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 363

enstatite + liquid C. In

Fig. 12.13 , liquid compositions may be divided into those which fractionate toward
the undersaturated eutectic (phlogopite + kalsilite + sanidine + liquid) and those
which fractionate toward the (phlogopite + sanidine + quartz + liquid) eutectic.
Comparison of Figs. 12.12 and 12.13 illustrates the strong effect of phlogopite
fractionation in deriving liquids with normative olivine and leucite from fractionation
toward the undersaturated minimum under dry conditions or trend to the quartz-
bearing minimum in the presence of water-rich

phlogopite + sanidine, is close to Fo 4 Ks 44 Qz 52 and 1,010 °

⇌

fluid.
If the data of Fig. 12.13 are applied to model melting of phlogopite-bearing
harzburgite, then it may be predicted that under water-saturated (i.e. activity of
water 1) conditions, initial melts should be olivine and hypersthene-normative

and would be quite rich in olivine. Crystal fractionation of phlogopite + enstatite,
without olivine, would drive liquids to more silica-rich compositions. Comparison
with Fig. 12.12 , shows that the presence of water at these pressures plays a major
role in diverting the fractionation paths of olivine or (olivine and leucite)-normative
liquids away from the SiO 2 -undersaturated minimum toward the silica-oversatu-
rated minimum. This contrasts with the analogous system, forsterite

*

-
quartz (Gupta et al. 1987), where the join enstatite-albite remains a thermal divide at
2.8 GPa under both anhydrous and water-saturated conditions and liquids formed
by water-saturated melting of an albite-bearing model harzburgite will fractionate
toward a forsterite - jadeite- enstatite invariant point (Gupta et al. 1987).
Composition of the phase diagram for forsterite

- nepheline

quartz under water-
saturated conditions at lower pressure (Luth 1967; Wendlandt and Eggler 1980a, b)
shows an expansion of the liquidus

kalsilite

- -

field of phlogopite with increasing pressure and
a persistence of the incongruent melting of phlogopite to at least 2.8 GPa. However,
the invariant point (forsterite + enstatite + phlogopite + liquid) lies at approximately
Fo 7 Ks 37 Q 56 and

1,060 ° C under 0.3 GPa and marks the peritectic reaction,

*

forsterite þ liquid enstatite þ phlogopite ;

i.e. olivine rather than enstatite is in reaction relation with hydrous melt. In terms of
partial melting of a phlogopite-bearing harzburgite, liquids at the solidus are

`rhyolitic and strongly quartz-normative at <0.3 GPa, but are picritic and olivine

and hypersthene-normative at 2.8 GPa (cf., Wendlandt and Eggler 1980a, b).
The carbonation reaction forsterite + CO 2 ⇌

'

enstatite + magnesite lies below

1,200 C at 2.8 GPa (Newton and Sharp 1975). As all the experiments at 2.8 GPa

are at temperatures greater than 1,250

°

C, forsterite remains stable with CO 2 . The
results are projected from CO 2 apex on to the forsterite

°

quartz plane. It
may be noted however that the depression of the liquidus surface by CO 2 is very
pronounced, implying a high (CO = ) solubility in the melt phase. Although mag-
nesite is absent at the solidus at 2.8 GPa, and high CO 2 contents (>17 wt% CO 2 )
were used in the capsules, it is not certain that all charges were CO 2 -saturated. An
investigation of CO 2 (CO = ) solubility in liquids in the forsterite

kalsilite

- -

quartz
join would have enlarged the investigation considerably but Gupta and Green noted
a close approach of the experimental conditions to the carbonation reactions and

kalsilite

- -