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 354

Sa

Lc L
Fo

Sa En
Fo L

Sa

En L

S 5

Sa + En + Fo + L

S 4

Sa + Fo + L

S 3

Sa Fo

Sanidine + Forsterite
Sanidine + Enstatite
Enstatite + Liquid
Leucite + Liquid
Forsterite + Liquid
Liquid

En L

Sa Lc

Fo L

1300 1325 1350 1375 1400

o

Temperature( C)

Fig. 12.6 Detail of reactions associated with singular points, S 3 ,S 4 and S 5 in the join
KAlSiO 4 - Mg 2 SiO 4 - SiO 2 . The boxes represent runs on the compositions, En 16 Sa 84 and Fo 20 Sa 80 .
Phase abbreviations are the same as Fig. 12.5 . (after Wendlandt and Eggler 1980b)

forsterite join and sanidine starts to melt congruently. The melt in equilibrium with
sanidine + enstatite + forsterite with or without vapour becomes leucite-normative.
They further established that at 2.75 GPa in the CO 2 -saturated system and at
3.45 GPa in volatile-absent system, the solidus of the assemblage sanidine + ensta-
tite + forsterite intersects the reaction, 4 enstatite + kalsilite

sanidine + 2 for-
sterite, this marks the upper pressure stability limit of sanidine in the presence of
forsterite. At these pressures, primary melt compositions are extremely silica-
undersaturated and is kalsilite-normative. According to them at pressures above
2.9 GPa, in presence of CO 2 the primary liquids are more undersaturated and
magnesite-normative.
Wendlandt and Eggler noted that at pressures below 1.8 GPa in the KAl-
SiO 4 -

⇌

CO 2
join, silica undersaturated leucite-normative liquid can fractionate to silica-saturated
sanidine-normative melts by crystallisation of forsterite and leucite. They further
concluded that at pressures greater than 2.0 GPa in the volatile-absent join and at
1.5 GPa in the CO 2 -bearing join olivine and sanidine-normative liquid can frac-
tionate to leucite-normative liquids by crystallisation of enstatite and sanidine.
The schematic liquidus projections for the joins kalsilite

Mg 2 SiO 4 - SiO 2 join and less than 1.3 GPa in the KAlSiO 4 - Mg 2 SiO 4 -

- forsterite-SiO 2 and

kalsilite CO 2 are shown in Fig. 12.9 . In these diagrams the mag-

nesia contents of liquids are exaggerated by Wendlandt and Eggler, except for
carbonate-rich liquids for which MgO contents may be extremely low. They found
that statistical characterization of 835 leucite- normative, Cenozoic volcanic rocks
constitute two distinct subgroups. The rocks of

- forsterite-SiO 2 -

first group are characterised by high