to 0.151‰ with an abrupt step increase at the base of Lower Zone c (LZc) within LS with the appearance of cumulus magnetite and subsequent decline accompanying FeTi oxide fractionation. The lowest δ56Fe values are found near the Upper Zone b (UZb) to c (UZc) boundary followed by a sharp rise across UZc approaching the Sandwich Horizon. Magnetite–ilmenite separates straddle bulk rock compositions with fractionation factors (Δ56Femt-ilm) of 0.081‰ to 0.239‰, consistent with subsolidus equilibration. Granophyric rocks occurring as pods, sheets and wispy layers from the upper zone and UBS equivalents and having unradiogenic Sr similar to gabbroic rocks of Skaergaard, are isotopically heavier than their host ferrodiorites (Δ56Fegranophyre-ferrodiorite ≥ 0.1‰) reaching a maximum δ56Fe of 0.217‰ in UBS. A fused xenolith from UBS has δ56Fe = 0.372‰. This range in δ56Fe spans much of that reported for terrestrial igneous rocks, and like the global dataset, shows a pronounced increase in δ56Fe with inferred silica content of modeled Skaergaard liquids.

   Forward modeling of closed system fractional solidification was undertaken to account for Fe isotope systematics, first by testing published liquid lines of descent (LLD), and then by exploring improvements and considering the impacts of liquid immiscibility, crustal contamination, fluid exsolution and diffusional processes. Our modeling relies on published Fe+2 and Fe+3 force constants for magmatic minerals and silicate glasses, and the most reliable estimates of the average bulk composition and mass proportions of the well-defined subzones of the intrusion. We show that the increase in δ56Fe across the LZb–LZc boundary is readily explained by the increased incorporation of Fe+3 into the crystallizing solid including magnetite. We further demonstrate that the classic Fenner LLD, involving strong Fe enrichment at nearly constant silica, does not lead to a rise in δ56Fe toward the end stages of evolution, while a Bowen-like LLD, with little Fe enrichment and strong Si enrichment, also underestimates enrichment in heavy Fe isotopes in the ferrodiorites of UZc. A LLD following an intermediate path involving modest Fe and Si enrichment, followed by Fe depletion best explains the observations. We predict ~3.5% (by mass) residual liquid after crystallization of UZc having a composition similar to felsic segregations in pegmatitic bodies found in the intrusion. While liquid immiscibility may have been encountered within fractionating mush at the margins of the intrusion, the Fe isotope systematics do not support liquid phase separation of the bulk magma. Crustal contamination, fluid exsolution, hydrothermal alteration and thermal diffusion are also shown to have no resolvable effect on the Fe isotope composition of the gabbroic and granophyric rocks. We conclude that the Fe isotope systematics documented in the Skaergaard intrusion reflect the dominant role of fractionating Fe-rich minerals from gabbroic through ferrodioritic to rhyolitic liquids. The success of our model to account for the observed Fe isotope systematics for Skaergaard demonstrates the utility of Fe+2 and Fe+3 force constants determined at ambient conditions to model magmatic conditions and gives critical insights into plutonic processes fractionating Fe isotopes complementary to the volcanic record.

    

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5. Terrestrial planet compositions controlled by accretion disk magnetic field

   https://doi.org/10.1186/s40645-021-00429-4  

   McDonough, William F. ; Yoshizaki, Takashi ( July 2021 , Progress in Earth and Planetary Science)

   Terrestrial planets (Mercury, Venus, Earth, and Mars) are differentiated into three layers: a metallic core, a silicate shell (mantle and crust), and a volatile envelope of gases, ices, and, for the Earth, liquid water. Each layer has different dominant elements (e.g., increasing iron content with depth and increasing oxygen content to the surface). Chondrites, the building blocks of the terrestrial planets, have mass and atomic proportions of oxygen, iron, magnesium, and silicon totaling ≥ 90% and variable Mg/Si (∼ 25%), Fe/Si (factor of ≥2), and Fe/O (factor of ≥ 3). What remains an unknown is to what degree did physical processes during nebular disk accretion versus those during post-nebular disk accretion (e.g., impact erosion) influence these planets final bulk compositions. Here we predict terrestrial planet compositions and show that their core mass fractions and uncompressed densities correlate with their heliocentric distance, and follow a simple model of the magnetic field strength in the protoplanetary disk. Our model assesses the distribution of iron in terms of increasing oxidation state, aerodynamics, and a decreasing magnetic field strength outward from the Sun, leading to decreasing core size of the terrestrial planets with radial distance. This distribution enhances habitability in our solar system and may be equally applicable to exoplanetary systems.

    

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