STABILITY OF PHASE-D AT HIGH-PRESSURE AND HIGH-TEMPERATURE

We have determined the stability of the dense hydrous magnesium silica te phase D in a Mg2SiO(4) + 20.5 wt % H2O composition between 16 and 2 5 GPa at 900 degrees-1400 degrees C. Phase D coexists with superhydrou s phase B and a Mg-rich liquid to temperatures of 1000 degrees C at 17 GPa and 1400 degrees C at 26 GPa. Experiments in an externally heated diamond-anvil cell confirm that phase D is stable to pressures of at least 50 GPa at 930 degrees C. From static compression measurements, t he zero-pressure bulk modulus of phase D was determined as 200+/-7 GPa . We also present evidence that phase D is identical to phase F [Kanza ki, 1991] and phase G [Kudoh et al., 1997a; Ohtani et al., 1997]. The high-pressure and high-temperature stability of phase D makes it emine ntly suitable as a hydrous phase which is stable within subducting lit hosphere, in the transition zone, and in the lower mantle. The positiv e pressure-temperature stability slope of phase D, determined in this study, indicates that it is potentially stable at temperatures much gr eater than 1400 degrees C in the lower mantle.