P-T Stability of Phlogopite, K-Richterite and Phengite, as a Source of Potassium in the Mantle - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 367

3 kbar

Phl

En

Fo

San

HO
2

CH 4

E n

dry

CO 2

F

Phl

Ks Fo

Fig. 12.15 Part of the join kalsilite-forsterite-quartz showing relative positions of the enstatite-
forsterite phase boundary in the presence of various volatile species. The boundary for CH 4 is
estimated from its position in the nepheline-forsterite-quartz system, taken from Gupta et al.
(1987). The position with HF is uncertain, but can be expected to be close to the CH 4 position. The
data for the water-saturated kalsilite-forsterite-quartz join at 0.3 GPa are taken from Luth (1967)
(after Foley et al. 1986). Abbreviations as in Fig. 12.14

enstatite + forsterite phase boundary is in a similar position to that in the CO 2 -
saturated join (Fig. 12.12 ) of Gupta and Green, 1988. The position of this phase
boundary is frequently taken as the degree of polymerization of the melt (Eggler
1974; Kushiro 1975). Expansion of the enstatite phase volume at the expense of
forsterite with the addition of

fluorine causes
polymerization of the structure. The assemblage phlogopite + enstatite + forste-
rite + liquid in the simple join is similar to that of a phlogopite harzburgite. The
position of this point may indicate that partial melting of a phlogopite harzburgite in
H 2 O-free conditions but

fluorine therefore, suggests that

fluorine should produce a silica-
undersaturated magma. Such a melt should lie to the silica-poor side of the for-
sterite-sanidine join at 1,480

in the presence of

° C. In contrast, in the water-bearing system (Gupta and

Green 1988), the first melt would lie in the more silica-saturated part of the system,

within the triangle, de C.

Foley et al. (1986) conducted several experiments with composition 1
(Fig. 12.14 ) with 10 % F 2 O − 1 . They observed that the near-liquidus runs contained
a very minor amount of immiscible liquid phase rich in Mg and F. The immisci-
bility may extend above the liquidus, but this could not be ascertained because of
the abundance of quench crystals in these near liquidus runs. They further observed
that with 10 % F 2 O − 1 , composition 1 lies just inside the primary phase volume of
phlogopite, indicating expansion of the phlogopite phase volume with increasing

ned by forsterite, enstatite and sanidine at 1160 °

fluorine, They think that because of the apparent immiscibility, which occurs at
high fluorine contents, it is unlikely that phlogopite will melt congruently at any

fluorine content.