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 370

Fig. 12.17 P(H 2 O)-T
stability of sodium-richterite.
The data below 0.1 GPa are
from Charles (1975).
Breakdown of richterite to
various phases below 140 bar
is not shown (after Gupta and
Venkatesh 1993)

above this pressure however, hydroxy potassium richterite has slightly higher P-T
stability. The fluor-potassium richterite breaks down under much higher P-T con-

ditions than richterite and hydroxy potassium richterite (Fig. 12.17 ), whereas
sodium-richterite breaks down to orthopyroxene + forsterite + diopside + liquid,
potassium richterite yields diopside + forsterite + liquid. The study of Gupta and
Venkatesh (1993) indicates that sodium-richterite is stable also at 3.0 GPa and
1185

° —

fluorine-bearing
potassium richterite. Experimental study of Gupta and Venkatesh (1993) indicates
that alkali-richterite could be a source of Na 2 O in the upper mantle. Experimental
study of Gilbert and Briggs (1974) suggests that partial melting of a potassium-
richterite should result in producing a liquid in equilibrium with wherlite — rich —
fraction (clinopyroxene + forsterite-rich olivine), whereas an upper mantle with
pockets of

C but at higher pressure it is less stable than hydroxy and

fluor-potassium richterite after partial melting, should produce a lam-
proitic liquid.
Konzett et al. (1997) studied the stability of potassium-richterite in a MARID
(mica- amphibole-rutile-ilmenite-diopside) assemblage. They compared the results
with phase relations and compositions of natural MARIDs to assess possible
mechanism of formation for MARID type rocks. Their experimental study on the
stability of potassium-richterite on the MARID assemblage is shown in Fig. 12.18 .
They found that potassium-richterite is stable in a wide range of bulk K/Na ratios in
the MARID assemblage to 8.5 GPa and 1,300

° C. In this assemblage the amphibole