Chapter
9
A
laminated
lumpy
mantle
What
can
be
more
foolish
than
to
think
that
all
this
rare
fabric
of
heaven
and
earth
could
come
by
chance,
when
all
the
skill
of
art
is
not
able
to
is done.
Now
an
o
ys
ter
has
hardly
any
more
reasoning
power
than
a man
has,
so
it
is
probable
this
one
jumped
to
the
conclusion
that
the
nineteen
make
an
oyster!
million
years
was
a preparation
for
Anatole
France
him.
That
would
be
just
like
an
oyster.
Even
so,
it
was
99,970,000
years
getting
ready
..
. because
God
first
had
to
make
the
oyster.
You
can't
make
an
oyster
out
of
nothing,
nor
you
can't
do
it
in
a day
. You've
got
to
start
with
a vast
variety
of
invertebrates,
belemnites,
trilobites,
jebusites,
amalekites,
and
that
sort
of
fry,
and
put
them
into
soak
in
a
primary
sea
and
observe
and
wait
what
will
happen.
Some
of
them
will
turn
out
a disappointment;
the
belemnites
and
the
ama
lekites
and
such
.
..
but
all
is
not
lost,
for
the
amalekites
will
develop
gradually
into
encrinites
and
stalactites
and
blatherskites,
and
one
thing
and
another
.
..
and
at
last
the
first
grand
stage
in
the
preparation
of
the
world
for
man
stands
completed;
the
oyster
Mark
Twain
Gravitation
structures
the
earth
in
concentric
shells,
or
geospheres,
according
to
their
specific
gravity.
Gal
-
or
Standard
geochemical
and
geodynamic
models
of
the
mantle
involve
one
or
two
layers.
Global
tomographic
models
tend
to
be
fairly
simple;
they
show
continental
roots,
slabs,
shallow
mid-
ocean
ridge
structures
and
a few
very
large
fea-
tures
in
the
deep
mantle
.
Regional
and
high
-
reso
l
ution
seismological
models
of
the
mantle
are
more
complex.
High-resolution
seismic
tech-
niques
involving
reflected
and
converted
phases
show
about
10
discontinuities
in
the
mantle,
not
all
of
which
are
easily
explained
by
solid-solid
phase
changes
.
They
also
show
some
deep
low-
velocity
zones,
which
may
be
eclogite
layers
.
The
upper
mantle
is
highly
attenuating
,
anisotropic
and
scatters
short
period
seismic
energy.
TI1e
opposite
extreme
of
a
well-stirred
homogenous
mantle
is
a
mantle
that
is
strat-
ified
by
intrinsic
density.
Convection
can
be
expected
to
homogenize
the
mantle
if
the
various
110
A
LAMINATED
LUMPY
MANTLE
components
do
not
differ
much
in
intrinsic
den-
sity,
of
the
order
of
2
or
3%,
depending
on
depth.
The
Earth
itself
is
stratified
by
composition
and
density
(atmosphere,
hydrosphere,
crust,
mantle,
core)
and
the
crust
and
upper
mantle
are
strati-
fied
as
well.
The
region
at
the
base
of
the
mantle-
D"
-
appears
to
be
iron-rich
and
intrinsically
dense
.
There
may
be
a
buoyant
refractory
(melt
depleted)
discontinuous
layer
at
the
top
of
the
mantle
-
the
perisphere.
The
perisphere
may
never
get
cold
enough
to
subduct
and
D"
may
never
get
hot
enough
to
rise
.
These
are
only
the
most
obvious
candidates
for
chemical
layers;
internal
layers
will
likely
be
subtle
and
they
need
not
be
continuous
or
flat
.
Figure
9.1
shows
the
shear
velocity
in
a vari-
ety
of
rocks
and
minerals,
at
STP
(standard
tem-
perature
and
pressure)
arranged
according
to
increasing
zero-pressure
density
. This
represents
a
stably
stratified
system.
Many
of
the
chemically
dis
ti
nct
layers
differ
little
in
seismic
properties
and
sometimes
a
denser
layer
has
lower
seismic
velocity
than
a
less
dense
overlying
layer.
The
densities
range
from
2.6
to
4.2
gfcm
3
;
the
den-
sity
scale
is
monotonic
but
nonlinear.
Severa
l esti-
mates
of
depth
are
given,
calibrated
according
to
estimates
of
mantle
uncompressed
density.
Given
enough
time
in
a
low
enough
viscosity
mantle
this
is
the
stable
density
stratification.
