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and St Helena in the Atlantic and the Austral and
Ballemy Islands in the South Pacific, for example, all
plot in the 'St Helena-type' cluster; Pitcairn Island and
Tristan da Cunha plot with Ascension Island. Other
hotspots plot in a more general continuum in between.
(h) The wide spread of Indian Ocean MORB compos-
itions from the depleted quadrant into the enriched
quadrant is the clearest sign in Figure 10.8 of the
importance of
mixing
(either between mantle reser-
voirs themselves or between their partial melting
products).
the oscillations will be. The same applies to the
atoms in a molecule (Figure 9.7.1), where chemical
bonds take the place of the spring: introducing a
heavier isotope into a molecule of H
2
O, for example,
slows the thermal vibration (Chapter 1) that
stretches the O-H bond, and thereby lowers the
molecule's internal energy. The presence of a heav-
ier isotope - whether
2
H in
1
H
2
H
16
O or
18
O in
1
H
2
18
O -
subtly alters the water molecule's thermodynamic
and kinetic properties, and thereby affects its distr-
ibution between coexisting phases (between water
and vapour, for instance), potentially leading to
slight fractionation in the isotope ratio between the
coexisting phases.
Such
mass-dependent isotope fractionation
is insignif-
icant for heavier elements like Sr and Nd (at least in
natural equilibria)
8
because the relative differences in
mass number between the isotopes involved are min-
ute (little more than 1% in the case of
86
Sr and
87
Sr).
For lower-
A
elements like hydrogen, carbon and ox-
ygen, however, the mass difference between isotopes
assumes greater significance: the relative mass differ-
ence between
16
O and
18
O, for instance, is (18 − 16)/
[0.5 × (18 + 16)] = 2/17 ≈ 12%. Using modern high-
precision mass spectrometry, measurable variations
in the isotopic composition of the low-
A
elements H,
C, N, O and S (Figure 10.9) can be observed between
various natural reservoirs, fractionations which pro-
vide important insights into the workings of a range of
Earth processes. This research field is known as
stable
isotope geochemistry
.
The continuum of basalt compositions in Figure 10.8
can be seen as the product of mixing together, in differ-
ent circumstances and in varying proportions, a number
of mantle reservoirs of contrasting composition: think of
each basalt composition as being derived from a hetero-
geneous mantle 'cake' prepared from a common set of
'ingredients' (reservoirs) that were not fully blended
before baking. Five reservoir compositions postulated in
the literature are shown by the star symbols in Figure 10.8.
Where are these supposed reservoirs located and
how have they formed? Answers to these questions
are unavoidably speculative. There is a broad consen-
sus equating the depleted MORB source with the duc-
tile, convecting asthenosphere, which wells up and
undergoes decompression melting beneath oceanic
spreading centres (Figure 2.5.1b) across the globe. The
uniformity of this reservoir reflects mixing associated
with convective circulation, and its depleted charac-
ter is thought to arise from the extraction of the cont-
inental crust - much earlier in Earth history - by
partial melting of originally homogeneous primitive
mantle.
Many OIB centres/hotspots are associated with
mantle plumes (Figure 2.5.1b) rising from - and tap-
ping mantle reservoirs located within - the lower man-
tle. Their diverse compositions are believed to reflect
'contamination' of these parts of the primordial mantle
by recycled materials such as subducted terrigenous
sediment or altered oceanic crust.
Notation
Variations in a stable isotope ratio (e.g.
18
O/
16
O) seen in
Nature mostly amount to only a few parts in a thous-
and. To optimize analytical precision and minimize
inter-laboratory variations, the standard analytical
practice is to alternate repeated measurements on an
unknown sample with measurements on a universally
available standard of known isotopic composition/
ratio. The isotope ratio measurement for each sample
Stable isotope systems
Sr and Nd isotopes are, however, susceptible to mass fraction-
ation in certain artificial circumstances, such as in evapora-
tion from a hot filament during thermal ionization mass
spectrometry (for which a correction has to be applied).
8
A body suspended from a spring oscillates up and
down with a natural frequency that depends upon
the body's mass: the heavier the body, the slower
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