Geoscience Reference
In-Depth Information
Chapter 16
Noble gas isotopes
3
He is not here, for
3
He is risen!
high concentrations of He (written [He]), high
3
He/
4
He ratios (written R) and 'solar' neon are
found. Mantle-derived basalts and xenoliths hav-
ing similar
primordial
or
solar
characteristics are
generally attributed to plumes from the
unde-
gassed primitive
lower mantle reservoir. This is
based on the assumption that high
3
He/
4
He and
36
Ar/
40
Ar ratios in basalts require high
3
He and
36
Ar contents in their sources, rather than low
4
He and
40
Ar, and that the whole upper mantle is
characterized by low ratios and low abundances.
The critical issues are when and how the 'pri-
mordial' noble gases entered the Earth, where
they are stored and whether transport is always
outwards and immediate upon melting, as often
assumed. It is therefore important to understand
the noble-gas budget of the Earth, both as the
Earth formed and as it aged. The major part of
terrestrial formation probably occurred at high
temperature from dry, volatile-poor, degassed
particles. During the early stages of planetary for-
mation, core differentiation and massive bolide
impacts heated the Earth to temperatures that
may have been high enough for a substantial part
of it to have melted and vaporized. This is sug-
gested by the fact that a large fraction (30---60%)
of the incompatible elements (e.g. U, Th, Ba, K,
Rb) are in the Earth's crust, which is consistent
with an extensively differentiated mantle.
Some of the components now in the man-
tle fell to Earth in a
late veneer
, mostly around
3.8 Ga --- the period of late bombardment --- that
added material to the Earth after completion of
core formation. Late low-energy accretion prob-
ably introduced most of the volatiles and trace
Anon.
Overview
The group of elements known as the rare, inert
or noble gases possess unique properties that
make them important as geodynamic tracers. The
daughter isotopes fractionate readily from their
parents, they are inert, and they differ from other
geochemical tracers in being gases and diffus-
ing relatively rapidly, at least at low pressure
and high temperature. Nevertheless, they can
be trapped in crystals for long periods of time.
They give information about the degassing his-
tory of the mantle and magmas, the formation
of the atmosphere, and about mixing relation-
ships between different mantle components. Iso-
topes made during the Big Bang are called the
'primordial' isotopes. Some of the noble gas iso-
topes are cosmogenic, nucleogenic or radiogenic
and these were made at later times. The pres-
ence of
3
He in a rock or magma has often been
taken as evidence for the existence of a
primordial
reservoir
in the mantle, one that had not previ-
ously been melted or degassed. A
primordial reser-
voir
is different from a
primordial component
.Pri-
mordial and solar components are still raining
down on Earth. The most primordial materials
on Earth, in terms of noble gases, are on moun-
tain tops, in the stratosphere and in deep-ocean
sediments.
Evidence for recent additions of noble gases to
the Earth comes from deep-sea sediments where
