Isotopes as clues to the origin and earliest differentiation history of the Earth

Abstract

Measurable variations in182W/183W,142Nd/144Nd,129Xe/130Xe and136XePu/130Xe in the Earth and meteorites provide a record of accretion and formation of the core, early crust and atmosphere. These variations are due to the decay of the now extinct nuclides182Hf,146Sm,129I and244Pu. Thel82Hf–182W system is the best accretion and core-formation chronometer, which yields a mean time of Earth's formation of 10 Myr, and a total time scale of 30 Myr. New laser shock data at conditions comparable with those in the Earth's deep mantle subsequent to the giant Moon-forming impact suggest that metal–silicate equilibration was rapid enough for the Hf–W chronometer to reliably record this time scale. The coupled146Sm–147Sm chronometer is the best system for determining the initial silicate differentiation (magma ocean crystallization and proto-crust formation), which took place atca4.47 Ga or perhaps even earlier. The presence of a large129Xe excess in the deep Earth is consistent with a very early atmosphere formation (as early as 30 Myr); however, the interpretation is complicated by the fact that most of the atmospheric Xe may be from a volatile-rich late veneer.