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the precipitation of phlogopite (above 2.5 GPa). They established that clinopy-
roxene is diopsidic with low alumina and titanium content and phlogopite is alu-
mina-de
cient (up to 12 wt%). Figure 13.7 demonstrates that olivine is not a
liquidus phase at pressures above 0.7 GPa, and orthopyroxene is not stable at any
pressure up to 3.0 GPa, which suggests that madupite was not probably produced
by melting of a hydrous lherzolite or a garnet lherzolite in the upper mantle. They
concluded that madupitic melt might have been derived by partial melting of a
phlogopite pyroxenite or a phlogopite peridotite. Barton and Hamilton pointed out
that the chemistry of some of the potassium-rich volcanic rocks might have been
affected by volatile transfer and other such processes during eruption and the
experimental studies on materials affected this way may have little bearing upon the
genesis of potassic magmas.
13.2.5 Investigation on a Leucite Lamproite from Gaussberg,
Antarctica
Experimental study on a olivine leucitite from Gaussberg was studied under buf-
fered (low f(O 2 )) conditions in presence of an aqueous
fluid having minor amount
of CH 4 (Foley 1989, Fig. 13.8 a). Spinels present in the lamproite suggested reduced
mantle conditions for the formation of this lamproitic magma. He therefore, added
CH 4 to the aqueous
fluid. His experimental results are summarized in Fig. 13.8 a,
which shows the presence of low temperature liquidus
field for olivine. Fig-
ure 13.8 b shows high pressure liquidus for orthopyroxene. This is expected, if the
source rock is mica harzburgite but the intervening
field of phlogopite does not
explain this concept. Foley considers that the likely explanation is related to
addition of excess amount of water as an experimental convenience so that f(O 2 )
can be suitably buffered. If the amount of water present was less, the phlogopite
field should have shrunk and olivine could have appeared as an incongruent melting
phase of phlogopite. The experimental study of Foley (1985) may be compared
with that of Barton and Hamilton (1982). The chemistry of the two lamproites is
similar, but the two starting materials have different silica content and mg-number,
and the experiments were conducted in case of the Leucite Hills lamproite in
presence of water-de
cient condition, whereas the Gaussberg lamproite was studied
in presence of excess water.
It is observed that orendite with higher silica concentration has a four-phase
point at the liquidus at 2.7 GPa (Fig. 13.8 a). The assemblage at this point is similar
to a lherzolite. The Gaussberg lamproite does not show the presence of olivine at
pressures greater than 0.5 GPa. These two studies are comparable if allowance is
made for the uncertainty about the presence of olivine. Barton and Hamilton (1982)
in the study of lamproites had dif
culty in recognizing olivine in runs above
1.2 GPa, because the grain size of olivine was small and microprobe analyses could
not be conducted. Thus if the assumption is made that olivine is stable only below
 
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