of 21
Theory of the Earth
Don L. Anderson
Chapter 10. Isotopes
Boston: Blackwell Scientific Publications, c1989
Copyright transferred to the author September 2, 1998.
You are granted permission for indi
vidual, educational, research and
noncommercial reproduction, distribution, display and performance of this work
in any format.
Recommended citation:
Anderson, Don L. Theory of the Earth.
Boston: Blackwell Scientific Publications,
1989.
http://resolver.calt
ech.edu/CaltechBOOK:1989.001
A scanned image of the entire book may
be found at the following persistent
URL:
http://resolver.caltech.edu/CaltechBook:1989.001
Abstract:
The various chemical elements have diffe
rent properties and can therefore be
readily separated from each other by igneous processes. The various isotopes of a
given element are not so easily separat
ed. The abundances of the radioactive
isotopes and their decay products are not constant in time. Therefore, the
information conveyed by the study of isotopes is different in kind than that
provided by the elements. Each isotopic system contains unique information,
and the various radioactive isotopes a
llow dating of various processes in a
planet's history. The unstable isotopes mo
st useful in geochemistry have a wide
range of decay constants, or half-lives, and can be used to infer processes
occurring over the entire age of the Eart
h (Table 10-1). In addition, isotopes can
be used as tracers and in this regard
they complement the major- and trace-
element chemistry of rocks and magmas.
Isotopes
The
Earth
has
a spirit
of
growth.
-LEONARD0
DA
VINCI
T
he
various chemical elements
have
different properties
and
can therefore
be
readily separated from each other
by
igneous processes. The various isotopes
of
a given
ele
-
ment are not
so
easily separated. The abundances
of
the
radioactive isotopes and their
decay
products are
not
con
-
stant in time. Therefore, the information
conveyed
by
the
study
of
isotopes is
different
in
kind than
that
provided by
the elements. Each isotopic
system
contains unique infor
-
mation,
and
the various radioactive isotopes
allow
dating
of
various processes in a
planet's
history. The unstable iso
-
topes
most useful
in
geochemistry
have
a wide range
of
decay
constants,
or
half
-
lives,
and
can
be
used
to infer
pro
-
cesses occurring
over the
entire age
of
the Earth
(Table
10
-
1).
In
addition, isotopes can
be
used as
tracers
and in
this regard
they
complement the major
-
and
trace
-
element
chemistry
of
rocks
and
magmas.
Studies
of
isotope ratios
have played an important role
in
constraining mantle
and
crustal evolution, convective
mixing and
the long
-
time isolation
of
mantle
reservoirs.
Isotope studies derive their
power
from the existence
of
suitable pairs
of
isotopes
of
a given element, one a
"
pri
-
mordial
"
isotope present
in
the Earth since its formation,
the other a radiogenic daughter isotope
produced
by
radio
-
active
decay
at a
known
rate throughout geological time.
The isotopic composition
of
these isotope
pairs
in
different
terrestrial
reservoirs
-
for
example, the atmosphere, the
ocean,
and
the different parts
of
the crust
and
mantle
-
are
a function
of
the transport
and mixing
of
parent
and
daugh
-
ter elements
between
the reservoirs.
In some
cases the par
-
ent and daughter
have
similar geochemical characteristics
and
are
difficult
to separate
in geological processes.
In
other
cases the parent
and
daughter
have
quite
different
proper
-
ties,
and
isotopic ratios contain information
that
is
no
longer
available
from studies
of
the
elements themselves.
For
example the
s7Sr/86Sr
ratio
gives
information
about the
time
-
integrated
RbISr
ratio
of
the rock
or its reservoirs.
Since rubidium is a volatile element
and
separates from
strontium
both
in
preaccretional
and
magmatic processes,
the isotopic ratios
of
strontium in various products
of
man
-
tle differentiation,
combined with mass
-
balance
calcula
-
tions, are our
best
guide
to the
rubidium content,
and
vola
-
tile content,
of
the
Earth.
Lead
isotopes
can be similarly
used
to constrain the
U/Pb
ratio, a
refractory/volatile
pair,
of
the mantle.
In
other cases, such
as
the
neodymium-
samarium pair,
the
elements in question are
both
refractory,
have
similar geochemical characteristics
and
are
probably
in
the Earth
in
chondritic ratios. The
neodymium
isotopes
can therefore
be used
to infer
ages
of
reservoirs
and
to
dis
-
cuss whether these reservoirs are enriched
or
depleted,
in
terms
of
NdISm,
relative
to
chondritic
or undifferentiated
reservoirs. The
Rb/Sr
and
Nd/Sm
ratios
of
a reservoir are
changed
when
melt is
removed
or
added
or
if it is
mixed
with
or
contaminated
by
sediment, crust
or
seawater.
