Geoscience Reference
In-Depth Information
CRB
MORB
OIB
IAB
BAB
CRUST
DM
DM
EM
EM
EM
EM
DM
DM
LM (BARREN)
major elements and mineralogy, possibly the
result of crystallization and gravitational separa-
tion. Partial melting of primitive mantle followed
by crystallization and gravity separation gives
upper-mantle source regions that, at least ini-
tially, have LIL ratios, including Rb/Sr and Sm/Nd
ratios, similar to primitive mantle. Residual flu-
ids in a cooling Earth, or a cooling cumulate,
become more fractionated with time.
Partial melting of the mantle during accre-
tion, melt separation, crystal fractionation and
formation
Fig. 25.4
Possible locations of geochemical components
(DM, depleted mantle; EM, enriched mantle; LM, lower
mantle; CRB, continental rift basalts; MORB, midocean-
ridge basalts; OIB, oceanic-island basalts; IAB, island-arc
basalts; BAB, back-arc basalts). In this model EM is
heterogeneous and probably not continuous. It is isotopically
heterogeneous because it has been enriched at various times.
LM does not participate in plate tectonic or hotspot
volcanism.
primitive-mantle growth curve (Figure 25.6). The
U/Pb ratio appears to behave similarly to the Rb/Sr
ratio unless sulfides are involved. For a simple
crystallization history of the depleting reservoir,
the fractionation factor of the melt increases
rapidly with time, for example as an exponential
or power law of time (Figure 25.5).
The distribution of oceanic ridges and
hotspots suggests that a large part of the upper
mantle is still above or close to the solidus.
The normalized Rb/Sr ratios of a melt and
residual crystals can be written where
of
cumulate
layers
is
one
model
+
1.5
+
1.0
+
0.5
Rb
Sr
−
1
F
+
(1
−
F
)
D
Sr
(Rb
/
Sr)
m
=
0
F
+
(1
−
F
)
D
Rb
Sm
Nd
−
=
(Rb
/
Sr)
res
(
D
Sr
/
D
Rb
)
=
f
m
+
1
1
−
0.5
the
D
are solid--melt partition coefficients and
F
is the melt fraction. As the cumulate freezes, con-
tinuously or episodically losing its fluids to the
overlying or underlying layer, it contains less of
a more enriched fluid. The net result is a nearly
constant
f
for the cumulate as it evolves. Most
of the fractionation that a crystallizing reservoir
experiences occurs upon the removal of the first
batch of melt.
Depleted reservoir become depleted by the
removal of fluids representing late-stage inter-
stitial fluids or small degrees of partial melting
(Figure 25.1). The enriched fluid is not necessar-
ily removed to the continental crust; it can also
enrich the uppermost mantle and lithosphere.
The enriched and depleted layers may differ in
1.0
−
0
20
40
60
80
100
Percent crystallized
Fig. 25.5
Variation of the normalized Rb/Sr and Sm/Nd
ratios in the melt fraction of a crystallizing eclogite cumulate.
Equilibrium crystallization is assumed. The fractionation
factor in the melt increases as freezing progresses. If melt
extracts from this layer are the enriching fluids for
upper-mantle metasomatism, then enrichment will increase
as crystallization proceeds. Extents of melting and
crystallization depend on depth and lateral locations as well
as on time. Aggregated melts from a melting regime will have
a variety of ratios. The crystalline residue is assumed to be
50:50 garnet: clinopyroxene.
