J. Konzett et al., POTASSIUM AMPHIBOLE STABILITY IN THE UPPER-MANTLE - AN EXPERIMENTAL-STUDY IN A PERALKALINE KNCMASH SYSTEM TO 8.5 GPA, Journal of Petrology, 38(5), 1997, pp. 537-568
Experiments were performed from 1.0 to 8.5 GPa in a peralkaline system
K2O-Na2O-CaO-MgO-Al2O3-SiO2-H2O (KNCMASH) to investigate the stabilit
y and composition of richteritic amphiboles in the MARID (mica-amphibo
le-rutile-ilmenite-diopside) assemblage amphibole + phlogopite + clino
pyroxene. The results were compared with phase relations and the compo
sition of natural MARIDs to assess possible mechanisms of formation fo
r MARID-type rocks. K-richterite is stable in a wide range of bulk K/N
a ratios in the MARID assemblage to 8.5 GPa and 1300 degrees C. In thi
s assemblage the amphibole can accomodate significant amounts of K on
the M(4)-site and shows a systematic increase in the K/Na ratio with i
ncreasing pressure. At P > 7.0 GPa, K-richterite can coexist with garn
et. Phase relations of K-richterite in a natural MARID composition are
consistent with those in the simplified system and confirm the potent
ial stability of K-richterite and K-richterite + garnet within the dia
mond stability field. The assemblage K-richterite + phlogopite + clino
pyroxene is incompletely buffered in the KNCMASH system, resulting in
a systematic relation between bulk- and mineral compositions observed
in the experiments. Such a correlation, however, cannot be observed in
natural MARIDs. Therefore, MARID-type rocks do not represent the bulk
composition from which they formed and, hence, must be products of an
open-system crystallization.