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 360

They concluded that at pressures in excess of 5.0 GPa, phlogopite ceases to be a
solidus phase. This pressure corresponds to the breakdown of phlogopite at sub-
solidus conditions and to the approximate intersection of the continental and
Lesotho geotherms with the phlogopite + enstatite + magnesite solidus. Partial
melting at higher pressures de

ned by the geotherm will not involve phlogopite.

12.5 Investigation of the System Forsterite Kalsilite-SiO 2

at 2.8 GPa under Dry or Volatile Present Conditions,
(in Presence of H 2 OorCO 2 )

-

Liquidus phase relation of the join Mg 2 SiO 4 - SiO 2 was also studied under

dry condition and in presence of H 2 O and/or CO 2 by Gupta and Green (1988).
Their experimental results at 2.8 GPa with or without CO 2 are summarised in
Fig. 12.12 , which is based on quenching data and compositions of liquids (as
determined by electron microprobe analyses) in equilibrium with liquid at different
temperatures. Comparison of Fig. 12.12 with that of Schairer

KAlSiO 4 -

' s one atmosphere

phase diagram (1954) suggests that the field of leucite is absent in 2.8 GPa isobaric

polythermic section, and the eutectic point, characterised by the assemblage, leu-
cite + forsterite + kalsilite + liquid is missing. The study of Scarfe et al. (1966)
established that leucite breaks down to kalsilite and sanidine at high pressure and
the study of Lindsley (1966) in the system kalsilite-SiO 2 at different pressures
suggests that leucite may be stable at 2.8 GPa in the system as an incongruent
melting phase of sanidine. In this study the coexistence of sanidine and kalsilite
(

± liquid) suggests that the stability of leucite is restricted to pressures of <2.8 GPa.

Phase equilibria study of the silica-undersaturated portion of the forsterite

-
kalsilite-SiO 2 join under dry conditions (Fig. 12.12 ) shows that there are two
invariant points: (1) one occurring at Fo 5 Ks 62 Qz 33 and 1460

C (forsterite + sani-
dine + orthopyroxene ss + liquid) and (2) the other at Fo 6 Ks 69 Qz 25 and 1440

°

C
(kalsilite + forsterite + sanidine + liquid). The invariant point Fo 5 Ks 62 Qz 33 lies
within the forsterite- kalsilite-sanidine join and is thus a peritectic point at which the
reaction, enstatite + liquid

°

forsterite + sanidine occurs. Enstatite crystals in runs
near the peritectic point are too small (and contained quench overgrowths) for
accurate electron microprobe analysis, but if the enstatite contains MgAl 2 SiO 6 solid
solution as is probable at this high temperature and pressure, then the liquids along
the forsterite + enstatite cotectic would be mildly peralkaline, and would lie on the
K 2 O-rich side of the forsterite

→

kalsilite-SiO 2 plane. The peralkaline character dis-
appears at the peritectic point as enstatite reacts out and liquids in equilibrium with
forsterite- sanidine lie on the forsterite

-

kalsilite-SiO 2 plane.
The invariant point Fo 6 Ks 69 Qz 25 lies within the forsterite

-

eld
and is thus a eutectic point marking the low temperature goal of liquids crystallising
in this system. It is noteworthy that the minimum melt composition lies close to the
forsterite

kalsilite-sanidine

-

leucite join and has an extremely undersaturated composition. The join
enstatite-sanidine acts as a thermal divide so that all olivine-normative liquids

-