Abstract
Because of the high energies involved, giant impacts that occur during planetary accretion cause large degrees of melting. The depth of melting in the target body after each collision determines the pressure and temperature conditions of metal-silicate equilibration and thus geochemical fractionation that results from core-mantle differentiation. The accretional collisions involved in forming the terrestrial planets of the inner Solar System have been calculated by previous studies using N-body accretion simulations. Here we use the output from such simulations to determine the volumes of melt produced and thus the pressure and temperature conditions of metal-silicate equilibration, after each impact, as Earth-like planets accrete. For these calculations a parametrised melting model is used that takes impact velocity, impact angle and the respective masses of the impacting bodies into account. The evolution of metal-silicate equilibration pressures (as defined by evolving magma ocean depths) during Earth's accretion depends strongly on the lifetime of impact-generated magma oceans compared to the time interval between large impacts. In addition, such results depend on starting parameters in the N-body simulations, such as the number and initial mass of embryos. Thus, there is the potential for combining the results, such as those presented here, with multistage core formation models to better constrain the accretional history of the Earth.
Highlights
Accretion of the Earth took place over a time period on the order of 100 million years (e.g. Jacobson et al 2014)
Model description The volume of melt produced during an accretional collision depends on the kinetic energy of the impactor, as determined by its mass and velocity, and the impact angle which determines how much of the kinetic energy is transferred into the target body
Our results show that metal-silicate equilibration pressures during core formation are likely to depend strongly on the rate of magma ocean crystallisation compared to the time interval between impacts
Summary
Accretion of the Earth took place over a time period on the order of 100 million years (e.g. Jacobson et al 2014). (2) From this sea of planetesimals, planetary embryos (lunar- to Mars-mass bodies) emerged from runaway growth processes either due to gravitational focusing (Greenberg et al 1978) or pebble accretion (Lambrechts and Johansen 2012). A major differentiation process that occurred during the early history of the Earth led to the formation of its iron-rich core and silicate mantle. Core formation involved the separation of metal from silicates for which high temperatures were essential. Most likely both the metal and the silicate had to be in a molten state for core formation to occur efficiently (Stevenson 1990; Rubie et al 2003, 2015a). Following the decay of shortlived 26Al, the heat required for core-mantle differentiation of the Earth resulted primarily from high-energy impacts with other planetary bodies. The postulated impact between the proto-Earth and a large de Vries et al Progress in Earth and Planetary Science (2016)3:7
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