Chapter
5
The
eclogite
engine
The
World's
great
age
begins
anew,
The
golden
years
return,
The
Earth
doth
like
a snake
renew
Her
winter
weeds
outgrown
Shelley
The
water
cycle
drives
geological
processes
at
the
surface.
TI1e
fact
that
water
coexists
as
fluid,
vapor
and
solid
is
crucial
in
shaping
the
Earth's
surface
.
The
fact
that
conditions
in
the
upper
mantle
can
readily
convert
eclogite
to
magma
to
basalt,
and
back,
with
enormous
density
changes,
is
crucial
in
global
magmatism
and
tectonics.
Phase
changes
in
the
mafic
components
of
the
upper
mantle
are
larger
than
thermal
expansion
effects
and
they
drive
the
eclogite
engine.
There
are
several
ways
to
generate
massive
melting
in
the
mantle;
one
is
to
bring
hot
mate-
rial
adiabatically
up
from
depth
until
it
melts
;
the
other
is
to
insert
low-melting
point
fertile
material
-
delaminated
lower
arc-crust,
for
exam-
ple
-
into
the
mantle
from
above
and
allow
the
mantle
to
heat
it
up.
Both
mechanisms
may
be
involved
in
the
formation
of
large
igneous
provinces
-
LIPs
.
The
timescale
for
heating
and
recycling
of
lower-crust
material
is
much
less
than
for
subducted
oceanic
crust
because
the
for-
mer
starts
out
much
hotter
and
does
not
sink
as
deep
.
TI1e
standard
petrological
models
for
magma
genesis
involve
a
homogenous
pyrolite
mantle,
augmented
at
times
by
small-scale
pyroxenite
veins
or
recycled
oceanic
crust.
It
is
increasingly
being
recognized
that
large
blocks
of
eclogite
in
the
mantle
may
be
an
important
fertility
source.
Delaminated
continental
crust
differs
in
many
important
respects
from
recycled
MORB.
It
does
not
go
through
subduction
zone
and
seafloor
pro-
cessing,
it
starts
out
hotter
than
MORB,
it
may
occur
in
bigger
blobs,
and
it
is
not
accompanied
by
the
same
amount,
if
any
,
of
buoyant
infer-
tile
harzburgite.
Figure
5.1
illustrates
the
lower
crust
delamination
cycle.
The
crust
thickens
by
tectonic
or
igneous
processes
,
eventually
forming
dense
eclogite
that
detaches
and
sinks
into
the
mantle.
It
reaches
a level
of
neutral
buoyancy
and
starts
to
warm
up.
Eventually
it
rises
and
forms
a
warm
fertile
patch
in
the
mantle
.
If
the
overlying
continents
have
moved
off,
a
midplate
magmatic
province
is
the
result.
Thermal
expansion
is
the
main
source
of
buoyancy
in
thermal
convection
of
simple
flu-
ids.
Phase
changes
can
be
more
important
in
the
mantle.
When
basalt
converts
to
eclogite,
or
when
it
melts,
there
are
changes
in
density
that
far
exceed
those
associated
with
thermal
expansion.
Removal
of
the
dense
lower
continen-
tal
crust
is
an
important
element
of
plate
tecton-
ics
that
complements
normal
subduction
zone
and
ridge
processes.
It
causes
uplift
and
magma-
tism
and
introduces
distinctive
materials
into
the
mantle
that
are
dense,
fertile
and
have
low
melt-
ing
points.
After
delamination,
eclogitic
lower
crust
sinks
into
the
mantle
where
it
has
rela-
tively
low
seismic
velocities
and
melting
point
(Figures
5.2
and
5.4)
compared
with
normal
man-
tle
peridotite.
These
fertile
mafic
blobs
sink
to
various
depths
where
they
warm
up
,
melt
and
ROOT
FORMATION
DELAMINATION
ridge
SPREADING
heating
UPWELLING
4
SPREADING
5
@ji
Th
e
delamination
cycle.
return
to
the
surface
as
melting
anomalies,
often
at
ridges.
