HR: 09:00h
AN: MR31D-05    [Abstracts]
TI: Keeping mush mushy at the core-mantle boundary: the role of internal convection and secular cooling
AU: * Hernlund, J W
EM: hernlund@eos.ubc.ca
AF: Dept. of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, CA V6T 1Z4, France
AU: Jellinek, M
EM: mjellinek@eos.ubc.ca
AF: Dept. of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, CA V6T 1Z4, France
AB: Williams and Garnero (1996) proposed that thin (5-40 km thick) patches of dramatically decreased seismic velocity above the core-mantle boundary (ultralow-velocity zones, or ULVZ) could best be explained by the presence of partial melt, and this remains the favored mechanism to explain the anomalous seismic properties. However, simple estimates for compaction and expulsion of melt from a porous solid on the order of 1 Gyr require an effective bulk viscosity that is probably much larger than realistic, and therefore melt should have separated long ago from the interstices it occupies in the matrix. In more detail, however, this layer is subject to a more complicated style of internal stirring governed by the combined influences of motions induced by flow in the overlying mantle and motions arising in response to the compaction-driven drainage of interstitial melt, which depend critically on the melt fraction. In the simplest scenario, analogous to the sedimentation of solids from a convecting slurry, this circulation may enhance expulsion of fluid from the mush. Additional important factors include the possibility of melting and freezing in different parts of the layer due, for example, to small thermal gradients, slow secular cooling at the top of the core, chemical flux to or from the core, and internal compositional stratification. We use numerical models of compaction and flow in a two-phase medium in equilibrium according to a simple binary phase diagram to better understand the evolution of porosity in a churning mush. Flow is driven by convection in the overlying mantle along with internal buoyancy forces due to phase and composition variations, with the former becoming more important when the tendency is toward a gravitationally stable stratification of the mush. Flux of light elements to or from the core is also studied by imposing composition at the lower boundary. A central aim of this work is to identify plausible sets of conditions in which thin partially molten layers can be maintained at the core-mantle boundary over time scales of order the age of the Earth.
DE: 1037 Magma genesis and partial melting (3619)
DE: 3621 Mantle processes (1038)
DE: 4465 Phase transitions
DE: 8121 Dynamics: convection currents, and mantle plumes
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting