Chapter
13
Making
an
Earth
Thus
God
knows
the
world,
because
He
conceived
it
in
His
mind,
as
if
from
the
outside,
before
it
was
created,
and
we
do
not
know
its
rule,
because
we
live
inside
it,
having
found
it
already
made.
Brother
William
of
Baskerville
Overview
Attempts
to
estimate
mantle
composition
fall
into
two
broad
categories.
Cosmochemical
approaches
take
meteorites
as
the
basic
building
blocks
;
these
materials
are
processed
or
mixed
in
order
to
satisfY
such
constraints
as
core
size,
heat
flow
and
crustal
ratios
of
certain
elements.
These
models
constrain
the
bulk
chemistry
of
the
Earth
rather
than
that
of
the
mantle
alone.
The
only
primitive
meteorites
that
match
satisfactorily
both
the
stable
isotope
and
redox
characteristics
of
the
Earth
are
the
enstatite
chondrites.
The
actual
material
forming
the
Earth
is
unlikely
to
be
repre-
sented
by
a
single
meteorite
class.
It
may
be
a
mixture
ofvarious
kinds
of
meteorites
and
the
composition
may
have
changed
with
time.
The
oxidation
state
of
accreting
mate-
rial
may
also
have
changed
with
time.
Petrological
models
take
the
present
continen-
tal
crust,
basalts
and
peridotites
as
the
basic
building
blocks.
Since
b
asa
lts
represent
melts,
and
peridotites
are
thought
to
be
residues,
some
mixture
of
these
should
approximate
the
composition
of
the
upper
mantle.
Peridotites
are
the
main
mantle
reservoirs
for
elements
such
as
magne-
sium,
chromium,
cobalt,
nickel,
osmium
and
iridium.
The
continental
crust
is
an
important
reservoir
of
the
crustal
elements;
potassium
,
rubidium,
barium
,
lanthanum,
uranium
and
thorium.
Enriched
magmas,
such
as
kimberlites,
are
rare
but
they
are
so
enriched
in
the
crustal
elements
that
they
cannot
be
ignored
.
Thus
,
each
of
these
components
plays
an
essential
role
in
deter-
mining
the
overall
chemistry
of
the
prim-
itive
mantle.
An
alternate
approach
is
to
search
for
the
most
'primitive'
ultramafic
rock
or
component
and
attribute
its
compo-
sition
to
the
whole
mantle
or
the
original
mantle
,
or
to
take
the
most
depleted
MORB
and
attribute
its
source
region
to
the
whole
upper
mantle.
The
earliest,
and
simplest,
petrological
nwdels
tended
to
view
the
mantle
or
the
upper
mantle
as
homogenous,
and
capable
of
providing
primitive
basalts
by
p
artia
l
melting.
When
the
petrological
data
are
combined
with
isotopic
and
geophysical
data
,
and
with
considerations
from
accretional
calculations,
a
more
complex
multi-stage
evolu-
tion
is
required.
Similarly,
simple
evolutionary
models
have
been
constructed
from
isotope
data
alone
that
conflict
with
the
broader
database
.
There
is
no
conflict
between
cosmochemical,
geo-
chemical,
petrological
and
geophysical
data
,
but
154
MAKING
AN
EARTH
a
large
conflict
between
models
that
have
been
constructed
by
specialists
in
these
separate
fields.
The
mantle
is
heterogenous
on
all
scales
and
there
are
vast
regions
of
the
Earth
that
are
unsampled
today
because
they
were
isolated
by
gravitational
stratification
during
accretion
of
the
planet.
The
composition
of
the
crust
and
the
upper
mantle
are
the
results
of
a
series
of
melting
and
fractionation
events,
including
the
high-
temperature
accretion
of
the
planet
.
Attempts
to
estimate
upper-mantle
chemistry
usually
start
from
the
assumption
that
it
initially
was
the
same
as
bulk
s
il
ic
at
e
Ea
rth
(ESE)
and
dif-
fers
from
it
only
by
the
extraction
of
the
crust
,
or
that
the
most
depleted
midocean-ridge
basalts
(MORB)
plus
their
refractory
residues
constitute
the
entire
upper
mantle
.
