HR: 14:35h
AN: T33F-04 INVITED     [Abstracts]
TI: Sedimentation and Compaction at the Top of Earth's Core
AU: * Buffett, B
EM: buffett@geosci.uchicago.edu
AF: University of Chicago, 5734 S. Ellis Avenue, Chicago, IL 60637 United States
AB: Multiphase flow in the core is most likely to occur at the boundaries of the liquid outer core. Growth of the inner core by solidification is expected to produce a porous, two-phase mush that compacts under the influence of gravity. A porous, two-phase mixture may also develop at the top of the core. In this case the deformable solid phase is deposited in response to changes in chemical equilibrium between the liquid core and the base of the mantle as inner-core growth accumulates light elements in the outer core and as the liquid iron alloy cools. Solid sediments may precipitate directly from the supersaturated liquid or evolve through chemical reactions at the base of the mantle, in analogy to chemical weathering at the Earth's surface. In either case the sediments are expected to be buoyant relative to liquid iron and accumulate at the top of the core. While the composition of the sediment is presently unknown, the possibilities include various iron alloys and silicate minerals. We favor the deposition of silicate sediments because a solid metallic phase of iron alloy more than 10 km thick would excessively screen short-period variations in the internal magnetic field. A layer of silicate sediments could also have a high electrical conductivity by virtue of the interstitial liquid iron. However, the zone of high conductivity is limited to regions where the porosity is large and the liquid iron is interconnected. We investigate the structure and evolution of a sediment layer at the top of the core by coupling the processes of sedimentation and compaction to a model for the thermal evolution. Using sediment properties based on values for mantle minerals, we estimate that the conductive region is confine to a layer about 1 km thick. Sediments that are buried more than 1 km from the core-sediment interface have bulk properties that become indistinguishable from those of the lower mantle as most of the liquid iron is expelled. However, the anomalies in density, elastic moduli and electrical conductivity within 1 km of the core-sediment interface are large enough to have observable consequences for geodetic and seismic measurements. We also show that the accumulation of sediments at the top of the core has a small feedback on the thermal evolution of the core, which may increase the power available to drive the geodynamo.
DE: 8115 Core processes (1213, 1507)
DE: 8125 Evolution of the Earth (0325)
DE: 8147 Planetary interiors (5430, 5724, 6024)
SC: Tectonophysics [T]
MN: Fall Meeting 2005