Chapter
16
Noble
gas
isotopes
3
He
is
not
here,
for
3
He
is
risen!
high
concentrations
of
He
(written
[He])
,
high
Anon.
3
Hef
4
He
ratios
(written
R)
and
'solar'
neon
are
Overview
The
group
of
elements
known
as
the
rare
,
inert
or
noble
gases
possess
unique
properties
that
make
them
important
as
geodynamic
tracers
.
The
daughter
isotopes
fractionate
readily
from
their
parents,
they
are
inert,
and
they
differ
from
other
geochemical
tracers
in
being
gases
and
diffus-
ing
relatively
rapidly
,
at
least
at
low
pressure
and
high
temperature.
Nevertheless
,
they
can
be
trapped
in
crystals
for
long
periods
of
time.
They
give
information
about
the
degassing
his-
tory
of
the
mantle
and
magmas,
the
formation
of
the
atmosphere,
and
about
mixin
g
relation-
ships
between
different
mantle
components
. Iso-
topes
made
during
the
Big
Bang
are
called
the
'primordial'
isotopes.
Some
of
the
noble
gas
iso-
topes
are
cosmogenic,
nucleogenic
or
radiogenic
and
these
were
made
at
later
times.
The
pres-
ence
of
3
He
in
a
rock
or
magma
has
often
been
taken
as
evidence
for
the
existence
of
a
primordial
reservoir
in
the
mantle,
one
that
had
not
previ-
ously
been
melted
or
degassed.
A
primordial
reser-
voir
is
different
from
a
primordial
compon
ent.
Pri-
mordial
and
solar
components
are
still
raining
down
on
Earth.
The
most
primordial
materials
on
Earth,
in
terms
of
noble
gases,
are
on
moun-
tain
tops,
in
the
stratosphere
and
in
deep-ocean
sediments
.
Evidence
for
recent
additions
of
noble
gases
to
the
Earth
comes
from
deep-sea
sediments
where
found
.
Mantle
-
derived
basalts
and
xenoliths
hav-
ing
similar
primordial
or
solar
characteristics
ar
e
generally
attributed
to
plumes
from
the
und
e-
gas sed
primitiv
e
lower
mantle
reservoir
. This
is
based
on
the
assumption
that
high
3
He
/
4
He
and
36
Arr
0
Ar
ratios
in
basalts
require
high
3
He
and
36
A.r
contents
in
their
sources,
rather
than
low
4
He
and
40
Ar,
and
that
the
whole
upper
mantle
is
characterized
by
low
ratios
and
low
abundances
.
The
critical
issues
are
when
and
how
the
'pri
-
mordial'
noble
gases
entered
the
Earth
,
where
they
are
stored
and
whether
transport
is
always
outwards
and
immediate
upon
melting,
as
often
assumed.
It
is
therefore
important
to
understand
the
noble
-gas
budget
of
the
Earth,
both
as
the
Earth
formed
and
as
it
aged
.
The
major
part
of
terrestrial
formation
probably
occurred
at
hi
gh
temperature
from
dry,
volatile-poor,
degassed
particles
.
During
the
early
stages
of
planetary
for-
mation,
core
differentiation
and
massive
bolide
impacts
heated
the
Earth
to
temperatures
that
may
have
been
high
enough
for
a
substantial
part
of
it
to
have
melted
and
vaporized.
This
is
su
g-
gested
by
the
fact
that
a
large
fraction
(30-60
%)
of
the
incompatible
elements
(e.g
.
U,
Th,
Ba,
K,
Rb)
are
in
the
Earth's
crust,
which
is
consistent
with
an
extensively
differentiated
mantle
.
Some
of
the
components
now
in
the
man-
tle
fell
to
Earth
in
a
lat
e ven
ee
r,
mostly
around
3.8 Ga
-
the
period
of
late
bombardment
-
th
a t
added
material
to
the
Earth
after
completion
of
core
formation.
