HR: 17:30h
AN: U44A-07 INVITED    [Abstracts]
TI: The Basal Magma Ocean
AU: * Hernlund, J W
EM: hernlund@eos.ubc.ca
AF: Dept. of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, V6T 1Z4, Canada
AU: Labrosse, S
EM: stephane.labrosse@ens-lyon.fr
AF: Ecole Normale Superieure de Lyon, 46 Allee d'Italie, Lyon, 69364, France
AU: Coltice, N
EM: coltice@univ-lyon1.fr
AF: Université de Lyon, 43 bd de 11 Novembre 1918, Villeurbanne, 69100, France
AB: The presence of partial melt at the base of Earth's mantle today implies a more extensive basal melt layer in its hotter past, stabilized by its enrichment in Fe and small or inverted molar volume difference relative to solids at high pressure. The low viscosity of such a basal magma ocean formed early in Earth's history ensures it was subject to vigorous convection and nearly isentropic conditions, and it will have undergone slow fractional crystallization from the top down at a rate determined by secular cooling and its phase diagram, likely enriching it in Fe-bearing components and thus further increasing its density. The basal magma ocean also became further enriched in other incompatible elements as it crystallized, and its evolution can be constrained by 1) the systematic differences in \varepsilon¹⁴²Nd of Earth samples compared to chondrites 2) the initial thickness of about 1000 km to provide for the hidden reservoir of heat producing elements, 3) coupled mantle and core thermal evolution models, and 4) the present day ULVZ thickness of order several km. Plausible thermal evolutions yield an approximate exponential decrease in thickness with time, and all of the estimates consistently converge on e-folding time scales for decay of around 1 Gyr. The remaining 4-6 TW of heat produced in this layer at the present time decreases the amount of core cooling required of the high temperature gradients in D" implied by post-perovskite-related seismic discontinuities, and core thermal evolutions predict a decreased cooling rate in the past such that a geodynamo may not have been viable in Earth's early history.
DE: 3630 Experimental mineralogy and petrology
DE: 3924 High-pressure behavior
DE: 7207 Core (1212, 1213, 8124)
DE: 7208 Mantle (1212, 1213, 8124)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Union [U]
MN: 2007 Fall Meeting