Geology Reference
In-Depth Information
°
Temperature
( C)
Fig. 6.11 Plots of
log f(O
2
) versus temperature for selected Latera pumice and scoria samples
calculated from whole-rock data; also shown are buffer curves for magnetite
−
hematite,
-
fayalite
amphibole. Open square trachyte, open triangle
evolved phonolite, solid triangle unevolved phonolite; solid square tephriphonolite (after
Turbeville 1993)
magnetite
quartz and stability of biotite
-
-
-
clinopyroxene, biotite, magnetite, apatite and sphene in the groundmass. On the
basis of Fe
3+
/Fe
2+
ratio of the whole rock, he calculated the oxygen fugacity of the
rocks. His plot of the rocks from Latera Caldera in a
log f(O
2
) versus temperature
diagram shows that oxygen fugacity of these Latera Pumice plots above FMQ but
below biotite
−
amphibolite curve.
-
6.2.3 Oxide Phases as an Indicator for f(O
2
) Condition
of Formation of Potassic Rocks
The oxygen fugacity condition of formation of an ilmenite-bearing minette, was
calculated by Eggler (1987). According to him the f(O
2
) data cluster between QFM
and MW buffers at the estimated pressures of origin (2.5 GPa and above).
The analyses of lamproites suggests that they are relatively more H
2
O-rich but
CO
2
-poor. Crystallization of phlogopite from lamproitic magma suggests that the
genesis of the rock is related to hydrous melting. The presence of diamond in some
olivine lamproites, led Foley (1985) and Foley et al. (1986b) to believe that oxygen
fugacity condition of the source rock for some lamprophyres at least is best rep-
resented by CW (carbon
water) buffer (Fig.
6.12
). The buffer curve is the locus of
points on the carbon-saturation surface, where X(H
2
O) is maximum. This buffer
curve lies roughly mid-way between IW and WM buffers at temperatures and
pressures likely to represent diamond stability curve. They think that it is possible
that melt generation is induced by dehydration at the beginning and introduction of
a water-rich volatile phase, which may lead to a condition more oxidizing than CW
-
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