Late
low-energy
accretion
prob-
ably
introduced
most
of
the
volatiles
and
trace
siderophiles
(iron-lovers.
such
as
Os
and
Ir)
that
ar
e
in
the
upper
mantle
today
. However
,
what
fraction
of
the
noble
gases
in
mantle
basalts
is
truly
primordial
vs.
a
late
veneer
-
or
even
later
additions
of
cosmic
dust
-
or
recent
contamina-
tion
is
unknown
.
The
hot
-
accretion
model
of
Earth
origin
and
evolution
contrasts
with
the
'standard
mode
l'
of
noble-gas
geochemistry
that
assumes
that
the
bulk
of
the
mantle
accreted
in
a
primor-
dial
undegassed
state
with
chondritic,
or
cos-
mic,
abundances
of
the
noble-gas
elements
.
Such
material
is
widely
assumed
to
make
up
the
deep
mantle
today
and
to
be
accessible
to
surface
volcanoes
.
He
lium
f u
ndamentals
,
the
di
s-
trib
u
tion
o f
th
e l
igh
t
er
noble
gases
-
hel
i
um
,
neo
n a n d a
r
gon
-
and
the
r o l e o
f
n
oble
gases
in
g
eoc
h e m
ist
r y
and
cos
m
o-
che
m
istry
are
extensively
discussed
in
mono
-
g
raphs
and
review
articles
and
on
web
sites
[mantleplu
m
es,
he
l
ium
paradoxes
].
Components
and
reservoirs
The
noble
gases
do
not
necessarily
enter
the
mantle
in
the
same
way
or
reside
in
the
same
components
or
reservoirs
as
other
geochemical
tracers.
Noble
gases
reside
in
bubbles,
along
crystalline
defects,
in
deep-sea
sediments
,
and
in
rocks
exposed
to
cosmic
rays
. Olivine-rich
cumu-
late
s
in
the
mantle
may
trap
C0
2
and
helium
,
but
little
else.
U,
Th
and
K
have
quite
differ-
ent
fates
and
reside
in
quite
different
places
than
He
and
Ar
.
The
noble
gases
are
fractionated
from
their
parents
and
from
each
other
by
melt
-
ing,
transport
and
degassing
processes.
Therefore
they
cannot
be
treated
as
normal
LIL-elements.
Solubility
in
magmas
is
an
additional
issue
for
gases.
Isotope
ratios
In
c
ommon
with
much
of
isotope
g
eochemistry
,
the
noble
gases
are
discussed
in
terms
of
ratios
.
The
absolute
abundances
of
the
noble
gases
in
a
rock,
magma
or
the
mantle
ar
e
seldom
known
or
believed
.
This
comp
l
icates
matters
since
normal
ISOTOPE
RATIOS
199
statistics
and
mixing
re
l
ations
cannot
be
applied
to
ratios.
The
most
usefu
l
light-nobl
e-gas
isotopic
ratios
,
in
petro
logy
are
shown
below
(*
sig nifies
rad
i
ogenic
or
nucleogenic
; t h
ere
may
have
be
e n
some
4
He
and
4 0
Ar
in
primordial
matter
but
most
of
it
in
the
Earth
was
created
later,
and
this
is
des
i
gnated
4
He
*
and
40
Ar
*
to
distinguish
them
from
primordial
isoto
p es,
if
the
distinction
is
necessary)
:
3
He
r He
*
4
He
*F'
Ne
*
zo
Ne
F 2
Ne
z
'Ne
*Fz
Ne
40
A.r
•e6
Ar
3
He
F
2
Ne
• He
•
t•o
A.r
*
For
convenience
,
these
will
sometime
s
be
abbre-
viated
as
3 /
4,
3 /
22
, 21
/
22
and
so
on
,
and
s
ome-
times
written
as
3
Hef
4
He
,
or
R,
20
Ne/
22
Ne
and
so
on.
Abso
l
ute
concentrations
will
be
written
[H
e
].
[
3
He]
and
so
on
.
Helium
Helium
is
ver
y
rare
in
the
Earth
a
nd
it
contin
-
uously
esc
a
pes
from
the
atmosphere
.