Tradition-
ally,
geochemists
have
assumed
that
the
lower
mantle
is
still
undifferentiated
ESE
.
On
the
other
hand,
large-scale
melting
and
differentiation
upon
accretion
probably
pre-enriched
the
upper
mantle
with
incompatible
elements
,
including
the
radioactive
elements
;
the
crust
and
the
vari-
ous
enriched
and
depleted
components
sampled
by
current
melting
events
were
probably
already
in
the
upper
mantle
shortly
after
accretion
and
solidification
.
Midocean-ridge
basalts
represent
large
degrees
of
melting
of
a
large
source
volume,
and
blending
of
magmas
having
different
melting
histories.
The
central
limit
theorem
explains
many
of
the
differences
between
MORB
and
other
kinds
of
melts
that
sample
smaller
volumes
of
the
heterogenous
mantle
.
Observed
isotopic
arrays
and
mixing
curves
of
basalts,
including
ocean-island
basalts
(OIB),
can
be
generated
by
various
stages
of
melting,
mixing,
melt
extrac
-
tion,
depletion
and
enrichment
and
do
not
require
the
involvement
of
unfractionated
,
prim-
itive
or
lower,
mantle
components.
However,
the
first
stage
in
building
an
Earth
-
the
accretional
stage
-
does
involve
large
degrees
of
melting
that
essentially
imparted
an
unfractionated
-
but
enriched
-
chondritic
REE
pattern
to
the
upper
mantle.
Small-degree
melts
from
this
then
serve
to
fractionate
LIL.
Mass-balance
and
box-model
calculations
can
go
just
so
far
in
constrain-
ing
the
chemistry
of
the
mantle
. Geophysics
provides
information
about
physical
properties
and
boundaries
in
the
mantle.
Recycling
of
crust
into
the
upper
mantle
is
an
important
process.
It
is
possible
to
estimate
the
composition
of
the
f e r
ti
l e
u
pper
m
an-
t l e
by
combining
known
components
of
the
upper
mantle-
basalts,
peridotites,
recycled
crust
and
so
on
-
in
such
a
way
as
to
satisfY
cosmic
ratios
of
the
lithophile
refractory
elements
.
Other
elements
are
not
necessarily
concentrated
into
the
crust
and
upper
mantle.
The
MORB
source
is
just
part
of
the
upper
mantle
and
it
is
not
the
only
LIL
depleted
part
of
the
mantle.
It
is
not
necessarily
convectively
homogenized
.
Attempts
to
establish
an
average
composi-
tion
for
the
upper
mantle
have
focused
on
MORB
because
of
the
assumption
the
whole
upper
mantle
is
the
MORE-reservoir
.
This
pro-
cedure
involves
major
assumptions
about
melt
generation,
melt
transport
and
differentiation
processes
that
have
affected
these
melts,
and
the
sources
of
non-MORB
melts
.
The
d e
pl
e
te
d
u
pper
ma n t l e ,
that
part
of
the
mantle
that
is
assumed
to
provide
MORB
by
partial
melt-
ing
is
variously
called
DUM,
DM,
DMM
and
the
co
n
vec
tin
g
upper
mantl
e.
Simplified
mass
balance
calculations
suggested
to
early
workers
that
this
depleted
mantle
constituted
~
30
%
of
the
mantle;
the
650-670-km
discontinuity
was
adopted
as
the
boundary
between
DUM
and
'
th
e
primiti
ve
u n d e
plet
ed
undegas
se
d l
ower
man
t l e '.
The
starting
condition
for
the
upper
mantle
(UM)
was
taken
as
identical
to
primi
-
tiv
e
ma
ntl
e
(PM)
and
the
present
lower
man-
tle
(LM)
.
There
are
many
estimates
of
PM
and
ESE
based
on
various
cosmological
and
petrologi-
cal
considerations
.
The
primitive
upper
mantle
-
crust
plus
DUM
- is
labeled
PUM.
It
was
further
assumed
that
the
upper
mantle
was
vigorously
convecting,
well-stirred
and
chemically
homo
ge-
nous,
and
extended
from
the
base
of
the
plate
to
650-km
depth
.
Thus,
this
part
of
the
mantle
was
also
called
the
convecting
mantle.
