Time of Formation of the Earth's Core

Abstract

ACCORDING to several theories^1–3, the Earth was formed by accretion from a mixture of silicate particles and metal particles, something like those in chondritic meteorites. The accreting material may also have contained relatively small quantities of other components such as iron meteorites⁴ and carbonaceous chondrites⁵. Most advocates of these theories have maintained that accretion occurred relatively slowly (over a period of about 10⁸ yr), so that the gravitational potential energy was efficiently radiated away and the Earth formed in a relatively cool and unmelted condition. Subsequently, heating by long lived radioactive elements occurred, leading to melting of the metal phase and segregation of the core. Differentiation and formation of the core may thus have occurred much later than the primary accretion of the Earth. It has even been argued that core formation might not yet have proceeded to completion^1,6,7. On the other hand, alternative theories of origin of the Earth^8–10,11 have maintained that much of the accretion of the Earth occurred in high temperature conditions and that core formation occurred either during accretion or very soon afterward. Clearly, an independent determination of the time interval ΔT_c between accretion and core formation would have a crucial bearing on these contrasting theories.