Out
g
assing
of
4
He
from
the
mantle
does
not
occur
at
a
high
enough
rat
e
to
correspond
to
the
outflow
of
heat
generated
by
U
+
Th
decay
. This
is
known
as
the
h e l
ium
heat
- f l
ow
paradox
.
Since
it
is
probably
more
difficu
lt
to
degas
the
mantle
now
than
durin
g
the
early
,
high-temperature
period
of
Earth
hi
s
tory
,
it
is
likely
that
the
stable
noble
gas
isotopes
-
those
that
have
not
increased
over
time
-
ar
e
more
degassed
than
the
radio
g
enic
and
nucleo
g
enic
ones
e.g .
4
He
*
and
40
Ar
*.
There
are
few
constraints
on
how
much
helium
may
be
in
the
Earth
or
when
it
arrived
.
It
is
readily
lost
from
the
atmosphere
via
e
scape
into
space
.
It
is
therefore
not
very
useful
for
studying
the
long
-
term
outgassing
history
of
the
Earth
.
Neon
and
ar
g
on
are
potentially
more
use
-
ful
in
this
re
g
ard
since
they
do
not
es
cape
as
readily;
th
ey
accumulate
in
the
a
tmospher
e . W
e
know
, for
example,
that
most
(
~
70
%
)
of
the
a r g
on
produc
ed
in
the
Earth
is
now
in
th
e a
tmo
s
phere,
even
thou
g h
much
of
it
was
not
in
th
e
mantle
durin
g
the
earliest
hi
g
h-temperature
phase
s
of
accretion
,
de
g
assing
and
evolution.
Thi
s
ar
g
ue
s
against
a l
a r ge
undegassed
reservoir
in
the
Earth
.
On
the
other
hand
,
the
imbalance
between
heat
200
NOBLE
GAS
ISOTOPES
Ta
ble
16
.1
f
Statistics
of
helium
in
MORE
[statistical
tests
helium
reser-
voirs]
Mean
Standard
deviation
n
All
ridge
9.14
3.59
503
Atlantic
9.58
2.94
236
Pacific
8.13
0.98
245
Indian
8.49
1.62
177
OIB
7.67
3.68
23
flow
and
4
He
flux
from
the
mantle
implies
that
He
is
trapped
in
the
upper
mantle;
it
is
not
escap-
ing
as
fast
as
it
is
produced.
Apparently,
4
He
is
accumulating
in
the
mantle.
Helium
is
more
sol-
uble
in
magma
than
the
heavier
noble
gases
and
may
be
trapped
in
residual
melts
and
cumulates.
Helium
trapped
in
residual
melts
or
fertile
man-
tle
will
evolve
to
low
R
because
of
the
presence
of
U
and
Th.
Helium
trapped
in
olivine,
olivine-
rich
cumulates
or
depleted
restites
will
main-
tain
nearly
its
original
isotopic
ratio.
Thus,
high
3
Hej
4
He
samples
may
retain
an
ancient
frozen-in
ratio
rather
than
a
current
primordial
ratio.
Such
material
is
probably
intrinsic
to
the
upper
mantle
and
upper
mantle
processes,
such
as
degassing
of
ascending
magmas
.
The
reference
isotope
for
helium
is
3
He
and
the
ratio
3
HeWHe
+
4
He
*)
is
usually
writ-
ten
3
He
j
4
He
or
R
and
this
in
turn
is
refer-
enced
to
the
current
atmospheric
ratio
Ra
(or
R
A).
and
written
R/Ra
(or
R/RA).
The
atmospheric
value
involves
degassing
from
the
mantle
and
crust
-
which
have
quite
different
values
and
time
scales
-
a
contribution
from
interplane-
tary
dust
particles
(IDP),
and
the
rate
of
escape
from
the
atmosphere.
These
all
change
with
time
and
the
atmospheric
ratio
is
also
expected
to
change
with
time.
