Abstract:
An experimental study of the CaMgSi2O6-Ca3Al2Si3O12-K2Mg(CO3)2, CaMgSi2O6-Mg3Al2Si3O12-K2Ca(CO3)2 and CaMgSi2O6-KAlSi2O6 joins of the carbonate-silicate system CaMgSi2O6-(Ca,Mg)3Al2Si3O 12-KAlSi2O6-K2(Ca,Mg)(CO3 )2 was conducted at pressures of 5-7 GPa and temperatures of 950-2000°C in order to model phase equilibria in the diamondiferous Mg-rich garnet-clinopyroxene-carbon-ate rocks of the Kokchetav Complex. Phase diagrams are constructed on the basis of the experiments. The diagrams reflect the crystallization sequence of phases from ultrapotassic Mg-rich carbonate-silicate and aluminosilicate melts. The crystallization field of potassium-bearing clinopyroxene (characteristic for the Kokchetav Complex) with Ca-rich garnet and carbonates is outlined. Clinopyroxene containing more than 1 wt % K2O is a liquidus mineral in the diopside-rich (above 50 mol %) portions of these systems. Garnet becomes a liquidus mineral at lower diopside content. The experimental results show that ultrapotassic carbonate-silicate and aluminosilicate melts can be favorable media for the initial crystallization of garnet-clinopyroxene-carbonate rocks of the Kokchetav Complex at depths of >150 km.