Geoscience Reference
In-Depth Information
16
14
12
OIB
10
8
Arc
6
Plagioclase
4
Olivine,
clinopyroxene
2
0
0.5
1
1.5
2
2.5
3
5
Plagioclase
4.5
Olivine
4
Clinopyroxene
3.5
3
OIB
2.5
2
1.5
1
Arc
0.5
0
0
0.5
1
1.5
2
2.5
3
FeO/MgO
Figure 11.6
Continued from
Fig. 11.5
.
nearly zero until magnetite reaches saturation. In contrast with MORB, cumulates of both
ocean island and arc magmas are plagioclase-free and therefore:
D
Fe
2
+
MORB
<
D
Fe
2
+
OIB
+
arc
(11.3)
The Fe
3
+
/Fe
2
+
ratios must therefore be smaller in MORB than in any ocean island and
arc magma of similar
f
. This behavior is seen by petrologists as indicating an evolution
towards more oxidizing conditions (higher oxygen fugacity) in the latter. It is actually a