Note
that
shear
ve
l
ocity
is
not
a
monotonically
increasing
function
of
density
. A
stable
density
stratification
has
an
irre
g
ular
complex
shear
velocity
struc-
ture.
Even
if
the
mantle
achieves
this
stable
strat-
ification
it
will
not
be
permanent.
The
differ-
ent
lithologies
have
different
melting
points
and
thermal
properties
,
phase
changes
and
can
rise
or
sink
as
the
temperature
changes.
Figure
9.2
is
a
similar
plot,
with
some
of
the
minerals
and
rocks
identified.
Eclogites
occur
at
various
depths
because
they
come
in
a
variety
of
compositions;
eclogite
is
not
a
uniform
rock
type.
Arclogites
are
garnet
clinopyroxenites
that
occur
as
xenoliths
in
arc
magmas.
The
deeper
eclogite
layers
in
the
fig-
ures
are
low
-ve
locity
zones
relative
to
similar
density
rocks.
Cold
dense
eclogite
melts
as
it
warms
up
to
ambient
mantle
temperature,
and
becomes
buoyant.
The
stable
stratification
of
a
chemically
zoned
mantle
is
only
temporary.
This
kind
of
mantle
convects
but
it
is
a
different
kind
D
E
N
s
I
T
y
400
km
500
km
650
km
3
SHEAR
VELOCITY
(P
=
0)
VS
4
CRUST
5
5
UPPER
MANTLE
6
6
~
·
••
Chemical
stratification
of
the
mantle
if
mantle
rocks
and
minerals
arrange
themselves
in
the
gravity
field
according
to
intrinsic
density
(density
increases
downward
but
is
not
tabulated)
.
The
velocities
(horizontal
axis)
and
densities
(vertical
axis)
are
appropriate
for
Standard
Temperature
and
Pr
essure
(ST
P) conditio
n
s.
Approx
Depth
reflecto
rs
(km)
60
80
90
130
200
280
330
400
500
650
800
900
Rock
type
SHEAR
VELOCITY
(P
=
0 )
Vs
(km
/
s)
STP
density
Vs
3
4
(gl ee)
(kmls)
pyroxenite
3 .23
4.4
3
Avg.ul
trama
f
ic
rock
3 .29
4.68
harz
bur
gi
te
3.30
4.90
du
nite
3.
31
4.84
PHN1
569
3 .3 1
4.87
sp
.p
eri
do
tit
e
3.35
4.52
G t.Lhz
.
3 .35
4.83
py
ro
lite
3.38
4.82
PH
N
1611
3 .42
4.76
arclogite(
h
igh
MgO)
3.45
4.60
eclog
it
e
3.46
4.77
Hawaii
Lhz
.
3.47
4.72
arclogote(hogh
MgO)
3.48
4.68
majo
r
ite
(mj)
3.52
5 .06
gamma
-spinel
3.55
5 .79
garnet
3 .57
5.08
Mgo
3.58
6.05
beta
-spinel
(.
t F eO)
3.59
5.54
py
rolite( 41
Okm)
3.60
5.33
arclogi
te
3.60
4.93
arcl
ogi
te
3.63
4.84
gam
ma- spine l
(.
1
FeO)
3.68
5 .59
pyrolit
e(500km)
3.67
5.40
arclog
ite
3.70
4.91
ar
clog
i
te
3.74
4.93
arclogite(low
MgO)
3.75
###
MO
RB(mj
+st)
3.75
5 .6 +
'
i1me
n
ite
'(.1
FeO)
3 .
92
5.71
mw(
Mg .8)
4.07
5 .08
pv
4.
11
6.62
ca
pv
4 .
13
5 .50
pv( .
1Fe0)
4.22
6.44
MO RB(pv
+st)
4.23
6.6
+
mw(
.
2Fe0
)
4.26
5 .08
Same
as
Figure
9.1
with
additional
information
and
types.
of
convection
than
the
homogenous
mantle
usu-
ally
treated
by
convection
modelers
or
geodynam-
ici
st
s.
Convection
in
the
mantle
is
mainly
driven
by
the
differences
in
density
between
basalt,
melt
and
eclogite.
Note
that
sinking
eclogite
can
be
trapped
above
the
various
mantle
phase
changes
,
giving
low-velocity
zones.
Although
mantle
strat-
ification
is
unlikely
to
be
as
extreme
or
ideal
as
Figure
9.1
it
is
also
unlikely
to
be
as
extremely
homogenous
or
well
-
mixed
as
often
assumed.
Crustal
type
reflection
seismology
is
required
to
see
this
kind
of
structure.
