We propose this scenario:
Layered Mantle Convection
potential
hotspot
upper mantle
lower
mantle
Based on illustration by Lidunka Vočadlo,
University College London
Location of the
boundary can be
adjusted to suit
different needs.
It does not need to
be at a constant
depth globally.
Whole mantle convection: Where's the
radiogenic heat?



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Whole mantle convection means the
mantle should be relatively
homogeneous.
But the MORB (mid ocean ridge basalt)
has Th/U ratio of ~2.5 (Turcotte 2001)
vs chondritic (“primitive”) value of 4.
Also, it's depleted in heat production 510 times over chondritic values (Kellogg
1999).
This means that if it was representative
of the mantle, we'd only get 2-6 TW of
heat production: Not enough!
From Lay et al. 2008
Layering


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There's an easy
explanation for all of
this: The mantle is
differentiated (i.e.
layered convection).
Upper mantle is
depleted in uranium,
crust enriched.
Lower mantle is close to
the chondritic value.
Primitive Th/U ratio.
From Turcotte 2001
What drives plate tectonics?
Henri Bénard and Lord Rayleigh, 1900
cited in Anderson 2001
A pan of heated whale oil, circa 1900
[Side view] Plate tectonics (green) are the surface
expression of the convective cells in the upper
mantle (black)
What drives plate tectonics?
Henri Bénard and Lord Rayleigh, 1900
cited in Anderson 2001
A pan of heated whale oil, circa 1900
(the burner)
[Side view] Plate tectonics (green) are the surface
expression of the convective cells in the upper
mantle (black).
Boundary conditions
* Heating from below
(radiogenics in lower mantle)
*Cooling from above
(room temperature)
(earth’s surface / radiation)
What drives plate tectonics?
Henri Bénard and Lord Rayleigh, 1900
cited in Anderson 2001
A pan of heated whale oil, circa 1900
Shown for
the Bénard
pan in 1958
by Pearson
(the burner)
[Side view] Plate tectonics (green) are the surface
expression of the convective cells in the upper
mantle (black).
Boundary conditions
* Heating from below
(radiogenics in lower mantle)
*Cooling from above
(room temperature)
(earth’s surface / radiation)
What drives plate tectonics?
Standard model: Upwellings and
flow anomalies in the mantle drive
plate tectonics.
Alternative model: Cooling of
the plates themselves drives
plate tectonics.
(The hot boundary condition matters.)
(The cold boundary condition matters.)
What does self-controlled plate tectonics
say about mantle convection?
• Plates are not required to drive full-mantle convection by making the difficult
crossing of the 660km boundary (see Brooke’s section); any convection they
do incite can be limited to the upper mantle (Anderson 2001)
• The lower mantle can be stratified
• its low Rayleigh number (102-103, estimated by tomography) suggests
stratification (Anderson 2001)
• chemistry arguments suggest a gradual fractionation due to density at
depth (Anderson 2001)
3. A boundary at 660 km depth exists that inhibits material
transfer between the upper and lower mantle
Hamilton, 2003
What is observed in the transition zone??
Dense
Denser
Densest
Fowler, The Solid Earth
Jumps in seismic wave speed  olivine phase transformations  density increases
What does this mean for subduction?
Negative P-T slope at 670 km
Positive P-T slope at 410 km
Surface
Fowler, The Solid Earth
depth
410 km
670 km
Slab transitions shallower,
aids subduction
Mantle transitions shallower,
hinders subduction
Research to be done to investigate
the validity of layered mantle convection
• Re-examine seismic tomography for evidence of slabs plating out on the 660.
• Better data transparency from the tomography community
• Develop chemistry techniques to pinpoint the source regions of OIB
(highpoints on mantle boundary layer? core-mantle boundary?)
• Develop models of plate motions that are not driven by mantle
inhomogeneities derived from the geoid or from elsewhere. Do these models
recreate plate motions better than existing, mantle-driven models? (Anderson 2001)
• Examine the effects of phase transitions at the transition zone in models of
mantle convection. Results from early models suggest that including the
phase transition requirement creates layered convection, while leaving out the
phase transition requirement allows full-mantle convection.
• Look at plate motions from alternate reference frames framing hinge roll back
and ridge migration. Both imply a self-driven tectonic system rather than a
static system sitting atop mantle dynamics. (Hamilton 2003)
1. Mantle rocks have distinct chemical signatures,
suggesting separate (upper/lower) reservoirs
2. Plate tectonics are driven at the surface, not from
below – so full-mantle convection is not required.
3. Subducting slabs do not penetrate 670 km depth due
to upward acting buoyancy forces
 There is no convective material transfer between
upper and lower mantle
 Convection is confined to upper/lower layers