Geoscience Reference
In-Depth Information
TECTONIC
SHIELD
RIDGE
ISLAND
ARC
CRUST
HARZBURGITE
>
90% o
+
opx
+
o
PERIDOTITE
PERIDOTITE
>
70% o
+
opx
gt
+
cpx
+
o
+
o
+
mj
Solidus
PICLOGITE
gt
+
cpx
+ b
<50 %
o
gt s.s.
+ g
pv
+
mw
PEROVSKITITE
<40 %
o
Picritic melts
Tholeiitic
Eclogite Transition
Piclogite Solidus
"
cpx
gt
o
AOB
+
+
"
thickens and cools, it becomes denser at its
base than the underlying mantle and a poten-
tial instability develops. If the crust in island
arcs, batholiths or compressional mountain belts
gets thicker than about 50 km it is prone to
delamination. Similarly, if the temperature in
a deep garnet-rich layer or blob exceeds the
solidus, the density may become less than the
overlying layer. The large density changes asso-
ciated with partial melting, delamination and
the basalt--eclogite phase change may be more
important in driving mantle convection than
thermal expansion. In the deep mantle, chemi-
cal variations are more important than tempera-
ture variations in controlling density and density
contrasts.
Fig. 25.3 A possible configuration of the major rock types
in the mantle. Peridotite accumulates in the shallow mantle
because of its low density, particularly residual or infertile
peridotite. Fertile peridotite, eclogite, olivine eclogite and
piclogite are denser and are, on average, deeper. Partial
melting, however, can reverse the density contrast.
Upwellings from fertile layers are shown under ridges, islands
and tectonic continents; some are passive, some are
entrained and some are caused by melt-induced bouyancy.
The continental lithosphere is stable and is dominantly
infertile harzburgite. It may have been enriched by upward
migration of fluids and melts. The shallow mantle is
predominantly infertile peridotite (stippled) with blobs of
fertile eclogite. Deep fertile layers or blobs can rise into the
shallow mantle if they become hot, partially molten or
entrained. Melts can underplate the lithosphere, giving a
dense, eclogite-rich base to the oceanic lithosphere, which
can recycle into the transition region. The top parts of the
subducted slab rejuvenate the shallow mantle with LIL. Some
slabs are confined to the upper mantle, being too buoyant to
sink into the lower mantle. Lower continental crust can also
be an eclogite cumulate.
Enriching fluids
The composition of a residual fluid in equilib-
rium with eclogite, as a function of crystalliza-
tion, is shown in Figures 25.5 and 25.6. The frac-
tionations increase rapidly as the residual melt
fraction drops below about 20% (that is, above
80% crystallization).
Kimberlites may represent such late-stage or
residual fluids. They appear to have been in
equilibrium with eclogite and often contain
eclogite xenoliths. They also have LIL patterns
that are complementary to MORB. Isotopic evo-
lution in regions enriched by expelled small-
degree melts will deviate significantly from the
are such that they cannot be reversed by ther-
mal expansion and the kinds of temperature
differences normally encountered in mantle con-
vection. However, phase changes such as par-
tial melting and basalt--eclogite involve large
density changes. A picritic or pyroxenitic crust
or lithosphere, for example, will be less dense
than fertile peridotite at depths shallower than
about 50 km where it is in the plagioclase or
spinel stability field. As the lithosphere or crust
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