HR: 1340h
AN: P33A-1010    [Abstracts]
TI: Magma Ocean Overturn: Implications for The Creation of Large Scale Mantle Heterogeneities and Influences on Planetary Evolution
AU: * Zaranek, S
EM: Sarah\_Zaranek@brown.edu
AF: Brown University, Box 1846 , Providence, RI 02912 United States
AU: Elkins-Tanton, L T
EM: Linda\_Elkins\_Tanton\@brown.edu
AF: Brown University, Box 1846 , Providence, RI 02912 United States
AU: Parmentier, E
EM: EM\_Parmentier@brown.edu
AF: Brown University, Box 1846 , Providence, RI 02912 United States
AB: Global scale asymmetries on terrestrial planets date from very early on in their evolution. A better understanding of crystallized magma ocean dynamics could clarify possible mechanisms to create lateral chemical heterogeneities in planets' early mantles. These heterogeneities (or the processes the create these heterogeneities) could aid the creation of global scale asymmetries. The converted kinetic energy from accretion of planetesimals or the potential energy release from the formation of a core is thought to be large enough to melt a significant part of a terrestrial planet's mantle interior. This melted region would then form a magma ocean that would then subsequently recrystallize. Density profiles for the crystallized magma oceans of Mars and the Moon have been obtained through models that treat the fractional crystallization of a magma ocean. These profiles predict the compositional density stratification of the crystalline cumulate mantle. This compositional stratification is gravitationally unstable (heavy material on top) and would result in overturn. The subsequent theorized Rayleigh-Taylor overturn to a stable configuration has important consequences for the future evolution of the planet. First, inverting of the initial solidus temperature profile could result in large scale melting, perhaps producing an early crust. The thermal structure after overturn brings cold material to the base of the mantle, possibly creating an early magnetic field. Second, the compositional density profile is also inverted, resulting in a stable density stratification. This stable density configuration could also control the subsequent evolution of the planet, in terms of the distribution of heat producing elements, chemical composition controls density and physical properties, controlling the extent (depth) of subsequent thermal convection. Our numerical experiments are used to characterize the overturn mechanism. We solve the conservation equations for mass, momentum and energy assuming a viscous rheology in an infinite Prandtl number fluid. In particular, we are interested in exploring the mechanisms that allow early creation of lateral chemical heterogeneities in the mantle. Two types of lateral heterogeneity can be formed. The first type of heterogeneity is a consequence of pressure-dependent rheology. In overturn with pressure-dependent viscosity, the time for lateral heterogeneities caused by the initial large-scale convective overturn to relax is much longer because of the increased viscosity of downwellings. This effect would be more pronounced in planets with deeper magma oceans. The second heterogeneity results from considering initial density profiles that are not monotonically decreasing with depth. In overturn with non-montonically decreasing density structures, lateral compositional heterogeneity is possible with no lateral density variation. These lateral heterogeneities consist of mantle material that initially crystallized at different depths of the magma ocean and therefore are expected to have different bulk compositions including different amounts of heat producing elements. These variations, unlike those caused by increased viscosity, do not decay away with time. Currently, we are using linear stability analysis and numerical models in spherical coordinates to determine dominate wavelengths of overturn to see the expected horizontal length scales of these heterogeneities.
DE: 8147 Planetary interiors (5430, 5724)
DE: 5430 Interiors (8147)
DE: 5455 Origin and evolution
DE: 6225 Mars
DE: 3210 Modeling
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting