Geoscience Reference
In-Depth Information
seawater from which it precipitates; as the mantle melts,
143
Nd
144
Nd is the same in the
molten liquid as in the residue. There are, admittedly, processes where melting occurs in
disequilibrium, but even if thermodynamic fractionation persisted at such high tempera-
tures it would be corrected out by the internal standardization procedure (see box). In the
rest of the discussion, we will therefore ignore mass-dependent fractionation.
/
Why radiogenic tracers are insensitive to phase changes and normalization values
The question often arises of how “are we so sure that these tracers are not upset by mag-
matic, metamorphic, or sedimentary processes just as oxygen and carbon isotopes are?”
A common answer, albeit quite incorrect, is that the atomic weights of these tracers are
heavy enough to make such fractionation negligible. Fractionation of the stable isotopes of
strontium (A
200) are well established, both in nature and during
mass-spectrometric analysis. The correct answer is that stable isotope fractionation is care-
fully separated from the fractionation induced by radiogenic decay by internal normalization
to a reference isotope ratio. To measure isotopic ratios of interest for radiogenic isotopes, e.g.
the
87
Sr/
86
Sr ratio, both isotopic instrument bias and natural thermodynamic and kinetic frac-
tionation are eliminated by standardization to an arbitrary stable isotope ratio (
Fig. 4.2
)using
the theory developed for stable isotopes. For strontium, the universal choice is
88
Sr/
86
Sr
≈
88) and mercury (
≈
=
be inferred from the measurement of this ratio:
88
Sr
86
Sr
8.3752
1
2
f
meas
=
+
(4.8)
and the resulting value of
f
introduced into the similar expression for the
87
Sr/
86
Sr ratio:
87
Sr
86
Sr
87
Sr
86
Sr
1
1
f
normalized
=
+
(4.9)
meas
⎡
⎣
⎤
⎦
88
Sr
86
Sr
meas
−
87
Sr
86
Sr
/
8.3752
1
2
=
meas
×
1
+
(4.10)
8.3752
This completes the internal isotopic normalization which removes any isotope fractionation
that is not due to radioactive decay. Note the values
1 for the mass differences
between the masses at the numerator and denominator of the isotopic ratios.
For technical and historical reasons, the standardization of isotopic ratios of certain ele-
ments such as neodymium may refer to different isotopic values: the
143
Nd/
144
Nd ratios
taken from the literature must therefore be compared with utmost care. We saw in the pre-
vious chapter that with the delta notation, the problem of bias calibration among different
laboratories or using different reference ratios disappears as soon as the isotope composi-
tions are taken with respect to a common reference sample. For neodymium and hafnium,
it is common practice to compare the sample isotopic compositions to that of the mean of
+
2and
+
