Stabilities of NAL and Ca-ferrite-type phases on the join NaAlSiO4-MgAl2O4 at high pressure

Stabilities of hexagonal new aluminous (NAL) phase and Ca-ferrite-type (CF) phase were investigated on the join NaAlSiO₄-MgAl₂O₄ in a pressure range from 23 to 58 GPa at approximately constant temperature of 1,850 K, on the basis of in situ synchrotron X-ray diffraction measurements in a laser-heated diamond-anvil cell. The results show that NAL is formed as a single phase up to 34 GPa, NAL + CF between 34 and 43 GPa, and only CF at higher pressures in 40%NaAlSiO₄-60%MgAl₂O₄ bulk composition. On the other hand, both NAL and CF coexist below 38 and 36 GPa, and only CF was obtained at higher pressures in 60%NaAlSiO₄-40%MgAl₂O₄ and 20%NaAlSiO₄-80%MgAl₂O₄ composition, respectively. These results indicate that NAL appears only up to 46 GPa at 1,850 K, and CF forms continuous solid solution at higher pressures on the join NaAlSiO₄-MgAl₂O₄. NAL has limited stability in subducted mid-oceanic ridge basalt crust in the Earth’s lower mantle and undergoes a phase transition to CF in deeper levels.