Geoscience Reference
In-Depth Information
but relatively poor in Na, Al and Ca. Kimberlitic
and some other enriched magmas have a comple-
mentary relationship to MORB.
The extreme enrichments of kimberlitic mag-
mas in incompatible elements are usually
attributed to low degrees of melting and/or
metasomatized source compositions. The obser-
ved enrichment of
kimberlitic magmas
with rare earth elements (REE) can be
explained in terms of melt migration
through source rocks having the compo-
sition of normal mantle
. The resulting satu-
rated REE spectrum is practically independent of
source mineral composition, which may explain
the similarity of kimberlites from different geo-
graphic localities. Kimberlite is thus an impor-
tant mantle component and can be used as such --
component Q -- in mass-balance calculations.
Table 13.1
Estimates of average composi-
tion of the mantle
Oxide
(1)
(2)
(3)
(4)
(5)
SiO
2
45.23
47.9
44.58
47.3
45.1
Al
2
O
3
4.19
3.9
2.43
4.1
3.9
MgO
38.39
34.1
41.18
37.9
38.1
CaO
3.36
3.2
2.08
2.8
3.1
FeO
7.82
8.9
8.27
6.8
7.9
TiO
2
—
0.20
0.15
0.2
0.2
Cr
2
O
3
—
0.9
0.41
0.2
0.3
Na
2
O
—
0.25
0.34
0.5
0.4
K
2
O
—
—
0.11
0.2
(0.13)
(1) Jacobsen and others (1984): extrapolation
of ultramafic and chondritic trends.
(2) Morgan and Anders (1980): cosmochemi-
cal model.
(3) Maal
/
e and Steel (1980): extrapolation of
lherzolite trend.
(4) 20 percent eclogite, 80 percent garnet
lherzolite (Anderson, 1980).
(5) Ringwood and Kesson (1976, Table 7):
pyrolite adjusted to have chondritic
CaO/Al
2
O
3
ratio and Ringwood (1966) for
K
2
O.
Chemical composition of the mantle
Considerations from cosmochemistry and the
study of meteorites permit us to place only very
broad bounds on the chemistry of the Earth's
interior. These tell us little about the distribu-
tion of elements in the planet. Seismic data tell
us a little more about the distribution of the
major elements. General considerations suggest
that the denser major elements will be toward
the center of the planet and the lighter major
elements, or those that readily enter melts or
form light minerals, will be concentrated toward
the surface. To proceed further we need detailed
chemical information about crustal and mantle
rocks. The bulk of the material emerging from
the mantle is in the form of melts, or magmas.
It is therefore important to understand the chem-
istry and tectonic setting of the various kinds of
magmatic rocks and the kinds of sources they
may have come from.
Midocean-ridge basalt represents the most
uniform and voluminous magma type and is an
end member for LIL concentrations and many iso-
topic ratios. This is usually taken as one of the
components
of the mantle, even though it itself
is an average or a blend. Most mantle magma
compositions can be approximated with a mix-
ture of a depleted MORB-component and one or
more enriched components, variously called EM1,
EM2, HIMU, DUPAL and Q. The resulting magmas
themselves are called NMORB, EMORB, OIB, AOB,
CFB and so on. The refractory residue left after
melt extraction -- the
restite
-- is usually consid-
ered to be a peridotite, dunite or harzburgite, all
ultramafic rocks
(UMR). All of the above, plus
con-
tinental crust
(CC), are candidate components for
primitive mantle.
The MORB source appears to have been
depleted by removal of a component that is
rich in LIL but relatively poor in Na and the
clinopyroxene-compatible elements (such as Al,
Ca, Yb, Lu and Sc). Kimberlitic magmas have the
required complementary relationship to MORB,
and I adopt them in the following as a possible Q
component. Some elements, such Nb, Ta, Ti and
Zr are extraordinarily concentrated into specific
minerals -- rutile and zircon, for example -- and
estimates of these elements in rocks can be
highly variable and dependent on the amount of
these minerals. Peridotites and sulfides are the
main carriers of elements such as magnesium,
