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marked depression of the solidus and liquidus by CO
2
near this reaction boundary
(Wendlandt and Eggler 1980a, b).
In the CO
2
-bearing join the forsterite
enstatite cotectic moves to much lower
SiO
2
conditions such that the olivine + orthopyroxene + sanidine or leucite
invariant point occurs below 1,260
-
°
C and at compositions within the forsterite
-
kalsilite
field (Wendlandt and Eggler 1980a, b). The experiments of Gupta
and Green do not de
leucite
-
ne the exact nature or position of the invariant point as it must
be noted that the P-T conditions approach the carbonation reaction so that the
character of the melt may change rapidly from silicate- dominated with dissolved
(CO
=
) to carbonate dominated with dissolved (SiO
=
).
The composition of enstatite along the forsterite
enstatite cotectic is close to
-
ideal enstatite with
0.5 wt% Al
2
O
3
in solid solution. Liquids are therefore, not
appreciably peralkaline in character in the presence of CO
2
in this join.
The forsterite
≤
quartz join at 2.8 GPa is a simple system analogue for
highly potassic liquids under mantle conditions and for melting of mantle harz-
burgite with a minor potassic phase. The study of the join at 2.8 GPa, illustrates the
effect of pressure in moving phase boundaries, particularly the important forsterite-
enstatite boundary, from quartz-normative to strongly leucite-normative at high
pressures (Wendlandt and Eggler 1980a, b). The effects of the common volatiles
H
2
O and CO
2
are also large with H
2
O playing a particularly important role in
stabilising the low-SiO
2
, high Mg phase phlogopite at liquidus temperatures. While
an H
2
O-rich
kalsilite
-
-
fluid moves liquids in equilibrium with harzburgite away from leucite-
normative to olivine- and hypersthene-normative compositions, a CO
2
-rich
uid
has the opposite effect producing liquids of extremely silica-undersaturated char-
acter from a harzburgite source. If H
2
O:CO
2
ratios are variable in a relatively
oxidised mantle (i.e. f(O
2
) condition >MW; Taylor 1985; Foley et al. 1986b) then
phlogopite stability may extend to a temperature greater than 1160
°
C and the
position of the peritectic point (phlogopite + enstatite
forsterite + liquid) may
move to leucite-normative compositions. Analogous results were obtained for an
olivine leucitite composition (Ryabchikov and Green 1978) and in the forste-
rite
→
quartz system by Wendlandt and Eggler (1980a, b, c). Foley et al.
(1986 a, b) have shown that
kalsilite
-
-
fluorine substituting for (OH)
−
in
fluorphlogopite has a
dramatic effect in expanding the phlogopite
Cat
2.8 GPa) and moving the forsterite-enstatite cotectic to leucite-normative compo-
sitions. Natural phlogopite from high pressure xenoliths contain both
field to higher temperatures (1480
°
fluorine
(Foley et al. 1986b) and variable TiO
2
contents which will stabilisze them to higher
temperatures.
The experimental study of
quartz illustrates a
mechanism by which, even in this simple system, the availability of C
the join forsterite
-
kalsilite
-
uid
may produce diverse liquids at very low degrees of melting of a model phlogopite,
harzburgite source rock. In addition, small differences in H
2
O/CO
2
ratio may
control the appearance of phlogopite during fractionation of highly potassic liquids
in the upper mantle. The early appearance of phlogopite will drive liquids toward
silica-oversaturated derivative melts, whereas the same parent magma following a
P-T path avoiding the phlogopite
-
H-O
field, will fractionate to the silica-undersaturated
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