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Abstract Planetary formation involves highly energetic collisions, the consequences of which set the stage for the ensuing planetary evolution. During accretion, Earth's mantle was largely molten, a so‐called magma ocean, and its oxidation state was determined by equilibration with metal‐rich cores of infalling planetesimals through redox buffering reactions. We test two proposed mechanisms (metal layer and metal droplets) for equilibration in a magma ocean and the resulting oxidation state (Fe³⁺/ΣFe). Using scaling laws on convective mixing, we find that the metal layer could promote oxidation of a magma ocean, but this layer is too short‐lived to reproduce present‐day mantle Fe³⁺/ΣFe (2%–6%). Metal droplets produced by the fragmentation of impactor cores can also promote oxidation of a magma ocean. We use Monte Carlo sampling on two possible accretion scenarios to determine the likely range of oxidation states by metal droplets. We find that equilibration between silicate and metal droplets tends toward higher mantle Fe³⁺/ΣFe than presently observed. To achieve present‐day mantle Fe³⁺/ΣFe and maintain the degree of equilibration suggested by Hf‐W and U‐Pb systematics (30%–70%), the last (Moon‐forming) giant impact likely did not melt the entire mantle, therefore leaving the mantle stratified in terms of oxidation state after main accretion completes. Furthermore, late accretion impacts during the Hadean (4.5–4.0 Ga) could generate reduced domains in the shallow upper mantle, potentially sustaining surface environments conducive for prebiotic chemistry. 

Abstract Photoevaporation is thought to play an important role in early planetary evolution. In this study, we investigate the diffusion limit of X-ray- and ultraviolet-induced photoevaporation in primordial atmospheres. We find that compositional fractionation resulting from mass loss is more significant than currently recognized, because it is controlled by the conditions at the top of the atmosphere, where particle collisions are less frequent. Such fractionation at the top of the atmosphere develops a compositional gradient that extends downward. The mass outflow eventually reaches a steady state in which the hydrogen loss is diffusion-limited. We derive new analytic expressions for the diffusion-limited mass-loss rate and the crossover mass. 

Highly siderophile elements (HSEs; namely Ru, Rh, Pd, Re, Os, Ir, Pt, and Au) in Earth’s mantle require the addition of metals after the formation of Earth’s core. Early, large collisions have the potential to deliver metals, but the details of their mixing with Earth’s mantle remain unresolved. As a large projectile disrupts and penetrates Earth’s mantle, a fraction of its metallic core may directly merge with Earth’s core. Ensuing gravitational instabilities remove the remaining projectile’s core stranded in Earth’s mantle, leaving the latter deprived of HSEs. Here, we propose a framework that can efficiently retain the metallic components during large impacts. The mechanism is based on the ubiquitous presence of a partially molten region in the mantle beneath an impact-generated magma ocean, and it involves rapid three-phase flow with solid silicate, molten silicate, and liquid metal as well as long-term mixing by mantle convection. In addition, large low-shear-velocity provinces in the lower mantle may originate from compositional heterogeneities resulting from the proposed three-phase flow during high-energy collisions. 

The isotopic characteristics of ocean island basalts have long been used to infer the nature of their source and the long-term evolution of the Earth’s mantle. Anticorrelation between tungsten and helium isotopic signatures is a particularly puzzling feature in those basalts, which no single process appears to explain. Traditionally, the high 3 He/ 4 He signature has been attributed to an undegassed reservoir in the deep mantle. Additional processes needed to obtain low 182 W/ 184 W often entail unobserved ancillary geochemical effects. It has been suggested, however, that the core feeds the lower mantle with primordial helium, obviating the need for an undegassed mantle reservoir. Independently, the tungsten-rich core has been suggested to impart the plume source with anomalous tungsten isotope signatures. We advance the idea that isotopic diffusion may simultaneously transport both tungsten and helium across the core–mantle boundary, with the striking implication that diffusion can naturally account for the observed isotopic trend. By modeling the long-term isotopic evolution of mantle domains, we demonstrate that this mechanism can account for more than sufficient isotopic ratios in plume-source material, which, after dynamical transport to the Earth’s surface, are consistent with the present-day mantle W-He isotopic heterogeneities. No undegassed mantle reservoir is required, bearing significance on early Earth conditions such as the extent of magma oceans. 

Abstract Exposed continents are one of Earth's major characteristics. Recent studies on ancient ocean volume and exposed landmasses suggest, however, that early Earth was possibly a water world, where any significant landmass was unlikely to have risen above sea level. On modern Earth, the thickness of continental crust seems to be controlled by sea level and the buoyancy of continental crust. Simply applying this concept to the Archean would not explain the absence of exposed continents, and we suggest that a third element that is currently insignificant was important during early Earth: the strength of continental upper crust. Based on the pressure imbalance expected at continent-ocean boundaries, we quantified the conditions under which rock strength controls the thickness of continental crust. With the level of radiogenic heat production expected for early Earth, continents may have been too weak to have maintained their thickness against a deep ocean.