lower mantle

upper mantle

poten

tial

hotsp

otpotential hotspot

Based on illustration by Lidunka Vočadlo, University College London

We propose this scenario: Layered Mantle Convection

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?

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 5-10 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 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

A pan of heated whale oil, circa 1900

What drives plate tectonics?

[Side view] Plate tectonics (green) are the surface expression of the convective cells in the upper

mantle (black)

Henri Bénard and Lord Rayleigh, 1900cited in Anderson 2001

A pan of heated whale oil, circa 1900

What drives plate tectonics?

[Side view] Plate tectonics (green) are the surface expression of the convective cells in the upper

mantle (black).

Boundary conditions* Heating from below

(the burner) (radiogenics in lower mantle)

*Cooling from above (room temperature) (earth’s surface / radiation)

Henri Bénard and Lord Rayleigh, 1900cited in Anderson 2001

A pan of heated whale oil, circa 1900

What drives plate tectonics?

[Side view] Plate tectonics (green) are the surface expression of the convective cells in the upper

mantle (black).

Boundary conditions* Heating from below

(the burner) (radiogenics in lower mantle)

*Cooling from above (room temperature) (earth’s surface / radiation)

Henri Bénard and Lord Rayleigh, 1900cited in Anderson 2001

Shown for the Bénard pan in 1958 by Pearson

What drives plate tectonics?Standard model: Upwellings and flow anomalies in the mantle drive plate tectonics. (The hot boundary condition matters.)

Alternative model: Cooling of the plates themselves drives plate tectonics. (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 materialtransfer between the upper and lower mantle

Transiti

on Zone

Hamilton, 2003

Jumps in seismic wave speed olivine phase transformations density increases

D e n s e

Denser

Densest

What is observed in the transition zone??

Fowler, The Solid Earth

What does this mean for subduction?

Positive P-T slope at 410 km

Negative P-T slope at 670 kmCo

ld S

lab

Hot M

antle

dens

ede

nser

dens

est

MantleSlab

Slab transitions shallower,aids subduction

Mantle transitions shallower,hinders subduction

410 km

670 km

Surface

Fowler, The Solid Earth

dept

h

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