figs for ambient mantle

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Shear-driven magma segregation
Super-adiabatic boundary layer
REGION B
Hawaii source
Thermal max
300 km
Tp decreases with depth
Narrow downwellings
Broad passive upwellings
MORB source
TRANSITION ZONE (TZ)
600 km
600 km
(RIP)
200 Myr of oceanic crust accumulation
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ridge
hotspots
Tp
LIL
Sheared mélange
200 400 km
LIL
LVZ
UPPER MANTLE
Ancient eclogite cumulates
TZ
Modern slab fragments
cold
THE NEW PARADIGM
the canonical box
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Density wavespeed
Harzburgite (Hz) with 1-2 melt
Vs
piclogite
red but not hot
Accumulated oceanic crust
This is the stratigraphy for a density stratified
mantle
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Ocean Island
LITHOSPHERE
LID
MORB
LVZ
220 km
MORB
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island arc basalts backarc basin basalts
underplate
Ridge suction
Sheared boundary layer
LLAMA
Low wavespeeds
TZ
Secondary downwellings
Passive upwellings (not self-driven)
Athermal explanation of tomographic results
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RIDGE
The plate tectonic cycle Harzburgite (Hz) stays
in or is returned to the shallow boundary layer
(re-used lithosphere) MORB source is displaced
entrained up (passive upwelling) Dense cold
eclogite stays at bottom of TZ
Hz
VERY COLD
LLAMA Harzburgite
Hz
TZ piclogite
Hz
Au Revoirsevoir
Oceanic crust accumulates at base (au
revoir) Harzburgite rises out as it heats
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eclogite
harzburgite
410
cold
650
cold
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RIDGE
Shear wavespeed
Temperature
OIB
1
1600 C adiabat
BL
VSHgtVSV
Observed Seismic profile
High-T conduction geotherm
2
5
220 km
1600 C
VSVgtVSH
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3
Vs for self-compressed solid along adiabat
Subadiabatic geotherm
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7
Tp1300 C
650 km
disconnect
A mantle circulation model based on anisotropy,
anharmonicity, absolute wavespeeds gradients,
allows for, and predicts, non adiabaticity
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European, African, Asian (Changbai), Yellowstone
most continental hotspots are underlain by
slabs
Cold slab
Cooled mantle
CAN BOTH UPPER MANTLE LOWER MANTLE BE COOLED BY
LONG-LIVED FLAT (STAGNANT) SLABS?
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Central Pacific Ritzwoller
The laminated upper mantle
T
Vs (T)
G1
Vs(T,f)
f
SH
G2
Boundary layer
VSHgtVSV
SV
Vs(T)
L
Not pyrolite
1600oC
fV2/V1lt 2
VSVVSH (slow)
Decrease of Vs with depth due to high conduction
thermal gradient and the variation of melt-rich
layer thicknesses and number
?(VSH)2G1 , ?(VSV)2G2/f
Note contrary to some petrologists, there is
nothing wrong with Tp1600 C at 200 km if the
boundary layer is harzburgite with 2 melt
rather than pyrolite.
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ridge
Ridge-normal profile
OIB
B
TZ
MORB source
Birchs Transitional Layer
Density barrier
D
D
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The idea that ridges may be sourced deeper than
OIB based on fixity (Wilson) and geochemistry
(Tatsumoto) is more than 30 years old.
Tatsumoto Model (1978)
LLAMA Model
ridge
OIB source
FERTILE DEPLETED MORB SOURCE
BARREN LOWER MANTLE
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Along-ridge profile
R I d g e
ridge
Ridge-normal profile
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SUMMARY
Ridges are fed by broad 3D upwellings plus
lateral flow along toward ridges
ridge
OIB
LID
LVZ
LLAMA
200 400
subadiabatic
Mesosphere (TZ)
km
Cold slabs
Intraplate (delamination, CRB, Deccan, Karoo,
Siberia) magmas are shear-driven from the 200 km
thick shear BL (LLAMA)
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TATSUMOTO
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large relative delay times in BL
comparable to crustal delays
Seismology of LLAMA
S late
Laminated Lithologies Aligned
Melt Accumulations
SKS very late
S early
underplate
teleseismic rays Large lateral variations in
relative delay times due to plate LVZ
structure, subplate anisotropy bleed into
deep mantle