With
time, the isotopic ratios
of
such reservoirs diverge. The
iso
-
topic ratios
of
the crust
and
various
magmas
show
that
mantle differentiation
is ancient
and
that
remixing
and
ho
-
mogenization is secondary
in
importance to separation
and
isolation.
On
the other
hand
we
know
that these isolated
reservoirs can
readily provide
material
to
the surface.
Al
-
though isotopes cannot tell
us where
these reservoirs are, or
their bulk chemistry, their long
-
term isolation
and
lack
of
homogenization
plus
the temporal
and
spatial
proximity
of
their products suggests that, on average,
they
occur at dif
-
ferent depths. This in turn suggests
that
the reservoirs
differ
in intrinsic density
and
therefore
in bulk
chemistry
and
min
-
eralogy. Melts from the reservoirs,
however,
are
buoyant
TABLE
10
-
1
Radioactive Nuclides
and
their
Decay
Products
Radioactive
Decay
Half
-
life
Parent
Product
(billion years)
238
u
206
Pb
4.468
232Th
208
Pb
14.01
Ii6Lu
li6
Hf
35.7
147
Sm
L43
Nd
106.0
87
Rb
*'Sr
48.8
235
U
207
Pb
0.7038
40
K
40Ar,
40Ca
1.250
1291
lz9Xe
0.016
26
A1
26Mg
8.8
x
relative to the
shallow
mantle.
It may
be that
basalts origi
-
nate
primarily from
only
one
of
these reservoirs
and
that the
trace
-
element
and
isotopic diversity is acquired
when they
traverse
the shallow mantle
on
their
way
to the surface.
The crust
is
extremely enriched
in
many
of
the so
-
called incompatible elements, generally the large ionic ra
-
dius or high
-
charge elements that
do
not readily
fit
into the
lattices
of
the
"
major
mantle
minerals,
"
olivine
and
ortho-
pyroxene. The crust
is not particularly enriched
in elements
of
moderate charge
having
ionic radii
between
those
of
Ca
2
+ and
A1
3
+. This suggests
that the
mantle
has
retained
elements
that
can
be accommodated
in
the garnet
and
cli-
nopyroxene
structures. The crust is also
not
excessively en
-
riched
in lithium, sodium, lead,
bismuth
and
helium.
Some
mantle rocks and magmas have high
concentra
-
tions
of
the incompatible elements
and
isotopic ratios
that
reflect
long
-
term enrichment
of
an
appropriate incompat
-
ible
-
element parent. The
common
geochemical prejudice is
that
the crust
is somehow involved
in
the evolution
of
these
magmas, either
by
crustal contamination prior to
or
during
eruption or
by
recycling
of
continent
-
derived sediments.
Usually, however,
it is impossible
to
distinguish this pos
-
sibility from
the alternative that all potential crust
-
forming
material
has not
been
removed
from
the
upper mantle or
that
the
crust formation process,
efficient
as
it seems
to
be,
is
not 100
percent
efficient
in
removing
the incompatible
elements from the mantle. Ironically, a parallel geochemical
prejudice
is that
some magmas
represent
melts
from
"
primi
-
tive
"
mantle,
which has
survived from the accretion
of
the
Earth
without
any
melting
or melt extraction, the apparent
reasoning being that part
of
the mantle
provided
the present
crust,
with
100 percent
efficiency, and the rest of
the mantle
has
been
isolated,
again with
100 percent
efficiency. In
this
scenario,
"
depleted
7
' magmas
are derived from a reservoir,
complementary to the continental crust,
which has
experi
-
enced
a two
-
stage history (stage one involves
an
ancient
removal
of
a small melt
fraction, the crust; stage
two
in
-
volves
a recent extensive
melting
process,
which
generates
MORB).
"
Enriched''
magmas
(also called
"
primitive,
"
"
more primitive,
" "
less depleted,
" "
hotspot
"
or
"
plume
"
magmas)
are single
-
stage
melts
from a
"
primitive
"
reser
-
voir
or depleted
magmas that have
experienced some
type
of
crustal contamination.
There is
no
room
in
these
models
for
an
ancient en
-
riched
mantle
reservoir. This
"
box
model
"
of
the
Earth
contains three boxes: the continental crust, the
"
depleted
mantle
"
(which
is equated to the upper mantle
or
MORB
reservoir)
and
"
primitive
mantle
"
(which
is equated to the
lower mantle)
with
the
constraint that primitive mantle is
the sum
of
continental crust
and
depleted mantle.
With
these simple rules
many
amusing
games can
be played with
crustal recycling rates
and mean
age
of
the crust.
When
contradictions appear
they
are explained
by
hiding material
in
the
lower
crust, the continental lithosphere
or
the
core,
or
by
storing subducted material somewhere
in
the mantle
for
long
periods
of
time. The products
of
mantle differentia
-
tion are
viewed
as readily
and efficiently
separable but, at
the same time, storable for long periods
of
time
in
a hot,
convecting mantle.
