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Percent crystal fractionation
Lines of constant
mixing proportions
5 10% EM
99.9
99 95
90
50
0
25
0.6
1% EM
20%
20
Depleted
Mantle
99.9%
15
5% EM
alkalics
Hawaii
Iceland
Columbia River
ALKALICS
Hawaii
99%
MORB
0.5
10
Iceland
95%
1%
5
Bouvet
Siberia
Enriched
Mantle
(EM)
0%
MORB
Crystallized
0
Kerguelen
BCR-1
Brazil
MIXING LINES
THOLEIITES
Hawaiian Basalts
0.4
5
5% EM
kimberlites,
lamproites
Enriched Mantle (EM)
10
15
0.3
Midocean Ridge Basalts
Depleted Mantle (DM)
20
0
0.1
0.2
0.3
0.4
0.5
Sm
/
Nd
0.702
0.710
0.720
Fig. 17.6 143 Nd/ 144 Nd (relative to primitive mantle) versus
Sm/Nd for mixtures of enriched mantle (EM) and residual
melts resulting from high-pressure crystal fractionation of
MORB-like depleted magma.
87 Sr / 86 Sr
Fig. 17.5 La/Ce versus Sr-isotopes for the fractionation--
contamination model, compared with Hawaiian basalts.
Unfractionated MORB (DM) has La / Ce = 0 . 265. La/Ce of
the depleted end-member increases as crystal fractionation
proceeds. The enriched end-member (EM) has La / Ce = 0 . 5,
in the range of kimberlitic magmas. Hawaiian tholeiites can be
modeled as mixes ranging from pure MORB plus 2--7% of an
enriched component to melts representing residuals after
95% clinopyroxene-plus-garnet crystal fractionation and 5--8%
enriched component. Alkali basalts involve more crystal
fractionation, or smaller-degree partial melts, and more
contamination. In this and subsequent figures solid curves are
mixing lines between EM and melts representing fractionating
depleted magmas. Dashed curves are trajectories of constant
mixing proportions. Note that some basalts are not
intermediate in La/Ce to the end-members. Similar
'discrepencies' in other geochemical ratios are often called
paradoxes .
Neodymium isotopes versus 87 Sr/ 86 Sr
Isotopic ratios for ocean-island and continen-
tal basalts are compared with mixing curves
(Figure 17.2). These basalts can be interpreted as
mixes between a fractionating depleted magma
and an enriched component. The value for
primitive mantle is also shown. The primitive
mantle value of 87 Sr/ 86 Sr is unknown and cannot
be
inferred
from
basalts
that
are
themselves
mixtures.
Neodymium isotopes versus Sm/Nd
The mixing--fractionation curves for Nd isotopes
versus Sm/Nd are shown in Figure 17.6. High-
Sm/Nd basalts from Iceland, Hawaii, Siberia,
Kerguelen, and Brazil all fall near the curve for
unfractionated MORB with slight, 1--5%, contam-
ination. Alkalics from large oceanic islands with
thick crust (Hawaii, Iceland and Kerguelen) are
consistent with large amounts of crystal frac-
tionation and moderate (5--10%) amounts of con-
tamination. The interpretation is that the more
voluminous tholeiites are slightly fractionated
and contaminated, while the alkalics have expe-
rienced sublithospheric crystal fractionation and
contamination or magma mixing prior to erup-
tion. MORB itself has about 1% contamination,
similar to that required to explain lead isotopes.
and equilibrium, or batch, crystallization are
the same. Therefore, large degrees of crystalliza-
tion of a MORB-like melt or small amounts of
partial melting of a depleted source are implied
by an inverse relationship between La/Ce (or
Rb/Sr, La/Yb, Nd/Sm, and so on) and 87 Sr/ 86 Sr
(or 143 Nd/ 144 Nd) such as observed at many mid-
plate environments. The apparently contradic-
tory behavior of magmas with evidence for
current enrichment and long-term depletion
is often used as evidence for 'recent mantle
metasomatism.' Figure 17.5 illustrates an alterna-
tive explanation. Note that, with the parameters
chosen, Hawaiian alkalics have up to 10% con-
tamination by an enriched component.
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