HR: 0800h
AN: U41A-0729    [Abstracts]
TI: Terrestrial Magma Ocean and Hadean Crust Formation: Insights From $^{146}$Sm-$^{142}$Nd and Trace Element Partitioning in High-Pressure Mantle Minerals
AU: * Caro, G
EM: caro@ipgp.jussieu.fr
AF: Laboratoire Geochimie-Cosmochimie, IPGP, 4, place Jussieu, Paris cedex 05, 75252 France
AU: Bourdon, B
EM: bourdon@ipgp.jussieu.fr
AF: Laboratoire Geochimie-Cosmochimie, IPGP, 4, place Jussieu, Paris cedex 05, 75252 France
AU: Wood, B J
EM: B.J.Wood@bristol.ac.uk
AF: Dept. of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's road, Bristol, BS8 1RJ United Kingdom
AU: Corgne, A
EM: a.corgne@gl.ciw.edu
AF: Dept. of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's road, Bristol, BS8 1RJ United Kingdom
AU: Corgne, A
EM: a.corgne@gl.ciw.edu
AF: Geophysical Laboratory, CIW, 5251 Branch Road NW, Washington, DC 200015 United States
AB: The effect of the formation of a magma ocean during terrestrial accretion on the composition and stratification of the Earth mantle remains a major unanswered question. Although partial or complete melting of the mantle appears unavoidable as a result of heating by giant impacts, core segregation, and possibly a blanketing atmosphere, ratios of refractory lithophile elements in modern upper mantle peridotites do not show the characteristic fingerprint of magma ocean crystallization. A likely interpretation is that convective mixing would have hampered efficient segregation of liquidus mineral phases during the early stages of cooling. However, differentiation may have proceeded by segregation of a residual melt during the final stage of solidification, when convection would have slowed down in the crystallizing mush. Here, we address this issue using short-lived $^{146}$Sm-$^{142}$Nd chronometry. $^{146}$Sm decays to $^{142}$Nd with a half-life of 103 Myr, such that $^{142}$Nd is a selective tracer of early mantle differentiation. Using ultrahigh-precision Nd isotope measurements, we show that 3.6-3.8 Gyr old rocks from West Greenland carry positive $^{142}$Nd anomalies ranging between 8 and 15 ppm. This indicates that the Earth mantle underwent large-scale differentiation near the final stage of terrestrial accretion, at 4.35-4.5 Gyr. Using trace element partition coefficients for high-pressure mantle minerals, we show that the observed Nd isotopic signature of early Archaean rocks and combined Hf signature of Hadean zircons are consistent with segregation of small amounts of melt from a crystallizing mush at upper mantle pressure. To match our observations, the protocrust resulting from crystallization of this residual melt must have had a lifetime of ca. 500 Ma. This suggests that crustal recycling was less efficient during the Hadean era, despite vigorous mantle convection. Alternatively, the earliest terrestrial crust may have undergone efficient recycling followed by long-term storage at the core-mantle boundary.
DE: 4830 Higher marine organisms
DE: 5407 Atmospheres--evolution
SC: Union [U]
MN: 2004 AGU Fall Meeting