Geoscience Reference
In-Depth Information
Table 25.2
Composition of mantle, upper mantle, picrites and eclogites
Material
SiO
2
Al
2
O
3
FeO
MgO
CaO
TiO
2
Na
2
O
2
O
Mantle and Upper Mantle Compositions
1. Bulk mantle
48.0
5.2
7.9
34.3
4.2
0.27
0.33
2. Residual mantle
48.3
3.7
7.1
37.7
2.9
0.15
0.15
3. Pyrolite
45.1
3.3
8.0
38.1
3.1
0.2
0.4
Possible Picritic Parent Magmas
4. Eclogite extract
46.2
13.9
9.3
16.3
11.9
0.81
1.29
0.02
5. Oceanic crust
47.8
12.1
9.0
17.8
11.2
0.59
1.31
0.03
6. Tortuga dikes
47.3
13.6
9.8
17.6
9.6
0.79
0.89
0.06
7. High-MgO
46.2
12.6
11.0
16.6
10.5
0.69
1.18
0.02
tholeiites
46.3
13.0
11.3
15.5
10.9
0.71
1.26
0.03
Kimberlite Eclogites
8. Average
47.2
13.9
11.0
14.3
10.1
0.60
1.55
0.84
9. Roberts Victor
46.5
11.9
10.0
14.5
9.9
0.42
1.55
0.85
1. Bulk mantle composition (Ganapathy and Anders, 1974).
2. Residual after 20 percent extraction of primitive magma (line 1, Table 11.1).
3. This is an estimate of shallow mantle composition (Ringwood, 1975).
4. Possible eclogite extract from primary magma (O'Hara and others, 1975).
5. Average composition of oceanic crust (Elthon, 1979).
6. High magnesia Tortuga dike NT-23 (Elthon, 1979).
7. High magnesia-tholeiites.
8. Average bimineralic eclogite in kimberlite.
9. Eclogite, Roberts Victor 11061 (O'Hara and others, 1975).
mantle is more-or-less uniformly enriched in
these elements (Figure 25.1). As the magma
layer or magma ocean cools, cumulates, contain-
ing intercumulus fluids, form. Peridotitic cumu-
lates at shallower depths. Cumulate layers can
have near-primitive ratios of Rb/Sr and Sm/Nd
if they contain a moderate amount of intersti-
tial fluid. Transfer of late-stage melts (KREEP or
kimberlite) is one mechanism by which parts of
the mantle become depleted and other regions
enriched. For this type of model, the isotopic
ratios will be a function of the crystalliza-
tion (fractionation) history of the upper mantle
and the history of redistribution of LIL-enriched
fluids.
A mechanism for creating an LIL-depleted
but still fertile reservoir involves an early thick
basalt layer in the relatively cold surface thermal-
boundary layer. Small degrees of melt can be
removed and still leave the basalt fertile. As the
Earth cools this basalt layer converts to eclogite
and sinks into the mantle, creating a depleted
but fertile reservoir.
What is the fate of eclogite in the mantle?
Some models assume that it sinks to the
core--mantle boundary and is removed from the
system; others assume that it is in the transition
region or at the base of over-thickened crust.
Estimated densities as a function of depth for
eclogite and garnet peridotite are shown in
Figure 25.2. Some eclogites and garnetites (garnet
solid solutions) are denser than peridotite to
depths at least as great as 500 km. On the
other hand the post-spinel phases of olivine and
the perovskite form of orthopyroxene are denser
than garnetite or ilmenite eclogite. Eclogite-rich
cumulates, or subducted eclogitic lithosphere,
are therefore unlikely to sink below 650 km
unless they are very garnet-rich and very cold.
Whether eclogite can sink below 500 km depends
on composition, temperature and the com-
pressibility and thermal expansivity relative to
