The Deep Earth and Planets: D and ULVZs

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The Deep Earth and PlanetsD and ULVZs

• See http//slamdunk.geol.ucl.ac.uk/brodholt
• for the papers we will discuss and the ppt file.

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Bullen (1942) - divided the planet into 7
layers. A Crust (0 - 33 km) B Upper mantle
(33 - 413) C Upper mantle (413 - 984 km) D
lower mantle (984 - 2898 km) E Outer core (2898
- 4982 km) F Outer core (4982 - 5121 km) G
Inner core (5121 - 6271 km) In 1946 he realised
that he needed an anomalous layer at the bottom
of the mantle (about 200 km thick) and simply
divided D into D and D (F-layer is sometimes
talked about but may not be there.)
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PREM (Today) Crust (0 - 33 km) Upper Mantle (33 -
410) Transition zone (413 - 670 km) Lower mantle
(670 - 2798 km) D (2798 - 2998 km) Outer core
(2898 - 5121 km) F-layer ?? Inner core (5121 -
6271 km)
Bullen (1940) A Crust (0 - 33 km) B Upper
Mantle (33 - 413) C lower zone (413 - 984
km) D lower mantle (984 - 2798 km) D lower
mantle (2798 - 2998 km) E Outer core (2898 -
4982 km) F Outer core (4982 - 5121 km) G
Inner core (5121 - 6271 km)
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Jeffreys-Bullen Travel Times
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AK135, Kennett 95
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First modern study of D is by Lay and
Helmberger in GJI (1983) A lower mantle S-wave
triplication and the shear velocity structure of
D Able to give an estimate of size of
reflector and where it was. Cited 198 times
(which is good but should really be better for
such a classic piece of work).
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• How to study it?
• Seismology
• Large-scale whole Earth models
• Regional
• Local
• How to interpret it?
• Seismology mineral physics.

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Note anti-correlation of Vc and Vs
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Agrees with more regional or local studies of 0.5
to 3 increase across D
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• In D SH gt SV (horizontally polarised waves are
  faster)

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0.4-2.0 Anisotropy. Azimuthal anisotropy
required in Pacific and under Caribbean. Radial
anisotropy viable other places.
Garnero, McNamara, Lay in prep, 2005
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Double reflectors.
Thomas et al, EPSL, 2004
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D - Some things we know
D separates a liquid core (Fe Ni light
elements) from a solid silicate mantle
(Mg,Fe)SiO3 (Mg,Fe)O
Strong vertical and horizontal thermal gradients.
(From melting of Fe and adiabatic gradient of
mantle.)
Expect a rich geology there.
Reactions between core and mantle?
Perhaps approaching eutectic melting curve of the
mantle.
As much as 5 to 15 TW of heat passing through it.
Some slabs perhaps end up there.
Perhaps hot spots originate from there.
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The Standard Model?
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Paper 0
Sidorin 1999
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Paper 1
Murakami 2004
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Paper 2
Tsuchiya 2004
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Paper 3
Oganov 2004
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Paper 4
Hernlund 2005
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Paper 5
Wookey 2005
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Umemoto et al, Science 2006
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ULVZs
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Ultra-Low-Velocity Zones (ULVZs) Standard model
(from about 1995 onwards) Thin layer or discreet
mounds of very low velocity material located just
above the CMB 5 - 40 km thick Vp reductions of
5 to 10 Vs reductions of 10 to 30 ULVZs
possibly correlated with the location of
hotspots Reduction of Vs to Vp of 31 is
consistent with partial melt Ross et al (2004)
Peaceful Nuclear Explosion Program (120) Vp
reduction of 25 Vs reduction of 40 Rost et al
(2005, 2006) Vp reduction of 8 Vs reduction of
25 Density increase of 10 Avants et al (2007)
Central Pacific 24 to 30 km thick Vs reduction of
3.3 to 7.4 Vp reduction of 4 (Russell et al
2001)
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So, gt5 Vp and gt10 Vs variations seen in ULVZs
are large
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Mori Helmberger, JGR (1995)
Best fit is 10 km and 5-10 reduction in Vp
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lt 1 chance that the correlation is
random (Williams et al, Science 1998)
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Paper 8
Rost 2005
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Paper 6
Petford 2005
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Paper 7
Mao 2004
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Paper 9
Dobson Brodholt 2005
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Paper10
Labrosse et al Nature 07
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Lay et al, Science 2006
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Finish
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• Project titles
• Melting in the deep Earth past present
• Seismic anisotropy in the Earth where is it,
  what is it and what causes it?
• How chemically homogeneous is the Earths mantle?

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The End
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D Discontinuity Waveform Stacking Beneath the
Central Pacific
Avants, Lay, Russell JGR, 2005, in revison
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