The
non-MORB
basalts
that
occur
at
the
initiation
of
spreading
and
at
various
locations
along
the
global
spread-
ing
system
were
attributed
to
plumes
from
the
lower
mantle.
Most
or
all
of
the
mantle
needs
to
be
depleted
and
degassed
to
form
the
crust
and
upper
mantle
and
the
40
Ar
in
the
atmosphere
(e .g.
the
first
edition
of
Th
eo
r y o
f
t h e
Earth
,
or
TOE).
Depletion
of
the
upper
mantle
alone
cannot
explain
the
continental
crust
(CC);
the
MORB
reservoir
and
the
CC
are
not
exactly
comple-
mentary.
There
must
be
other
components
and
processes
beyond
single-stage
small-degree
melt
removal
from
part
of
the
primordial
mantle
to
form
CC.
There
are
other
enriched
components
in
the
mantle,
probably
in
the
shallow
man-
tle.
Other
depleted
components
or
reservoirs,
in
addition
to
the
MORB-source,
are
required
by
mass-balance
calculations.
TI1e
upper
mantle
can-
not
be
treated
as
if
its
composition
can
be
uni-
quely
determined
from
the
properties
of
depleted
MORB
-
NMORB
or
DMORB
-
and
depleted
peri-
dotites,
continental
crust,
and
an
undifferential
starting
condition.
Both
the
cosmochemical
and
petrological
approaches
utilize
terrestrial
and
meteoritic
data.
The
common
theme
is
that
the
Earth
should
have
an
unfractionated
chondri
tic
pattern
of
the
refractory
elements.
This
can
be
used
as
a
formal
a
priori
constraint
in
geochemical
model-
ing
of
the
composition
of
the
Earth.
This
mass
balance
approach
is
consistent
with
the
idea
that
most
of
the
radioactive
elements,
and
other
crustal
elements,
are
in
the
crust
and
upper
man-
tle.
Some
investigators,
however,
decouple
their
models
of
the
Earth
from
meteorite
composi-
tions
. Two
extreme
positions
have
been
taken:
(1)
the
Earth
is
a
unique
body
and
is
not
related
to
material
currently
in
the
solar
system;
upper
mantle
rocks
are
representative
of
the
whole
mantle
and
Earth-forming
material
is
not
to
be
found
in
our
meteorite
collections;
(2)
only
part
of
the
Earth
has
been
sampled
and
the
upper
mantle
is
not
representative
of
the
whole
mantle
.
There
are
several
variants
of
the
second
option
:
(1)
the
mantle
is
extensively
differentiated
and
the
deepest
layers
are
the
dense
residues
of
this
differentiation
,
which
is
irreversible;
(2)
only
the
upper
mantle
has
been
processed
and
differ-
entiated;
the
deeper
mantle
is
'
primordial'
. TI1ere
are
numerous
petrological
reasons
why
the
Earth's
upper
mantle
should
be
distinct
from
a
ny
known
type
of
meteorite
and
why
no
p a
rt
of
the
mantle
should
have
survived
in
a
homogenou
s
primitive
state
.
However
,
some
ar
g
ue
that
it
is
PETROLOGICAL
BUILDING
BLOCKS
ISS
more
reasonable
that
the
Earth
accreted
from
material
with
major-element
compositions
that
were
distinct
from
primitive
meteorite
types
than
to
accept
chemical
stratification
and
petrological
differentiation
of
the
mantle.
'Primitive
mantle'
or
PM
is
the
silicate
frac-
tion
of
the
Earth
,
prior
to
differentiation
and
removal
of
the
crust
and
any
other
parts
of
the
present
mantle
that
are
the
result
of
differen-
tiation,
or
separation,
processes.
This
is
called
bulk
silicate
Earth
(ESE).
In
geochemical
models,
which
were
popular
until
very
recently
,
it
was
assumed
that
large
parts
of
the
Earth
escaped
partial
melting,
or
melt
removal,
and
are
ther
e-
fore
still
'primitive'
.
Some
petrological
models
assumed
that
melts
being
delivered
to
the
Earth
's
surface
are
samples
from
previously
unprocess
ed
material.
It
is
difficult
to
believe
that
any
part
of
the
Earth
could
have
escaped
processing
dur-
ing
the
high-temperature
accretional
process
.