Recycled
oceanic
crust,
one
kind
of
eclogite,
will
have
a
particularly
high
density
if
it
can
sink
below
about
720
km
because
the
high
silica
content
of
MORE
gives
a
large
stishovite
con-
tent
to
MORE
-
eclogite.
On
the
other
hand,
cumu-
late
gabbros
,
the
average
composition
of
the
oceanic
crust
and
de
l
aminated
continental
crust
have
much
lower
silica
contents
and
this
reduces
their
high
-
pressure
densities.
The
controversy
A
LAMI
N A
TE
D
LU
M PY
MA
NTLE
Ill
regarding
the
fate
of
eclogite
in
the
mantle
involves
this
point
.
The
arrangement
in
Figure
9 .2
approximates
t h e
situation
in
an
idea
lly
chemically
stratified
mantle.
The
densities
of
peridotites
vary
from
about
3.3
to
3.47
gfcm
3
while
measured
and
theoretical
eclogite
densities
range
from
3.45
to
3.75
gfcm
3
.
The
latter
is
comparab
le
to
the
inferred
STP
density
near
650
km
and
about
10
%
less
dense
than
the
lower
mantle.
The
lower
den-
sity
eclogites
(high-MgO
,
low-Si0
2
)
have
densities
less
than
the
mantle
below
410
km
and
will
there-
fore
be
trapped
at
that
boundary,
even
when
cold
.
There
are
several
things
to
note
. Eclogites
come
in
a
large
variety
of
compositions,
densi-
ties
and
seismic
velocities
.
There
is
not
a
one-
to-one
correlation
of
seismic
velocity
and
den-
sity
in
mantle
rocks,
and
shear
velocity
is
not
a
monotonic
increasing
function
of
density
or
depth.
Some
chemically
distinct
layers
have
sim
-
ilar
seismic
velocities
.
The
velocities
are
quan-
tized
at
about
the
4 %
level,
a
typical
variation
observed
in
the
shallow
mantle
globally,
and
under
hotspots
in
particular
;
such
variations
on
the
slow
side
are
usually
attributed
to
partial
melting
or
high-temperature.
The
shear-velocity
quantum
step
is
equivalent
to
a
temperature
vari-
ation
of
1000
°C
at
constant
pressure
and
about
t h e
size
of
the
correction
to
be
made
to
STP
velocities
to
account
for
ambient
mantle
tem-
perature
and
pressure.
Pressure
and
temperature
effects
may
change
the
ordering
and
the
velocity
and
density
jumps
at
depth
. Eclogite
can
settle
to
various
levels,
depending
on
composition;
the
eclogite
bodies
that
can
sink
to
greater
depths
because
o f
their
density
have
low-velocity
com-
pared
to
similar-density
rocks.
Some
eclogites
have
densities
intermediate
to
the
low-
and
high-
pressure
asemblages
at
the
various
peridotite
phase
boundaries
(410
km
,
500
km,
650
km);
they
will
be
trapped
at
these
boundaries,
affect-
ing
the
seismic
properties,
and
changing
them
from
the
ideal
phase-change
conditions.
Cold
eclogites
with
STP
densities
between
3.45
and
3.6
g j
cm
3
may
be
trapped
above
the
olivine-beta-
spine
l
phase
boundary
near
410
km
depth,
giving
a low-velocity
zone
(LVZ)
at
this
depth.
The
obser-
vations
of
a
L
VZ
at
o p
th
e
41
0
km
d
isco
n -
t
inuit
y
are
u
sually
interpreted
in
terms
of
par-
t ial
me
l
ting
. A
perched
eclogite
fragment
will
112
A
LAMINATED
LUMPY
MANTLE
heat
up
by
conduction
from
the
surrounding
hot-
ter
mantle
and
will
eventually
melt
.
MORE-eclogite
contains
stishovite
at
high
pressure
and
may
sink
below
500
km.
Some
low-MgO-arc
eclogites
have
comparable
densities.
Note
that
the
deeper
eclogite
layers
form
substan-
tial
low-velocity
zones.
The
stratification
shown
in
the
figure
is
only
temporary.
Subducted
or
delaminated
material
warms
up
by
conduction
of
heat
inward
from
ambient
mantle
. Eclogites
have
much
lower
melting
points
than
peridotites
and
will
eventually
heat
up
and
rise,
even
if
they
are
not
in
a
TBL,
creating
a
sort
of
yo-yo
or
lava-lamp
tectonics.
The
ilmenite
form
of
garnet
and
enstatite
is
stable
at
low
temperature
but
will
convert
to
more
buoyant
phases
as
it
warms
up.
