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 CaAl2S i2O8 (anorthite composition) undergoes subsolidus disproportionation i nto grossular Ca3Al2Si3O12, stishovite SiO2 and a calcium aluminosilic ate phase of composition CaAl4Si2O11. This last phase (GAS) was first synthesised by Irifune et al. (1994, Earth Planet. Sci. Lett., 126: 35 1-368) in a continental crust composition, at P,T conditions appropria te for the Transition Zone. In the system CaO-Al2O3-SiO2, a new three- phase assemblage (grossular + stishovite + GAS) encountered at 14GPa r eplaces the grossular + stishovite + kyanite assemblage known to be st able at around 10 GPa because of an univariant reaction between grossu lar and kyanite that yields stishovite and GAS. By 1600 degrees C anot her 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 pressur es.