'Primitive
mantle
', as
used
here,
is
a
hypothetic
a l
material
that
is
the
sum
of
the
present
crust
and
mantle
.
Some
petrological
models
assume
that
it
is
a
mixture
of
the
MORB-source
and
conti-
nental
crust.
'Primitive
magma
' is
a
hypotheti-
cal
magma,
the
parent
of
other
magmas,
which
formed
by
a
single-stage
melting
process
of
a
par-
ent
rock
and
has
not
been
affected
by
loss
of
material
(crystal
fractionation)
prior
to
sampling
.
It
is
much
more
likely
that
magmas
are
the
result
of
a
multi-stage
process
and
that
they
represent
blends
of
a
variety
of
melts
from
various
depth
s
and
lithologies
.
Petrological
building
blocks
Most
mantle
magmas
can
be
matched
by
mix-
tures
of
depleted
MORB
(DMORB)
and
enriched
components
(Q).
Midocean-ridge
basalt
(MORB)
represents
the
most
uniform
and
voluminou
s
magma
type
and
is
often
taken
as
an
end-member
for
large-ion
lithophile
(LIL)
concentrations
and
isotopic
ratios
in
other
basalts
.
The
uniformity
of
MORB,
however
,
may
be
the
result
of
samplin
g,
and
the
central
limit
th
e
orem
.
In
this
case,
MORB
is
a
good
average,
but
not
a
good
end-member
.
The
MORB
source
has
been
depleted
by
removal
of
a c
omponent
-
Q-
that
must
be
ri
ch
in
LIL
156
I
MAKING
AN
EARTH
but
relatively
poor
in
Na,
AI
and
Ca.
Kimberlitic
and
some
other
enriched
magmas
have
a
comple-
mentary
relationship
to
MORB
.
The
extreme
enrichments
ofkimberlitic
mag-
m a s
in
incompatible
elements
are
usually
attributed
to
low
degrees
of
melting
andjor
metasomatized
source
compositions.
The
obser-
ved
enrichment
of
kimb
er
liti
c
magmas
wi
th
r
are
earth
e l e me
nt
s
(REE)
c
an
be
e
xp
l
ai
n e d
i n t
e
rm
s o
f
me
l t
mi
g
rati
o n
thr
o
ugh
so
ur
c e r
o
ck
s
ha
v
in
g
th
e
co
mp
o-
si
ti
o n o f n
o
rmal
ma
nt
le
.
The
resulting
satu-
rated
REE
spectrum
is
practically
independent
of
source
mineral
composition,
which
may
explain
the
similarity
of
kimberlites
from
different
geo-
gr
a
phic
localities
.
Kimberlite
is
thus
an
impor-
tant
mantle
component
and
can
be
used
as
such-
component
Q-
in
mass-balance
calculations
.
Chemical
composition
of
the
mantle
Considerations
from
cosmochemistry
and
the
study
of
meteorites
permit
us
to
place
only
very
broad
bounds
on
the
chemistry
of
the
Earth
's
interior.
These
tell
us
little
about
the
distribu-
tion
of
elements
in
the
planet.
Seismic
data
tell
us
a
little
more
about
the
distribution
of
the
major
elements
.
General
considerations
suggest
that
the
denser
major
elements
will
be
toward
the
center
of
the
planet
and
the
lighter
major
elements,
or
those
that
readily
enter
melts
or
form
light
minerals,
will
be
concentrated
toward
the
surface
.
To
proceed
further
we
need
detailed
chemical
information
about
crustal
and
mantle
rocks
.
The
bulk
of
the
material
emerging
from
the
mantle
is
in
the
form
of
melts
,
or
magmas
.
It
is
therefore
important
to
understand
the
chem-
istry
and
tectonic
setting
of
the
various
kinds
of
m
ag
matic
rocks
and
the
kinds
of
sources
they
1nay
have
come
from.
Midocean-ridge
basalt
repres
e
nts
the
most
uniform
and
voluminous
ma
g
ma
typ
e
and
is
an
end
member
for
LIL
concentrations
and
many
iso-
topic
ratios.
