Early Differentiation and Its Long-Term Consequences for Earth Evolution

Several first order features of Earth owe their origin to processes occurring before, during, and within a few hundred million years of Earth formation. Arguably the most significant expression of these early events is the bulk composition of Earth. Earth's depletion in some volatile elements likely was inherited from the materials from which it formed. This is most easily attributed to Earth's accumulation from planetesimals formed in the inner Solar System where the temperatures were hot enough, for long enough, to keep many volatile elements in the gas phase until after the solids had accumulated into at least planetesimal-sized objects. Improved understanding of the processes of planetary accretion makes it increasingly clear that the main fraction of Earth's mass was accumulated through violent collisions with large planetesimals, not by gentle accumulation of primitive bodies. The accreted planetesimals likely had already experienced global differentiation to separate core from mantle and crust, and suffered additional volatile loss by gravitational escape of any atmosphere formed through this early differentiation on the small planetesimal. The short-lived Hf-182-W-182 system indicates that the metal-silicate separation associated with core formation began on planetesimals within a million years or less and on Earth within tens of millions of years of the start of Solar System formation. Metal-silicate separation left Earth's mantle deficient in siderophile elements relative to their abundances in bulk chondrites. Mantle abundances of moderately siderophile elements suggest high-pressure and temperature equilibrium between metal and silicate, consistent with metal-silicate segregation occurring during largely or entirely molten stages of early Earth history. By contrast, the mantle abundances of highly siderophile elements are most easily reconciled with addition of approximately half a percent of Earth's mass of material with chondritic composition after chemical exchange between mantle and core had stopped. Evidence for early differentiation of the silicate Earth, as would be expected for a terrestrial magma ocean, is remarkably subdued, but is now being extracted from information provided by short-lived radioactive systems such as I-129-Xe-129, Sm-146-Nd-142, and Hf-182-W-182. For example, Xe-129 and Nd-142 heterogeneities in the mantle point to a major terrestrial differentiation event occurring between circa 4.4 and 4.45 Ga, which is most easily attributed to the time of the Moon-forming giant impact. What little evidence remains for the nature of Earth's crust that formed immediately after the resulting magma ocean suggests the presence of a primitive mafic crust that did not become reworked into substantial felsic continental crust until 3.8 to 4.0 Ga.