Geoscience Reference
In-Depth Information
enormous spread of isotopic ratios, overlapping
the values for MORB samples. Many OIB sam-
ples have
3
He/
4
He ratios much lower than MORB
samples but there is no obvious dividing point;
the data for MORB and OIB are a continuum.
If MORB samples larger volumes of the man-
tle than OIB then they will have a more con-
stant mean, a smaller standard deviation, and
fewer outliers; these are simple consequences of
the central limit theorem. The outliers will be
eliminated by the averaging. Nevertheless, the
two-reservoir model for mantle noble gases is
widely accepted and is the cornerstone of the
standard model of mantle geochemistry. The two
reservoirs in the standard model are a homoge-
nous degassed upper mantle with low
3
He/
4
He
ratios, and an undegassed lower mantle with
high
A shallow origin for the 'primi-
tive' He signature
in ocean-island basalts
reconciles the paradoxical juxtaposition of
crustal, seawater, and atmospheric signatures
with inferred 'primitive' characteristics. High
238U/204Pb components in ocean-island basalts
are generally attributed to recycled altered
oceanic crust. The low
238
U/
3
He component may
be in the associated depleted refractory mantle.
High
3
He/
4
He ratios are due to low
4
He, not
excess
3
He, and do not imply or require a deep
or primordial or undegassed reservoir.
40
Ar in
the atmosphere also argues against such models.
[www.pnas.org/cgi/content/abstract/95/
9/4822]
3
He/
4
He ratios;
3
He/
4
He is used as a proxy
Sampling and recycling
A useful way to look at the noble gas data from
mantle samples is the following. There are man-
tle and recycled
components
with a large range
of helium isotopic ratios --- because of the dis-
tribution in ages and
3
He/U ratios --- rather than
two
reservoirs
with distinctive compositions. High
20/22, 'solar' neon and high
3
He/
4
He components
have several possible sources including IDP, cos-
mic rays and 'old' gas trapped in depleted U
for [
3
He].
The He/Ne ratios of MORB and OIB, in con-
trast to
3
He/
4
He, define two distinct fields, but
the implications are the opposite from the stan-
dard models. It is MORB that appears to have
been degassed and OIB that has inherited sec-
ondary gases from a degassed magma and from
the atmosphere.
A challenge for the two-reservoir model is to
explain the respective concentrations of helium
andotherraregasesinthereservoirs.IfOIBs
come from an undegassed source, they would
be expected to contain more helium than MORB
glasses from the degassed upper mantle. How-
ever, OIB glasses typically have ten times less
3
He than MORB. This has been explained as
degassing, but that cannot explain the higher
He/Ne in MORB than in OIB. This observation
is one of the
helium paradoxes
,andthere
are several. The dynamics of mantle melting and
melt segregation under ridges must be different
from under oceanic islands and seamounts. Ridge
magmas probably collect helium from a greater
volume of mantle and blend magmas from vari-
ous depths, during the melting and eruption pro-
cess. This would eliminate the extremes in MORB
basalts. Most workers favor the two-reservoir
model for the noble gases, although the evidence
is far from definitive, and the assumption of an
undegassed lower mantle causes more problems
that it solves.
Th
rocks, minerals and diamonds. Low 3/4 and high
21/22 materials are plausibly from high-U and
high-Th environments, and older environments.
Air and seawater contamination lower the 4/40,
20/22 and 21/22 ratios.
Several sources of high
R
have been suggested;
frozen in ancient values from normal mantle
processes, and subduction of IDP and other late
veneer material. IDP accumulate in ocean-floor
sediments and raise the Os, Ir, He and Ne con-
tents of the sediments (Figure 16.6). Cosmic dust
has very high
3
He/
4
He ratios and [
3
He] and some
of it falls to Earth without burning up in the
atmosphere. In this way ocean-floor sediments
develop a 'primordial' helium isotope signature,
with high
R
and high [
3
He].
Basalts degas as they rise and crystallize. If
this gas is trapped in olivine crystals, or cumula-
tes or the depleted lithosphere, away from U and
Th, it will have, in time, a high
R
compared with
the parent magma and LIL-rich sources. This com-
ponent has high
R
and high [He].
+