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 361

KAlSiO 4
(kal s i lite)

KAlSi 26
(leucite)

KAlSi 38
( san idine)

SiO 2
(quartz)

6

8

Sa

Ks

2

o

o 1460-20

1440-10

3 5

1

4

En

Fo

7

phlogopite

Mg Si O
226
(enstatite)

Mg Si 2 4
(forsterite)

Fig. 12.12 Phase relations for liquidus surface of forsterite-kalsilite-quartz at 2.8 GPa under
vapour-absent, anhydrous conditions (solid lines) and under CO 2 -saturated conditions (dashed
line). The shaded area includes all compositions which fractionate toward the silica-undersat-
urated minima at 2.8 GPa under both dry and CO 2 -saturated conditions. Filled circle indicates the
composition used. Square indicates analysed cotectic glass compositions (after Gupta and Green
1988)

fractionate toward the forsterite kalsilite-sanidine eutectic (Wendlandt and Eggler

-

1980a, b).
The anhydrous joins at 2.8 GPa contrast with that at 1 atmosphere (Schairer
1954) in the contraction of the forsterite stability

field in the former and the large

stability field for leucite in the latter. At one atmosphere, the peritectic forste-

rite + liquid → enstatite + leucite, is at Fo 1 Ks 43 Qz 56 and 1200 + 20 ° C, and the

eutectic lies at Fo 20 Ks 65 Qz 15 and 1456 + 10 C.

Application of phase diagram to model partial melting of a harzburgite with
minor potassium-rich phase (sanidine) shows that melting at low pressure will
produce silica-oversaturated liquids, but at 2.8 GPa, initial liquids are strongly
silica-undersaturated (leucite-normative) but become olivine- and enstatite-norma-
tive at temperatures greater than 1560 ° C. At 2.8 GPa, under dry conditions, liquids

°