HR: 0800h
AN: U11A-0008    [Abstracts]
TI: Earth's Basal Magma Ocean and the Delayed onset of the Geodynamo
AU: * Labrosse, S
EM: stephane.labrosse@ens-lyon.fr
AF: Ecole Normale Superieure de Lyon, Universite de Lyon, 46, Allee d'Italie, Lyon, 69364, France
AU: Hernlund, J W
EM: hernlund@eos.ubc.ca
AF: University of British Columbia, Dept. of Earth and Ocean Sciences, 6339 Stores Road, Vancouver, BC V6T 1Z4, France
AU: Coltice, N
EM: coltice@univ-lyon1.fr
AF: Universite de Lyon, Ecole Normale superieure de Lyon, 43, bd du 11 novembre 1918, Villeurbanne, 69100, France
AB: Partial melt has been proposed to explain the presence of thin ultralow-velocity zones in the lowermost mantle. The cooling of the core necessary to have maintained the geodynamo for at least 3.2 Gyr implies a greater amount melt in the past and a possibly large magma ocean at the base of the mantle following Earth's formation. The gravitational stability of such a melt layer can be explained by the small volume change upon melting at high pressure (Stixrude and Karki, 2005) and a larger amount of FeO in the melt than in solids at equilibrium. The crystallization of the magma ocean leads to progressive enrichment in FeO in both the melt and the solid, leading to formation of dense piles at the bottom of the mantle. Solving the thermal-chemical evolution of the deep Earth involving a basal magma ocean, we show that, to first order, the mass of melt decreases exponentially with time. Many parameters involved in this evolution are poorly known but can be constrained using geochemical observations. Indeed, the basal magma ocean is the perfect "hidden" reservoir that can store 20 to 30% of the Earth inventory in incompatible elements, provided its initial mass is around 10²⁴ kg. In addition, the systematic difference in ¹⁴²Nd between terrestrial samples and chondrites (Boyet and Carlson, 2005) can be used to constrain the time scale of crystallization. Because of the large amount of radiogenic and latent heat to extract from the basal magma ocean early in Earth's history, the heat extracted from the core can initially be lower than that conducted along an adiabat. Models most readily satisfying geochemical constraints predict that the geodynamo would not start before 3.4-4 Gyr ago, in agreement with lunar (Ozima et al, 2005) and terrestrial data (Tarduno et al, 2007).
UR: http://perso.ens- lyon.fr/stephane.labrosse
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1025 Composition of the mantle
DE: 1507 Core processes (1213, 8115)
DE: 8125 Evolution of the Earth (0325)
DE: 8147 Planetary interiors (5430, 5724, 6024)
SC: Union [U]
MN: 2007 Fall Meeting