Magmas having
similar geochemical
characteristics are given a
variety
of
origins.
"
Enriched
"
magmas are variously attributed to continental
contarnina-
tion,
recycling
and
a
"
lower
-
mantle
primiti.ve
reservoir.
"
Isotopes are extremely
useful
as probes
of
planetary
processes, and
they
are
even more
useful
when used
in
conjunction
with
other petrological, geochemical
and
geo
-
physical
data.
They are
also
much more
interesting
as
tracers
of
the
Earth's
dynamics
than
is implied
by
static
box
models involving
crust, mantle
and
core
-
the
main
features
of
the Earth
acknowledged
by
isotope geochemists.
Isotopes
make
it possible to rule out conventional ideas
that relate various
basalts by
different degrees
of
partial
melting at different depths (the pyrolite hypothesis) or
by
crystal fractionation (the parent
magma
hypothesis). These
mechanisms
may
still
be
important, but
they must be
com
-
bined with
contamination or
mixing of
materials from iso
-
topically distinct reservoirs.
LEAD
ISOTOPES
Lead has
a unique position
among
the radioactive nuclides.
Two
isotopes, lead
-
206
and
lead
-
207, are
produced
from
radioactive parent isotopes
of
the same element,
uranium-
238
and
uranium
-
235. The simultaneous use
of
coupled
parent
-
daughter systems allows one
to avoid
some
of
the ambiguities associated
with
a single parent
-
daughter
system.
In
discussing
the
uranium
-
lead system,
it is convenient
to
normalize all isotopic abundances to that
of
lead
-
204, a
stable nonradiogenic lead isotope. The
total
amount
of
lead-
204 in the Earth
has been
constant since the Earth
was
formed; the uranium parents
have been
decreasing
by
radio
-
active
decay
while lead
-
206
and
lead
-
207
have been
in
-
creasing. The
U/Pb
ratio
in
various parts
of
the Earth
changes
by
chemical fractionation
and
by
radioactive de
-
cay.
The
238U/204Pb
ratio, calculated
as
of
the present, can
be used to remove
the
decay
effect
in
order
to
study the
chemical fractionation
of various
reservoirs.
If no chemical
separation
of
uranium from lead occurs, the
238U/204Pb
ratio
for the
system
remains constant. This ratio is called
the
p
of
the system.
The
decay
schemes are
where
A
and
X'
are the
decay
constants
and
the second terms
on the right are the initial ratios (at
t
=
t,).
By
combining these equations
we
can write
an
expres
-
sion
for
t,
in
terms
of
isotopic ratios
of
lead
and
uranium:
207Pb/204Pb,
206Pb/204Pb
and
235U/238U.
This is the basis for
determining
the
lead
-
lead age
of
the
Earth.
Use
of
this equa
-
tion
does
not
involve measurements
of
the absolute concen
-
tration
of
either the uranium
or
the
lead
in
the rock,
nor
does
t,
depend
on
the absolute concentrations
of
these ele
-
ments. The
ratio
235U1238U
is
very nearly
constant
(11137.9)
in
natural uranium. For uranium
-
rich minerals,
207Pb/204Pb
and
206Pb/2wPb
are high,
and
the equations do
not much
depend
on
the initial ratios. These equations
were used
by
Patterson (1956)
and
Houtermans (1947)
to
calculate
the
age
of
the Earth,
using
initial ratios inferred from meteor
-
itic
data.
The coupled equations can
be
used
to calculate
206Pbl
2wPb
and
207Pb/204Pb
for
a given
238U/204Pb
ratio,
as
a func
-
tion of
time.
If
the initial ratios
and
p
are known,
or
as
-
sumed from meteorite
data,
then
the locus
of
points so cal
-
culated is called the
geochron.
The lead
-
isotopic ratios can
therefore be calculated for
any
time, including the present,
for unfractionated reservoirs.
For
p
=
8
and
to
=
4.6
Ga,
values
thought to
be
appropriate for the Earth,
206Pb1204Pb
is about
17.6
and
207Pb/204Pb
is
about
15.45.
If
the Earth
fractionated
at any time, giving
high
U/Pb
in the
melts and
low
U/Pb
in the residue,
then
present
day
lead
-
isotopic ra
-
tios will
be
greater
and
less,
respectively,
than
present
-
day
ratios inferred
for
primitive mantle.
Most lead
-
isotopic results can
be
interpreted as
growth
in a primitive reservoir for
a certain period
of
time
and then
growth
in reservoir
with
a different
p-value
from that
time
to the present.
By
measuring
the isotopic ratios
of
lead
and
uranium in
a rock, the time
at
which the
lead ratios
were
the same as inferred for the primitive reservoir can
be
de
-
termined, thus giving the lead
-
lead age
of
the rock. This
dates the age
of
the uranium
-
lead fractionation event, as
-
suming
a
two
-
stage growth
model.
