Download Eclogite Engine

Eclogite
Engine
Don L. Anderson
Caltech
Can we bridge
geophysics,geochemistry &
geodynamics?
 Continents; the missing link
 The lower crust is transient
 It is recycled 6 times faster
than upper crust
 Recent arc growth estimates
are 5X previous estimates!
 Therefore, a huge previously
unaccounted for flux
THE UPPER MANTLE
ECLOGITE CYCLE
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All the components of so-called hotspot
or plume magmas originate in crust,
lithosphere, cumulates or mantle wedge
The Isotope Zoo
EM1, EM2, HIMU, DUPAL, LONU,
High 3He/4He, C-, FOZO, Os…
Mantle is a Top-Down System
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
delamination
Sources of dissipation
The top boundary condition is not simple
Many ways to get big chunks of
mafic matter into upper mantle
Subduction of seamount chains,
aseismic ridges
Delamination of island arcs
Delamination of compressional
mountain belts
Estimating mantle sources & sinks, continental addition & loss, through time,
depends critically on when deep subduction, got underway
1 Gyr of oceanic crust subduction=70 km of eclogite
But lots of other things get put into the mantle
OIB
~2
Km3/yr
20 km3/yr
David Scholl
Piles up @ 70
km/Gyr
MASS BALANCE
Recycling rate of oceanic crust (basaltic + gabbroic) ~20 km3/yr
Midplate extrusive magmatism
1-2 km3/yr
(4 to 8 current Hawaii’s; 10x more average Hawaii’s)
(Factor of 10 to 20 mismatch for Hofmann & White recycling mechanism)
Underside erosion of continents at marine margins
2.5 km3/yr
Erosion plus delamination at continental colli sions
2-3 km3/yr
Production rate of magmas in arcs
3-9 km3/yr
[Scholl (2006) gives
4-5 km3/yr]
Growth rate of arc garnet pyroxenite cumulates
1.5-6 km3/yr
Island arcs and oceanic plateaus (and Hawaii’s) can also delaminate
Midplate volcanisms & arcs can be upper mantle
RIDGE-TRENCH ANNIHILATION
Abandonment of young oceanic crust and
mantle wedge
BAB
Future
suture
EM
EM12
HIM
U
Trapped crust
FOZ
O
HI
3HE/U
LOMU
Mantle
wedge
All of the isotope and trace element signatures of OIB
are manufactured in the subduction factory, wedge &
crust
ROOT FORMATION
Delamination
cycle

dense roots
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fall off
warm up in
ambient mantle
rise
possible mechanism
for Atlantic & Indian
ocean plateaus &
DUPAL anomaly
1
DELAMINATION
2
SPREADING
3
heating
UPWELLING
4
SPREADING
5
ridge
mafic melt
ERUPTABLE
MELTS
50 km
LMP
ERUPTABLE
100 km
MELTS
200 km
BUOYANT MELT
PONDED MELT
LMP
300 km
ultramafic melt
jadeite
eclogite
mafic melt
ultramafic melt
UMR AVERAGE
dunite
sp.perid.
Gt.Lhz.
mafic melt
ultramafic melt
PYROLITE
eclogite
Hawaii Lherzolite
densit SHEAR
y(-0.68) VELOCITY
(-0.40)
3.20
3.24
(-0.40)
(-0.15)
3.29
3.30
3.35
(-0.18)
(+0.00)
3.38
3.43
3.47
4.82
4.28
4.68
4.90
4.52
4.83
4.58
4.72
ECLOGITE AT DEPTH HAS LOW SHEAR VELOCITY
Non-uniform heat leakage from the top
& peeling off of crust
Dual eruptions
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
unstable
Qui ck Time™ and a
TIF F ( Uncompres sed) dec ompres s or
are needed to s ee this pic ture.
The eclogite engine, as in any
engine, involves changes in volume
due to compression, heating and
phase changes.One cannot
consider T but not P.
The cycle is cooling, phase change
(eclogite), compression (sinking),
heating, phase change (melting),
decompression (rising), heat
exchange…
[The continent may move, rather than
the blob.]
Delaminated roots warm
quickly
• will start to melt
before reaching
same T as
surrounding mantle
• already in TBL, so
starts off warm
• when 30% melt,
garnet mostly gone
& will start to rise
Peridotite melts
Eclogite
melts
Dry peridotite can only melt in very shallow mantle;
hence adiabatic ascent at ridges
Eclogite can melt much deeper, and much more,
even when colder; hence, “midplate magmatism”
Delamination rate of arc cumulates 1.5-6 km3/yr is non-negligible. It is also
likely that larger chunks are inv olved than would be the case with
subduction of normal oceanic crust. They are also hotter. Thus, this material
may be responsible for fertile melting anomalies, in addition to contributing
trace element and isotopic signatures to their melts. To the arc lower crustal
delamination rates must be added the colli sional mountain belt delamination.
The breakup of Gondwana and the uncovering of the
Indian and Atlantic oceans provides the best opportunity
for seeing the re-emergence of these fertile blobs, after
they have been heated by ambient mantle. It has been
proposed that the Indian ocean and the south Atlantic
plateaus and island chains may be due to these mafic
patches that were injected into the mantle from the base
of the Gondwana crust (Anderson, 2005).
LIPs are associated with
continental breakup
• reconstruction at
~ 30 Ma
• dual volcanism
– on breakup
– ~ 30 Myr later
• oceanic plateaus form
~ 1,000 km offshore
• = rising of
delaminated root?
MANTLE
DELAMINATION
WEDGE
COUNTERFLOW
CHANNEL
EROSION
Slow upwelling
Slow upwelling
Broad upwellings
NO PLUME
NECESSARY
Input into asthenosphere: delamination, seamount chains,
broad upwelling, abandoned mantle wedges
Fertile blob & asthenospheric counterflow model
^
LVZ
3
2
1
4


