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1. Deep Earth carbon reactions through time and space

   https://doi.org/10.2138/am-2020-6888CCBY  

   McCammon, Catherine; Bureau, Hélène; Cleaves, James H.; Cottrell, Elizabeth; Dorfman, Susannah M.; Kellogg, Louise H.; Li, Jie; Mikhail, Sami; Moussallam, Yves; Sanloup, Chrystele; et al (January 2020, American Mineralogist)

   Abstract Reactions involving carbon in the deep Earth have limited manifestations on Earth's surface, yet they have played a critical role in the evolution of our planet. The metal-silicate partitioning reaction promoted carbon capture during Earth's accretion and may have sequestered substantial carbon in Earth's core. The freezing reaction involving iron-carbon liquid could have contributed to the growth of Earth's inner core and the geodynamo. The redox melting/freezing reaction largely controls the movement of carbon in the modern mantle, and reactions between carbonates and silicates in the deep mantle also promote carbon mobility. The 10-year activity of the Deep Carbon Observatory has made important contributions to our knowledge of how these reactions are involved in the cycling of carbon throughout our planet, both past and present, and has helped to identify gaps in our understanding that motivate and give direction to future studies. 

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2. Water‐Induced Diamond Formation at Earth's Core‐Mantle Boundary

   https://doi.org/10.1029/2022GL098271  

   Ko, Byeongkwan; Chariton, Stella; Prakapenka, Vitali; Chen, Bin; Garnero, Edward J.; Li, Mingming; Shim, Sang‐Heon (August 2022, Geophysical Research Letters)

   Abstract The carbon and water cycles in the Earth's interior are linked to key planetary processes, such as mantle melting, degassing, chemical differentiation, and advection. However, the role of water in the carbon exchange between the mantle and core is not well known. Here, we show experimental results of a reaction between Fe₃C and H₂O at pressures and temperatures of the deep mantle and core‐mantle boundary (CMB). The reaction produces diamond, FeO, and FeH_x, suggesting that water can liberate carbon from the core in the form of diamond (“core carbon extraction”) while the core gains hydrogen, if subducted water reaches to the CMB. Therefore, Earth's deep water and carbon cycles can be linked. The extracted core carbon can explain a significant amount of the present‐day mantle carbon. Also, if diamond can be collected by mantle flow in the region, it can result in unusually high seismic‐velocity structures. 

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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. Density of Fe‐Ni‐C Liquids at High Pressures and Implications for Liquid Cores of Earth and the Moon

   https://doi.org/10.1029/2020JB021089  

   Zhu, Feng; Lai, Xiaojing; Wang, Jianwei; Amulele, George; Kono, Yoshio; Shen, Guoyin; Jing, Zhicheng; Manghnani, Murli H.; Williams, Quentin; Chen, Bin (March 2021, Journal of Geophysical Research: Solid Earth)

   Abstract The presence of light elements in the metallic cores of the Earth, the Moon, and other rocky planetary bodies has been widely proposed. Carbon is among the top candidates in light of its high cosmic abundance, siderophile nature, and ubiquity in iron meteorites. It is, however, still controversial whether carbon‐rich core compositional models can account for the seismic velocity observations within the Earth and lunar cores. Here, we report the density and elasticity of Fe₉₀Ni₁₀‐3 wt.% C and Fe₉₀Ni₁₀‐5 wt.% C liquid alloys using synchrotron‐based X‐ray absorption experiments and first‐principles molecular dynamics simulations. Our results show that alloying of 3 wt.% and 5 wt.% C lowers the density of Fe₉₀Ni₁₀liquid by ∼2.9–3.1% at 2 GPa, and ∼3.4–3.6% at 9 GPa. More intriguingly, our experiments and simulations both demonstrate that the bulk moduli of the Fe‐Ni‐C liquids are similar to or slightly higher than those of Fe‐Ni liquids. Thus, the calculated compressional velocities (v_p) of Fe‐Ni‐C liquids are higher than that of pure Fe‐Ni alloy, promoting carbon as a possible candidate to explain the elevatedv_pin the Earth's outer core. However, the values and slopes of both density andv_pof the studied two Fe‐Ni‐C liquids do not match the outer core seismic models, suggesting that carbon may not be the sole principal light element in Earth's outer core. The highv_pof Fe‐Ni‐C liquids does not match the presumptivev_pof the lunar outer core well, indicating that carbon is less likely to be its dominant light element. 

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5. The Lithophile Element Budget of Earth's Core

   https://doi.org/10.1029/2021GC009986  

   Chidester, B. A.; Lock, S. J.; Swadba, K. E.; Rahman, Z.; Righter, K.; Campbell, A. J. (February 2022, Geochemistry, Geophysics, Geosystems)

   Abstract The relative composition of Earth's core and mantle were set during core formation. By determining how elements partition between metal and silicate at high pressures and temperatures, measurements of the mantle composition and geophysical observations of the core can be used to understand the mechanisms by which Earth formed. Here we present the results of metal‐silicate partitioning experiments for a range of nominally lithophile elements (Al, Ca, K, Mg, O, Si, Th, and U) and S to 85 GPa and up to 5400 K. With our results and a compilation of literature data, we developed a parameterization for partitioning that accounts for compositional dependencies in both the metal and silicate phases. Using this parameterization in a range of planetary growth models, we find that, in general, lithophile element partitioning into the metallic phase is enhanced at high temperatures. The relative abundances of FeO, SiO₂, and MgO in the mantle vary significantly between planetary growth models, and the mantle abundances of these elements can be used to provide important constraints on Earth's accretion. To match Earth's core mass and mantle composition, Earth's building blocks must have been enriched in Fe and depleted in Si compared with CI chondrites. Finally, too little Mg, Si, and O are partitioned into the core for precipitation of oxides to be a major source of energy for the geodynamo. In contrast, several ppb of U can be partitioned into the core at high temperatures, and this energy source must be accounted for in thermal evolution models. 

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