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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.
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Primordial Helium‐3 Exchange Between Earth's Core and Mantle
Abstract Volatiles from the solar nebula are known to be present in Earth's deep mantle. The core also may contain solar nebula‐derived volatiles, but in unknown amounts. Here we use calculations of volatile ingassing and degassing to estimate the abundance of primordial3He now in the core and track the rate of3He exchange between the core and mantle through Earth history. We apply an ingassing model that includes a silicate magma ocean and an iron‐rich proto‐core coupled to a nebular atmosphere of solar composition to calculate the amounts of3He acquired by the mantle and core during accretion and core formation. Using experimentally determined partitioning between core‐forming metals and silicate magma, we find that dissolution from the nebular atmosphere deposits one or more petagrams of3He into the proto‐core. Following accretion,3He exchange depends on the convective history of the coupled core‐mantle system. We combine determinations of the present‐day surface3He flux with estimates of the present‐day mantle3He abundance, mantle and core heat fluxes, and our ingassed3He abundances in a convective degassing model. According to this model, the mantle3He abundance is evolving toward a statistical steady state, in which surface losses are compensated by enrichments from the core.
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- Award ID(s):
- 1953992
- PAR ID:
- 10446271
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geochemistry, Geophysics, Geosystems
- Volume:
- 23
- Issue:
- 3
- ISSN:
- 1525-2027
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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