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Fig. 14.4 Trace-element
concentrations in kimberlite relative
to KREEP. Elements that are
fractionated into olivine, plagioclase,
garnet and jadeite are noted. The
solid/liquid partition coefficient for
garnet is shown. If kimberlite has
been in equilibrium with garnet, or
an eclogite cumulate, the
concentrations in kimberlite will be
proportional to the reciprocal of the
garnet partition coefficient.
Refractories
Zn
10
Volatiles
Garnet
Plag
Olivine
Ni
Cu
Co
Mn
Plag
Fe
Cr
Metals
Jadeite
Garnet
partition
coefficient
0.1
0.01 Li Na K Rb Cs P S Ca Ti Nb Ta Th U Sr Ba La Ce Nd Sm Eu Tb Dy Ho Er Yb Lu
and are early when the lithosphere is thinning
(rifting) and late when the lithosphere is thicken-
ing (flanks of ridges, downstream from midplate
volcanic centers).
On Kerguelen, the third largest oceanic
island, and on Ascension Island, the youngest
basalts are alkalic and overlie a tholeiitic base.
In Iceland, tholeiites dominate in the rift zone
and grade to alkali basalts along the western
and southern shores. Thus, there appears to be
a relation between the nature of the magma-
tism and the stage of evolution. The early basalts
are tholeiitic, change to transitional and mixed
basalts and terminate with alkaline composi-
tions. It was once thought that Hawaii also fol-
lowed this sequence, but the youngest volcano,
Loihi, is alkalic and probably represents the earli-
est stage of Hawaiian volcanism. It could easily be
covered up and lost from view by a major tholei-
itic shield-building stage. Thus, oceanic islands
may start and end with an alkalic stage.
In continental rifts, such as the Afar trough,
theRedSea,theBaikalriftandtheOslograben,
the magmas are alkalic until the rift is mature
and they are then replaced by tholeiities, as in a
widening oceanic rift. A similar sequence occurs
in the Canary Islands and the early stages of
Hawaii. The return of alkalics in the Hawaiian
chain may occur when the islands drift away
from a fertile blob. Seamounts on the flanks of
midocean ridges, may be tapping the flanks of
a mantle heterogeneity, where crystal fractiona-
tion and contamination by shallow mantle are
most prevalent. In all cases lithospheric thick-
ness appears to play a major role. This plus the
long duration of alkalic volcanism, its simultane-
ity over large areas and its recurrence in the same
parts of the crust argue against the simple plume
concept.
Alkali-rich basalts generally have trace-
element concentrations intermediate between
MORB and highly alkalic basalts such as nepheli-
nite. In fact, they can be treated as mixtures of
MORB, or a picritic MORB parent, and nephelin-
ite. Alkaline rocks are the main transporters of
mantle inclusions.
Continental flood basalts
Continental flood basalts (CFB), the most copi-
ous effusives on land, are mainly tholeiitic
flows that can cover very large areas and are
typically 3--9 km in thickness. These are also
called traps, plateau basalts and fissure basalts,
and large igneous provinces (LIPs). Examples are
the Deccan traps in India, the Columbia River
province in the USA, the Parana basin in Brazil,
the Karoo province in South Africa, the Siberian
traps, and extensive flows in western Australia,
Tasmania, Greenland and Antarctica. They have
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