Geoscience Reference
In-Depth Information
components with differing He/U ratios, can
diverge
in situ
in their isotopic ratios, remov-
ing the requirement for ancient or large gas-rich
reservoirs. The component that carries the high
R
signature may be a peridotite, or gas bubbles in a
depleted olivine-rich cumulate. There are no con-
sistent global correlations between He-isotopes
and other isotopes. The helium in ocean island
basalts may be contained in recycled lithosphere
or may be acquired during ascent through a U---
Th depleted cumulate [
large scale melting
and averaging SUMA
].
OIB
CUMULATES
4
He
3
U
MORB
2
Helium history
Because of radioactive decay and nucleogenic pro-
cesses the isotopic ratios of the noble gases in the
mantle change with time. If the helium content
of part of the mantle is very high, the isotopic
ratios change slowly with time. If
3
He/
4
He is also
high, it will remain high. This is the reasoning
behind the standard undegassed mantle hypothe-
sis of noble gas geochemistry. Fertile parts of the
mantle have high U and Th concentrations and
these generate large amounts of
4
He over time.
Today's MORB has much higher
3
He/
4
He ratios
than ancient MORB. Ancient MORB, ascending
and degassing, generated gases with high
3
He/
4
He
ratios and some of this may have been trapped in
the shallow mantle. The present upper mantle
contains materials that melted and degassed at
various times. The helium-heatflow paradox and
the amount of
40
Ar in the atmosphere suggests
that noble gases can be trapped in the mantle,
probably the upper mantle. Figure 16.3 illustrates
the various stages in the helium history of the
mantle, starting at the lower right.
1
10
30
3
He
4
He (
R
/
R
A
)
/
Fig. 16.3
The primordial mantle curve is the growth of
3
He/
4
He in a hypothetical primordial reservoir from Big Bang
values to present (4.5Gyr). This is what the lower mantle
should provide in the standard model. At various times,
including during accretion, magma is extracted from accreting
material; this evolves faster because the magma gets the U
and Th and gas is lost (and some is retained by residual
mantle). Details aside, the idea is that differentiation and
degassing changes the
3
He/(U,Th) ratio of the products and
field labelled MAGMAS reflects this partitioning, containing
higher U/He ratios than primordial mantle. The magmas lose
gas as they rise to shallow depths, decreasing the He/U ratio
and increasing the He/Ar and He/Ne ratios of the residual
magma. The gas ends up in cumulates and gas inclusions in
peridotites, giving them high He/U ratios and
3
He/
4
He ratios
of the magmas. Ancient
3
He/
4
He ratios get frozen in, in
whatever material the degassed gas finds itself, while the
residual degassed magma continues to evolve to low values.
The trajectories of He (and CO
2
) and of U (and Th) upon
melting and degassing events are shown schematically. Each
melting/degassing event gives this kind of fractionation. The
dashed lines are simply an indication that ancient
3
He/
4
He
ratios are frozen in as He is removed from U and Th and
stored in olivine-rich mantle, while magmas go elsewhere.
The MORB and OIB histograms are plotted at arbitrary
positions on the time axis; they represent current values, and
involve mixtures of partially degassed magmas, previously
degassed gases and air, of a variety of ages. MORB is a cross
section across the magma field, today. The OIB field is a mix
of magmas and gases of various ages, in cumulates etc....and
is closer to the actual mantle distribution because it is
sampled in smaller batches than MORB. By convolving OIB
with itself we have a more MORB-like distribution. Note that
the low values of MORB do not have to evolve from high
mantle values or high OIB values.
Neon
AtmosphericNeisbelievedtohavebeensignif-
icantly depleted in the atmosphere by intense
solar irradiation. Ne
3-isotope plots
allow atmo-
spheric contamination of basalts to be monitored
for this isotope. On the other hand, the heavy
rare gases (argon, krypton and xenon) have accu-
mulated in the atmosphere over Earth history,
and air, or seawater, contamination is a serious
problem, even for mantle samples.
Neon isotope ratios for mantle rocks are typ-
ically displayed on a
3-isotope plot
of
20
Ne/
22
Ne vs.
