Phase relations in the systems CaAl2O4-MgAl2O4 and NaAlSiO4-CaAl2O4 up to top-lower-mantle conditions

An alumina-rich silicate phase of calcium ferrite-type (CF) phase is a major mineral in crustal rocks subducted into the lower mantle. However, its chemical composition with increasing pressure and temperature remains unclear yet. In this study, phase relations in the CaAl 2 O 4 -MgAl 2 O 4 system were determined at 15–25 GPa and 1100–2000 °C by using a multi-anvil apparatus. In the pressure-temperature range, CaAl 2 O 4 -rich CF phase coexisted with another alumina-rich silicate of hexagonal (NAL) phase in the (1-x)CaAl 2 O 4 -xMgAl 2 O 4 system (x = 5–70 mol%). The solubility of the MgAl 2 O 4 component in the CF phase increases with pressure and temperature, from 3.8 mol% to 23.9 mol%, while the composition of the NAL phase was almost constant. In the MgAl 2 O 4 -rich system, we found that almost no CaO was incorporated into MgAl 2 O 4 phases in the present pressure-temperature range. MgAl 2 O 4 spinel (Sp) transformed to decomposition phases of periclase (Per) or Mg 2 Al 2 O 5 (mLd) phase and corundum (Crn), and then the CF phase formed at higher pressures, showing phase assemblages of NAL + Sp, NAL + Crn + Per or mLd, and NAL + CF with increasing pressure. An additional experiment in the NaAlSiO 4 -CaAl 2 O 4 system at 20 GPa and 1400 °C showed both Na- and Ca-rich CF phases incorporated Ca 2+ and Na + , respectively. These results together with previously determined phase relations in NaAlSiO 4 -MgAl 2 O 4 can explain limited CaO contents of CF phases in crustal materials subducted into the lower mantle. In addition, pressure-temperature changes in the MgAl 2 O 4 solubility of CF-type CaAl 2 O 4 can be used as a new pressure calibrant in the multi-anvil experiments.