Geoscience Reference
In-Depth Information
uation for He, where
4
He/
3
He is usually much
higher in the atmosphere than in either OIB or
MORB as a result of the preferential escape of the
lighter
3
He atom. Much of the He in the atmo-
sphere entered it in the last million years and is
therefore dominated by
4
He, older atmospheric
and crustal helium having already escaped to
space.
The concept of a relatively undegassed man-
tle reservoir has been strongly contested. There is
strong evidence from the heavy noble gases that
OIB are contaminated by air or ocean sources;
low
40
Ar/
36
Ar can be a result of atmospheric or
seawater contamination. Seawater has 2---4 orders
of magnitude more
36
Ar than mantle basalts
and two orders of magnitude less
3
He. Hence,
Ar isotope ratios in magmas can be significantly
changed by seawater without affecting their He
signature. Measured [
36
Ar] covers the same range
for MORB and OIB, and [
3
He] is higher for MORB
than for OIB. This also argues against OIB aris-
ing from a reservoir much less degassed than the
MORB reservoir. This model for Ar is at odds with
the standard model for He that considers the
lower mantle to be undegassed, little degassed or
less degassed than the MORB source. The lower
ratios for OIB can be explained by more extreme
atmospheric contamination, or a potassium-poor
or younger source. Usually, however, unradio-
genic ratios are attributed to high abundances of
the primordial isotope, i.e. to undegassed or pri-
mordial reservoirs. The
20
Ne/
22
Ne ratios for man-
tle rocks that are significantly higher than atmo-
spheric indicate that the source of noble gas in
both OIB and MORB cannot be mainly present-
day atmosphere.
would contribute nothing besides He and CO2
and perhaps some Ne and Os, thereby decoupling
He from other isotopic tracers.
Argon
The main isotopes of argon found on Earth are
40
Ar,
36
Ar, and
38
Ar. Naturally occurring
40
Kwith
10
9
years, decays to stable
40
Ar.
We have estimates of the amount of potassium
in the Earth so we can estimate the efficiency of
degassing of the crust and mantle from the argon
content of air.
The initial amount of
40
Ar contained in the
Earth is negligible compared with the amount
that was produced subsequently. This makes it a
useful tool with which to study mantle degassing.
The amount of K in the crust and mantle may
be estimated by calculating the mix of mantle
and crustal components that satisfies the cosmic
ratios of the refractory elements. The amount of
K in the silicate Earth (the mantle and crust) so
determined is 151 ppm of which 46% is in the
crust. The amount of
40
Ar in the atmosphere rep-
resents 77% of that produced by the decay of
this amount of potassium over the age of the
Earth. This implies that either 23% of the Earth
is still undegassed or that the degassing process
is not 100% efficient, i.e. that there is a delay
between the production of
40
Ar and its release
into the atmosphere.
Ar may be compatible
at depths
a half-life of 1.25
×
150 km
and may therefore not
be easily extracted from the deeper Earth by vol-
canism. However, most of the K is probably in the
crust and shallow mantle. The values of
40
Ar/
36
Ar
(or
40
Ar
∗
/
36
Ar) measured, or estimated, in various
materials are:
>
∼
Air
∼
300
The ' two-reservoir' model
A two-reservoir model for the mantle was origi-
nally proposed to explain argon and helium iso-
tope systematics in basalts and xenoliths but the
samples were subsequently shown to be contam-
inated with atmospheric argon and cosmogenic
helium; they had nothing to do with the man-
tle. Some samples of ocean-island basalts, most
notably from Hawaii, Iceland and the Galapa-
gos have elevated
3
He/
4
He values compared with
most MORB samples, after the latter are 'filtered
for plume influence.' These islands also have an
OIB
atmospheric to
∼
13 000
MORB
atmospheric to
∼
44 000
In the standard model, which assumes a lower-
mantle source for OIB, the high ratios for OIB
and MORB, compared with air, have been taken
to indicate that both the upper (MORB) and the
lower mantles (OIB) have been degassed such that
little
36
Ar remains; but large quantities of
40
Ar
have been produced by radiogenic decay and are
retained. The higher
40
Ar/
36
Ar in mantle rocks
than in the atmosphere contrasts with the sit-
