Dominant process change
Composition change
Not many crossing rays here
Problems w/ tomography
• Source/receiver locations
– New innovations:
• OBS (ocean bottom seismometer)
• Noise tomography
• Errors in arrival time picks
– Use cross/correlation instead of single picks
• Errors in EQ locations
– Wrong location = assign velocity to wrong cell
• Can’t resolve sharp velocity contrasts
– Need info from different wave types
– Can only resolve features ~ order of wavelength
• Very computationally expensive!
• Inverse theory issues: may introduce “artifacts”, or false features
that are caused by processing or noise
– Difficult to interpret result - at least 4D!
Convection and the mantle
• Overview
– Discontinuities at 410 and 660 km
– Tomography: fuzzy images of continental
keels, plumes, slabs
– Convection: expected in viscous material
heated from bottom or cooled from top
• Depends on viscosity, changes in density,
etc
• Whole-mantle or layered convection
cells?
Convection and the mantle
• Are there changes to the structure of
minerals (phase transitions) expected
at the depths where we see velocity
jumps?
• Necessary, not sufficient condition
• Phase changes and their
dependence on pressure/temperature
• Claudius-Clapeyron equation
• How are phase transitions affected by
lateral temperature changes?
• How do phase transitions affect
convection?
Convection - what controls it?
Heating from bottom
Cooling at top
+ gravity
Buoyancy forces (thermal expansion), i.e.
gravity+density
vs.
Resistance to flow, i.e., viscosity
Useful: “non-dimensional” numbers that compare these two
L. Kellogg, Hager, van der Hilst (1999),
Compositional Stratification in the Deep Mantle,
Science
Eh Tan,
Caltech
Carbon!
Increase in Pressure
Decrease in Volume
Release/Require heat
Change in properties
~2.2 g/cm3
~3.5 g/cm3
Side note- Carbonado or black diamonds
Not this
This!
• Found in stream deposits in Africa and Brazil
• Gravels 2.6-3.8 billion years old
• Some show glassy exteriors, vesicles, shock features
Main theory behind Carbonados - impact
1.
Meteorite hits carbonate-rich rocks, shocks the carbon to
diamonds, sprays into air to generate aerodynamic
globules
–
–
–
2.
(Smith & Dawson, 1985)
Problem - no other shocked minerals found
Doesn’t look like manmade diamond (hexagonal)
Meteorite has diamonds already in it
–
–
(Brunini, 2006)
Timing is right - Methane may convert to diamond at
conditions found in gas giants, and the impact was during a
period where the orbits of the outer planets are thought to
have been perturbed.
Graphite&diamonds
consistent with
whole mantle
convection
One chemical
composition,
Pressure-dependent
change in structure
Velocity jump
x
x
x x
High-velocity
kittens
“Kitten-free”
layer
“Dead Kitten”
layer
x x
Whole mantle vs. multi-layer convection
• Are there phase transitions expected at the depths
where we see velocity jumps?
– i.e., can we explain the velocity jumps with one material?
– Necessary, not sufficient condition
• Do we expect them to enhance or impede
convection? (exothermic or endothermic?)
– Phase diagrams
• Diamond/graphite
• Ice/water/water vapor
• Olivine
– Clausius-Clapeyron equation (Clapeyron slope)
Shock-wave
synthesis
More phase diagrams: H2O
dP/dT is negative!
More phase diagrams: H2O
Mg2 SiO4: Forsterite (Olivine), ~60% of mantle
410

Olivine
520?
660
->  ->  -> perovskite
spinel
Mg2 SiO4: Forsterite (Olivine), ~60% of mantle
Positive Clapeyron slope
Negative Clapeyron slope
Note- other minerals (pyroxene
garnet, etc) also go through
transitions
Phase boundary (blue dashed)
At constant depth, pressure, T
Plume deflects T. P stays the same (black dash)
dP/dT > 0
(exothermic)
dP/dT < 0
(endothermic)
Subducting slabs
Plumes
From plumes.org
Phase changes and Clapeyron slopes
• So far – In convecting mantle, expect +/- slopes to predict
variations in depth of transition zones
– How do the density changes go along with it?
• Once an instability gets started, will it continue?
410
Pressure
Olivine
Spinel
dP/dT>0
410
Pressure
Olivine
Spinel
dP/dT>0
410
Pressure
Olivine
Spinel
dP/dT>0
660
Pressure
Spinel
Perovskite
dP/dT<0
Perovskite
660
Pressure
Spinel
What causes
topography in
transition zone?
•Plumes and slabs should cause
deflection of transition zone
depths
• But a thin transition zone is not
seen (seismically) at all plume
locations
• Multiple minerals undergo
transitions at different depths
Causes complications?
From:Duess, 2005