Large
melting
anomalies
that
form
on
or
near
ridges
and
triple
junctions
may
be
due
to
the
res
urfacin
g
of
fertile
blobs,
including
delam-
inated
continental
crust.
This
is
expected
to
be
especially
prevalent
around
the
'passive'
margins
of
former
supercontinents:
Bouvet,
Kerguelen,
Broken
Ridge,
Crozet,
Mozambique
ridge,
Marion,
Bermuda,
Jan
Mayen,
Rio
Grande
rise
and
Walvis
ridge
may
be
examples;
the
isotopic
signat
ur
es
of
these
plateaus
are
expected
to
reflect
lower
crustal
components.
The
eclogite
cycle
Cold
oceanic
crust
and
warm
lower
continen-
tal
crust
are
continuously
introduced
into
the
mantle
by
plate
tectonic
and
delamination
pro-
cesses.
These
materials
melt
at
temperatures
MANTLE
STRATIGRAPHY
59
higher
than
their
starting
temperatures,
but
lower
than
normal
mantle
temperatures
.
They
heat
up
by
conduction
from
the
surrounding
mantle
(Figure
5.2);
they
are
fertile
blobs
in
the
mantle
,
in
spite
of
being
cold
.
Trenches
and
continents
move
about
on
the
Earth
's
surface,
refertilizing
the
underlying
man-
tle.
Ridges
also
migrate
across
the
surface,
sam-
pling
and
entraining
whatever
is
in
the
mantle
beneath
them.
Sometimes
a
migrating
ridge
will
override
a
fertile
spot
in
the
asthenosp
her
e;
a
melting
anomaly
ensues,
an
interval
of
increas
ed
magmatic
output.
Ridges
and
trenches
also
die
and
reform
elsewhere.
What
is
described
is
a
form
of
convection,
but
it
is
quite
different
from
the
kind
of
convection
that
is
usually
treated
by
geodynamicists.
It
is
more
akin
to
fertilizing
and
mowing
a
lawn.
The
mantle
is
not
just
convect-
ing
from
a
trench
to
a
ridge;
the
trenches
and
rid
ges
are
visiting
the
various
regions
of
the
man-
tle.
The
fertile
dense
blobs
sink,
more-or-less
ver-
tically
into
the
mantle
and
come
to
rest
where
they
are
neutrally
buoyant,
where
they
warm
up
and
eventually
melt
(Figures
5.3
and
5.4).
They
will
represent
more-or-less
fixed
points
relative
to
the
overlying
plates
and
plate
boundaries.
They
are
chemically
distinct
from
'norma
l '
mantle.
Eclogites
are
not
a
uniform
rock
type;
they
come
in
a
large
variety
of
flavors
and
densities
and
end
up
at
various
depths
in
the
mantle.
Mantle
stratigraphy
The
densities
and
shear-velocities
of
crustal
and
mantle
minerals
and
rocks
are
arranged
in
order
of
increasing
density
(Figure
5.2)
. Given
enough
time,
this
is
the
stratification
toward
which
the
mantle
will
evo
lve
-
the
neutral-density
profile
of
the
mantle.
Such
density
stratification
is
already
evident
in
the
Earth
as
a
whole
,
and
in
the
crust
and
continental
mantle.
This
stratification,
at
least
of
the
upper
mantle,
is
temporary,
however
,
even
if
it
is
achieved.
Cold
eclogite
is
below
the
melting
point
but
eclogite
melts
at
much
lower
temperatures
than
the
surrounding
peridotite
.
As
eclogite
warms
up,
by
conduction
of
heat
from
the
surrounding
mantle,
it
will
melt
and
become
buoyant,
creating
a
form
of
yo-yo
tectonics
.
60
THE
ECLOGITE
ENGINE
depth
reflectors
(km)
Rock
type
granodiorite
13
gneiss
anorthosite
serpentinite
20
metabasalt
60
gabbro
amphibolite
granulite
-mafic
amphibol
e
pyroxenite
eclogite
den
sity
Vs
3
(gl ee)
km
/s
2 .
68
3 .
68
2 .
79
3 .
57
2 .
80
3.73
2.
81
3 .
83
2.87
3.28
2.95
3.