In
some cases multistage
or
continuous differentiation models are used.
A
melt
removed
from
the
primitive reservoir at
to
will
crystallize to
a rock
composed
of
minerals with
different
,u
values.
If
these minerals can
be
treated
as
closed sys
-
tems,
then they will have
distinctive lead ratios that
plot
as
a straight line on
a
207Pb/2wPb-206Pb/204Pb
plot
(Figure
10
-
1).
This line
is
an
isochron
because
it is
the
locus
of
points all
of
which
experienced fractionation at
the
same
time
to
form minerals
with
differing
U/Pb
ratios.
The
re
-
sidual
rock will
also
plot
on
this
line,
on
the
other side
of
the geochron. The time at
which
the
rock
was
fractionated
can
be
calculated from
the
slope
of
the isochron.
Mixing
lines
between
genetically
unrelated magmas
will
also
be
straight lines,
in
which
case the age
will be
spurious
unless
both magmas
formed at the same time.
In
the uranium
-
lead
decay
system,
the
curve represent
-
ing the
growth
of radiogenic
lead
in
a closed
system has
marked
curvature.
This
is because uranium
-
238
has
a half
-
life
(4.47
Ga) comparable to the age
of
the Earth,
whereas
uranium
-
235
has
a much
shorter half
-
life
(0.704 Ga).
In
early Earth history lead
-
207,
the
daughter
of
uranium
-
235,
is formed at a
higher rate
than
lead
-
206.
For
a late fraction
-
ation event
2°7Pb/2wPb
changes
slowly with
time.
For
isotopic
systems
with
very long
half
-
lives,
such
as
samarium
-
142
(106
Ga)
and rubidium
-
87
(48.8
Ga), the
analogous closed
-
system geochrons
will be nearly
straight
lines.
On the
other hand, isochrons
and
mixing
lines for
other systems,
in
general, are
not
straight lines.
They
are
straight
in
the
uranium
-
lead system
because
238U/204Pb
and
235U1204Pb
have
identical fractionation factors,
and
mix
-
ing
lines for ratios are linear
if
the ratios
have
the
same
denominator.
The initial lead
-
isotopic composition
in iron meteorites
can be obtained
since
these bodies are essentially free
of
uranium.
Galenas
are also high in lead
and low in
uranium
and
therefore nearly preserve
the
lead
-
isotopic ratios
of
their parent at the time
of
their birth.
Galenas
of
various
ages fall close
to
a unique single
-
stage
growth
curve. The
small departures can
be interpreted
as
further fractionation
events.
The equations describing the evolution
of
a given set
of
lead
-
isotope compositions from
a single
common com
-
position
in
systems
that
may
lose
lead
or
gain
or lose ura
-
nium
are
If
p
changes discontinuously at
various
times,
then
the
above
equations can
be
written
as
the appropriate sums.
The
p
values
for
basaltic
magmas are
usually
quite
high, 15
-
45,
compared to
primitive mantle. Their
lead-
isotopic ratios
will
therefore
grow
more
rapidly
with
time
than
the primitive mantle,
and the
206Pb/204Pb
and
207Pb/
204Pb
ratios of
such
magmas are high. Oceanic islands
have
such high lead
-
isotopic ratios that
they must have
come
from ancient enriched reservoirs or contain,
as
a compo
-
nent, ancient enriched material.
MORB
thought
to
come
from
an
ancient depleted reservoir, also
have
ratios
in
ex
-
cess
of
the geochron. This suggests either the
mantle
(or
200
ISOTOPES
15.5
15.0
Enriched
or
contaminated
14.5
13
15
17
19
2
1
'06pb/
'04pb
FIGURE
10
-
1
Lead isotope
diagram.
Age
of
Earth is taken as
4.57
Ga. Straight
lines labeled with letters
are values
for
oceanic islands. Black dots
are
the inferred primary isotopic
ratios
if the
island data are interpreted
as
secondary isochrons. Growth
curves for
238U/2MPb
(p,)
values
of
7.0
and
8.0
and
primary isochrons
at
1 Ga
intervals are shown.
The
primary mantle
reservoir appears to have a
p,
of
7.9.
Oceanic
-
island basalts appear to have evolved in
en
-
riched reservoirs ranging in age
from
1
to
2.5
Ga
with
p,
(the second
-
stage
p
value) values
ranging from
9
to
20.
A point is shown
for
a two
-
stage model with
p,
=
7.9
before
1.5
Ga
and
p2
=
15
subsequently.
The
black bar represents the range
of
values
for
depleted reser
-
voirs with
p2
=
7.0
and a range of depletion ages from 1 to
2.5
Ga.
The
range
for
mid
-
ocean
-
ridge basalts could be
due
to
growth in
an
enriched reservoir
or due
to contamination
by
enriched
magmas.
