New Research Opportunities in the Earth Sciences - New Research Opportunities in the Earth Sciences - page 16

Figure 2.3 Schematic evolution of progressive crystallization of surface and basal
magma oceans (yellow) following Earth accretion and core formation, based on the
assumed deep-mantle density crossover between melt and solid, leading to upward
segregation of melts in the upper mantle and downward migration of melts in the lower
mantle. Core-forming metals are shown in orange; solid mantle is shown in gray, with
circulation indicated by arrows. SOURCE: Labrosse et al. (2007). Reprinted by
permission from Macmillan Publishers Ltd.
Core Formation
In addition to the energy acquired from impacts, the segregation of the core
released enormous amounts of gravitational potential energy into the Earth system.
Isotope evidence generally points to early core formation (Yin et al., 2002), which is
consistent with the magma ocean hypothesis, wherein growth of the core essentially
kept pace with growth of the mantle. There are several theories on how the core
formed that are compatible with large impacts and the existence of magma oceans.
One theory assumes that impacting cores fell through the magma ocean as large metal
masses, directly merging with Earth's core (Halliday, 2006). Another assumes that
dispersed metal rained down through the magma ocean, collected at its base, then
descended through the underlying crystalline mantle by several possible mechanisms,
including fracture propagation, large metal diapirs, or metal-silicate plumes (Ricard et
al., 2009). The measured abundances in the mantle of moderately siderophile
elements such as Nickel (Ni) and Cobalt (Co) indicate that some degree of chemical
equilibration between core-forming metals and mantle silicates took place, possibly at
elevated pressure and temperature conditions (Chabot et al., 2005; Wood et al., 2006).
Additional geochemical and petrological constraints, better resolution of its timing
and duration, and a fuller picture of the possible dynamics are needed to constrain the
core segregation process.
Early Earth's Surface Environments
Evidence indicates that the accretion and major differentiation of Earth,
including core formation, were largely complete within about the first 100 megayears