Geology Reference
In-Depth Information
6.2 Partial Pressure of Oxygen Related to Genesis
of K-Rich Volcanic Rocks
6.2.1 Oxygen Fugacity Related to Stability of Annite
The stability of annite was studied by Eugster and Wones (1962). Their study is
summarized in Fig. 6.10 , which shows the coexistence of iron-rich olivine, leucite,
magnetite and vapour in equilibrium with iron-rich mica. In natural K-rich rocks,
mica coexisting with leucite, olivine, magnetite and vapour is phlogopite-rich con-
taining annite molecule. Phlogopite containing 20
30 mol% annite will have higher
-
P
T stability and hence, the assemblage, olivine + leucite + magnetite + vapour in
equilibrium with mica will be stable at oxygen fugacity possibly between MW and
QFM buffer.
-
6.2.2 The Fe 3+ /Fe 2+ Ratio for Determination of Oxygen
Fugacity in Potassic Rocks
Turbevile (1993) determined the oxygen fugacity of Latera Caldera rocks
(Fig. 6.11 ) containing phenocrysts of sanidine
±
leucite with plagioclase,
Fig. 6.10 Phase relation of
annite (+vapour) bulk
composition as a function of
oxygen fugacity (after Eugster
and Wones 1962)
-10
San
+
Mt
+
V
San
+
Hem
+
V
-15
Fa + Lc + M + V
C 1
C 2
Ks+Lc
+Fa+V
D
N
B 1
B 2
Fa+Lc+l+V
-20
E
Ann
+
V
-25
San
+
Fe
+
V
-30
A
-35
400
500
600
700
800
Temperature ( C)
°
 
 
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