Geoscience Reference
In-Depth Information
1000
Melt
Residue
D
= 0.005
100
D
= 0.02
10
D
D
= 10
1
C
D
= 0.02
C
0.02
/
C
0.0
05
(liqu
i
d)
0.1
D
= 10
D
= 0.005
0.01
0.001
0.0001
0.001
0.01
0.1
1
Melt fraction
100
10
D
= 0.005
D
= 0.02
1
C
0.02
/
C
0.005
(liquid)
B
A
0.1
0.01
D
= 10
0.001
0.01
0.1
1
Fraction of cumulate removed
Figure 2.10
Comparison of the effect of fractional crystallization (bottom) and partial melting (top) on the
concentration in magmatic melts of elements with different compatibilities. The diagrams show
the case of bulk partition coefficients for elements with
D
0.005, 0.02, and 10. Curve A: Even
the removal of a few percent of a mineral hosting a very compatible element, e.g. Ni in olivine
(
D
=
10), changes concentration in the residual melt drastically, whereas this change is hardly
noticeable with highly incompatible elements such as Th, Ba (
D
≈
0.02). Curve
B: Even extreme mineral fractionation does not change the ratio of two incompatible elements
such as Th/La in the residual melt. Curve C: For small to moderate fractions of melt, compatible
elements remain buffered by the solid. Curve D: In contrast, the ratio of two incompatible
elements is fractionated until the fraction of melt exceeds the largest of the two partition
coefficients. At this stage, the bulk of the incompatible elements has been transferred to the melt
and their relative concentrations are proportional to their abundances in the source. The dashed
lines labelled
C
0.02
/
C
0.005
represent the ratio of two elements with partition coefficients of 0.02
and 0.005, respectively.
≈
0.005), or La (
D
≈
