Potassium amphibole stability in the upper mantle: An experimental study in a peralkaline KNCMASH system to 8.5 GPa

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 stability and composition of richteritic amphiboles in the MARID (mica-amphibole-rutile-ilmenite-diopside) assemblage amphibole + phlogopite + clinopyroxene. The results were compared with phase relations and the composition of natural MARIDs to assess possible mechanisms of formation for MARID-type rocks. K-richterite is stable in a wide range of bulk K/Na ratios in the MARID assemblage to 8.5 GPa and 1300 degrees C. In this 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 increasing pressure. At P > 7.0 GPa, K-richterite can coexist with garnet. Phase relations of K-richterite in a natural MARID composition are consistent with those in the simplified system and confirm the potential stability of K-richterite and K-richterite + garnet within the diamond stability field. The assemblage K-richterite + phlogopite + clinopyroxene 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.