Geoscience Reference
In-Depth Information
65
60
Arc
55
MORB
50
45
OIB
40
35
0
0.5
1
1.5
2
2.5
3
18
Arc
16
14
12
Olivine,
clinopyroxene
10
Plagioclase
8
6
0
0.5
1
1.5
2
2.5
3
FeO/MgO
Figure 11.5
Changes in the behavior of some major elements in basalts from different tectonic settings as a
function of plagioclase saturation. The arrows represent the effect of removing a particular
mineral on the major element content of the residual melt. As shown by the evolution of SiO
2
,
the
x
-axis coordinate (FeO/MgO) represents the extent of differentiation: lavas become more
differentiated from left to right. Plagioclase saturation in MORB as a result of low-pressure
(
5 kb) differentiation makes Al compatible and Fe and Ti more incompatible. Plagioclase
undersaturation as a result of high pressure fractionation (OIB) or high water pressure (arc
magmas) has the opposite effect: Al becomes incompatible in contrast to Fe and Ti, which
become more compatible.
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