Isotopic
ratios for
xenoliths and kimberlites
are
shown along the
axes. Xenoliths are primarily
from
the
shallovv
mantle and many
are enriched.
KI
is kim
-
berlite. Diagram
is modified
from
Chase (1981).
upper mantle)
has
continuously lost lead, relative to ura
-
nium, to the
lower
crust or core (or lower mantle),
or
that
MORB
basalts
have been
contaminated
by
material
with
high
isotopic
ratios,
prior to
eruption.
Oldenberg (1984) performed
a detailed analysis
of lead
-
isotopic data from
conformable ore deposits to obtain infor
-
mation about
the
movement
of
uranium, thorium
and
lead
into or
out
of
the
mantle
source region.
He
showed that
p
(t)
=
238U(t)/204Pb
increased from
8.03
+
0.3
in
the
first
billion
years
after Earth formation
to
9.8
+
0.3
between
2.0
and
3.0
Ga
ago.
The ratio
p
apparently decreased in the
time interval
2 to
1 Ga
ago,
perhaps representing removal
of
uranium
and
lead
from the
mantle
to the continental crust
or the preferential sampling
of
a relatively depleted reser
-
voir during
that
period
of
time. The
ratio
5
=
232Th/204Pb
also increased
with
time from
about 35 to
38,
also
with
a
dip at
2
-
1 Ga
ago. The ratio
k(t)
=
t(t)lp(t)
thus
main
-
tained
a value near
4.0
since the Earth
formed.
In
a cooling, crystallizing mantle the
p
of
the residual
melt will
increase
with
time, assuming that solid silicates
and
sulfides
retain lead more effectively
than
uranium.
Modeling
shows
that
most of
the mantle
had solidified
prior
to
3.8 Ga,
close
to the age
of
the oldest known rock. Olden
-
burg estimated that
the
residual, depleted solid mantle
might
have
p
of
about
6.0.
Basalts from oceanic islands
have
apparently experienced secondary
growth
in reservoirs
with
p
from
about
10 to 20, after
a long
period
of
growth
in
a more
"
primitive
"
reservoir
(
p
=
7.9).
Leads from basaltic suites in
many
oceanic islands
form linear areas on
206Pb/204Pb
versus
207Pb/204Pb
diagrams
(Figure 10
-
1). These could represent either
mixing
lines or
secondary isochrons. Two
-
stage histories indicate that the
STRONTIUM
AND
NEODYMIUM
ISOTOPES
201
leads from each
island were
derived from
a common pri
-
mary reservoir
(
p
=
7.9)
at different times from
2.5
to
1.0
Ga ago.
Alternatively, the
magmas
from each island
could
represent mixtures
between an
enriched end
-
member and
a
less enriched
or
depleted end
-
member.
In
either case the
ocean
-
island basalts
involve
a source region with ancient
UiPb
enrichment. One mechanism for such enrichment is
removal
of
a melt
from
a primitive reservoir
to
another part
of
the mantle that subsequently provides
melts to
the oce
-
anic islands or contaminates MORB. The
most
logical stor
-
age place
for
such
a melt
is the
shallow
mantle. The enrich
-
ment
event
must have been
ancient, older
than
1
Ga.
To
explain the various trends
of
the individual islands
by mix
-
ing, the enriched end
-
member
must
come from parts
of
the
mantle that
were
enriched at different times
or
that
have
different time
-
integrated
UiPb
ratios.
In
a crystallizing cu
-
mulate or
magma
ocean, the
UiPb
ratio
of
the remaining
fluid probably increases with time,
and
regions
of
the
man
-
tle that
were
enriched
by
this melt
would have
variable
p
depending on
when
and how
often
they
were enriched.
If
the enriched reservoir is global, as indicated
by
the global
distribution
of
enriched magmas,
it is
plausible that differ
-
ent parts
of
it were
enriched at different times.
STRONTIUM
AND
NEODYMIUM ISOTOPES
Strontium
-
isotope measurements are
now
almost
as
routine
as
trace
-
element measurements
and
are
used
in similar
ways
in
petrological investigations. Magmas that are genetically
related
have
the same isotopic ratios. Midocean
-
ridge
ba-
salts
have
87Sr/86Sr
less
than
0.703,
and
"
pure
"
MORB
may
have values
of
0.702
or
less.
Ocean
-
island, island
-
arc
and
continental
flood
basalts are generally
much
higher
than
0.703,
commonly higher
than
0.7
1. Primitive mantle
values
of
87Sr/86Sr
are
unknown
because the mantle
RbiSr
is
un
-
known,
but
it probably falls
between
0.704
and
0.705.
