Outline

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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

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Composition of the lunar surface
- Mare basalt volcanism at 3.9 Gyr to 2.5 Gyr
long after MO solidification - Basalts generated
at gt400 km depth olivine-pyroxene multiple
saturation - Mantle source composition residual
to anorthositic crust crystallization - Global
asymmetry in emplacement of basalts and the PKT
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Timescales and mixing in terrestrial planetary
accretion
Chambers, EPSL 2004.
Chambers, Icarus, 2001.
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Magma ocean formation due to a large impact
Tonks and Melosh JGR 1993
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Tonks and Melosh JGR 1993
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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.
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Effect of atmosphere on cooling and
solidification of 500 km deep MO
non-convecting grey atmosphere following
Abe (1979)
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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
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The double diffusion problem of melt migration
in a convecting, compacting, permeable matrix
Buoyancy sources matrix density melt
distribution
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Does solidification occur by freezing or
squeezing (i.e. compaction)?
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Permeability dependence on f
b
Buoyant rise of liquid in pore space
L compaction length
(Kmsolid /mliquid)1/2
Wark and Watson, 2003
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Buoyant rise of liquid in pore space
L compaction length
(Kmsolid /mliquid)1/2
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Melt-solid fractionation during the first 100 Myr
of Earth evolution
Boyet and Carlson (2005)
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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
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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.
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Convective instability in a continuously
stratified fluid layer
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How long could stable stratification be preserved?
Some numbers g.25x10-6 /m
a10-5/oC give R10-1 f 200 mW/m2
k 3 W/m-oK Then z500 km after 4 Gyr
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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.
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Densities of solids and coexisting liquid
Stolper et al. (1981) Walker and Agee (1988)