HR: 17:00h
AN: U44A-05    [Abstracts]
TI: A Bound on Heat Flow Below a Double Crossing of the Perovskite-Postperovskite Phase Transition
AU: * Buffett, B A
EM: buffett@geosci.uchicago.edu
AF: University of Chicago, 5734 S Ellis Avenue, Chicago, IL 60637, United States
AB: A double crossing of the perovskite-postperovskite phase transition in (Mg,Fe)SiO₃ has been proposed to explain seismic reflections from structures near the base of the mantle. Estimates of temperature inferred from the phase transitions can be extrapolated to the core-mantle boundary (CMB) using a simple model for the thermal boundary layer. However, a complication arises when flow is driven by the (negative) buoyancy of the postperovskite phase. Latent heat release and advective transport increase the temperature gradient and heat flow below the region of postperovskite. A minimum bound on the temperature gradient at the CMB is obtained by noting that the temperature gradient at the base of the postperovskite region must be at least as steep as the transition temperature with depth. Both the temperature and heat flow at the CMB depend on the vertical velocity at the phase transition. A representative velocity of 1 mm/yr gives a local heat flux of 160 m W m^-2. Even higher heat flow is predicted as the velocity increases. Such high heat flow is unlikely, so the heat flow bound can be used as a constraint on both the viscosity of the lower mantle and the geometry of the postperovskite layer.
DE: 8115 Core processes (1213, 1507)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
DE: 8130 Heat generation and transport
SC: Union [U]
MN: 2007 Fall Meeting