64
3.07
4.30
3.10
4.05
3.20
3 .23
3.24
SHEAR
VELOCITY
(P
=
0)
4
5
6
Avg
.ultramafic
rock
3.
29
3.80
4.43
4.28
4 .
68
4 .
60
4 .
87
4.52
4.83
4.90
4.76
4.58
4.77
4.72
5.73
4.95
5.
43
5.
79
5 .
08
continental
moho
cpx
80
PHN1569
sp.pe
rid
.
90
G1.Lhz
.
eclogite
130
PHN1611
eclogite
eclogite
200
Hawaii
Lhz
.
220
~-spinei(O
FeO)
eclogite
280
majorite
330
y-
spinel
garnet
400
P
·
spine~.1
FeO)
410
gr
.
garnet
eclogite
eclogite
mj
eclogite(cold)
500
y·
spine~
.
1
FeO)
py.
garnet
'
ilmenite
~.
1
FeO)
mj
650
mw(
Mg.8
)
710
perovskite(pV)
1000
3 .
30
3 .
31
3 .35
3.35
3.37
3.42
3.43
3.
46
3.47
3.47
3.49
3 .
53
3 .
55
3 .57
3.59
3.60
3.
60
UPPER
MANTLE
LVZ
TZ
3.61
5.
54
5.45
4.
86
4.69
5.65
4.90
LVZ
3.61
3.67
3
4
5
6
3 .
68
5 .
59
3 .
71
5 .
01
3 .
92
5.
71
4 .
00
5 .
63
4.10
6.11-..._
LOWER
MANTLE
4.07
5.08
Density
and
shear-ve
locity
of
crustal
and
mantle
minerals
and
rocks
at
STP,
from
standard
compilations.
The
ordering
approximates
the
situation
in
an
ideally
chemically
stratified
mantle.
The
materials
are
arranged
in
order
of
increasing
density.
The
STP
densities
of
peridotites
vary
from
3.3
to
3.47
g/cm
3
;
eclogite
densities
range
from
3.45
to
3.75
g/cm
3
.
The
lower
densicy
eclogites
(high-MgO.
low-Si02)
have
densities
less
than
the
mantle
below
41
0
km
and
will
therefore
be
trapped
at
that
boundary.
even
when
cold,
creating
a
LVZ.
Eclogites
come
in
a
large
variecy
of
compositions,
densities
and
seismic
velocities.
They
have
much
lower
melting
points
than
peridotites
and
will
eventually
heat
up
and
rise,
or
be
entrained.
If
the
mantle
is
close
to
its
normal
(peridotitic)
solidus
,
then
eclogitic
blobs
will
eventually
heat
up
and
melt
.
Cold
oceanic
crust
can
contain
perovskite
phases
and
can
be
denser
than
shown
here.
Regions
of
over-thickened
crust
can
transform
to
eclogite
and
become
denser
than
the
underlying
mantle.
The
upper
mantle
may
contain
peridotites,
lherzolit
es
and
some
of
the
less
garnet-rich,
high-MgO,
eclogites.
.0
"'
Ui
0
SLAB
EQUILIBRIUM
Eclogite
Melts
20
40
60
80
Time
since
Subduction
(Myr)
Heating
rates
of
subducted
sla
bs
due
to
conduction
of
heat
from
ambient
mantle.
Delaminated
continental
crust
starts
hot
and
will
melt
quickly.
The
total
recycle
time,
including
reheating,
may
take
30-75
Myr.
If
delamination
occurs
at
the
edge
of
a
continent
, say
along
a
suture
belt,
and
the
continent
moves
off
at
3.3
em/year
,
the
average
opening
velocity
of
the
Atlantic
ocean,
it
will
have
moved
I
000
km
to
2500
km
away
from
a
vertically
sinking
root.
One
predicts
paired
igneous
events,
one
on
land
and
one
offshore
(see
Figure
5.5).
8
Q)
::;
Iii
Q)
a.
E
~
Pressure
(GPa)
Melting
relations
in
dry
lh
erzo
lite
and
eclogite
based
on
laboratory
experiments.
