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 349

period at moderate pressure to produce differentiates of biotite pyroxenite series,
which was followed by their emplacement at shallow depth. Because of the pres-
ence of leucite (pseudoleucite), olivine and clinopyroxene and absence of phlog-
opite, he considered that the rocks of Shonkin Sag Laccolith were produced under
low water pressures at shallow depth. He suggested that the presence of mica along
with leucite, clinopyroxene, and amphibole is indicative of the fact that the rocks of
West Kimberly were produced by a long cooling process of a potassic magma at a
moderate depth. From the rarity of forsterite and presence of large amounts of
phlogopite, he considered that the rocks of the Leucite Hills were produced at a
relatively greater depth.

12.2 P-T Stability of Phlogopite

Yoder and Kushiro (1969) studied the following reactions pertaining to the stability
of phlogopite up to a pressure of 3.75 GPa:

1. phlogopite + vapour ⇌ forsterite + liquid,

2. phlogopite forsterite + liquid (vapour-absent condition),

3. forsterite + orthorhombic kalsilite + leucite + vapour

⇌

phlogopite (Luth 1967;

⇌

Wones 1967), and
4. phlogopite forsterite + leucite + kalsilite + liquid.

⇌

Their study is summarised in Fig. 12.3 . Reactions (1), (2), (3) and (4) are
denoted by the curves A, C, L and G in Fig. 12.3 ; curve B refers to the beginning of
melting of the vapour-absent assemblage, forsterite + phlogopite + kalsilite
(orthorhombic) + liquid. The curve D denotes minimum liquidus in the presence of
a vapour phase in the phlogopite- H 2 O join. The curves above the invariant point
involving phlogopite, forsterite, orthorhombic kalsilite, leucite, liquid, and vapour
at about 1160 C and 0.1 GPa, indicate that the breakdown of phlogopite occurs

according to the reaction, phlogopite + vapour

°

forsterite + orthorhombic
kalsilite + liquid. Under vapour-absent equilibrium conditions, phlogopite melts
incongruently according to reaction, phlogopite

⇌

forsterite + leucite + orthorhom-
bic kalsilite + melt (up to 0.17 GPa), above this pressure it melts incongruently
according to the reaction, phlogopite

⇌

forsterite + liquid (Fig. 12.3 ).
The study of Yoder and Kushiro (1969) showed that the assemblage forste-
rite + phlogopite was stable to at least 3.75 GPa at 1,200

⇌

C (Fig. 12.3 ). Kushiro
et al. (1967) established that phlogopite is stable to 7.2 GPa at 1000

°

° C. The

phlogopite + enstatite assemblage are in equilibrium up to 3.0 GPa