Whole
man-
tle
convection
or
vigorous
mantle
convection,
and
entrainment,
are
not
necessary
in
order
to
bring
fertile
material
into
the
shallow
mantle.
Multiple
mantle
discontinuities
The
classical1D
seismological
models
of
the
man-
tle
include
a
TR
between
400
and
1000
km
-
separating
the
upper
and
lower
mantles
-
that
was
attributed
to
phase-changes
. Early
investi-
gators,
using
reflected
phases
and
breaks
in
teleseismic
travel
time
curves
identified
many
discontinuities
and
some
of
the
early
seismic
models
consisted
of
layers
rather
than
smooth
variations
with
depth
.
Whitcomb
and
Anderson
(1970),
using
precursors
to
the
seismic
phase
P' P' ,
identified
about
six
seismic
discontinuities
in
the
mantle;
a
major
one
near
630
km
depth
and
oth-
ers
at
280,
520,
940,
410
and
1250
Ian.
Although
the
attention
of
geochemists
and
geodynamicists
has
been
focused
on
the
better
known
410
and
650
km
features,
there
are
about
10
discontinu-
ities
in
the
mantle
that
have
now
been
identified
by
seismologists
by
a
variety
of
high-resolution
or
correlation
techniques.
Systematic
searches
for
mantle
discontinuities
have
yielded
reflections
or
conversions
from
depths
of
220,
320,
410,
500-520,
650-670,
800,
860,
1050,
1150-1160
and
1320
km.
A
survey
of
many
such
studies
shows
that
most
of
the
reflections
and
conversions
occur
in
the
depth
intervals
of
60-90,
130-170
,
200-240,
280-320,
400-415,
500-560
,
630-670,
800-940,
1250-1320
and
2500-2700
Ian.
Some
of
0.
01
0
- 200
Precur
sors
to
P'P'
P'660P
'
P'410P
'
- 150
long
-period
reflection
amplitudes
- 100
Time
relative
to
P'P'
(sec)
- 50
Stacked
seismograms
of
P' P'
precursors
showing
the
major
seismic
discontinuities
and
many
minor
ones
(Xu
e t
of
..
2003).
these
are
probably
chemical
in
nature
and
some
apparently
are
either
highly
variable
in
depth
or
are
independent
scatterers
(the
reports
of
reflec-
tions
or
scatterers
between
800
and
1320
kn1may
all
be
due
to
a s
in
g le
highly
irregular
interface)
.
The
shallow
features
may
represent
boundaries
between
depleted
and
fertile
peridotites
and
par-
tial
melt
zones.
Deeper
ones
may
represent
slabs
that
have
been
trapped
at
various
depths
because
they
are
too
buoyant
to
sink
further.
A
number
of
possible
underside
reflections
are
evident
in
Figure
9 .3.
Deuss
and
Woodhouse
(2002)
found
a
large
number
of
reflections
from
a
depth
of
220
kn1
beneath
both
oceans
and
conti-
nents
(see
Figure
9.4).
Altogether,
there
are
more
reflections
reported
below
650
kn1
and
between
410
and
650
lrn1
than
are
reported
at
410
and
650
lrn1
.
These
are
not
so
evident
in
reflection
histograms
because
they
occur
over
a
wide
range
of
depths
as
expected
for
chemical
interfaces.
Deep
low-velocity
zones
Figure
9.2
predicts
the
presence
of
low-v
elocity
zones
in
a
petrologically
stratified
mantle.
Some
eclogites
create
a
LVZ
having
velocities
about
2-5
%
lower
than
the
surrounding
mantle
or
man-
tle
of
the
same
density.
The
deeper
eclogite
LVZs
2
00
3
00
400
500
600
7 0 0
8 0 0
90
0
1000
11
00
12
00
Depth
(km)
Robust
reflections
from
the
mantle
(Deuss
and
Wc>orlihoiJ<<'
~
2002).
are
about
9 %
slower.
Mixing
with
normal
mantle
will
reduce
these
differences;
heating
and
melt
-
ing
will
increase
them.
The
main
point
is
that
lateral
and
radial
reductions
in
seismic
veloc-
ity
of
order
2-10
%
can
have
a
simple
petrolog-
ical
explanation.
Shallow
LVZ
may
be,
in
part,
due
to
adiabatic
upwelling
of
displaced
astheno-
sphere
but
this
also
need
not
be
particularly
hot.
