HR: 08:42h
AN: MR41A-04 INVITED     [Abstracts]
TI: At the frontiers of the Earth's fluid core
AU: * Le Mouel, J
EM: lemouel@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4, place Jussieu, Paris, 75252 France
AU: Poirier, J
EM: poirier@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4, place Jussieu, Paris, 75252 France
AB: The fluid core exchanges energy and momentum with the mantle at the Core-Mantle Boundary (CMB) and crystallizes at the Inner core Boundary (ICB). The dynamics of the core depends on the structure of both boundaries. We investigate here the roughness of the CMB, possibly conducive to dissipation, as well as the existence of a mush at the ICB. The roughness of the CMB, on the scale of centimeters to tens of meters, was modelled using a cellular automata method. Cubic cells, of the size of grains of the mantle material, on a 3-D grid, can be in one of three states, corresponding to mantle silicate or oxide, core fluid saturated in light element and unsaturated core fluid. We assumed a stationary stochastic process of evolution, without memory. Transitions of doublets of cells from one state to another are governed by parameters representing the rates of physical processes : dissolution and crystallization of mantle material at the CMB and diffusion of the light element in the core fluid. With reasonable values of the parameters, it is found that the boundary roughens on the scale of grains, and a boundary layer of saturated fluid, a few tens of centimeters thick, soon appears at the interface. An undulation with dominant wavelength of the order of a few tens of meters eventually appears. An order of magnitude of the resulting dissipation in the fluid flow, due to the roughness, is given. At the ICB, it is currently believed that a mushy layer develops, while the outer core material freezes onto the inner core. However, this view is derived from metallurgical or analog experiments on solidification carried out in the laboratory, in conditions widely different from those obtaining in the core. At the ICB, the temperature gradients are negative and many orders of magnitude smaller than the positive gradients in the laboratory, while the velocity of the solid-liquid interface is about 0.3 mm/year instead of, typically, 0.01 to 0.1 mm/s. Using a modified Mullins-Sekerka linear stability analysis, we investigated the conditions in which constitutional supercooling and instability of the interface (with dendrite and mush formation) coexist. We found that a mushy layer at the ICB is likely to exist if the liquidus slope (melting T vs concentration of light element) is of the order of 100 K or higher (as is currently assumed). However, if it is of the order of 0.1 K, which might occur if the major light element is oxygen, the interface might be stable and the mush would be replaced by a slurry, with a drizzle of solid particles onto the inner core.
DE: 8115 Core processes (1507)
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting