I/Pu reveals Earth mainly accreted from volatile-poor differentiated planetesimals

Area of Science:

• Geochemistry
• Planetary Science
• Earth Science

Background:

• Mid-ocean ridge basalts (MORBs) and ocean island basalts (OIBs) exhibit distinct iodine/plutonium ratios, suggesting differences in Earth's accretion and mantle evolution.
• The geochemical behavior of iodine and plutonium during core formation is crucial for interpreting these isotopic differences but remains poorly understood.

Purpose of the Study:

• To quantify the metal-silicate partition coefficients of iodine and plutonium during core formation using first-principles molecular dynamics.
• To model multistage core formation to assess its impact on iodine/plutonium ratios in Earth's mantle reservoirs.
• To determine whether observed iodine/plutonium variations are solely due to core formation or indicate heterogeneous accretion.

Main Methods:

• First-principles molecular dynamics simulations to calculate partition coefficients.
• Multistage core formation modeling incorporating calculated partition coefficients.
• Analysis of iodine and plutonium behavior during metal-silicate segregation.

Main Results:

• Both iodine and plutonium exhibit partial partitioning into the metallic liquid during core formation.
• Core formation alone cannot account for the observed ~3× higher iodine/plutonium ratio in MORBs compared to OIBs.
• The results strongly support a heterogeneous accretion model for Earth.

Conclusions:

• Earth's accretion was heterogeneous, involving an early phase of volatile-poor material followed by late accretion of volatile-rich chondrites.
• This heterogeneous accretion explains the distinct iodine/plutonium ratios observed in different mantle reservoirs.
• Earth inherited a significant portion of its volatiles, including water, from late-stage chondritic accretion.