Velocity
reversals,
or
low-velocity
zones
(LVZ)
have
been
identified
in
regional
studies
at
depths
near
100,
185,
380,
410
,
460-480
,
570-600,
610
and
720
km
(Nolet
&
Zielhuis,
1994
;
Vinnik
et
al.,
2003).
The
velocity
reduction
in
these
LVZ
is
gen-
erally
between
2-5
%.
These
LVZ
are
almost
invari-
ably
attributed
to
the
effects
of
water,
partial
melting
or
high
temperature.
These
LVZ
are
in
addition
to
those
that
occur
in
the
upper
200
to
350
km
in
tectonic
and
volcanic
regions
such
as
Yellowstone
,
Iceland
,
western
North
America
and
near
oceanic
ridges
.
The
LVZ
that
occur
just
above
the
major
phase-change
boundaries
at
410
and
650
km
are
particularly
instructive
since
these
are
the
places
where
one
expects
to
find
barri-
ers
to
certain
kinds
of
subducted
or
delaminated
materials.
Tomographic
studies
suggesting
that
some
slabs
cross
the
650
km
mantle
discontinuity
do
not
imply
that
all
do
.
The
transition
zone
may
act
as
a
petrological
filter
. Recycled
mate-
rial
can
also
be
trapped
at
other
depths-
deeper
and
sha
ll ower;
thick
,
co
ld
slabs
can
sink
further
and
take
longer
to
warm
up;
younger
slabs
or
those
with
t h
ick
crust
tend
to
underplate
con-
tinents
.
The
dry
and
dep
l
eted
residual
phases
-
A
LAMINATED
LUMPY
MANTLE
113
peridotites
and
eclogites
-
equilibrate
at
va
ri
-
ous
depths
and
the
removed
material
metasoma-
tizes
the
shallow
mantle
(the
mantle
wedge
,
the
perisphere
and
the
plate).
Young
oceanic
p l
ates,
delaminated
lower
crust,
subducted
seamount
chains
and
plateaus
thermally
equilibrate
and
m.
elt
at
depths
different
from
older
thicker
plates.
The
650
km
discontinuity,
with
its
ne
g
a-
tive
Clapyron
slope,
is
a
temporary
barrier
to
cold
sinking
material
of
the
same
composi-
tion
,
but
such
material
may
eventually
break
through.
A
different
material,
with
higher
pre
s-
sure
phase-changes
, e .g.
eclogit
e,
can
be
stranded
by
phase-chan
ges
in
peridotite
.
Eclogite
can
density
-
equilibrate
at
depths
above
600
km
(Fi
g-
ure
9 .
2)
.
Chemical
discontinuities,
even
those
with
very
small
density
jumps
,
can
be
a
barri
e
r-
or
filter
-
to
through-goin
g
convection
.
Delaminated
continental
crust
is a
particu-
larly
potent
source
of
mantle
heterogen
eity,
low-
ve
l
ocity
zones
and
meltin
g
anomalies;
it
starts
out
warmer
and
equilibrates
fast
er
than
sub-
ducted
oceanic
crust
.
It
is
also
low
in
Si0
2
,
which
means
it
has
more
buoyancy
below
some
400
km
where
subducting
MORE
may
have
a
high
dense
stishovite
component.
Large
fertile
low
meltin
g-
point
blobs
trapped
in
the
upper
mantle
may
be
responsible
for
'
meltin
g
anomalies'
and
LVZs
.
These
rec
y
cling,
filtering
and
samplin
g
pr
o-
cesses
can
explain
many
g
eo
c
hemical
observa-
tions
while
avoidin
g
the
pitfalls
associat
e d
with
iso
l
ated
m a
ntle
reservoirs
and
deep
penetration
of
all
slabs
and
all
components
.
High-resolution
seismolo
gical
techniques
involving
reflected
and
converted
phase
a
nd
scat-
tering
are
starting
to
reve
a l
the
re
a l
complexity
of
the
mantle
.
Abrupt
seismic
discontinuities
are
not
necess
a rily
isotope
or
reservoir
boundaries
a
nd
the
d e
eper
layers
are
not
neces
s
arily
acce
s-
sib
le
to
surface
volcanoes
.
Pl a
te
tectoni
cs
and
geochemical
cycles
may
be
entirely
restricted
to
the
upper
~
1000
km,
where
thermal
expansion
is
high
and
melting
point
s, viscosity
and
therm
a l
conductivity
are
low
.
The
seismic
velocities
of
plausib
le
materials
in
the
mantle
differ
little
from
one
another
,
even
if
th e
density
contrasts
are
adequate
to
perma-
nently
sta
b
ilize
the
layering
against
convective