Geoscience Reference
In-Depth Information
Fig. 14.3
Chondrite-normalized
trace-element compositions for
kimberlite and lunar KREEP.
Volatiles
Refractories
10
3
KREEP
10
2
10
1
Kimberlite
1
10
−
1
Eclogite elements
10
−
2
Li
Na
K
Rb
Cs
P
Nb
Ta
Th
U
Sr
Ba
La
Ce
Nd Sm Eu
Yb
Y
Zr
Hf
Sc
requires another process, such as removal of a
jadeite component by eclogite fractionation.
Chromium, manganese and iron are approx-
imately one-third the chondritic level in both
KREEP and kimberlite. This is about the level in
the mantles of these bodies. Nickel and cobalt in
kimberlite are about the same as estimated for
the Earth's mantle. These elements are extremely
depleted in KREEP, indicating that they have
been removed by olivine or iron extraction from
the source region.
The similarity of kimberlite and KREEP is
shown in Figure 14.4. For many elements (such
as Ca, Ba, Nd, Eu, Nb, Th, U, Ti, Li and P) the con-
centrations are identical within 50%. Kimberlite
is enriched in the volatiles rubidium, potassium
and sulfur, reflecting the higher volatile content
of the Earth. Kimberlite is also relatively enriched
in strontium and europium, consistent with a
prior extraction of plagioclase from the KREEP
source region.
Kimberlite and lunar KREEP are remarkably
similar in their minor- and trace-element chem-
istry. The main differences can be attributed to
plagioclase fractionation in the case of KREEP
and eclogite fractionation in the case of kimber-
lite. KREEP has been interpreted as the residual
fluid of a crystallizing magma. In a small body
the Al-content of a crystallizing melt is reduced
by plagioclase crystallization and flotation. In a
magma ocean on a large body, such as the Earth,
the Al
2
O
3
is removed by sinking or residual gar-
net. Kimberlite is depleted in eclogite elements
including HREE and sodium. Kimberlite may rep-
resent the late-stage residual fluid of a crystalliz-
ing terrestrial magma ocean. A buried eclogite-
rich cumulate layer is the terrestrial equivalent
of the lunar anorthositic crust.
Removal of a kimberlite-like fluid from a
garnet-clinopyroxene-rich source region gives a
crystalline residue that has the appropriate trace-
element chemistry to be the reservoir for LIL-
depleted magmas such as MORB. Enriched fluids
permeate the shallow mantle and are responsible
for the LIL-enrichment of island-arc and oceanic
island basalts.
Alkali basalt
Continental alkaline magmatism may persist
over very long periods of time in the same region
and may recur along lines of structural weakness
after a long hiatus. The age and thickness of the
lithosphere play an important role, presumably
by controlling the depth and extent of crystal
fractionation and the ease by which the magma
can rise to the surface.
The non-random distribution of alkaline
provinces has been interpreted alternatively as
hotspot tracks and structural weaknesses in the
lithosphere. What have been called
hotspots
may
be passive centers of volcanism whose location
is determined by pre-existing zones of weak-
ness, rather than manifestations of deep man-
tle plumes. Alkaline basalts both precede and
follow abundant tholeiitic volcanism. In general
alkaline basalts erupt through thick lithosphere
