AN EXPERIMENTAL-STUDY OF THE PRESSURE-TEMPERATURE STABILITY OF MGMGAL-PUMPELLYITE IN THE SYSTEM MGO-AL2O3-SIO2-H2O

The stability field of MgMgAl-pumpellyite, Mg5Al5Si6O21(OH)(7), was de termined in the system MgO-Al2O3-SiO2-H2O in reversal experiments at p ressures between 34 and 100 kbar and temperatures in the range of 597 to 1050 degrees C. Brackets were obtained on five breakdown reactions (in order of increasing pressure): [GRAPHICS] This phase becomes stabl e only at pressures of more than 34 kbar and temperatures up to 820 de grees C. Thus, MgMgAl-pumpellyite may be an H2O-containing phase at de pths greater than 100 km in the coldest parts of subduction zones.