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In-Depth Information
'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
40
Iceland
35
30
Hawaii
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/R
A
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.
+
25
Galapagos
20
15
Heard
/
Kerguelen
Samoa
Atlantic
Juan Fernandez
Cape Verde
Societies
10
Indian
Azores
Pacific
5
Cook-Australs
Canaries
African xenoliths
St. Helena
Gough
0
0
5 0 5 0
Maximum
R
/
R
A
25
30
35
40
45
50
Fig. 16.2
Maximum value of
R
vs. the spread in
values showing that high-
R
islands also have the largest
range in values (figure courtesy of A. Meibom,
www.mantleplumes.org
).
(1)
R
values higher than the MORB average reflect
high [
3
He].
(2) High
3
He/
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
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
large 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 gen-
erate most of the Earth's oceanic helium flux
only produces about 5---10% of the oceanic heat
flow; this is the
helium heat-flow para-
dox
. Helium, along with CO
2
,fromdegassing
magmas, may be trapped in the shallow man-
tle. Trapped CO
2
and
4
He along with high U
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 logical fallacies. There are
many other ratios that can be formed with
3
He,
e.g.
3
He/
22
Ne,
3
He/
20
Ne,
3
He/
36
Ar...andtheseare
all lower in OIB than in MORB except for
3
He/
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
depleted
degassed well-stirred upper mantle
reservoir
and a heterogenous undegassed pri-
mordial --- and accessible --- lower mantle reser-
voir, assuming that OIB and MORB represent
distinct isolated reservoirs, that the upper man-
tle 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
Th,
contributes to the low
3
He/
4
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