This
is
usually
taken
as
one
of
the
compon
ents
of
the
mantle,
even
though
it
itself
is
a n
average
or
a
blend
. Most
mantle
magma
compositions
can
be
approximated
with
a mix-
ture
of
a
depleted
MORE-component
and
one
or
more
e
nriched
components,
variously
called
EM1
,
EM2,
HIMU,
DUPAL
and
Q
The
resultin
g
magmas
Table
13.1
I
Estimates
of
average
composi-
tion
of
the
mantle
Oxi
de
( I )
(2)
(3)
(4)
(5)
Si0 2
45.23
47.9
44.58
47.3
45
.1
A I20 3
4
.1
9
3.9
2.43
4.
1
3.9
M
gO
38
.39
34.
I
41
. 18
37.9
38.
I
CaO
3.36
3.2
2.08
2.8
3.
I
F
eO
7.82
8.9
8.27
6.8
7.9
Ti 0 2
0.20
0
.1
5
0.2
0 .2
Cr20 3
0 .9
0.4
1
0.2
0 .3
N a20
0.25
0 .34
0.5
0.4
K20
0
.11
0.2
(0.
1
3)
(1)
Jac
ob
sen
and
oth
ers
(1984)
: e
xtrapolation
of
ultramafic
and
chondri
tic
tr
e
nds
.
(2)
Mor
g
an
and
Anders
(1980)
:
cosmoch
emi-
cal
mod
el.
(3) M
a al0e
and
Steel
(1980):
extrapolation
of
lh
e r z
olite
tr
e
nd.
(4)
20
pe
rc
e
nt
e
clogite
,
80
p e
rc
e
nt
g
arn
et
lh
er z
olit
e (Anderson,
1980)
.
(
5)
Rin
gw
ood
and
Kesson
(1976
, Tabl
e
7):
pyrolit
e
adjusted
to
hav
e
chondritic
Ca
0 /
Al
2
0
3
ratio
a
nd
Ringwood
(1966)
for
K2
0.
themselves
are
called
NMORB
,
EMORB,
OIB,
AOB,
CFB
and
so
on.
The
refractory
residue
left
after
melt
extraction
-
the
restite
-
is
usually
consid-
ered
to
be
a
peridotite,
dunite
or
harzburgite
,
all
ultramafi
c
ro
cks
(UMR)
.
All
of
the
above,
plus
con-
tin
ental
crust
(CC)
,
are
candidate
components
for
primitive
mantle
.
The
MORB
source
appears
to
have
been
depleted
by
removal
of
a
component
that
is
rich
in
LIL
but
relatively
poor
in
Na
and
the
clinopyroxene-compatible
elements
(such
as
AI,
Ca,
Yb,
Lu
a
nd
Sc).
Kimberlitic
magmas
have
the
required
complementary
relationship
to
MORB
,
and
I
adopt
them
in
the
following
as
a
possible
Q
component.
Some
element
s ,
such
Nb
, Ta,
Ti
and
Zr
a
re
extraordinarily
concentrated
into
specific
minerals
-
rutile
a
nd
zircon,
for
example
-
and
estimate
s
of
the
se
elements
in
rocks
ca n
be
highly
variable
and
dependent
on
the
amount
of
the
se
minerals
.
Peridotites
and
s ulfides
are
the
main
carri
e
rs
of
elements
such
a s
ma
g n e
sium
,
chromium
,
cobalt,
nickel
,
osmium
and
iridium,
some
of
the
so-called
compatible
elements
.
The
continental
crust
is
an
important
reservoir
of
potassium
,
rubidium,
barium,
lanthanum,
ura-
nium
and
thorium,
some
of
the
classical
LIL
ele-
ments
.
Thus,
each
of
these
components
plays
an
essential
role
in
determining
the
overall
chem-
istry
of
the
primitive
mantle
.
It
is
conventional
to
adopt
a
single
hypothet-
ical
mix
-
lherzolite
or
harzburgite
plus
basalt
-
as
the
dominant
silicate
portion
of
the
mantle;
this
has
been
called
pyrolite,
for
pyroxene-olivine
rock.
An
orthopyroxene-rich
component
(OPX)
is
also
present
in
the
mantle
and
is
required
if
such
major-element
ratios
as
Mg/Si
and
Al/Ca
ratios
of
the
Earth
are
to
be
chondritic
.
