Geology Reference
In-Depth Information
6.3 Determination of Oxygen Fugacity in Potassic Rocks
Based on the Presence of Picroilmenite
Picroilmenite has been reported to occur in phlogopite-bearing minette from Buell
Park, Arizona. Esperanca and Holloway (1987) observed that picroilmenite in these
minettes are stable only at QMF buffer and temperatures below 1,100
C. Under
this f(O
2
) condition at 1 GPa, it coexists with olivine, clinopyroxene and phlogo-
pite, and contain lower amounts of TiO
2
and FeO than at 1.5 GPa, where it coexist
with clinopyroxene and phlogopite.
Roden and Smith (1979) studied ilmenites and oxidized spinels from ring dykes
at Buell Park. They employed the method of Buddington and Lindsley (1964) and
obtained a temperature of 815
°
C and f(O
2
) condition of 10
−
10.4
. These values also
correspond to QM buffer condition, and subsolidus temperature of re-equilibration
condition. Titanomagnites also occur in Agathapa Park and the Thumbs minnetes
(Jones and Smith 1983). Luhr and Carmichael (1981) reported the occurence of
chromian spinel from K-rich basanites, and minettes of Colima graben, where it
occurs as an early crystallization phase. Titano-magnetite appears as a groundmass
phase. Estimated f(O
2
) values for Colima graben potassic suite range between NNO
and HM buffer conditions (Sack et al. 1980). Esperanca and Holloway thought that
partial re-equilibration in volatile-rich magmas at variable f(O
2
) conditions during
ascent could have been responsible for the scatter in the chemical composition of
micropheno crystal Fe-Ti
°
Ti oxides in the K-rich lavas of different localities.
-
6.4 Oxidation Path of a Leucitite Magma with Respect
to CO
2
Solubility
The solubility of CO
2
in a Ca-rich leucitite was determined by Thibault and
Holloway (1994). In this connection, they calculated the oxidation path (Fig.
6.13
)
of an ascending leucitite magma originated at a depth of 75
80 km (2.5 GPa). They
-
estimated a temperature of 1,300
C for the magma at the source region. They
considered an isothermal ascent at this temperature. If no carbon is dissolved and
there is no oxidation through degassing, then the magma will ascend through path
(a). If the magma generated again at 75
°
80 km and dissolves 5 wt% CO
2
(i.e. the
liquid is not saturated with respect to CO
2
), then it will follow path (b). If the
magma is saturated with respect to CO
2
after its generation at same depth, it would
then take an ascending path denoted by the curve (c). The path would then show
maximum deviation with respect to
-
log f(O
2
) NNO]. The
heavy lines denoted by NNO and CCO represent respectively the f(O
2
) values of
the NNO oxygen buffer and a graphite-saturated CO
NNO[log f(O
2
) CCO
Δ
−
fluid.
Search WWH ::
Custom Search