Geoscience Reference
In-Depth Information
MORB rather than being similar to those exhib-
ited by OIB or hotspots.
Melting temperatures can be lowered by the
presence of water, carbon dioxide, and eclog-
ite. Boninites are high-degree melts produced
at shallow depths. High degrees of melting in
hot upwellings necessarily must begin at great
depth; in the case of boninites, large extents
of melting occur at shallow depth because the
melting is facilitated by high water contents.
Fertile mantle, such as eclogite, also melts at
higher pressure and at lower temperature than
peridotite, and can therefore have a long melt-
ing
Table 14.2
Kimberlite composition com-
pared with ultrabasic and ultramafic rocks
Average
Average
Average
Oxide
Kimberlite
Ultrabasic
Ultramafic
SiO
2
35.2
40.6
43.4
TiO
2
2.32
0.05
0.13
Al
2
O
3
4.4
0.85
2.70
6.8
—
—
FeO
2.7
12.6
8.34
MnO
0.11
0.19
0.13
MgO
27.9
42.9
41.1
CaO
7.6
1.0
3.8
column,
and
produce
large
amounts
of
Na
2
O
0.32
0.77
0.3
basalt.
K
2
O
0.98
0.04
0.06
H
2
O
7.4
—
—
Kimberlite
Some rocks are rare but are so enriched in certain
key elements that they cannot be ignored in any
attempt to reconstruct the composition of the
mantle. Kimberlite is a rare igneous rock that is
volumetrically insignificant compared with other
igneous rocks. Kimberlite provinces themselves,
however, cover very broad areas and occur in
most of the world's stable craton, or shield, areas.
Kimberlites are best known as the source rock for
diamonds, which crystallized at pressures greater
than about 50 kilobars. They carry other samples
from the upper mantle that are the only direct
samples of mantle material below about 100 km.
Some kimberlites appear to have exploded from
depths as great as 200 km or more, ripping off
samples of the upper mantle and lower crust
in transit. The fragments, or xenoliths, provide
samples unavailable in any other way. Kimber-
lite itself is an important rock type that provides
important clues as to the evolution of the mantle.
It contains high concentrations of lithophile ele-
ments (Table 14.2) and higher concentrations of
the most incompatible trace elements (Pb, Rb, Ba,
Th, Ce, La) than any other ultrabasic rock. Kim-
berlite is also enriched in elements of ultramafic
affinity (Cr and Ni).
Diamond-bearing kimberlites are usually
close to the craton's core, where the lithosphere
may be thickest. Barren kimberlites (no dia-
monds) are usually on the edges of the tecton-
ically stable areas. Kimberlites range in age from
Precambrian
CO
2
3.3
0.04
—
P
2
O
5
0.7
0.04
0.05
Wederpohl and Muramatsu (1979).
Dawson (1980).
been subjected to kimberlite intrusion over long
periods
of
geological
time,
suggesting
litho-
spheric control.
Compared to other ultrabasic rocks --
lherzolites, dunites, harzburgites -- kimberlites
contain high amounts of K, Al, Ti, Fe, Ca, C, P
and water. For most incompatible trace elements
kimberlites are the most enriched rock type;
important exceptions are elements that are
retained by garnet and clinopyroxene. Carbon-
atites may be enriched in U, Sr, P, Zr, Hf and the
heavy REEs relative to kimberlites. Since kimber-
lites are extremely enriched in the incompatible
elements, they are important in discussions of
the trace-element inventory of the mantle. Such
extreme enrichment implies that kimberlites
represent a small degree of partial melting of a
mantle silicate or a late-stage residual fluid of a
crystallizing cumulate layer. The LIL elements in
kimberlite show that it has been in equilibrium
with a garnet--clinopyroxene-rich source region,
possibly an eclogite cumulate. The LIL contents
of kimberlite and MORB are complementary.
Removal of a kimberlite-like fluid from an
eclogite cumulate gives a crystalline residue that
has
to
Cretaceous.
Some
areas
have
the
required
geochemical
characteristics
