Stability field of phase Egg, AlSiO3OH at high pressure and high temperature: possible water reservoir in mantle transition zone

High–pressure and high–temperature experiments were conducted to determine the stability field of phase Egg, AlSiO3OH, in the pressure range of 16.5–23.5 GPa and in the temperature range of 800–1500 °C. We found that phase Egg decomposes to δ–AlOOH + stishovite below ~ 1200 °C (16.5 GPa and 800 °C; 20.6 GPa and 1000 °C) and to Al–phase D + corundum + stishovite above ~ 1200 °C (21.8 GPa and 1500 °C) at P–T conditions corresponding to the mantle transition zone. This indicates that phase Egg is unstable at the top of the lower mantle and can be a water reservoir only in the mantle transition zone. In addition, the present results imply that the superdeep diamonds that include phase Egg do not originate from the lower mantle but from the wet mantle transition zone.