Geoscience Reference
In-Depth Information
ratio of the Solar System is known, typically from meteorite work. The mean
147
Sm
144
Nd
/
143
Nd
144
Nd
147
Sm
144
Nd
143
Nd
144
Nd
1
λ
147
Sm
t
0
≈
today
−
(4.34)
initial
Let us first deduce the time-integrated
147
Sm
144
Nd ratio of the source of different sam-
/
ples from their
143
Nd
144
Nd values. Let us pick up a basalt from the East Pacific Rise
(0.5131) and a clay mud collected from the mouth of the Amazon (0.5108). The basalt
source is the upper mantle, whereas the clay is derived from erosion of South American
/
144
Nd
ratio of the upper mantle and that of the continental crust from the beginning of the Earth's
history. Of course, these are virtual ratios, as the crust and mantle have complex histories
that cannot be fully captured by a single parameter, but they give us useful information
about the nature of the fractionation processes that have affected these geological units.
Meteorite studies tell us that at the time the Solar System formed 4.56 billion years ago,
the
143
Nd
/
144
Nd ratio of the mantle in which
the neodymium evolved before passing into the basalt was therefore:
144
Nd ratio was 0.5067. The mean
147
Sm
/
/
147
Sm
144
Nd
mantle
=
0.5131
−
0.5067
10
9
)
=
0.215
(6.54
×
10
−
12
)
×
(4.56
×
For the continent that was the source of the clay, this calculation gives a
147
Sm
144
Nd ratio
/
of 0.138.
Similar calculations can be made for the
87
Rb-
87
Sr and U-Pb systems. Let us assume
that the
86
Sr ratio is 0.7023 for the basalt and 0.7140 for the clay, i.e. that this
increases when
143
Nd
87
Sr
/
144
Nd decreases. Using equations similar to those we just derived
for Nd and a chondritic initial
87
Sr
/
86
Sr ratio turns out to
be 0.046 for the upper-mantle and 0.22 for the continental-crust source of the clay. For
lead, the the linear approximation would have to be abandoned, which raises no serious
problem.
The Sm/Nd, Rb/Sr, and U/Pb ratios therefore differ between the continental crust and
the upper mantle, so that the more incompatible element of each pair (Nd, Rb, and U) is
more concentrated in the crust than the corresponding more compatible element (Sm, Sr,
and Pb). A process capable of fractionating these ratios as the continental crust forms
must therefore be imagined, for example, melting followed by selective extraction of
magmas.
By plotting on an isochron diagram the present-day
143
Nd
86
Sr ratio of 0.6992, the
87
Rb
/
/
144
Nd
ratios of any rock sample of continental crust and those of the average upper mantle (the
which this particular piece of continental crust could have separated from the upper man-
tle. This age, usually referred to as the Nd model age of this particular crustal sample,
is obtained by writing
(4.34)
once for the rock and once for the upper mantle, and by
eliminating the
143
Nd
144
Nd and
147
Sm
/
/
144
Nd
0
ratio between the two expressions:
/
