Accretion and core formation of the Earth: Constraints from high pressure experiments

The processes of accretion and core segregation are fundamental to understanding the evolution of our solar system and, indeed, set the stage for a planet's subsequent geologic evolution. Today, a remnant trace element signature in the Earth's mantle from these processes, "the excess siderophile element problem," fuels ongoing investigations. Two hypotheses have emerged: (1) Earth accreted heterogeneously from compositionally diverse material derived from varying annuli within the solar system. Core formation was an episodic process involving periods of metal-silicate equilibrium following impact of differentiated chondritic material. Mantle trace elements reflect only the very last episode of accretion, a late chondritic veneer. (2) Earth accreted from a narrow band of material around 1 AU. The mantle trace element signature was derived from metal-silicate equilibrium at high pressures and temperatures in the deep mantle. This study investigates (2) by conducting element partitioning and phase equilibria experiments at pressures, temperatures, and a composition relevant to the early Earth. Partitioning of Au between metal-sulfide liquid and silicate liquid [D(Au)met/sil] was chosen because almost no partitioning data exist for the high siderophile elements at the investigated conditions and new microanalytical techniques allow more thorough characterization of run products. Results of these experiments show D(Au)met/sil decreases with increasing pressure and temperature, suggesting core-mantle equilibrium occurred at an average depth of ∼21 GPa in a magma ocean. Results of phase equilibria studies suggest that perovskite may be the dominant hydrous phase in the lower mantle. The results support an equilibrium core formation process during a "wet" homogenous accretion.