Geoscience Reference
In-Depth Information
(a)
0.516
0.514
0.512
Mantle
0.510
0.508
Crust
0.506
0.504
0
2000
4000
Age (Ma)
(b)
[+8]
[+6]
MAGMATIC
ARC
INTRAPLATE
OCEANIC
ISLANDS
[0]
[+10]
MID-OCEAN
RIDGE BASALT
CONTINENTAL
FLOOD
BASALT
[--15]
CONTINENTAL
CRUST
UPPER MANTLE
[+12]
[+12]
[0]
600--700 km
LOWER MANTLE
CORE
Figure 6.7.
(a) The isotopic evolution of neodymium for the CHUR model for the
primitive mantle. The straight-line CHUR is defined by Eq. (6.51). If the CHUR
undergoes partial melting at, say, 3400 Ma, the melt fraction (solid line) has a lower
Sm/Nd ratio (and hence a lower
143
Nd
/
144
Nd ratio) than that of the CHUR, whereas
the solid residue (dashed line) has a higher Sm/Nd ratio (and hence a higher
143
Nd
/
144
Nd ratio) than that of the CHUR. Thus, the residual mantle plots above the
CHUR (positive epsilon) as it evolves, whereas the crust plots below (negative
epsilon). (b) A model of the Earth based on neodymium isotopic data. The present
ε
NA
value of the continental crust is about
−
15, and the value for the upper mantle is
+
12. The lower mantle may be chondritic (i.e.,
ε
Nd
=
0 there). (From DePaolo (1981).
The model of the primitive mantle called the chondritic uniform reservoir
(CHUR) is used as a standard for mantle isotopic geochemistry. The CHUR is
described by Eq. (6.51), and, since the half-life of
147
Sm is so long, a plot of
143
Nd
144
Nd ratio against time is nearly a straight line (Fig. 6.7(a)). Initial
the
/
143
Nd
/
144
Nd ratios are often quoted as deviations from the CHUR model, where
