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against the corresponding
147
Sm/
144
Nd ratios, we can
generate a Sm-Nd isochron from which the age of the
rock suite can be determined (see Exercise 10.2 for an
example). The equation for this isochron is:
therefore acquire
lower
parent/daughter (
147
Sm/
143
Nd)
ratios than the source rock, and this is the reason why
143
Nd/
144
Nd grows more
slowly
in the rocks derived
from them.
It follows from Figure 10.6c that continental crust is
characterized by low Sm/Nd and therefore by Nd
that is
less radiogenic
than primitive mantle, whereas
depleted mantle Nd is
more radiogenic
than primitive
mantle.
143
=
143
+
147
Nd
Nd
Nd
Nd
Sm
Nd
(
1
e
λ
t
−
(10.6)
Sm
144
144
144
t
0
t
The value of the decay constant
λ
Sm
is given in Table 10.1.
Because REEs like Sm and Nd are less mobile than Rb
and Sr under hydrothermal and low-grade metamor-
phic conditions, Sm-Nd dating is invaluable for dating
the eruption ages for moderately altered samples. The
longer half-life (lower
λ
- Table 10.1) of
147
Sm makes
Sm-Nd most suited to the dating of Precambrian rocks
(see Exercise 10.2).
Figure 10.6c shows how
143
Nd/
144
Nd evolves in the
scenario outlined above for Rb-Sr. By analogy with
Equation 10.5, the
143
Nd/
144
Nd growth equation may
be written:
Sr and Nd isotope signatures of young oceanic
volcanics - mapping geochemical reservoirs
in the mantle
Is the Earth's mantle homogeneous in composition, or
has it become segregated over geological time into
regions having distinct geochemical fingerprints?
Isotopic analysis of young basalts from the ocean
basins (where contamination by continental crust
cannot confuse the picture) offers a means to probe
present-day heterogeneity in the Earth's mantle.
Basalts inherit the radiogenic isotope ratios of their
source, and if the mantle feeding recent oceanic basalt
eruptions
7
were homogeneous in its content of radio-
active and radiogenic nuclides like
87
Rb and
87
Sr, we
would expect such basalts to cluster around a single
87
Sr/
86
Sr-
143
Nd/
144
Nd composition, equivalent to
pm
in
Figure 10.6. A glance at the global compilation of such
data in Figure 10.8 makes clear this is not the case.
Each data point in Figure 10.8 represents a basalt
analysis, and collectively they define not a single clus-
ter but an elongated array of basalt compositions, a
broad correlation between
143
Nd/
144
Nd and
87
Sr/
86
Sr
sometimes referred to as the 'mantle array'. Because
isotope ratios of heavy elements like Sr cannot be
altered by partial melting, the array in Figure 10.8
suggests that:
143
≈
143
+
147
Nd
Nd
Nd
Nd
Sm
Nd
λ
t
(10.7)
Sm
144
144
144
t
0
The first difference to note in Figure 10.6c is the greater
expansion of the
y
-axis compared with Figure 10.6a: in
both figures each division represents 1 in the third sig-
nificant figure. Different scaling is necessitated by the
fact that Sm and Nd are LREEs of very similar chemistry
and ionic radius, so they have closer incompatibility
and are fractionated (relative to each other) to a smaller
degree in melting and crystallization than is the case
for Rb and Sr. The need to measure smaller changes in
143
Nd/
144
Nd ratio also requires very precise mass spec-
trometric measurement.
Although
143
Nd/
144
Nd rises with time in a similar
fashion to
87
Sr/
86
Sr, aspects of Figure 10.6c appear
'upside down' relative to Figure 10.6a. When mantle
melting occurs at
n
, the
143
Nd/
144
Nd ratio of the crust
formed evolves at a
lower
gradient than the mantle,
while the depleted mantle left behind evolves with a
steeper
gradient, contrary to what we see with Rb-Sr.
The same differences arise when the crust itself under-
goes partial melting at
p
. The reason is that the parent
nuclide
147
Sm is
less
incompatible than the daughter
element Nd (whereas parent
87
Rb is
more
incompatible
than daughter
87
Sr). The melts formed at
n
and
p
(a) the mantle regions from which oceanic basalt mag-
mas are sourced must vary significantly in their
Rb/Sr and Sm/Nd ratios (to account for the
observed differences in
143
Nd/
144
Nd and
87
Sr/
86
Sr
as in Figure 10.6); and
(b) this chemical heterogeneity in the mantle must
have evolved long ago in Earth history (illustrated
by point
n
in Figures 10.6a and c), to allow sufficient
All terrestrial basalts are the product of the partial melting of
mantle peridotite.
7
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