Geoscience Reference
In-Depth Information
into the mantle, probably its shallow levels. The
3
He in OIB is a small fraction of the total that
is degassed from the mantle and in volume is
comparable to the small amount brought onto
the seafloor by IDP. However, much of the
3
He in
the mantle may have come in as part of the late
veneer; it is subduction, or the Archean equiva-
lent, at some 1---2 Ga that is relevant, not today's
flux. The
3
He/
20
Ne ratio in cosmic dust is one
to two orders of magnitude lower than that in
MORB. Helium has a much greater diffusivity
than Ne, which would promote its preferential
degassing from cosmic dust grains during sub-
duction, although this does not mean that it
immediately escapes the mantle. It appears that
subduction of cosmic dust
having the properties of
today's dust
cannot contribute more than a small
fraction of the mantle
3
He budget without caus-
ing excessive enrichment of
20
Ne in submarine
glasses. The preferred explanation for the high
R
components in both OIB and MORB mantle is,
therefore, evolution in low-U
IDP
1000 ppm
10
−
4
100 ppm
High
3
He/
4
He hotspots
10 ppm
~1 ppm
MORB
10
−
5
Low
3
He
4
He hotspots
/
3
He
4
He
PRESENT ATMOSPHERE
10
−
6
RADIOGENIC HELIUM
10
−
7
Th environments
such as olivine cumulates or depleted lithosphere
(Anderson, 1998a, b). High
3
He/
4
He and solar Ne
ratios, may have been isolated in [He]-poor, U
+
Continental Crust
Th-
poor environments rather than in [He]-rich ones.
[
mantle helium neon and argon
]
While some samples from Loihi, Iceland, Yel-
lowstone and the Galapagos have high
3
He/
4
He
components --- usually called the most primor-
dial He compositions --- some ocean islands such
as Tristan, Gough, islands in the SW Pacific
and the Azores, and some components (basalts,
inclusions, gases) in Yellowstone, Iceland and
the Galapagos, have
3
He/
4
He ratios lower (more
radiogenic) than MORB. If these are viewed as
samples from a global population, then this is
the expected result of the CLT. The low
3
He/
4
He
ratios require a component of radiogenic He from
a long-lived U
+
10
−
8
0
20
40
60
80
100
% Sediment
Fig. 16.6
Materials found at the surface of the Earth exhibit
a range of 4 orders of magnitude in
3
He/
4
He ratios.
Sediments on the seafloor collect interplanetary dust
particles (IDP) and these raise the helium content and helium
ratio of deep sea sediments in proportion to their
abundances of IDP. Continental sediments have very low
ratios because of high U and Th and age. Subduction of these
materials contribute to the isotopic heterogeneity of the
mantle. High
3
He/
4
He ratios in mantle materials can also be
due to trapping of ancient gases --- in U-poor environs --- from
degassing of rising magmas.
Many basalts with so-called primitive, primor-
dial or solar noble gas signatures show strong
evidence for seawater or atmospheric contamina-
tion. This combination of surface characteristics
with ones conventionally attributed to the deep
mantle is another paradox in the standard model
for noble gases.
If cosmic dust and oceanic sediments survive
the
subduction barrier
they have the potential to
deliver He and Ne with primordial signatures
Th-rich source such as recycled
crust or sediments in the mantle, as inferred
from lithophile isotope data. Parallel recycling
of refractory peridotite or olivine-rich cumulates
could be responsible for high
3
He/
4
He ratios and
these components will not have enriched sig-
natures for other isotopes. None of these com-
ponents need to be recycled very deep into
the mantle. Both high- and low-
3
He/
4
He compo-
nents can arise from recycling, but the extreme
+
