Accretion of Rocky Planetary Bodies: Chemical Constraints

All rocky materials originating from small and undifferentiated bodies of the Solar System have similar and close to solar abundances of refractory elements and are depleted to various degrees in moderately volatile elements. This depletion correlates with the respective condensation temperatures of the elements. In contrast, the element abundance pattern in bulk silicate Earth (BSE) is different. Its depletion in the moderately volatile lithophile elements does not show a correlation with their respective condensation temperature, rather these elements show chondritic relative abundances.

The element pattern of BSE can be modeled as a mixture of three distinct components with different volatile element depletions and oxidation states. Component A is volatile-element depleted and strongly reduced; Component B has chondritic abundances of the refractory and moderately volatile elements and is highly oxidized; Component C has element abundances similar to carbonaceous chondrites. These components make up ~85%, ~15% and ~0.4% of BSE, respectively, and require a specific sequence of events by which they were added. The major part of the Earth accreted early under strongly reducing conditions during which it underwent metal-silicate differentiation. Addition of the oxidized and volatile-richer second component was followed by a second core formation under more oxidizing conditions by segregation of a sulfide melt. The last component was added as a “late veneer”. Combining this evolution of element abundances in BSE with constraints from U-Pb and Rb-Sr systematics suggest the combination of Component A with B occurred at 4.50 Ga and corresponds to the Moon-forming event.