Geoscience Reference
In-Depth Information
was generated at the midocean ridge systems. The
pillow basalts that constitute the upper part of
the oceanic crust extend to an average depth of
1--2 km and are underlain by sheeted dikes, gab-
bros and olivine-rich cumulate layers. The aver-
age composition of the oceanic crust is much
more MgO-rich and lower in CaO and Al
2
O
3
than
the surface MORB. The oceanic crust rests on a
depleted harzburgite layer of unknown thickness.
In certain models of crustal genesis, the harzbur-
gite layer is complementary to the crust and is
therefore about 24 km thick if an average crustal
thickness of 6 km and 20% melting are assumed.
Oceanic plateaus and aseismic ridges have thick
crust (
Table 8.5
Estimates of the chemical compo-
sition of the crust (wt.%)
Oceanic
Continental Crust
Crust
Oxide
(1)
(2)
(3)
SiO
2
47.8
63.3
58.0
TiO
2
0.59
0.6
0.8
Al
2
O
3
12.1
16.0
18.0
Fe
2
O
3
—
1.5
—
FeO
9.0
3.5
7.5
MgO
17.8
2.2
3.5
CaO
11.2
4.1
7.5
Na
2
O
1.31
3.7
3.5
20 km in places), and the corresponding
depleted layer would be more than 120 km thick
if the simple model is taken at face value. Since
depleted peridotites, including harzburgites and
dunite, are less dense than fertile peridotite, or
eclogite, several cycles of plate tectonics, crust
generation and subduction would fill up the shal-
low mantle with harzburgite. Oceanic crust and
oceanic plateaus may in part be deposited on
ancient, not contemporaneous, buoyant under-
pinnings.
Estimates of the composition of the oceanic
and continental crust are given in Table 8.5;
another estimate that includes the trace
elements is given in Table 8.6. Note that the
continental crust is richer in SiO
2
,TiO
2
,A1
2
O
3
,
Na
2
OandK
2
O than the oceanic crust. This
means that the continental crust is richer in
quartz and feldspar and is intrinsically less
dense than the oceanic crust. The mantle under
stable continental-shield crust has seismic prop-
erties that suggest that it is less dense than
mantle elsewhere. The elevation of continents
is controlled primarily by the density and thick-
ness of the crust and the intrinsic density and
temperature of the underlying mantle.
It is commonly assumed that the seismic
Moho is also the petrological Moho, the bound-
ary between sialic or mafic crustal rocks and
ultramafic mantle rocks. However, partial melt-
ing, high pore pressure and serpentinization
can reduce the velocity of mantle rocks, and
increased abundances of olivine, garnet and
pyroxene can increase the velocity of crustal
rocks. High pressure also increases the velocity of
>
K
2
O
0.03
2.9
1.5
H
2
O
1.0
0.9
—
(1) Elthon (1979).
(2) Condie (1982).
(3) Tayor and McLennan (1985).
mafic rocks, by the gabbro--eclogite phase change,
to mantle-like values. The increase in velocity
from 'crustal' to 'mantle' values in regions of
thick continental crust may be due, at least
in part, to the appearance of garnet as a sta-
ble phase. The situation is complicated further
by kinetic considerations. Garnet is a common
metastable phase in near-surface intrusions such
as pegmatites and metamorphic teranes. On the
other hand, feldspar-rich rocks may exist at
depths greater than the gabbro-eclogite equilib-
rium boundary if temperatures are so low, or the
rocks so dry, that the reaction is sluggish.
The common assumption that the Moho is a
chemical boundary is in contrast to the position
taken with regard to other mantle discontinu-
ities. It is almost universally assumed that the
major mantle discontinuities represent equilib-
rium solid-solid phase changes in a homoge-
nous mantle. A notable exception to this view
is the geochemical model that attributes the
650 km discontinuity to a boundary separat-
ing the depleted 'convecting upper mantle' from
the undegassed primitive lower mantle. This is
strictly an assumption; there is no evidence in
support of this view. It should be kept in mind
that chemical boundaries may occur elsewhere in
