Phase relations for CaAl2Si2O8 (anorthite composition) in the system CaO-Al2O3-SiO2 at 14 GPa

Multi-anvil experiments at 14GPa and 1500 degrees C reveal that CaAl2Si2O8 (anorthite composition) undergoes subsolidus disproportionation into grossular Ca3Al2Si3O12, stishovite SiO2 and a calcium aluminosilicate phase of composition CaAl4Si2O11. This last phase (GAS) was first synthesised by Irifune et al. (1994, Earth Planet. Sci. Lett., 126: 351-368) in a continental crust composition, at P,T conditions appropriate for the Transition Zone. In the system CaO-Al2O3-SiO2, a new three-phase assemblage (grossular + stishovite + GAS) encountered at 14GPa replaces the grossular + stishovite + kyanite assemblage known to be stable at around 10 GPa because of an univariant reaction between grossular and kyanite that yields stishovite and GAS. By 1600 degrees C another univariant reaction between grossular and stishovite produces CAS plus liquid. Schreinemakers' analysis suggests that, given Ca,Al-rich lithologies, the CAS phase should be stable at transition-zone pressures.