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 371

Fig. 12.18 P-T diagram summarising experimental results for a natural MARID sample
(AJE137). Circles are run symbols for experiments made by Konzett et al. (1997), heptagons
denote experiments by Sweeney et al. (1993). Meaning of symbols are as follows: Cpx
clinopyroxne, Opx orthopyroxene, Ga garnet, Ol olivine, Kr potassium-richterite, phl phlogopite.
Numbers next to run symbols are approximate percentage of melt in the charge. The dashed line is
a 40 mW/m 2 geotherm according to Pollack and Chapman (1977);. ACM and PA are the average
current mantle adiabat for a subcontinental temperature of 1280 C and a plume adiabat as given

by Thompson (1992). Shaded areas span the P-T range obtained from clinopyroxene,
orthopyroxene-garnet geothermo-barometry (WE88, Waters and Earlank 1988), P-T for metaso-
matized peridotites (PH95, Phillips and Harris 1995), P-T for diamond inclusions from the
Kimberley areas: northern Lesotho: P-T derived from garnet peridotite data given by Nixon and
Boyd (1973). (after Konzett et al. 1997)

°

can accommodate signi cant amounts of K in the M site and shows a systematic

increase in K/Na ratio with increasing pressure. At P > 7.0 GPa, potassium-rich-
terite can coexist with garnet. This result contradicts the observation of Kushiro and
Erlank (1970). Phase relations of potassium-richterite in a normal MARID com-
position are consistent with those in the simpli rm the potential

stability of potassium-richterite and potassium-richterite + garnet within the dia-
mond stability

ed system and con

field. The assemblage K-richterite + phlogopite + clinopyroxene is
incompletely buffered in the KNCMASH system in a systematic relation between
bulk and mineral compositions observed in the experiments. Such a correlation,