Ba
-
salts
with
high
87Sr/86Sr
are often considered
to
be
contami
-
nated
by
crust or recycled material
or
seawater. They
may
also come from ancient enriched mantle reservoirs,
but
other isotope
and
trace
-
element data are required
to remove
the ambiguities. Attributing the properties
of
MORB to
"
normal mantle
"
and, more recently, to the whole upper
mantle, leaves crustal contamination, recycling or the lower
mantle
as
the only alternatives to explain ocean
-
island
and
other
"
plume
"
or
"
hotspot
"
basalts.
However,
the upper
mantle
itself
is probably inhomogeneous.
In
some cases
continental, sediment
or
seawater contamination
can be
ruled
out.
There is
no
evidence
that any magma
comes from
a primordial reservoir or from the lower mantle.
The radioactive isotope samarium
-
147 decays to
neo
-
dymium
-
143 with
a half
-
life
of
106
Ga.
143Nd/144Nd
ratios
are expressed in terms
of
deviations,
in
parts per
lo4,
from
the
value
in
a reservoir
that has
had
chondritic ratios
of
Sm/
Nd
for all time,
Clearly,
a chondritic unfractionated reservoir
has
E,,
=
0
at all times. Samarium
and
neodymium
are
both
refractory
rare
-
earth elements
and
should
be
in
the Earth
in
chondritic
ratios.
However, they
are separated
by
magmatic
process
and
thus record the magmatic or fractionation history
of
the
Earth.
Samarium has
a higher crystal
-
melt partition
coeffi
-
cient
than
neodymium,
and thus
the
SmiNd
ratio is smaller
in
melts than
in the original rock. The
143Nd/144Nd
ratio,
normalized as above,
will
therefore
be
positive
in reservoirs
from
which
a melt has
been
extracted
and
negative
in
the
melts
or regions
of
the
mantle that have been infiltrated
by
melts. The
SmiNd
ratio depends
on
the
extent
of
melting
and
the nature
of
the residual phases,
and
E,,
depends
on
the
SmINd
and
the age
of
the fractionation event.
Since
neodymium
and
samarium
are geochemically
similar
and
are
both
refractory elements,
the
neodymium-
isotope
system has
certain
advantages over
the strontium
and
lead systems:
1.
Rocks
are less sensitive
to
alteration,
in
particular sea
-
water
alteration.
2.
The
SmiNd
ratio
of
the Earth is probably
well
known
(chondritic).
3.
There is probably little separation
of
samarium
and
neo
-
dymium once
a rock has been
formed.
Nevertheless,
there
is
generally
good
correlation
between
neodymium
and
strontium isotopes.
Positive
E,,
correlates
with
low
87Sr186Sr
and
vice versa. Midocean
-
ridge
basalts
have high
E,,
and
low
87Sr/86Sr,
indicating time
-
integrated
depletions
of
Nd/Sm
and
RbiSr.
The isotopic ratios are so
extreme that
the
depletion
must have
occurred
in the
MORB
reservoir
more
than
1
Ga
ago, probably
more
than
2
Ga
ago.
The depletion
may
have
occurred at the time the continental
crust formed or
even
during the accretion
of
the Earth. The
measured
SmINd
and
RbISr
ratios
in
MORB
generally
would not
support
such
ancient ages,
but
the depletion
may
have been
progressive
and MORB
may
be
mixtures
of
a
more
depleted
and
an
enriched material.
Incompatible
-
element ratios
such
as
Rb/Sr
and
NdiSm
are increased
in
partial melts.
However,
for large fractions
of
partial
melting
the ratios are similar
to
the original rock.
Since elements
with
D
much
less
than
1 (such
as
Rb, Sr,
Nd
and Sm)
are not retained
effectively
by
residual crystals,
it is
difficult
to
change their ratio
in
melts, but the residual
crystals, although
low in
these elements,
have highly
frac
-
tionated ratios. Partial
melts
representing large degrees
of
partial
melting
from primitive
mantle will
also
have near
-
primitive ratios,
as
will regions
of
the mantle
invaded
by
202
ISOTOPES
FIGURE
10
-
2
E~,
versus
87Sr/86Sr
for mixtures involving
a
depleted
magma
or
residual fluids
from such
a magma
after crystal fractionation,
and
an
enriched
component
(EM)
(after
Anderson,
1985).
these melts.
If
the
melt
cools
and
crystallizes,
with
refrac
-
tory crystals
being removed and
isolated, the
SmINd
ratio
changes. Thus,
it is dangerous to infer that a melt came
from a primitive reservoir simply because the
'43Nd/144Nd
ratio appears primitive. Similarly,
magmas
with
s,,
near
0
can
result
from
mixtures
of
melts,
with
positive
and
nega
-
tive
EN,.
Figure
10
-
2 shows
the
s,,
versus
strontium isotope
correlation for a
variety
of
materials.
