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evidence for early mantle differentiation and atmosphere formation that are indicative
of a magma ocean environment (Moynier et al., 2010). What is more certain,
however, is that terrestrial magma oceans and the early atmosphere provided highly
dynamical environments in which a wide variety of chemical and physical processes
were active, ranging from shock-wave heating to fracturing and fragmentation,
turbulent convection, percolation, mixing, and a host of possible redox reactions.
Understanding the evolution of a terrestrial magma ocean requires answers to such
basic questions as:
•
What is the relationship between impact and magma ocean sizes?
•
What is the lifetime of a magma ocean and how is it coupled to the early
atmosphere?
•
Does a terrestrial magma ocean crystallize from the bottom up or from the top
down?
•
Was there a deep-mantle abyssal magma ocean?
•
Do deep melts rise or sink in the early mantle?
•
What sequence of crystals form in a cooling magma ocean?
•
As a magma ocean crystallizes, is it stably stratified, or will it overturn?
•
How did metals and silicates mix and then segregate in magma oceans?
•
What was the nature of mantle dynamics following magma ocean
solidification?
Providing answers to these questions will probably require geodynamical
modeling constrained by improved understanding of the petrology of melts and
element partitioning at high pressures and temperatures, in parallel with
interpretations of present-day seismic images of mantle heterogeneity in the deep
mantle, where the chances are best of finding relics of this process still preserved. In
addition, many of the issues raised by these questions are linked together, requiring
cross-disciplinary expertise. For example, separation of immiscible liquids (in this
case, iron from silicate melts) with greatly different densities happens rapidly in a
low-viscosity magma ocean, whereas buoyancy-driven segregation of silicates
depends on the environmental conditions. Because the moon's interior spans a small
range of pressures, the crystallization sequence of a silicate lunar magma ocean is
reasonably well understood (Shearer, 2006). As is the case for many shallow layered
mafic intrusions on Earth, buoyancy-driven separation of lower-density Ca- and Al-
rich plagioclase from denser Mg- and Fe-rich silicates occurs on the Moon. On Earth,
however, the greater range of internal pressures introduces the likelihood of liquid-
solid density crossovers (Mosenfelder et al., 2007; Stixrude et al., 2009), so magma
oceans may stabilize at both the top and the base of the mantle (Labrosse et al., 2007),
as shown in Figure 2.3, significantly complicating their evolution.
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