Geoscience Reference
In-Depth Information
1000
F
(melt
fraction)
100
Melt
0%
5%
1%
15%
10
(Source)
1
0%
5
1%
15%
0.1
0.01
La
Ce
Nd
Sm
Gd
Dy
Er
Yb
Lu
Figure 11.8
Distribution of rare-earth elements in melts from the mantle and in their residues for different
degrees
F
of melting. Concentrations were calculated using equilibrium batch melting equations
similar to
(11.4)
and then normalized to the values in the mantle source. The residues were
assumed to contain 15% clinopyroxene with the following mineral-liquid partition coefficients:
La: 0.03, Ce: 0.085, Nd: 0.19, Sm: 0.29, Dy-Yb: 0.44 and 85% of rare-earth free olivine and
orthopyroxene. Note that the highly incompatible elements (La, Ce) are extracted much more
efficiently from the residues than the more compatible elements (Yb, Lu). The tie-lines connect,
for Nd, liquid and solid pairs for a same
F
. Since MORBs are depleted in La, Ce with respect to Yb,
went through previous melt extraction events. This is why the mantle under the mid-ocean ridges
is usually referred to as the depleted mantle (DM).
fraction of melt prevailing during formation of MORB and
D
Yb
s
the partition coefficient of
/
l
Yb between solid residue and melt,
(2.17)
can be written:
C
Yb
source
C
Y
MORB
=
(11.4)
D
Yb
s
F
+
l
(1
−
F
)
/
As a first approximation, La is almost completely incompatible and its solid residue/melt
partition coefficient
D
La
s
is so small that it can be taken as zero. We can then approximate:
/
l
C
La
source
F
C
L
MORB
=
(11.5)
MORB
=
C
La
C
Yb
1
C
La
C
Yb
1
−
F
D
Yb
s
+
>
(11.6)
/
l
F
source
source
Because the term in the parentheses of the right-hand side is greater than unity, this equa-
tion shows that the La/Yb ratio must be greater in the magmatic liquid (MORB) than in the
source mantle. This approach is valid, of course, for all trace elements.
Figure 11.8
shows
the distribution of rare-earth elements in melts from the mantle and in their residues for
different degrees
F
of melting. The enrichment of incompatible elements in the melts with
