The stability of hydrous potassic phases in lherzolitic mantle - An experimental study to 9 center dot 5 GPa in simplified and natural bulk compositions

lo investigate the pressure stability limit of phlogopite and the pressure- temperature stability field of its breakdown product K-richterite, experime nts were conducted from 4.0 to 9.5 GPa and between 800 degrees C and 1400 d egrees C in a subalkaline system K2O-Na2O-CaO-MgO-Al2O3-SiO2-H2O (KNCMASH) and in natural phlogopite and K-richterite-doped lherzolite systems. In KNC MASH, phlogopite breaks down between 6.0 and 6.5 GPa at 800 degrees C and b etween 6.5 and 7.0 GPa at 1100 degrees C to form potassic amphibole by the reaction phlogopite + clinopyroxene + orthopyroxene = K-richterite + garnet + olivine + H2O. In the natural system, the stability field of amphibole i s shifted towards lower pressures by similar to 0.5 GPa. The high-temperatu re stability limit of K-richterite in KNCMASH was located between 1300 and 1400 degrees C at 8.0 GPa and at <13000C at 7.0 GPa. Thus, K-richterite can De stable in the mantle wedge above subduction zones below a depth of simi lar to 180-200 km. Because of the small difference in K/OH ratios between p hlogopite and K-richterite, only a snail amount of aqueous fluid is likely to be produced during phlogopite breakdown ill an average mantle lherzolite bulk composition, This fluid might be trapped by nominally anhydrous miner als before it can migrate to hotter portions of the mantle wedge. Phlogopit e breakdown therefore is unlikely to be a factor in inducing significant me lting of the wedge lending to are magmatism.