SYNCHROTRON-RADIATION STUDY ON THE HIGH-PRESSURE AND HIGH-TEMPERATURE PHASE-RELATIONS OF KALSI3O8

In situ X-ray diffraction study on KAlSi3O8 has been performed using the cubic type high pressure apparatus, MAX90, combined with synchrotron radiation. We determined the phase relations of sanidine, the wadeite-type K2Si4O9 + kyanite (Al2SiO5) + coesite (SiO2) assemblage, and hollandite-type KAlSi3O8, including melting temperatures of potassic phases, up to 11 GPa. Our data on subsolidus phase boundaries are close to the recent data of Yagi and Akaogi (1991). Melting relations of sanidine are consistent with the low pressure data of Lindsley (1966). The breakdown of sanidine into three phases reduces melting temperature, and wadeite-type K2Si4O9 melts first around 1500 degrees C in three phase coexisting region. Melting point of hollandite-type KAlSi3O8 is between 1700 degrees C and 1800 degrees C at 11 GPa. If these potassic phases host potassium in the earth's mantle, the true mantle solidus temperature will be much lower than the reported dry solidus temperature of peridotite.