Some Issues on Core‐Mantle Chemical Interactions: The Role of Core Formation Processes

Abstract

A model of core formation is reviewed a nd its consequences on mantle chemistry and core-mantle interaction are discussed. A growing planet forms a cold proto-core made of a mixture of various materials including primitive materials rich in highly siderophile elements (HSE) as well as volatiles. These primitive materials are squeezed out to the bottom of the magma ocean when Fe accumulated above the proto-core sinks to the center. After heated, the proto-core materials are mixed with the magma ocean. Consequently, mantle is made of a mixture of materials equilibrated with Fe at modest pressure and temperature and a small amount of primitive materials from the proto-core. This model explains the observed abundance pattern of all siderophile elements and predicts that most of HSE (+ volatiles) came from the proto-core formed early in the process of Earth formation rather than added in the later stage. The model implies that most of the core materials are in equilibrium with the mantle at the lower pressure and temperature than those of the current core-mantle boundary (CMB). Therefore, the core is undersaturated with volatile and siderophile elements at the CMB and, consequently, the core is likely a sink (not a source) of these elements even if the concentrations of these elements in the bulk of the core exceed those in the mantle. The transport of light elements from the mantle to the core provides a mechanism for a low-velocity layer on top of the core. The core-mantle disequilibrium promotes the migration of molten Fe into the mantle by the morphological instability in regions of the CMB where (Mg,Fe)O is interconnected. In these regions (presumably the ultralow-velocity regions), volatile (and siderophile) elements are carried by the molten Fe ∼tens km into the mantle providing a window for the core chemistry.