Geoscience Reference
In-Depth Information
paradox has a parallel in the carbon budget.
Carbon is depleted by about an order of magni-
tude in the exosphere, compared to other volatile
elements. On the other hand, the presence of dia-
monds and carbonatites in the mantle, and CO
2
in magmas, shows that the mantle is a long-time
repository for CO
2
and probably helium. The pres-
ence of
3
He in mantle magmas shows the same
thing.
3
He is a primordial isotope in the sense
that it is not created in substantial quantities by
reactions in the Earth (although some is brought
in or generated by cosmic particles). Some
3
He
may have been brought into the Earth by a late
veneer but in any case helium has been trapped
inthemantleforalongperiodoftime.The
most efficient location for bringing magmas to
the near-surface and degassing is along the global
spreading ridge system. Even in these locations
the presence of magma chambers and off-axis vol-
canoes suggest that gas exsolved from magmas
at low-pressure may be trapped in the shallow
mantle. However, gas can separate from parent
magma, which separates the gas from U and Th.
Helium trapped in olivine crystals in cumulates
or melt-depleted peridotite will therefore retain
its isotopic composition. Ridges migrate readily
over the mantle; in fact, ridge migration may
be essential to continuous magmatism. It takes
about 1 to 2 Gyr for ridges, at their present migra-
tion rates, to visit each part of the mantle. At
these times, trapped gas has another chance to
reach the surface. But the
3
He/
4
He ratios of some
of these gases will be more appropriate for man-
tle 1 to 2 Gyr older. Gases of various trapped ages
may co-mingle, especially in the ridge environ-
ment, but in other environments, e.g. seamounts,
oceanic islands, a diversity of components of dif-
ferent ages may be evident, including extreme
values which would be averaged out at ridges.
It is interesting, and instructive, that products
of U and Th decay (heat, lead and helium) have
their names attached to so many
geochemical
paradoxes and enigmas
.Thisisastrongsig-
nal that current geochemical models are inade-
quate. Nucleogenic neon is also a product of U
and He/Ne ratios are not completely understood.
It does not seem possible to use the
4
He budget
to usefully constrain the U and Th abundances in
the mantle. On the other hand, trapping of He
and CO
2
in the upper mantle may explain the
low outgassing rates of He and the low crustal
abundances of CO
2
. It would be interesting to
know if the mantle is currently a sink for CO
2
(subduction fluxes exceeding volcanic fluxes) as
is occasionally reported. If so, the shallow man-
tle may well be the repository of the missing CO
2
and
4
He and a storage vessel for
3
He as well. Some
midocean ridge basalts have large concentrations
of
3
He; the accompanying CO
2
causes the rocks
to explode or pop when they are removed from
depth by dredging.
Heat from the core
The existence of a geomagnetic field and a solid
inner core place constraints on the Earth's ther-
mal history. The solid inner core may be essen-
tial for the nature of the current field includ-
ing reversals. Any model of the Earth's evolution
must involve sufficient heat loss from the core to
power the dynamo, but not so much as to freeze
the core too quickly. These constraints are sur-
prisingly strong. The inner core probably grew
with time and may have started to freeze about
2 Ga. The magnetic field existed earlier than this
so freezing of the inner core -- and compositional
stirring of the outer core -- may not have been
involved in the generation of the early magnetic
field. Cooling from a high initial temperature
(superheat) may have driven the early dynamo.
Whether this could have been maintained for
∼
2 Gyr is a matter of debate. Convection in the
core is highly turbulent and the thermal gradi-
ent in the outer core is probably adiabatic. Since
the core is an excellent thermal conductor it can
conduct heat readily down this thermal gradient.
This places a constraint on the minimum amount
of heat that enters the mantle from below. This
minimum is 8.6 TW. Contraction due to cool-
ing and freezing adds a little to this. If the core
is still growing by interactions with the man-
tle there will be additional gravitational energy
terms. Radioactivity, secular cooling and thermal
contraction of the core must be minor compared
to the equivalent energy sources in the mantle.
This minor heat source, however, is the source of
heating in the plume hypothesis.
