HR: 10:45h
AN: U32A-02    [Abstracts]
TI: Microscale Deformation of (Post) Perovskite-Dominated Sediment in the Uppermost Outer Core
AU: * Petford, N
EM: n.petford@kingston.ac.uk
AF: Kingston University, Centre for Earth and Environmental Science Research, Kingston University, London, KT1 2EE United Kingdom
AU: Yuen, D
EM: davey@krissy.geo.umn.edu
AF: University of Minnesota, Department of Geology and Geophysics University of Minnesota , Minneapolis, 55455 United States
AU: Rushmer, T
EM: trushmer@zoo.uvm.edu
AF: University of Vermont University of Vermont, Department of Geology , Burlington, VT 05405 United States
AB: Seismic and nutational data hint strongly at a layer comprised of silicate sediments several km thick confined to the top of the liquid outer core, directly beneath the core-mantle boundary and equating with observed ultra low velocity zones (ULVZs)¹. Its origin is thought to be due to high pressure chemical reactions that take place between Fe-rich silicate in the lower mantle and liquid iron in the underlying outer core. We speculate that the Fe silicate is dominated by the newly discovered post-perovskite phase (ppv). Initial numerical investigations show that viscous compaction in the sediment layer will act to expel interstitial core metal liquid and reduce an initial 50% porosity to a residual value of < 0.1 on timescales of the order 80-100 Ma¹. Using a modified form of Biot's equations², we show that deformation of a poro-viscoelastic sediment pile will respond by drawing up Fe liquid metal into the layer from below at a rate proportional to the shear stress rate. Instead of a static, isolated residual porosity distribution in the most compacted upper regions of sediment, we envisage a more dynamic environment where fresh core liquid is emplaced periodically into the slowly accumulating pile. Estimates of the potential magnitude of the instability, including pressure changes, local fluid flow rates and timescales compare favourably with fluid motions in the convecting outer core. A key difference between both models, which are in fact complementary, relates to the rheology and microscale deformation behaviour of the assumed ppv-dominated sediment. The small grain size of the suspension is close to the limit of dilatant behaviour (c. 10^-6 m) in granular materials. More information is required on the chemical and physical behaviour of the post-perovskite phase at lengthscales characteristic of geophysically interesting colloidal suspensions under high P-T conditions. 1.Buffet, BA., Garnero, EJ & Jeanloz, R. 2000. Science, 290, 1338-1342. 2.Koenders, MA & Petford, N. 2000. Geophys. Res. Lett. 27, 1231-1234.
DE: 3909 Elasticity and anelasticity
DE: 3924 High-pressure behavior
DE: 5139 Transport properties
DE: 8115 Core processes (1507)
SC: Union [U]
MN: 2005 Joint Assembly