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E. M. Parmentier Department of Geological Sciences Brown University

Early planetary differentiation processes with implications for long term evolution (planetary evolution as an “initial value problem”). E. M. Parmentier Department of Geological Sciences Brown University.

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E. M. Parmentier Department of Geological Sciences Brown University

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  1. Early planetary differentiation processes with implications for long term evolution (planetary evolution as an “initial value problem”) E. M. Parmentier Department of Geological Sciences Brown University in collaboration with: Linda Elkins-Tanton; Paul Hess; Yan Liang

  2. Outline 1) Planetary accretion and magma oceans (MOs) - Moon is the type example – highly fractionated compositions - For the Earth - how many MOs and how deep? - Shallow vs. basal MO 2) Idealized fractional solidification of a MO - unstable stratification and overturn of solidified mantle - how realistic is fractional solidification idealization? solid state overturn during solidification buoyant liquid-solid segregation 3) Is there a hidden reservoir of heat and incompatible elements? 4) Convective heating and mixing of stably stratified fluid layer and the preservation of a hidden reservoir

  3. Composition of the lunar surface - Mare basalt volcanism at ~3.9 Gyr to 2.5 Gyr – long after MO solidification - Basalts generated at >400 km depth – olivine-pyroxene multiple saturation - Mantle source composition residual to anorthositic crust crystallization - Global asymmetry in emplacement of basalts and the PKT

  4. Timescales and mixing in terrestrial planetary accretion Chambers, EPSL 2004. Chambers, Icarus, 2001.

  5. Magma ocean formation due to a large impact Tonks and Melosh JGR 1993

  6. Tonks and Melosh JGR 1993

  7. Basal magma ocean Develops first 100 Myr and persists during the evolution of the Earth Due to heat generated during core formation Suggest that perovskite fractionation explains trace elements in continental crust + MORB mantle S. Labrosse, J. W. Hernlund & N. Coltice Nature 450, 866-869, 2007.

  8. Idealizations: • convection in liquid maintains adiabatic gradient and homogeneous liquid composition • crystal fraction >50% forms a stress-supported network and behaves as a porous solid • solid retains its solidus temperature and composition

  9. Effect of atmosphere on cooling and solidification of 500 km deep MO non-convecting grey atmosphere following Abe (1979)

  10. Time scale for solid state overturn Taking: m= 1018 Pa-s g = 2 x 10-4 kg/m3/m g = 10 m/sec2 d = 500 km Gives: tRT ≈ 0.1 Myr

  11. The “double diffusion problem” of melt migration in a convecting, compacting, permeable matrix Matrix density and flow Buoyancy sources matrix density melt distribution Melt retained against buoyant rise Pressure driven melt flow

  12. Idealizations: • convection in liquid maintains adiabatic gradient and homogeneous liquid composition • crystal fraction >50% forms a stress-supported network and behaves as a porous solid • solid retains its solidus temperature and composition Does solidification occur by freezing or squeezing (i.e. compaction)? region of compaction and melt-solid segregation

  13. K b2 f 3 Permeability: dependence on f b Buoyant rise of liquid in pore space: L = compaction length = (Kmsolid /mliquid)1/2 Wark and Watson, 2003

  14. Buoyant rise of liquid in pore space: L = compaction length = (Kmsolid /mliquid)1/2

  15. Melt-solid fractionation during the first 100 Myr of Earth evolution Boyet and Carlson (2005)

  16. Hidden reservoir Complement to continental crust and depleted MORB mantle For a chondritic earth – hidden reservoir would contain 20-30% of incompatible trace elements produce about this fraction of global heat flux (U, Th ,K) excess 40Ar (from decay of 40K over earth evolution) low 142Nd – requires formation in first ~100 Myr How would it form? Magma ocean is a prime candidate multiple shallow MOs followed by overturn deep, basal MO Could it be preserved? thermal convective mixing

  17. Farnetani, GRL, 24, 1583, 1997; Alley and Parmentier, PEPI 108, 15, 1998; Davaille, Nature, 402, 756, 1999; Hunt and Kellogg, JGR 106, 6747, 2001; Gonnermann, et al., GRL, 29, 1399, 2002; Samuel and Farnetani, EPSL 207, 39, 2003.

  18. Convective instability in a continuously stratified fluid layer

  19. How long could stable stratification be preserved? Some numbers: g=.25x10-6 /m a=10-5/oC give R=10-1 f = 200 mW/m2 k = 3 W/m-oK Then z*~500 km after 4 Gyr

  20. Planetary evolution is an “initial value problem”: the structure of the Earth today is not independent of how it formed and evolved in its first hundred Myr.

  21. horizontally averaged values idealized structure

  22. Densities of solids and coexisting liquid Stolper et al. (1981); Walker and Agee (1988)

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