Geoscience Reference
In-Depth Information
The most likely place to generate low-degree
melting in a low-melting point mantle with
CO
2
is in the outer reaches, probably in the
conduction layer or above 200 km. The EM1,
HIMU and FOZO components may all originate in
the upper and lower crust, the sub-continental
lithosphere and the shallow mantle. They can
enter the source region of OIB and carbonatites
by erosion, subduction or delamination. In the
absence of subduction, the lower crustal delam-
ination explanation is preferred. A wide-spread
but sporadic, and shallow depleted FOZO source
provides an explanation for carbonatites. The
'depleted mantle' signatures may be attributed
to differentiation events in the mantle that pro-
duced depleted sub-continental lithosphere and
continental crust. Carbonatite isotope data sug-
gest that a shallow source in the lithosphere
or perisphere could be FOZO mantle. It could
have existed for billions of years as an approxi-
mately closed system if it is buoyant and of high
viscosity.
Carbonatites may represent the initial melt-
ing of carbonated mantle peridotite and kimber-
lites may represent larger degrees of partial melt-
ing after the CO
2
is exhausted. There is also the
possibility that carbonated eclogite produces car-
bonatitic melt under various depth conditions in
the upper mantle and that this melt metasoma-
tizes mantle peridotite. The source of the eclogite
could be delaminated lower continental crust, or
subducted oceanic crust.
1000
ECLOGITE
COMPATIBLE
INCOMPATIBLE
100
KIMBERLITE
CFB
10
MORB
1
0.1
Cu
Ni
V
Na
Yb
Sr
Sm
Rb
La
Th
Co
Cr
Mn
Y
Zr
Nd
K
Ba
U
Fig. 14.1
Trace-element concentrations in MORB,
continental flood basalts (CFB) and kimberlites. The elements
to the right are incompatible in all major mantle phases
(olivine, pyroxene and garnet) while those to the left are
retained in the eclogite minerals (clinopyroxene and garnet).
Note the complementary pattern between MORB and
kimberlite and the intermediate position of CFB.
Concentrations are normalized to estimates of mantle
composition.
as the solid/liquid partition coefficient for that
element. This is illustrated in Figure 14.2. The
solid line is a profile of the MORB/kimberlite ratio
for a series of incompatible elements. The verti-
cal lines bracket the solid/liquid partition coef-
ficients for garnet and clinopyroxene. Although
MORB is generally regarded as an LIL-depleted
magma and kimberlite is ultra-enriched in most
of the incompatible elements, MORB is enriched
relative to KIMB in yttrium, ytterbium and scan-
dium, elements that have a high solid/melt parti-
tion coefficient for an eclogite residue. The trend
of the MORB/KIMB ratio is the same as the parti-
tion coefficients, giving credence to the idea that
enriched magmas, such as kimberlite, and MORB
are genetically related.
The LIL content of continental tholeiites and
alkali-olivine basalts suggest that they are mix-
tures of MORB and a melt from a more enriched
part of the mantle, or blends of high-degree melts
The Kimberlite--MORB connection
Kimberlites are enriched in the LIL elements,
especially those that are most depleted in MORB.
Figure 14.1 gives the composition of kimberlites,
MORB and continental tholeiites. The comple-
mentary pattern of kimberlite and MORB is well
illustrated as is the intermediate position of con-
tinental tholeiites (CFB). Note that kimberlite is
not enriched in the elements that are retained
by the eclogite minerals, garnet and clinopy-
roxene. This is consistent with kimberlite hav-
ing been a fluid in equilibrium with subducted
oceanic crust or an eclogite cumulate. If a resid-
ual cumulate is the MORB reservoir, the ratio of
an incompatible element in kimberlite relative to
the same element in MORB should be the same
