HIGH-PRESSURE MELTING OF CARBONACEOUS CHONDRITE

Phase equilibria of carbonaceous chondrite and phase density relations at high pressure govern a simple model for solidification of an initially molten chondritic Earth. The solidification can be described by differentiation stages, each characterized by a particular, experimentally predicted, petrologic process that dominates the distribution of chemical components during a specific evolution interval. The stages, though expressed here as discrete events for simplicity, may actually be overlapping segments in the cooling and crystallization continuum of Earth formation. We focus on five important differentiation stages of a chondritic Earth in the following chronological order: (1) sulfide, liquid-silicate liquid immiscibility leading to protocore formation; (2) magnesiowustite fractionation at the base of the early mantle; (3) perovskite flotation and fractionation in the midmantle; (4) olivine flotation in the shallow mantle; and (5) garnet flotation in the prototransition zone. Compositional layering produced by the five experimentally predicted differentiation stages obeys mass balance and is consistent with a shallow mantle source for peridotite xenoliths, a transition zone rich in garnet at approximately 540 km, a lower mantle with a superchondritic Si/Mg, and a core of Fe-Ni-S-O.