There
is
no
guarantee
that
ancient
ma
g
mas,
corrected
for
radioactive
decay,
had
the
same
relation
to
the
atmospheric
values
at
the
time
as
they
have
to
present-day
atmo-
spheric
ratios.
It
is
always
assumed
in
mantle
geoc
hemistry
calculations
that
the
atmospheric
ratio
is
invariant
with
time
and
that
the
atmo-
sphere
is
a
well-stirred
homogenous
reservoir.
It
would
be
useful
to
be
able
to
measure
the
time
variability,
just
as
it
is
useful
to
deter-
mine
the
variation
of
87
Srj
86
Sr
in
seawater
with
time.
The
helium
isotopic
ratio
in
MORE
is
usually
quoted
as
8
±
1
RA
but
this
represents
strongly
filtered
data,
filtered
to
'remove
any
plume
influence
'.
Unfiltered
data
gives
9 .1
±
3.6
RA
(Table
16.2)
.
Spreading
ridges
average
the
isotopic
ratios
from
large
volumes
of
the
mantle
and
it
is
fairly
straightforward
to
compile
meaningful
statistics.
Unfortunately,
the
helium
data
on
and
near
spreading
ridges
is
highly
selected
and
fil-
tered
prior
to
statistical
analysis.
Table
16.2
gives
both
filtered
and
unfiltered
estimates.
Ocean
island
basalt
data
is
harder
to
analyze
because
the
samples
are
not
collected
in
systematic
or
random
ways
and
anomalous
regions
are
over-
sampled
(the
reverse
of
the
situation
for
MORE
and
ridges).
Nevertheless,
one
can
compile
aver-
ages
of
all
samples
for
each
island
and
average
these
to
get
global
estimates
of
OIB
statistics.
The
statistics
and
the
distribution
of
helium
in
mantle
magmas
show
that
there
is
no
statistical
difference
in
3
Hej
4
He
(R)
ratios
between
the
available
data
for
midoceanic
ridge
basalts
(MOREs)
and
ocean
island
basalts
(OIBs)
.
It
is
usually
assumed,
however,
that
these
two
classes
of
basalts
require
the
existence
of
at
least
two
distinct
mantle
reservoirs
that
have
been
preserved
over
long
periods
of
the
Earth's
history
.
The
distribution
in
ocean
ridge
basalt
s
(Figure
16.1)
is
more
Gaussian
than
OIB.
The
variance
is
relatively
low
but,
importantly,
is
a
large
fraction
of
the
mean
.
OIBs
have
a
non-
Gaussian
distribution,
with
a
very
large
variance.
The
lar
gest
R
values
in
the
OIB
population
are
often
considered
to
be
dia
g
nostic
of
the
OIB
reservoir
and
lar
ge
values
in
MORE
are
consid-
ered
to
be
plume
contamination.
There
is
no
obvious
cut-off
between
MORE
values
and
OIB
values.
Model
calculations
suggest
that
the
sepa-
ration
between
the
low
R
ratio
components
and
the
hi
gh
ratios
found
in
some
OIB-type
compo-
nents
is
re
l
at
ively
recent
(Anderson,
1998a,
b;
Seta
et
al.,
2001).
The
variance
differences,
and
differences
in
extreme
values,
between
various
basalt
populations
is
a
consequence
of
the
aver-
aging
dwing
the
sampling
process.
The
extreme
3
Hej
4
He
r
at
ios
are
averaged
out
upon
melt-
ing
and
averaging
(
SUMA)
,
as
at
the
global
ISOTOPE
RATIOS
201
'Tradition!
'
MORB
Range
0
2
4
6
8
10
I
I-
I
I
I
12
14
Histogram
of
helium
isotopes
measured
in
samples
along
the
global
spreading
ridge
system
,
including
new
ridges,
back-arc
basins
and
near-ridge
seamounts.
In
most
compilations
much
of
the
right-hand
side
of
the
distribution
is
missing
since
values
higher
than
about
9
R/
Ra
are
filtered
out.