THE
LEAD
PARADOX
Both
uranium
and lead
are incompatible elements in sili
-
cates,
and
uranium
apparently enters the melt more readily
than lead. The
UlPb
ratio should therefore increase
in
melts
and
decrease
in
the solid residue. One
would
expect, there
-
fore, that the
MORB
reservoir should
be
depleted in
U/Pb
as
well
as
RbISr
and
NdISm.
A time
-
average depletion
would
give
206Pb/2MPb
and
zo7Pb/204Pb
ratios that fall
to
the
left
of
the primary geochron
and
below
the mantle
growth
curve.
Figure
10
-
1 shows,
however,
that
both MORB and
ocean
-
island tholeiites appear enriched relative to the pri
-
mary
growth
curve. This implies that
MORB
has been con
-
taminated
by
high
-
uranium or
high-UIPb
material before
being
sampled,
or that
lead
has
been
lost from the
MORB
reservoir.
Early lead loss
to the core, in sulfides, is possible,
but
the isotopic results,
if interpreted
in
terms
of
lead re
-
moval, also require
lead
extraction
over
the period
3
-
1 Ga.
Contamination
of
MORB
is
a
possible explanation, particu
-
larly
if
the depleted reservoir is deeper
than
the enriched
reservoir.
The
lead
reservoirs for oceanic islands are heteroge
-
neous but
may
have been
derived from a fairly uniform res
-
ervoir with
p
=
7.9
at times ranging from
2.5
to 10
Ga
ago
(Chase, 1981). There is
some
indication that the
more re
-
cently formed reservoirs
are
more enriched
in
UIPb.
This
is
consistent
with
progressive enrichment
of
the ocean
-
island
basalt
(OIB)
reservoir
over
time. Kimberlites from South
Africa appear to
have evolved
in a similar reservoir prior to
1.5
Ga and
in
an
enriched reservoir subsequent to that time
(Kramers,
1977). The uniform
mantle
reservoir
would have
206P'b/204Pb
of
about
17.5
if sampled
today.
After depletion,
this reservoir
would
evolve in a
low-p
environment
(p
<
7.9)
and would
generate
magmas with
206Pb/204Pb
values
less
than
17.5.
If
midocean
-
ridge
basalts
originate in the
depleted reservoir
and
are contaminated
by
magmas from
the complementary enriched reservoirs, then the
field
of
MORB
on
a lead
-
lead plot should fall on a
mixing
line
be
-
tween OIB
and
the uncontaminated magmas. This con
-
strains the
field of
these magmas
on
a lead
-
lead evolution
diagram. Assuming that the depleted
and
enriched reser
-
voirs have
the
same
range, 1
.O
-
2.5
Ga,
one can calculate
that
p
of
the depleted reservoir should
be
about 7 or less.
Magmas from depleted reservoirs that formed
over
the
above
time interval
would have
206Pb/204Pb
ratios
of
17.0
to
17.4.
In
the
MORB system itself
variations
of
UIPb,
RbISr
and
NdiSm
are correlated.
TABLE
10
-
2
Parameters
Adopted
for Uncontaminated
Ocean Ridge
Basalts
(1)
and
Contaminant
(2)
Enriched
Pure
MORB
Contaminant
Parameter
(1)
(2)
Pb
0.08
ppm
U
0.0085
ppm
UlPb
0.10
238
U/204Pb
7.0
206~b/204~b
17.2
Rb
0.15
ppm
Sr
50
PPm
87
SrIa6
Sr
0.7020
Sm
2
PPm
Nd
5
PPm
143
Ndl
lU
Nd
0.5134
Enrichment Factors
*
Pb
24.6
U/Pb
Sr
7.0
RbISr
Nd
6.0
SmINd
2
PPm
0.9
pprn
0.45
30.0
19.3,
21
28
PPm
350
pprn
0.7060
7.2
pprn
30
PPm
0.5124
4.5
26.7
0.60
(I)
Assumed
composition
of
uncontaminated
midocean
-
ridge
basalts.
(2)
Assumed
composition
of
contaminant.
This
is
usually
near
the
extreme
end
of
the
range
of
oceanic
-
island
basalts.
*Ratio
of
concentration
in
two
end
-
members.
In
order
to
test
if contamination
is a viable
explanation
for
the location
of
the
field
of
MORB
on lead
-
lead isotopic
diagrams,
we
must
estimate
the lead
content
of
uncon
-
taminated depleted magmas
and
the
lead
and
lead
-
isotopic
ratios
of
the
contaminant.
Table
10
-
2 lists
the
parameters
we
shall
use.
The
lead
content appropriate for
a
206Pb/204Pb
range
of
17.0
to
17.2
is 0.07 to
0.09
ppm
(Sun
and
others, 1975).
Let
us
adopt
0.08
ppm. The
UIPb
ratio for
basalt
glasses is
about
0.08
to 0.19.
Adopting
0.11,
we
derive a
uranium
content for
pure
MORB
basalts
of
0.0085 ppm.