The
dashed
lin
e
is
the
100
I
300-degee
mantle
adiabat.
showing
that
eclogite
will
melt
as
it sinks
into
normal
temperature
mantle,
and
upwellings
from
the
shallow
mantle
will
extensive
ly
melt
gabbro
and
eclogite.
Eclogite
will
be
about
70%
molten
before
dry
lherzolite
starts
to
melt
[compiled
by
J.
Natland,
personal
communication].
30
My
Reconstruction
ITIQ]
Break-up
(My)
90
Plateau
age
(My)
200
CFB
age
(My)
Distributions
and
ages
of
LIPs
in
the
Gondwana
hemisphere.
The
ages
of
continental
breakup
and
the
ages
of
volcanism
or
uplift
are
shown.
Delamination
of
lower
crust
may
be
responsible
for
the
LIPs
in
the
continents,
which
usually
occur
along
mobile
belts,
island
arcs
and
accreted
terranes.
If
the
continents
move
away
from
the
delamination
sites,
it may
be
possible
to
see
the
re-emergence
of
fertile
delaminated
material
in
the
newly
formed
ocean
basins
,
particularly
where
spreading
ridges
cause
asthenospheric
upwellings.
As
it
warms
up
by
conduction
it
will
rise,
adiabat-
ically
decompress
and
melt
more.
It
will
either
erupt,
causing
a
melting
anomaly
(Figure
5.5),
or
underplate
the
lithosphere,
depending
on
the
stress
state
of
the
plate.
Eclogites
have
a
wide
variety
of
properties,
depending
on
the
amount
of
garnet
,
and
the
Fe
and
Na
contents
. At
shallow
depths
eclogites
have
higher
seismic
velocities
than
peridotites
. At
greater
depth
,
eclogite
can
create
a
LVZ.
Cold
dense
sinking
eclogite
can
cre-
ate
a low-velocity
feature
that
could
be
mistaken
MANTLE
STRATIGRAPHY
61
for
a
hot
rising
plume.
As
eclogite
melts
it
will
have
even
lower
shear
velocities
.
There
are
two
main
sources
of
eclogite,
sub-
ducted
oceanic
crust,
and
delaminated
thick
crust
at
batholiths
and
island
arcs
(arclogites).
The
density
differences
between
basalts
and
gab-
bros,
eclogites
and
magmas
are
enormous
com-
pared
with
density
variations
caused
by
thermal
expansion
and
normal
compositional
effects.
The
dynamics
of
the
outer
Earth
will
be
strongly
influ-
enced
,
if
not
controlled,
by
this
'eclogite
engine
.'
Do
we
need
to
recycle
oceanic
crust?
If
current
rates
of
oceanic
crust
recycling
oper-
ated
for
1 Gyr,
the
total
oceanic
crust
subducted
would
account
for
2 %
of
the
mantle
and
it
could
be
stored
in
a
layer
only
70
lm1
thick.
The
surprising
result
is
that
most
subducted
oceanic
crust
need
not
be
recycled
or
sink
into
the
lower
mantle
in
order
to
satisfY
any
mass
balance
constraints
.
The
recycling
rate
of
lower
crustal
cumulates
(arclogites)
implies
that
about
half
of
the
conti-
nental
crust
is
recycled
every
0 .6
to
2.5
billion
years.
In
contrast
to
oceanic
crust
one
can
make
a
case
that
eroded
and
delaminated
continental
material
is
not
stored
permanently
or
long-term
or
very
deep
in
the
mantle;
it
is
re-used
and
must
play
an
important
role
in
global
magmatism
and
shallow
mantle
heterogeneity.
Sediments
,
altered
oceanic
crust,
oceanic
lithosphere
and
delaminated
continental
crust
probably
a ll
play
some
role
in
the
source
regions
of
various
m a n-
tle
magmas
,
including
midocean-ridge
basalts
,
ocean-island
basalts
and
continental
flood
basalt.
These
recycled
components
, however,
are
unlikely
to
be
stirred
efficiently
into
the
mantle
sources
of
these
basalts
.
The
melting
and
erup-
tion
process,
however,
is
a
good
homogenizer.