Stability of phase D at high pressure and high temperature

Abstract

We have determined the stability of the dense hydrous magnesium silicate phase D in a Mg2SiO4 + 20.5 wt % H2O composition between 16 and 25 GPa at 900°–1400°C. Phase D coexists with superhydrous phase B and a Mg‐rich liquid to temperatures of 1000°C at 17 GPa and 1400°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°C. From static compression measurements, the 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 [Kanzaki, 1991] and phase G [Kudoh et al., 1997a; Ohtani et al, 1997]. The high‐pressure and high‐temperature stability of phase D makes it eminently suitable as a hydrous phase which is stable within subducting lithosphere, in the transition zone, and in the lower mantle. The positive pressure‐temperature stability slope of phase D, determined in this study, indicates that it is potentially stable at temperatures much greater than 1400°C in the lower mantle.