Transcript Ambient Mantle - MantlePlumes.org

Shear-driven magma
segregation
Superadiabatic
boundary
layer
REGION B
Hawaii
source
Thermal max
300 km
MORB
source
600 km
Tp
decreases
with
depth
TRANSITION ZONE (TZ)
600 km
200 Myr of oceanic crust
accumulation
(RIP)
ridge
hotspots
Tp
LIL
Sheared mélange
200
LIL
LVZ
UPPER MANTLE
Ancient eclogite
cumulates
400
km
TZ
Modern slab fragments
‘cold’
THE “NEW” PARADIGM
“the canonical box”
UPPER harzburgite
MANTLE dunite
pyrolite
peridotite
B
arcl(highMgO)
eclogite
Hawaii Lhz.
arcl(highMgO)
TZ
C'
C"
Density
wavespeed
3.30
3.31
3.38
3.42
3.45
3.46
3.47
3.48
Vs
b-spinel(.1FeO)
(.12FeO)
pyrolite(410km)
majorite
arcl(lowMgO)
"
pyrolite(500km)
g-spinel(.1FeO)
MORB(mj+coe)
archlogites
(low MgO)
"
MORB(mj+st)
3.59
3.60
"
"
3.60
3.63
3.67
3.70
3.68
3.70
3.74
3.75
3.75
magnesiowustite
perovskite(pv)
4.04
4.11
UPPER
MANTLE
Vp= 8.4 km/s
Vp= 8.3 Km/s
stable
Harzburgite
(Hz) [with 12% melt]
eclogite
Vp=8.1 km/s
8.1 km/s
3
4
5
6
km/s
X
410 km
9.3 km/s
8.3km/s
9.7 km/s
eclogite
500 km
piclogite
8.6 km/s
ultra-stable
(when cold)
11 km/s
eclogite
‘red’ but not hot
oceanic crust Accumulated
oceanic crust
650 km
This is the stratigraphy for a density stratified mantle
Ocean Island
LITHOSPHERE
LID
MORB
LVZ
220 km
MORB
island arc basalts
backarc basin basalts
underplate
Ridge suction
Sheared
boundary
layer
LLAMA
Low wavespeeds
TZ
Athermal explanation of
tomographic results
Secondary
downwellings
Passive
upwellings
(not self-driven)
RIDGE
The plate tectonic
cycle
VERY COLD Hz
LLAMA Harzburgite
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
TZ piclogite
Hz
Au Revoirsevoir
Oceanic crust accumulates at base (au revoir)
Harzburgite rises out as it heats
eclogite
harzburgite
410
cold
650
cold
RIDGE
Shear wavespeed
OIB
1
Temperature
BL
VSH>VSV
Observed
Seismic
profile
6
High-T
conduction
geotherm
2
220 km
~1600 C
Vs for selfcompressed
solid along
adiabat
VSV>VSH
4
1600 C
adiabat
5
3
Subadiabatic
geotherm
7
Tp=~1300 C
650 km
disconnect
A mantle circulation model based on anisotropy,
anharmonicity, absolute wavespeeds & gradients, allows
for, and predicts, non adiabaticity
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?
Central Pacific
Ritzwoller
The laminated upper mantle
Vs (T)
Vs(T,f)
T
f
G1
SH
G2
Boundary
SV
VSH>VSV
layer
Vs(T)
Not
pyrolite
L
~1600oC*
f=V2/V1<
2%
VSV=VSH (slow)
Decrease of Vs with depth due to high conduction thermal gradient
and the variation of melt-rich layer thicknesses and number
(VSH)2~G1 ,
(VSV)2~G2/f
*Note: contrary to some petrologists, there is nothing wrong with Tp=1600 C at 200 km
if the boundary layer is harzburgite with ~2% melt rather than pyrolite.
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)
ridge
LLAMA
Model
OIB
source
FERTILE DEPLETED MORB SOURCE
BARREN LOWER MANTLE
Along-ridge profile
R I d g e
Ridge-normal profile
ridge
SUMMARY
Ridges are fed by broad 3D upwellings plus lateral
flow along & toward ridges
ridge
OIB
LID
LVZ
LLAMA
200
400
km
subadiabatic
Mesosphere (TZ)
Cold slabs
Intraplate (delamination, CRB, Deccan, Karoo, Siberia) magmas are
shear-driven from the 200 km thick shear BL (LLAMA)
TATSUMOTO
Seismology of
LLAMA*
large relative delay times in BL
=
comparable to crustal delays
*Laminated
Lithologies &
Aligned
Melt
Accumulations
SKS very late
S
late
S early
underplate
teleseismic rays
Large lateral variations in relative delay times
due to plate & LVZ structure, & subplate
anisotropy
…bleed into deep mantle