For the
contaminant,
we
adopt the values
in
Table
8
-
4.
These
are
close to the extreme
values measured on
ocean
-
island
tho
-
leiites.
For
the contaminant
206Pb/204Pb
ratio
we
adopt
19.3,
corresponding to
1.5
Ga
of
secondary
growth in
a reservoir
with
,u
=
15.
This
is a conservative
value
since ratios
as
high
as
21 are observed on oceanic
islands. The
effect
of
a
contaminant with this value
is
also shown
in
Figure
10
-
3.
The results
of
mixing
calculations are
shown
in Figure
10
-
3 for
the
'43Nd/144Nd,
87Sr/86Sr
and
206Pb/Z04Pb
systems.
The
differences
between
the lead
and
other
systems is
strik
-
ing. A
small amount
of
contamination, less
than
0.5
per
-
cent,
pushes
MORB compositions
into the enriched
field
for
lead but not
for
neodymium
or
strontium. In terms
of
single
-
stage evolution,
both
observed
(contaminated)
MORB
and
oceanic
-
island
basalt
will
appear to
have
future
ages on
a lead
-
lead
geochron
diagram. The
neodymium
and
stron
-
tium isotopic ratios
are
not
affected
as much,
and
contami
-
nated
MORB
will
appear to
come from depleted
reservoirs.
The
large increase
of
206Pb/204Pb
caused
by
a small
amount
of
contamination
is mainly
a function
of
the
choice
of
the
ratios
Pb(contaminant)/Pb(MORB)
and
206Pb/204Pb
of
the contaminant.
The lowest measured lead
concentra
-
tion
in
MORB
is about
0.2
ppm, a factor
of
2.5
times
greater
than
the
value adopted
for
uncontaminated
MORB
on
the basis
of
uranium
-
lead
systematics.
On
the
other
hand,
the value adopted
for the contaminant,
2
ppm,
is five
to
eight times
less than values commonly observed
for
oceanic
-
island basalts.
The
lead
enrichment
adopted
for
the
mixing
calculation,
24.6,
therefore
is conservative. The en
-
richment factors
for
strontium
and
neodymium
are
much
less,
and
this is the
main
reason
for
the
greater sensitivity
of
MORB
to
lead
contamination.
If
the
enriched
and
de
-
pleted
reservoirs
are chemically
distinct
layers
in
the
rnan-
tle,
they
will
experience
convection
with
scale
lengths
of
the order
of
one of
the layer
thicknesses. The individual
cells
may
experience
enrichmentldepletion
events
at differ
-
ent times. Thus,
the
reservoirs,
even
if
global in
extent,
need
not
be
homogeneous
in
trace
-
element
geochemistry.
In
the mantle differentiation
scheme
I
present here,
the
de
-
pleted source region is
below
the
enriched
source
region.
It
seems
likely
that
MORB
evolves
in
shallow mantle magma
chambers prior
to
eruption. Therefore, contamination
of
MORB
is probably unavoidable,
at least
at
the
low
levels
discussed here. The isotopes alone,
of
course, do
not
con-
L,
a
d
ENRICHED
0
N
.
n
Primitive
Mantle
a
m
DEPLETED
0
17
N
"0
0.2
0.4
0.6
0.8
1.0
Contaminant
fraction
FIGURE
10
-
3
Isotopic
ratios
versus contamination. Note that a small amount
of
contamination has a large effect
on
the lead system. Enriched
magmas and slightly contaminated depleted magmas will both
fall
in
the
"
enriched
"
field
relative to primitive mantle and will
both
give
"
future
"
ages
on
a single
-
stage
Pb
-
Pb
evolution dia
-
gram. Slight
contamination has less effect on
E,,
and
E,,,
and
MORB
will still appear depleted.
Curves
for two values
for
206Pb/
204Pb
for
the contaminant are shown.
The
correlation line cannot
be used
to estimate
the
primitive value
for
87Sr/8bSr
if
basalts
are
mixtures
(e.g.
Fig.
10
-
2).
strain
the
locations
of
the
depleted
and
enriched
reservoirs.
It is the
time
sequence
of
eruption
of
enriched
and
depleted
basalts,
the seismic
data for
the depth
of
the
MORB
reser
-
voir, the tendency
of
melts
to
migrate
upward and
the vul
-
nerability
of
the shallow mantle
to
contamination
by
sub
-
duction
and
trapped melts
that
suggest the stratification
fa
-
vored
here.
ISOTOPIC
EVOLUTION
OF
THE
MANTLE
Neodymium
-
isotope results
were
initially
interpreted
in
terms
of
simple
one
-
and
two
-
stage differentiation schemes,
with
a primary stage
characterized
by
growth in
a primitive
reservoir
followed
by
secondary growth
in
a reservoir
with
constant
Sm/Nd
ratio.
In
this scheme the change
in
Sm/Nd