Clinopyroxenites,
rather
than
fertile
peridotites,
may
be
impor-
tant
source
rocks
for
basalts.
Some
peridotites
appear
to
have
been
enriched
(metasomatized)
by
a
kimberlite-like
(Q)
component.
Seawater
is
an
important
repository
of
Cl
, I
and
Br.
The
atmo-
sphere
may
contain
most
of
the
heavier
rare
gases.
Mixtures
of
the
above
components,
plus
continental
crust,
can
be
expected
to
give
a first
approximation
to
the
composition
of
primitive
mantle
.
There
may
also
be
inaccessible
reservoirs
that
do
not
provide
samples
for
us
to
measure
.
The
so-called
mi
ss
ing
e l e me n t a
nd
i
so
t o p e
para
d
oxes
in
g
eoc
h e
mi
ca
l b
ox-
mo d e l s
suggest
that
some
material
is
hidden
away,
prob-
ably
in
deep
dense
layers
that
formed
during
the
accretion
of
the
Earth.
Ratios
such
as
CafAl,
Mg/Si
, U/Pb
and
U/Nb
and
some
isotope
ratios
imply
that
there
is
hidden
or
inaccessible
mate-
rial.
The
most
obvious
missing
elements
are
iron
and
other
siderophiles
,
such
as
Os
and
Ir.
These
are
in
the
core.
The
missing
silicon
is
probably
in
the
perovskite-rich
lower
mantle.
Other
missing
elements
are
S
and
C
and
other
volatiles
that
left
the
Earth
entirely
or
were
never
incorporated
into
it
.
There
are
numerous
par
a doxe
s
associated
with
U
and
Th
and
their
products
-
heat
, Pb-isotopes,
He-isotopes
and
Ne-isotopes
.
The
obvious
implication
is
that
we
are
missing
something;
the
mantle
may
be
chemically
stratified
and
we
are
sampling
only
the
outer
reaches
,
or
we
are
ignoring
certain
components
such
as
fluid-filled
or
melt
inclu-
PETROLOGICAL
BUILDING
BLOCKS
157
2 .5
2 .0
1.5
c:
1.0
0
-
~
c
"'
2 .0
u
c:
0
u
"0
1 .5
"'
.
!::!
rn
E
0
1 .0
c:
Ol
0
....J
0 .5
u
Trace-element
concentrations
in
the
continental
crust
(dots),
continental
basalts
and
midocean-ridge
basalts
(MORB),
normalized
to
average
mantle
compositions
derived
from
a
chondritic
model.
Note
the
complementary
relationship
between
depleted
basalts
(MORB)
and
the
other
materials
. MORB
and
continental
tholeiites
a
re
approximately
symmetric
about
a
composition
of
7 x
C
I.
This
suggests
that
about
14
%
of
the
Earth
may
be
basalt.
For
other
estimates
,
see
text
.
sions,
carbonatites
and
exotic
minerals
such
as
rutile
,
osmiridium
etc.
It
is
possible
that
cer-
tain
rock
types
such
as
lower-crustal
cumulates,
carbonatites,
recycled
material
and
island-arc
basalts
are
not
added
into
the
mix
in
appropriate
quantities.
Figure
13.1
shows
representative
composi-
tions
of
kimberlite,
crust
,
MORB
and
ultramafic
rock.
For
many
refractory
elements
kimberlite
and
crust
have
a
similar
enrichment
pattern
.
However,
the
volatile
/
refractory
ratios
are
quite
different,
as
are
ratios
involving
strontium,
hafnium,
titanium
,
lithium
,
yttrium
,
ytterbium
and
lutetium.
Kimberlite
and
MORB
patterns
are
nearly
mirror
images
for
the
refractory
elements
,
but
this
is
only
approximately
true
for
MORB
and
crust,
e
specially
for
the
HREE
,
and
the
small-
ion-high-charge
elements.
MORB
and
kimberlite
also
represent
extremes
in
their
strontium
and
neodymium
isotope
compositions
.
When
UL-rich
materials
(KIMB
,
lamproites)
are
mixed
with
a
depleted
magma
(MORB),
the
resulting
blend
c
an
have
apparently
paradoxical
g
eochemical
properties.
For
example,
the
hybrid