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1. Nickel isotopic evidence for late-stage accretion of Mercury-like differentiated planetary embryos

   https://doi.org/10.1038/s41467-020-20525-1  

   Wang, Shui-Jiong; Wang, Wenzhong; Zhu, Jian-Ming; Wu, Zhongqing; Liu, Jingao; Han, Guilin; Teng, Fang-Zhen; Huang, Shichun; Wu, Hongjie; Wang, Yujian; et al (December 2021, Nature Communications)

   Abstract Earth’s habitability is closely tied to its late-stage accretion, during which impactors delivered the majority of life-essential volatiles. However, the nature of these final building blocks remains poorly constrained. Nickel (Ni) can be a useful tracer in characterizing this accretion as most Ni in the bulk silicate Earth (BSE) comes from the late-stage impactors. Here, we apply Ni stable isotope analysis to a large number of meteorites and terrestrial rocks, and find that the BSE has a lighter Ni isotopic composition compared to chondrites. Using first-principles calculations based on density functional theory, we show that core-mantle differentiation cannot produce the observed light Ni isotopic composition of the BSE. Rather, the sub-chondritic Ni isotopic signature was established during Earth’s late-stage accretion, probably through the Moon-forming giant impact. We propose that a highly reduced sulfide-rich, Mercury-like body, whose mantle is characterized by light Ni isotopic composition, collided with and merged into the proto-Earth during the Moon-forming giant impact, producing the sub-chondritic Ni isotopic signature of the BSE, while delivering sulfur and probably other volatiles to the Earth. 

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2. Hafnium-tungsten evolution with pebble accretion during Earth formation

   https://doi.org/10.1016/j.epsl.2023.118418  

   Olson, Peter L; Sharp, Zachary D (November 2023, Earth and Planetary Science Letters)

   We combine calculations of pebble accretion and accretion by large and giant impacts to quantify the effects of pebbles on the hafnium-tungsten system during Earth formation. Our models include an early pebble accretion phase lasting 4–6 Myr with a global magma ocean and core segregation, a 20–50 Myr phase of large impacts, and a late giant impact representing the Moon-forming event. We consider various mass additions during each accretion phase, vary the metal-silicate partition coefficient for tungsten over a wide range, and track (180)Hf, (182)Hf, (182)W and (184)W in proto-Earth and impactor models over time using standard chondritic values for these isotopes in the pebbles. We find that an early phase of pebble accretion is compatible with the tungsten anomaly of Earth's early mantle as well as the present-day Hf/W ratio, but under restricted conditions. In particular, the pebble mass of proto-Earth is limited to 0.7 Earth masses or less, the average metal-silicate partition coefficient for tungsten is 30–50, and because the metal-silicate equilibration efficiency for giant impacts is low, the equilibration efficiency must be high for the large impactors. 

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3. Survival of Asteroid-sized Debris from the Moon-forming Impactor in Earth’s Deep Mantle with Implications for Its Solar System Provenance

   https://doi.org/10.3847/PSJ/ae1cbf  

   Yuan, Qian (December 2025, The Planetary Science Journal)

   Abstract As the largest terrestrial planet in the solar system, Earth experienced a prolonged major accretion, ending with the Moon-forming giant impact (MFGI), whereas the direct evidence and origin of the impactor Theia remain elusive. Recent computational studies indicate that parts of the impactor Theia mantle may persist above Earth’s core–mantle boundary as the large low-velocity provinces (LLVPs), yet it remains unclear how these results were affected by the initial size of Theia fragments after the MFGI. Here I explore such influence in whole-mantle convection simulations, assuming that the Theia debris size follows the size distribution of the main-belt asteroids, which provides a natural estimation of collision debris for the ill-constrained parameter during extreme impacts. The results demonstrate that the asteroid-sized Theia debris can survive Earth’s 4.5-billion-year convective history as large-scale thermochemical structures resembling the seismically observed LLVPs. The results also demonstrate that rheologically strong Theia fragments are more capable of long-term preservation compared to those with weaker compositions. The inferred viscosity of Theia fragments aligns with that proposed for LLVPs from noble gas isotope evidence for a dry plume mantle source and agrees with global mantle attenuation constraints from seismic normal modes. These findings provide insight into the physical mechanism of preserving ancient geochemical signatures in Earth’s mantle, support an inner solar system provenance for the impactor Theia, and further help explain the isotopic homogeneity between Earth and the Moon. 

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4. A magma ocean origin to divergent redox evolutions of rocky planetary bodies and early atmospheres

   https://doi.org/10.1038/s41467-020-15757-0  

   Deng, Jie; Du, Zhixue; Karki, Bijaya B.; Ghosh, Dipta B.; Lee, Kanani K. M. (April 2020, Nature Communications)

   Abstract Magma oceans were once ubiquitous in the early solar system, setting up the initial conditions for different evolutionary paths of planetary bodies. In particular, the redox conditions of magma oceans may have profound influence on the redox state of subsequently formed mantles and the overlying atmospheres. The relevant redox buffering reactions, however, remain poorly constrained. Using first-principles simulations combined with thermodynamic modeling, we show that magma oceans of Earth, Mars, and the Moon are likely characterized with a vertical gradient in oxygen fugacity with deeper magma oceans invoking more oxidizing surface conditions. This redox zonation may be the major cause for the Earth’s upper mantle being more oxidized than Mars’ and the Moon’s. These contrasting redox profiles also suggest that Earth’s early atmosphere was dominated by CO₂and H₂O, in contrast to those enriched in H₂O and H₂for Mars, and H₂and CO for the Moon. 

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5. Delivery of carbon, nitrogen, and sulfur to the silicate Earth by a giant impact

   https://doi.org/10.1126/sciadv.aau3669  

   Grewal, Damanveer S.; Dasgupta, Rajdeep; Sun, Chenguang; Tsuno, Kyusei; Costin, Gelu (January 2019, Science Advances)

   Earth’s status as the only life-sustaining planet is a result of the timing and delivery mechanism of carbon (C), nitrogen (N), sulfur (S), and hydrogen (H). On the basis of their isotopic signatures, terrestrial volatiles are thought to have derived from carbonaceous chondrites, while the isotopic compositions of nonvolatile major and trace elements suggest that enstatite chondrite–like materials are the primary building blocks of Earth. However, the C/N ratio of the bulk silicate Earth (BSE) is superchondritic, which rules out volatile delivery by a chondritic late veneer. In addition, if delivered during the main phase of Earth’s accretion, then, owing to the greater siderophile (metal loving) nature of C relative to N, core formation should have left behind a subchondritic C/N ratio in the BSE. Here, we present high pressure-temperature experiments to constrain the fate of mixed C-N-S volatiles during core-mantle segregation in the planetary embryo magma oceans and show that C becomes much less siderophile in N-bearing and S-rich alloys, while the siderophile character of N remains largely unaffected in the presence of S. Using the new data and inverse Monte Carlo simulations, we show that the impact of a Mars-sized planet, having minimal contributions from carbonaceous chondrite-like material and coinciding with the Moon-forming event, can be the source of major volatiles in the BSE. 

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