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of the depleted source region that provides
MORB. Adding a kimberlitic fluid to a depleted
basalt can make it similar to enriched magmas.
Carbonatites and other ultramafic alka-
line rocks, closely related to kimberlites, are
widespread. They are probably common in the
upper mantle, although they have been argued
to come from the lower mantle because they are
not MORB. Kimberlites provide us with a sam-
ple of magma that probably originated below
about 200 km and, as such, contain informa-
tion about the chemistry and mineralogy of
the mantle in and below the continental litho-
sphere. Kimberlites are anomalous with respect
to other trace-element enriched magmas, such
as nephelinites and alkali basalts, in their trace-
element chemistry. They are enriched in the very
incompatible elements such as rubidium, tho-
rium and LREE, consistent with their represent-
ing a small degree of partial melting or the
final concentrate of a crystallizing liquid. The
extreme enrichments of kimberlitic magmas in
incompatible elements (e.g. compared with a typ-
ical undepleted mantle) are usually attributed
to low degrees of melting and/or metasomatized
source compositions.
The observed enrich-
ment of kimberlitic magmas with rare
earth elements(REE)can be explained in
terms of melt migration through source
rocks having the composition of normal
mantle
.
Kimberlites are relatively depleted in the ele-
ments (Sc, Ti, V, Mn, Zn, Y, Sr and the HREE) that
are retained by garnet and clinopyroxene. They
are also low in silicon and aluminum, as well as
other elements (Na, Ga, Ge) that are geochemi-
cally coherent with silicon and aluminum. This
suggests that kimberlite fluid has been in equi-
librium with an eclogite residue. Kimberlites are
also rich in cobalt and nickel.
Despite their comparative rarity, dispropor-
tionately high numbers of eclogite xenoliths have
been found to contain diamonds. Diamond is
extremely rare in peridotitic xenoliths. Eclogitic
garnets inside diamonds imply a depth of ori-
gin of about 200--300 km if mantle tempera-
turesinthisdepthrangeareoftheorderof
1400--1600
◦
C. Seismic velocities in the transition
region are consistent with piclogite (eclogite plus
peridotite). The bulk modulus in the transition
zone for olivine in its spinel forms is higher than
observed.
Alkali basalts have LIL concentrations that are
intermediate to MORB and kimberlite. Although
kimberlite pipes are rare, there may be a
kimberlite-like component (Q) dispersed through-
out the shallow mantle. Indeed, alkali basalts can
be modeled as mixtures of a depleted magma
(MORB) and an enriched magma (kimberlite), as
shown in Table 14.3. Peridotites with evidence
of
secondary
enrichment
may
also
contain
a
kimberlite-like component.
Carbonatite
Carbonatites are igneous rocks that typically
occur in regions of lithospheric extension of
thick continental plates, i.e. continental rift
zones (e.g. East Africa) and on the margins of
continental flood-basalt provinces (e.g. Parana-
Etendeka, Deccan). They have also been found
on ocean-islands on continental margins (e.g.
Cape Verde and Canary Island carbon-
atites
). The close spatial and temporal asso-
ciation of kimberlites and carbonatites suggests
that these rocks are genetically related, although
diamond-bearing kimberlites may come from
deeper in the upper mantle. Kimberlites are
rare on oceanic islands. This may be because
the adiabatic ascent of the parent rock through
the asthenosphere generates large-degree melt-
ing, while small-degree melts can be better pre-
served if the eruption is through cold continental
mantle. Both carbonatites and kimberlites have
enriched trace-element patterns, consistent with
small-degree melts. Carbonatites have depleted
isotopic signatures while some kimberlites are
enriched. Kimberlites and carbonatites provide
an opportunity to study low-degree partial melt-
ing in the mantle. This is important since the
removal of low-degree partial melts seems to be
the mechanism for depleting the mantle. Since
much of this depletion apparently occurred in
early Earth history, when the mantle was much
hotter, the implication is that this melting and
melt removal occurred at shallow depths, prob-
ably in the thermal boundary layer and the
lithosphere. The solidi of CO
2
-bearing rocks is
much lower than volatile-poor rocks; this allows
