Geoscience Reference
In-Depth Information
Table 4.2
Solid/melt fractionation of the parent and daughter nuclides of common systems
and its effect on the long-term evolution of the isotope composition of the daughter element
Parent/daughter ratio
∗
Daughter element
∗
Parent-Daughter
In melt
In residue
In melt
In residue
87
Rb-
87
Sr
Higher
Lower
More radiogenic
Less radiogenic
147
Sm-
143
Nd
Lower
Higher
Less radiogenic
More radiogenic
176
Lu-
176
Hf
Lower
Higher
Less radiogenic
More radiogenic
187
Re-
187
Os
Higher
Lower
More radiogenic
Less radiogenic
232
Th-
208
Pb
Higher
Lower
More radiogenic
Less radiogenic
238
U-
206
Pb
Higher
Lower
More radiogenic
Less radiogenic
∗
With respect to source material
143
Nd
144
Nd
rock
−
143
Nd
144
Nd
mantle
/
/
1
λ
147
Sm
×
T
Nd
=
147
Sm
144
Nd
rock
−
147
Sm
144
Nd
mantle
(4.35)
/
/
If our Amazon mud has a
147
Sm
144
Nd ratio of 0.120, a value that is quite representative
of the continental crust in general, the model age for local separation of the crust and the
upper mantle is therefore:
/
1
0.5108
−
0.5131
10
9
years
T
Nd
=
10
−
12
×
0.215
=
3.7
×
0.654
×
0.120
−
Radiogenic isotopes are most commonly utilized, then, for tracing sources, which we
will examine in detail later. The principle is that when rock melts, the parent/daughter
ratios are affected in a way that depends on the residual mineral assemblage, while the
isotopic ratios remain unaffected. Thus, for mantle melting, Sm/Nd and Lu/Hf ratios are
higher in the melt residue than in the liquid, while the contrary is true of Rb/Sr, Re/Os, and
U-Th/Pb (
Table 4.2
).
4.5 Helium isotopes
Radiogenic helium
4
He is produced by the decay of the two natural isotopes of uranium
(
238
U and
235
U) and of the only long-lived isotope of thorium (
232
Th). We have seen earlier
that these decay processes are in fact the start of a chain of events. For example, in order
to pass from
238
U to its distant descendant
206
Pb, the initial nuclide must lose (238-206)/
4
=
8
α
particles, which, by capture of the rock matrix electrons torn off during particle
