title: Isotopes as clues to the origin and earliest differentiation history of the Earth.
abstract: Measurable variations in (182)W/(183)W, (142)Nd/(144)Nd, (129)Xe/(130)Xe and (136)XePu/(130)Xe 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 nuclides (182)Hf, (146)Sm, (129)I and (244)Pu. The (l82)Hf-(182)W system is the best accretion and core-formation chronometer, which yields a mean time of Earth's formation of 10Myr, and a total time scale of 30Myr. 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 coupled (146)Sm-(147)Sm chronometer is the best system for determining the initial silicate differentiation (magma ocean crystallization and proto-crust formation), which took place at ca 4.47Ga or perhaps even earlier. The presence of a large (129)Xe excess in the deep Earth is consistent with a very early atmosphere formation (as early as 30Myr); however, the interpretation is complicated by the fact that most of the atmospheric Xe may be from a volatile-rich late veneer.
authors.affiliation: Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA. jacobsen@neodymium.harvard.edu
authors.forename: Stein B
authors.initials: SB
authors.lastname: Jacobsen
authors.forename: Michael C
authors.initials: MC
authors.lastname: Ranen
authors.forename: Michael I
authors.initials: MI
authors.lastname: Petaev
authors.forename: John L
authors.initials: JL
authors.lastname: Remo
authors.forename: Richard J
authors.initials: RJ
authors.lastname: O'Connell
authors.forename: Dimitar D
authors.initials: DD
authors.lastname: Sasselov
mesh_terms: D001272:Atmosphere; D018534:Earth, Planet; D007554:Isotopes; D019110:Meteoroids; D016081:Moon
chemical_list: D007554:Isotopes