Geoscience Reference
In-Depth Information
10
Clinopyroxene
Lu
Yb
Er
Gd
1
Sm
Nd
Ce
La
0.1
0.01
0.90
0.95
1.00
1.05
1.10
1.15
1.20
Ionic radius (Å)
Figure 2.6
Variation of the partition coefficients of rare-earth elements between clinopyroxene and basaltic
control of the partition coefficient by the elastic properties of the crystalline lattice.
D
s
/
l
=
f
j
K
j
/
liq
(2.15)
solids
j
described as the weighted mean of individual partition coefficients
K
j
/
liq
of element
i
between mineral
j
and the liquid, the weights being the abundance of each mineral in
constant when the mineral fractions
f
j
change during the process. We will come across
geological settings, however, where
D
s
/
l
actually does remain constant. The coefficient
D
s
/
l
provides a measure of the element compatibility (or of its refractory character) in a
given environment, and it is standard practice in igneous geochemistry to represent the con-
centrations of elements of different magmatic rocks in order of increasing compatibility as
the mantle melts, the mean order being established by experiment. These “spidergrams”
entail normalization, i.e. division by concentrations of a reference rock or reservoir (chon-
drites, Bulk Silicate Earth, mid-ocean ridge basalts
=
MORB) and have become a standard
Let us apply the simple theory of chemical fractionation to the change in the concentra-
tion of an element
i
during partial melting of a source rock made up of several minerals.
This theory leads to the so-called equilibrium- or batch-melting equations. If the proportion
of liquid in the molten rock is
F
(melt fraction), the mass balance condition is written:
F
D
s
/
l
C
liq
C
source
=
FC
liq
+
(
f
j
C
j
1
−
F
)
=
+
(
1
−
F
)
(2.16)
solids
j
or
C
source
C
liq
=
(2.17)
D
s
/
l
F
+
(
1
−
F
)
