Accretion and core formation: constraints from metal-silicate partitioning

Area of Science:

• Geochemistry
• Planetary Science
• Earth Science

Background:

• Investigating Earth's accretion and core formation requires understanding metal-silicate partitioning.
• Chondritic ratios of refractory elements and magma ocean dynamics are key assumptions.
• Previous models often assumed fixed oxidation states during planetary growth.

Purpose of the Study:

• To explore geochemical consequences of various Earth accretion and core formation models.
• To reconcile observed elemental abundances in Earth's mantle and core with theoretical models.
• To determine the role of oxidation state changes during Earth's growth.

Main Methods:

• Utilized experimental metal-silicate partitioning data for elements like Ni, Co, V, Cr, Nb, Mn, Si, and W.
• Examined four distinct models of continuous metal segregation from a magma ocean.
• Assessed models involving chemically homogeneous and heterogeneous mantles, fixed and evolving oxidation states, and crystal-melt mush scenarios.

Main Results:

• Models with progressive oxidation during Earth's growth successfully match Ni, Co, W, V, Cr, and Nb abundances.
• A model of continuous segregation from a homogeneous, increasingly oxidized mantle predicts a core silicon content of 5.7%.
• Manganese is found to be depleted in Earth relative to chondritic abundances across all models.

Conclusions:

• Progressive oxidation during Earth's accretion is a critical factor in explaining core and mantle compositions.
• The magma ocean likely deepened significantly, reaching pressures up to 44 GPa.
• Observed elemental depletions, particularly for manganese, require further investigation.