Effect of Al on the stability of dense hydrous magnesium silicate phases to the uppermost lower mantle: implications for water transportation into the deep mantle.

We have systematically investigated the high-pressure and high-temperature stability of Al-bearing dense hydrous magnesium silicate phases (DHMSs) in natural chlorite compositions containing ~ 16 wt% H2O and ~ 14 wt% Al2O3 between 14 and 25 GPa at 800–1600 °C by an MA8-type multi-anvil apparatus. A chemical mixture similar to Fe-free chlorite was also investigated for comparison. Following the pressure–temperature (P–T) path of cold subduction, the phase assemblage of phase E + phase D is stable at 14–25 GPa. Superhydrous phase B is observed between 16 and 22 GPa coexisting with phase E + phase D. Following the P–T path of hot subduction, the phase assemblage of phase E + garnet is identified at 14–18 GPa coexisting with the melt. The phase assemblage of superhydrous phase B + phase D was found at 18–25 GPa, which is expected to survive at higher P–T conditions. We have confirmed that the presence of Al could enhance the stability of DHMSs. Our results indicate that, after chlorite decomposition at the shallow region of the subduction zone, the wide stability field of Al-bearing DHMSs can increase the possibility of water transportation into the deep lower mantle. [ABSTRACT FROM AUTHOR]