Geoscience Reference
In-Depth Information
+
15
Fig. 25.7
Neodymium and
strontium fractionation trends for
an eclogite cumulate (solid line) that
has been depleted at various times
by removal of a melt fraction. The
dashed line is the complementary
reservoir that has been enriched at
various times by the melt extract.
Reservoirs are unfractionated until
the enrichment/depletion event and
uniform thereafter. If present values
of enrichment/depletion have been
reached gradually over time or if the
magmas are mixtures, then ages
shown are lower bounds.
Eclogite cumulate
minus 5% late melt
Brazil
Hawaii
Mantle diopsides
MORB
+
10
ICELAND -
AZORES
+
5
Mt. Etna
Bouvet
Age of enrichment event (Ga)
0.5
1
2
3
0
2
1
Age of depletion event (Ga)
Kerguelen
Tristan da Cunha
−
5
Brazil
−
10
ENRICHED
RESERVOIRS
−
40
−
30
−
20
−
10
0
+
10
+
20
+
30
+
40
Sr
mixing relations are such that mixtures can be
enriched in U/Pb, Rb/Sr, Nd/Sm or
206
Pb/
204
Pb rel-
ative to primitive mantle, yet appear to have
time-integrated depletions in
143
Nd/
144
Nd and
87
Sr/
86
Sr. A small amount of contamination by
an enriched component or material from an
enriched reservoir can explain the lead results
for MORB (the 'lead paradox'). Depleted basalts
are more sensitive to lead than to rubidium or
neodymium contamination. Similarly, continen-
tal and oceanic-island basalts may represent mix-
tures of enriched and depleted magmas, or small-
and large-degree melts.
Figure 25.7 shows the correlation in differ-
entiation for MORB, oceanic islands, and some
continental basalts and mantle diopsides. The
depletion and enrichment ages are calculated
for a simple two-stage model for the develop-
ment of the enriched and depleted reservoirs
(Figure 25.8). The Rb/Sr and Sm/Nd ratios are
assumed to be unfractionated up to the age
shown and then fractionated to values appro-
priate for the depleted and enriched reservoirs.
Subsequent isotopic evolution occurs in these
fractionated reservoirs.
For this kind of model the MORB reservoirs
were apparently depleted and isolated at times
ranging from 1.5--2.5 Ga, and the enriched reser-
voirs (giving magmas in the lower-right quadrant)
were enriched between 0.5 and 1.8 Ga. If the
enrichment has been progressive, the start of
enrichment could have been much earlier. The
data shown in Figure 25.7 may be interpreted in
terms of mixtures of magmas from depleted and
enriched reservoirs. In fact, the compositions of
alkali olivine basalts, basanites and continental
tholefites are bracketed by MORB and potassium-
rich magmas such as nephelinites for most of
the major and minor elements as well as for the
isotopes. This supports the possibility that many
continental and oceanic-island basalt types are
mixtures.
The isotopic ratios of end-member MORB are
greatly affected by small degrees of contamina-
tion, contamination that is probably unavoidable
if MORB rises through, or evolves in, lithosphere
or enriched upper mantle. Basalts at anomalous
ridge segments show clear signs of contamina-
tion, as in T- and P-MORB (transitional and plume-
type MORB); normal MORB may simply show less
obvious signs of contamination. Magmas contain-
ing 10--20% contaminant will still appear isotopi-
cally depleted for Nd and Sr. Such mixtures will
appear to exhibit long-term enrichment in the
lead isotopic systems. Clearly, the mixing idea
can be extended to multiple components, and to
systems that are cooling and fractionating as they
mix.
