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 345

Chapter 12
P-T Stability of Phlogopite, K-Richterite
and Phengite, as a Source of Potassium
in the Mantle

The forsterite cant bearing

in the genesis of various potassium-rich igneous rock types. This plane has there-
fore, been the subject of numerous studies under variable P-T conditions in the
presence or absence of a

- kalsilite-SiO 2 - H 2 O plane lying in this system has signi

O system (Luth
1967; Yoder and Kushiro 1969; Wendlandt and Eggler 1980a, b, c; Gupta and
Green 1988; Seifert and Schreyer 1971; Foley et al. 1986). Experimental studies of
these investigators are described

fluid component belonging to the C H

- -

first followed by the discussion on the P-T stability
of potassium-richterite (Konzett et al. 1997) and phlogopite (Kushiro et al. 1967;
Yoder and Kushiro 1969), which are considered to be the possible source materials
for potassium-rich magmas. The assemblage diopside-phlogopite, found in many
lamproites, have also been studied at different

temperatures and pressures

(3.5 17 GPa) by Luth (1997). His results have been described later, followed by the

discussion on the P-T stability of phengite (Massonne 1992), which is also con-
sidered to be a source mineral for potassium in the mantle.

-

12.1 Phase Relations in the System Forsterite Kalsilite-

-

SiO 2 - H 2 O at Variable Temperatures up to 0.3 GPa

Luth (1967) studied the P(H 2 O)-T stability of 35 univariant reactions up to a pressure
of 0.3 GPa [P(H 2 O) = P(Total)]. These mineral assemblages related to numerous
univariant reactions have bulk compositions lying within the forsterite

- kalsilite-

SiO 2 - figure indicates the presence of nine

quaternary invariant points, where six phases are in equilibrium. Figure 12.1 also
shows the presence of four singular points, generated by the meeting of the following
curves: 14, 15 and 16; 4, 5, and 6; 8, 9, and 11; and 1, 9, and 10.
On the basis of Fig. 12.1 , Luth (1967) constructed a series of isobaric-
polythermal, polybaric-isothermal diagrams (Fig. 12.2 ) with inferred phase rela-
tions. These diagrams are produced by projection of saturation surfaces on to the
anhydrous base (Mg 2 SiO 4 -

H 2 O join (Fig. 12.1 ; Table 12.1 ). The

SiO 2 plane) of the tetrahedron from the H 2 O
apex. The saturation surfaces are the areas, where a vapour phase is in equilibrium

KAlSiO 4 -