Geoscience Reference
In-Depth Information
Table 17.2 Parameters adopted for uncon-
taminated ocean ridge basalts (1) and con-
taminant (2)
25
Depleted Mantle
100% melt
10% melt
0.1% melt
MORB
Hawaii
Ahaggar
melilites
Dreiser
Weiher
nephelinites
Kerguelen
Columbia River
Brazil
Snake River-Yellowstone
.05
20
15
.05
Enriched
10
Pure MORB
Contaminant
0.1
5
Parameter
(1)
(2)
St.
Helena
Primitive Mantle
Nd
=
0
0
Pb
0.08 ppm
2 ppm
0.2
U
0.0085 ppm
0.9 ppm
5
BCR-1
kimber lites
Lines of constant
mixing proportions
U/Pb
0.10
0.45
10
0.1
Enriched
Mantle
Iamproites
238 U/ 204 Pb
7.0
30.0
0.5
S.Nevada
0.2
0.5
15
Diopsides
206 Pb/ 204 Pb
Gaussberg
17.2
19.3, 21
Rb
0.15 ppm
28 ppm
0.701
0.705
0.710
0.715
0.720
87 Sr/ 86 Sr
Sr
50 ppm
350 ppm
87 Sr/ 86 Sr
0.7020
0.7060
Fig. 17.2 ε Nd versus 87 Sr/ 86 Sr for mixtures involving a
depleted magma or residual fluids from such a magma after
crystal fractionation, and an enriched component (EM). Plots
such as this are known as mantle arrays or multiple isotope plots .
Sm
2 ppm
7.2 ppm
Nd
5 ppm
30 ppm
143 Nd/ 144 Nd
0.5134
0.5124
Enrichment Factors
Pb
24.6
U/Pb
4.5
primitive reservoir simply because the isotopic
ratios appear primitive. Similarly, magmas with
ε Nd near 0 can result from mixtures of melts,
with positive and negative
Sr
7.0
Rb/Sr
26.7
Nd
6.0
Sm/Nd
0.60
ε Nd . Early claims of
evidence for a primitive unfractionated reser-
voir based on Nd isotopes overlooked these
effects. They also ignored the Pb-isotope evidence.
Figure 17.2 shows isotope correlation for a vari-
ety of materials and theoretical mixing curves
for fractionating melts.
(1)
Assumed
composition
of
uncontami-
nated midocean-ridge basalts.
(2) Assumed composition of contaminant.
This is usually near the extreme end of the
range of oceanic-island basalts.
Ratio
of
concentration
in
two
end-
members.
The lead paradox
There are a large number of paradoxes involving
U and its products (He, Pb and heat). Paradoxes
are not intrinsic to data; they exist in relation to
a model or a paradigm.
Both uranium and lead are incompatible ele-
ments in silicates but uranium enters the melt
more readily than lead. The U/Pb ratio should
therefore be high in melts and low in the solid
residue, relative to the starting material. One
would expect, therefore, that the MORB reservoir
should be depleted in U/Pb as well as Rb/Sr and
Nd/Sm. A time-average depletion would give Pb-
isotope ratios that fall to the left of the primary
geochron and below the mantle growth curve.
Figure 17.1 shows, however, that both MORB and
ocean-island tholeiites appear enriched relative
to the primary growth curve. This implies that
MORB has been contaminated by high-uranium
or high-U/Pb material before being sampled, or
that lead has been lost from the MORB reservoir.
Early lead loss to the core, in sulfides, is possible,
but the isotopic results, if interpreted in terms
of lead removal, also require lead extraction over
an extended period.
Contamination may have affected MORB or
MORB source rocks. In order to test if contami-
nation or magma mixing is a viable explanation
for the location of the field of MORB on lead-
lead isotopic diagrams, we need to estimate the
lead content of uncontaminated depleted mag-
mas and the lead and lead-isotopic ratios of possi-
ble contaminants. Table 17.2 lists some plausible
parameters.
The results of mixing calculations are shown
in Figure 17.3. The differences between the lead
and other systems is striking. A small amount
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