High
values
are
attributed
to
plume
contamination.
Anomalous
sections
of
the
global
ridge
system
are
a lso
avoided.
The
traditional
MORB
range
,
after
filtering
the
data
,
is
from
7-9
RI
RA
.
High
R
values
are
found
in
depleted
components
of
basalts
that
have
been
identified
as
the
most
common
mantle
component
,
possibly
peridotite.
The
high
R/
Ra
carriers
may
be
peridotites
or
olivines
with
low
U/
He
ratios.
The
helium
may
be
acquired
from
wall-rocks,
by
ascending
magmas
.
spreading
ridge
system
,
and
can
only
be
sam-
pled
by
smaller
scale
processes
,
such
as
individ-
ual
lava
flows
on
islands.
Estimates
of
R/R
A
for
OIB
and
for
filtered
ridge
samples
are
given
in
Table
16
.2 .
Ratios
that
are
sensitive
to
degassing
of
magma
,
and
atmospheric
contamination,
such
as
He
/Ne
and
He
/Ar,
show
a
clear
distinction
between
MORE
and
OIB
.
The
d
is
t
ri
buti
o n
of
the
l
ig
ht
er
n
ob
l e
gases
-
h
eliu
m,
a r
gon
and
n
eo
n
-
in
ma
ntle
m
ag
m
as
suggests
that
degassing
of
magmas
near
the
Earth's
surface,
trapping
of
volatiles,
and
possi-
bly
more
recent
atmospheric
contamination,
may
be
involved
in
the
differences
[m
a n
tle
plum
es
].
Th e
h
ea
vi
er
n o
bl
e
g
ases
i n m
a n t l e m
ag
-
m
as
m
ay
e
nti
re
l y
b e
du
e
t o a
tm
osp
h e
ri
c
an
d
sea
wat
e r c
o
ntamina
t i o n .
•
DEPTH
<
2 .5 km
0
All
near
-ridge
data
16
18
20
Table
16.2
1
Helium
isotope
data,
as
com-
piled
over
the
years.
The
year
of
the
compi-
lation
is
also
given
RIRA
All
ridges
9.06
'F
il
tered'
data
8.4
8
8.2
8.3
8.58
8.67
8.67
8.75
Back-arc
basi
n
basa
l
ts
8.4
Ocea
n i
sla
nd basa
lts
7.67
9
Seamou
n
ts
6.58
9.77
S.D.,
standard
deviation
.
S.D
326
0.36
2
0.2
0.3
1
.8
1
1.88
1.88
2.
14
3.2
3.68
3.9
1
.7
1
.4
Year
1
982
1
986
1991
1996
2000
2000
2000
2002
2002
1995
200
1
Most
workers
assume
a
lower-mantle
plume
source
for
high-R
magmas
-
and
a
very
high
[
3
He]
-
which
must
be
isolated
and
preserved
against
convective
homogenization
with
the
more
radiogenic
upper
mantle.
A
'primordial'
or
202
NOBLE
GAS
ISOTOPES
'relatively
undegassed'
reservoir,
however,
prob-
ably
does
not
exist.
The
mantle
is
degassed,
but
some
parts
(e.g.
'the
MORB
- reservoir')
are
more
enriched
in
radiogenic
helium
due
to
high-
[U+Th]
concentrations
combined
with
great
age.
Since
MORBs
are
LIL-depleted
and
have
low
R,
it
has
been
assumed
that
the
upper
mantle
is
degassed.
MORB,
however,
has
high
[
3
He]
com-
pared
with
other
basalts;
this
is
inconsistent
with
the
standard
model.
Primordial
He
may
be
pre-
sent
in
the
mantle
but
a
large,
coherent,
ancient,
primordial,
undegassed
region
is
unlikely
.
The
highest
R/RA
basalts
are
similar
to
MORB,
or
the
MORB
source,
in
their
heavy
isotopes
.