---------------------------------------------------------------------------
1 delaminated crust, 2 wedge, 3 broad upwelling,
4 young oceanic crust
Test of the model; lots of scattering in the upper
mantle
There is no mass balance
or geochemical reason for
anything to come from
below 1000 km
ECLOGITE ENGINE
0 Myr
40-80 Myr
CRUST
^
>>>>>>>>>
>>>
<<<
<<<
1
COLD
DENSE BLOB
10 Myr
2(z)
^
5
MAGMA-RICH
ZONE

RESIDUE
ASTHENOSPHERE
<1
Blobs deliver
heat & magma
to surface
50 Myr
S
L
4
PARTIALLY
MOLTEN
BLOB
A
B
HOT
^

13 Myr
2
HOT
WARM BLOB
PHASE CHANGE
HOT
COLD
BLOB
low U, Th

40 Myr
3

30 Myr
high U, Th

HOT

OLD SLABS
PHASE CHANGE

HOT
100+ Myr
NOTICE! Oceanic crust not involved
LOWER MANTLE
Blobs gain
heat from
mantle
TRI-CYCLING THROUGH THE EARTH
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RIDGE-TRENCH ‘CYCLE’ (the
escalator)
SUPERCONTINENT CYCLE (the
trolley)
SLAB-PLUME ‘CYCLE’ (the tube)
SUBTERRANEAN CYCLE
(delamination, orogenic cycle; the
elevator)
Elevated water contents in
Scale matters
>10-km blobs; gravity takes over; heating is slow
I ––––––10 cm or 10 km ? –– I
piclogite
QuickTime™ and a
TIFF (U ncompres sed) decompressor
are needed to see this picture.
Garnet and clinopyroxene
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Olivine and opx
Eclogite melts in the mantle react
with peridotite to form pyroxenite
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The olivine of mantle
peridotite is consumed by
reaction with melts derived
from recycled crust &
cumulates, to form a
secondary pyroxenitic
source
‘Hotspot’ basalts such as
Hawaii form from olivine-free
mantle
QuickTim e™ and a
TIFF (Uncom pres s ed) decom pres s or
are needed to s ee this picture.
QuickT ime™ and a
TI FF (Uncompressed) decompressor
are needed to see this picture.
GALILEO THERMOMETER
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Blobs sink to
level of neutral
buoyancy
But they don’t stay
there
In mantle, they heat up & melt
And bring mantle
heat & magma to
base of plate
Not core heat
Rates of generation of the
continental crust
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The rates at which basaltic magma is
added to the continental crust have
recently been revised upwards range to
3.7 km3 yr-1
The “plume affinity” of basaltic rocks in
juvenile crust has been used to support
deep-seated disturbances in the Earth, as
opposed to shallow-level processes. But
this evidence could also be used in
support of the delamination model.
The average residence times of the lower
crust may be at least five times less than
the upper crust
THE ECLOGITE CYCLE
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Mass balance seems o.k.
Petrology & major elements seem o.k.
Isotopes seem o.k. including stable
isotopes
Dynamics seems o.k.
Thermodynamics seem fine
Energetics seem o.k.
What caused these?
QuickTime™ and a
TIFF (U ncompressed) decompressor
are needed to see this picture.
LIPs CFB SDR NAVP CRB OPB BABB OJP
New parameters
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Scale (size)
Homologous temperature
Stress
Buoyancy parameter
Architecture
Fertility
Entrainability
MASS BALANCE
km3/yr
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TOTAL MIDPLATE MAGMA VOLUMES
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Growth rate of arc garnet pyroxenite cumulates
Underside erosion & delamination of continents
Production rate of magmas in arcs
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LOWER CRUSTAL LOSS = ‘HOTSPOT’ MAGMATISM
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CYCLE TIME 40-80 MYR
10-30 KM CHUNKS
3-6
1.5-6
4-6
4-5
KEY ELEMENTS
Mantle is source of heat
Thick basalt piles are the source
of material
True Top-Down system
Mantle is heterogeneous
Heterogenity washed out by
Central Limit Theorem (ridges,
tomography), not by mantle
convection
Mantle is gumbo, not fruit cake
Unification of geodynamics &
geochemistry of mantle
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Delamination of lower continental crust
& subduction of seamount chains
fertilize the mantle
These become low meltng point blobs
It is these, not plumes, that explain
‘anomalies’ such as midplate volcanism,
swells and ‘hotspots’