Accretion of differentiated planetesimals to the Earth

Abstract

It is argued here that the Earth and the other planets of the inner solar system accreted predominantly from volatile-depleted planetesimals and were already differentiated into metallic cores and silicate mantles. Metal-sulfide-silicate partitioning seems to have been present in the early dust components of the solar nebula, prior to chondrule formation, and proceeded efficiently during growth of planetesimals and accretion of the planets. Metal-sulfide-silicate equilibrium was thus mainly established at low pressures within relatively small planetesimals. Rapid core-mantle separation in the Earth of precursor metal and silicate makes their reequilibration at higher pressures less likely. Thus, the trace and minor element composition of the mantle and core will reflect the low pressures and oxygen fugacities of the small precursor bodies, rather than the megabar pressures of the deep terrestrial mantle. In this model, core formation on Earth was not a catastrophe affecting a fully formed planet, but more likely occurred during accetion of the previously differentiated planetesimals. Since metal-sulfide-silicate equilibrium was accomplished predominantly at low pressures, troilite will be the main accreting phase for sulfur, which thus remains a viable candidate for the light element in the Earth's core. -Authors