The
standard
model
of
mantle
noble
gas
geo-
chemistry
divides
the
mantle
into
a
depleted
degassed
upper
mantle
(DUM)
homogenized
by
convection,
and
an
undegassed
primordial
lower
mantle
(PM)
that
is
not
well
stirred.
This
model
is
based
on
a
string
of
assumptions.
(1)
R
values
higher
than
the
MORB
average
reflect
high
[
3
He].
(2)
High
3
Hej
4
He
imply
a
primordial
undegassed
reservoir
,
or
a
reservoir
more
primitive
and
less
outgassed
than
the
mantle
source
for
MORB,
assumed
to
be
the
upper
mantle
.
(3)
This
primordial
reservoir
must
be
isolated
from
the
upper
mantle.
(4)
It
is
therefore
deep
and
is
the
lower
mantle.
(5)
This
deep
isolated
reservoir
can
be
tapped
by
oceanic
islands.
The
first
two
items
are
lo
g ical fallacies.
There
are
many
other
ratios
that
can
be
formed
with
3
He,
e.g.
3
Hej
22
Ne,
3
He/
20
Ne,
3
He
e
6
Ar
. . .
and
these
are
all
lower
in
OIB
than
in
MORB
except
for
3
He
j
238
U.
The
latter
is
consistent
with
a
low
-U
source
, e .g.
peridotite,
for
high-R
OIB.
Complex
models
have
been
devised
to
explain
the
apparent
coexistence
of
a
dep
l e t
ed
d e
ga
ssed
we
ll
-stirred
upper
m
ant
l e
reservoir
and
a
hetero
g
enous
unde
g
assed
pri-
mordial
-
and
accessible
-
lower
mantle
reser-
voir
,
assuming
that
OIB
and
MORB
represent
distinct
isolated
reservoirs,
that
the
upper
man-
tl
e
can
only
provide
homogenous
MORB-like
materials,
and
that
low-R
implies
low-[
3
He]
.
The
rather
restricted
range
of
helium
isotope
ratios
in
MORB
is
more
plausibly
interpreted
as
a
result
0
5
1
0
1 5
20
25
30
35
40
Maximum
R! RA
•:n:.r.11o
..
Maximum
va
lu
e
of
R
vs.
the
spread
in
va
l
ues
showing
that
high-R
islands
al
so
have
the
largest
range
in
values
(figure
courtesy
of
A.
Meibom
,
www
.
mantleplumes.
org
).
45
50
of
the
vast
amount
of
averaging
that
midocean
ridges
perform
on
the
mantle
that
they
sample
.
Both
high-R
and
low-R
domains
may
coexist
in
the
upper
mantle
, as
long
as
they
are
larger
than
diffusion
distances;
these
can
be
commin-
gled
during
the
melting
process
or
sampled
sep-
arately
if
sampling
is
done
at
a
small
scale.
The
so-called
high-He
hotspots
exhibit
very
lar
ge
variance
(Figure
16
.
2).
This
is
suggestive
of
the
predictions
of
the
central
limit
theorem
and
implies
that
mid-ocean
ridges
sample
larger
vol-
umes
of
a
heterogenous
mantle
than
do
oceanic
islands
.
The
decay
of
uranium
and
thorium
produces
both
radiogenic
helium
(alpha
particles)
and
heat
.
The
amount
of
uranium
required
to
g
en-
erate
most
of
the
Earth's
oceanic
helium
flux
only
produces
about
5
-1
0 %
of
the
oceanic
heat
flow;
this
is
the
h e
lium
h
eat
-f
low
para-
dox.
Helium
,
along
with
C0
2
,
from
degassing
m.agm.as,
may
be
trapped
in
the
shallow
man-
tle
.
Trapped
C0
2
and
4
He
alon
g
with
high
U+ Th,
contributes
to
the
low
3
He
r He
and
high
[He]
of
some
upper
mantle
components,
and
ubiquitous
carbonatitic
metasomatism.
Helium
apparently
is
not
as
mobile
an
ele-
ment
as
generally
thought
,
opening
up
the
pos-
sibility
that
helium
in
various
upper
mantle