HR: 09:30h
AN: U31A-05    [Abstracts]
TI: Effects of the post-perovskite phase change on the thermal evolution of the Earth's core
AU: * Nakagawa, T
EM: takashi@eps.s.u-tokyo.ac.jp
AF: Department of Earth and Planetary Sciences, University of Tokyo, 7-3-1, Hongo, Bunkyo, Tokyo, 113-0033 Japan
AU: Tackley, P J
EM: ptackley@ess.ucla.edu
AF: Department of Earth and Space Sciences, and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, 3806 Geology Buildings BOX 951567, Los Angeles, CA 90095-1567 United States
AB: The heat flow through the core-mantle boundary is a key quantity for understanding the thermal evolution of the Earth??s core, as the geodynamo is presumably strongly affected by the temporal variation of CMB heat flow. A major challenge is to understand how this heat flux can have remained high enough to power the geodynamo over geological history without resulting in larger-than-observed cooling of the core and growth of the inner core. This problem has been approached using various coupled models of mantle convection and core heat balance: (1) Simple isochemical models using parameterized mantle convection and core heat balance [Buffett, 2002; Labrosse, 2003; Nimmo et al., 2004] have too rapid core cooling hence a too large inner core, (2) models with a global layer of dense material above the CMB [McNamara and van Keken, 2000] have a CMB heat flow that is too low for the geodynamo to occur, but (3) with discontinuous chemical layering [Nakagawa and Tackley, 2004], viable evolution solutions are obtained, with the best scenarios requiring 100-200 ppm radioactive potassium in the core [Nakagawa et al., 2004]. Recently, using high pressure experiments and ab initio calculations, a new perovskite to post-perovskite phase change was discovered near the CMB [Murakami et al., 2004; Oganov and Ono, 2004]. Dynamically, such a phase change results in small-scale instabilities in lower thermal boundary layer and higher CMB heat flow [Nakagawa and Tackley, 2004]. Furthermore, if the CMB is in the perovskite stability field then a double-crossing of the phase boundary may occur [Hernlund et al., 2005]. As the core and mantle cool with time, the location and thickness of the layer of post-perovskite phase will change [Nakagawa and Tackley, 2005]. In this study, a coupled model of thermo-chemical mantle convection including the post-perovskite phase change and a global heat balance in the core based on the entropy variation is used to assess the CMB heat flow, thermo-chemical structures in the D?? region (dense piles due to basaltic component and double-crossing due to the post-perovskite phase change) and their influence on the thermal history of the Earth??s core. The required amount of potassium in the core will be also discussed in this presentation.
DE: 1212 Earth's interior--composition and state (8105)
DE: 1213 Earth's interior--dynamics (8115, 8120)
DE: 8125 Evolution of the Earth
DE: 8130 Heat generation and transport
SC: Union [U]
MN: 2005 Joint Assembly