High pressure transitions in the system MgAl 2O 4–CaAl 2O 4: a new hexagonal aluminous phase with implication for the lower mantle

Abstract

Phase transitions in MgAl 2O 4 and CaAl 2O 4 and in the binary system MgAl 2O 4–CaAl 2O 4 were established at high pressures and high temperatures. MgAl 2O 4 spinel dissociates to periclase and corundum at 15–16 GPa and 1200°C–1600°C with an equilibrium boundary of P (GPa)=12.3+0.0023 T (°C). The mixture recombines into a phase with calcium ferrite structure at about 26.5 GPa at 1600°C. At 780°C–1400°C, CaAl 2O 4 crystallizes with the calcium ferrite structure at pressure above 8–9 GPa. At lower pressures, phases III and IV are stable. On the join MgAl 2O 4–CaAl 2O 4, solubility of MgAl 2O 4 component into CaAl 2O 4-rich calcium ferrite is limited to about 10 mol% at 20–21 GPa and 1200°C, and a new phase with composition of (Mg x ,Ca 1− x )Al 2O 4 ( x=0.66–0.8) was found to be stable at pressure above 15 GPa. Powder X-ray diffraction data show that the new phase possesses hexagonal symmetry. Moreover, it is interpreted that an Al-rich phase observed in high pressure assemblage of mid-ocean ridge basalt (MORB) above 25 GPa by Irifune and Ringwood [Irifune, T., Ringwood, A.E., 1993. Phase transformations in subducted oceanic crust and buoyancy relationships at depths of 600–800 km in the mantle. Earth Planet. Sci. Lett. 117, 101–110.] has the same structure as the hexagonal phase found in this study, rather than calcium ferrite structure. And another Al-rich phase synthesized at 24–27 GPa in high pressure experiments from natural pyrope-rich garnet starting material is also similar to the hexagonal phase. These results suggest that aluminous phase of the hexagonal structure would be one of major constituent minerals of subducted oceanic crust in the upper part of the lower mantle.