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10 000
F = 0
F = 0.001
1000
F = 0.01
100
F = 0.1
10
1/ F regime
1
1/ D regime
0.1
0.01
0.0001
0.001
0.01
0.1
1
10
100
Bulk solid/liquid partition coefficient, D
Figure 2.8
Enrichment/depletion factor during melting as a function of the bulk solid/liquid partition
coefficient D for different degrees F of melting. Note the two regimes separated by the dashed
line F
=
D :for F
<
D , the enrichment factor remains constant and equal to 1
/
D ,whilefor F
>
D ,
it varies as 1
/
F . Efficient fractionation between very incompatible elements requires small melt
fractions.
80
More compatible
60
40
20
0
Rb Ba Th
U
Nb
La
Ce
Pb
Sr
Nd Sm
Zr
Hf
Eu
Gd
Dy
Y
Er
Yb
Al
Ca
Si
Fe
Mg
Figure 2.9
The variability of concentrations of elements in basalts (measured by the relative standard
deviation) decreases with their compatibility (after Hofmann, 1988 ). This reflects the buffering of
the more compatible element concentrations in melts by the residual solid.
importantly still with water or CO 2 , which separate out in a vapor phase when magmas
ascend. At this point, the concentration of the element forming the major phase (e.g. Zr
or CO 2 ) remains fixed, we say buffered, by either the zircon or the vapor. The balance
equations have to be re-written to cover this new situation.
 
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