HR: 09:48h
AN: MR41A-09    [Abstracts]
TI: Timing And Processes Of Earth's Core Differentiation.
AU: * Allegre, C J
EM: allegre@ipgp.jussieu.fr
AF: Laboratoire Geochimie Cosmochimie IPGP-CNRS, 4 Place Jussieu T14-24, Paris Cedex 05, 75252 France
AU: Manhes, G
EM: manhes@ipgp.jussieu.fr
AF: Laboratoire Geochimie Cosmochimie IPGP-CNRS, 4 Place Jussieu T14-24, Paris Cedex 05, 75252 France
AU: Gopel, C
EM: gopel@ipgp.jussieu.fr
AF: Laboratoire Geochimie Cosmochimie IPGP-CNRS, 4 Place Jussieu T14-24, Paris Cedex 05, 75252 France
AB: Small $^{182}$W abundance excess of terrestrial W relative to W in bulk chondrites has been recently established (Yin et al. 2002, Kleine et al. 2002, Schoenberg et al. 2002). Rapid terrestrial accretion and early core formation, with completion of the bulk metal-silicate separation within less than 30 Myr have been proposed on this basis. These studies underline how much this $^{182}$W/$^{182}$Hf time scale agrees with dynamic accretion models (Wetherill, 1986) that predict a $\sim$10 Myr interval for the main growth stage of Earth's formation. This W model time scale for terrestrial accretion is shorter than current estimates based on Pb isotope systematics of mantle-derived basalts and terrestrial Xe isotope systematics. The end of metal-silicate differentiation and large scale mantle degassing has been defined $\sim$100 My after beginning of the accretion. These studies also indicate agreement of this time scale with dynamic accretion models that predict 100 My for the end of Earth's accretion. The Hf-W time scale for accretion and core formation assumes total equilibration of incoming metal and silicate of impactors with the bulk silicate Earth (BSE) during planet's growth. Recently, the assumption of incomplete equilibration of metal and silicate components with BSE has been investigated (Halliday, 2004). It is proposed that impacting core material has not always efficiently mixed with the silicate portions of the Earth before being added to the Earth's core Our approach also considers that equilibration between metal and silicate has not been complete in BSE during Earth's growth, and we argue that early part of the Earth's core has segregated through unmelted silicate material. When the baby Earth was large enough, the increase of the temperature induced Fe-FeS eutectic melting. The liquid metal segregated through the crystalline silicate matrix and formed the early part of the Earth's core. Experimental study (Yoshino et al. 2003) indicates the percolation threshold for molten iron-sulphur compounds of 5 vol% solid olivine, through channel on triple junction between minerals. This study allows us to reconsider the precedent proposition (Stevenson, 1990) based on experimental and theoretical considerations suggesting that percolation of metallic iron rich liquid through a mostly solid silicate matrix is largely prevented because of the high surface tension of iron. During formation and segregation of the Fe-FeS eutectic, W isotopic equilibration is limited by the diffusion through the solid silicate matrix. During the further Earth's growth, impact melting increased and has induced a progressive melting of BSE up to the formation of magma ocean at the end of the planet's accretion. Before the occurrence of the magma ocean, W equilibration between impactors and BSE has not been complete This incomplete isotopic exchange between terrestrial metal and metal originating from impactors with solid part of BSE during early accretion of the Earth leads to the observed excess of $^{182}$W of present BSE. It occurs when the $^{182}$W production in BSE is most significant, due to the short half-life of $^{182}$Hf. The change of segregation mechanisms of Earth's core during planet's growth and short-sightedness of Hf-W chronometer focused to the early segregation of Earth's core make the divergence with the U-Pb and I-Xe terrestrial records. Yin et al. 2002, Nature 418, 949-952. Kleine et al. 2002, Nature 418, 952-955. Schoenberg et al. 2002, Geochim. Cosmochim. Acta 66, 3151-3160. Wetherill 1986, in Origin of the Moon, eds Hartmann et al., LPI, 519-550. Yoshino et al. 2003, Nature 422, 154-157. Stevenson 1990, in Origin of the Earth, eds Newson et al., LPI, 231-249.
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
DE: 1213 Earth's interior--dynamics (8115, 8120)
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting