High-P behavior of anorthite composition and some phase relations of the CaO-Al2O3-SiO2 system to the lower mantle of the Earth, and their geophysical implications

Multianvil experiments with long experimental durations have been made with the anorthite composition CaAl2Si2O8 at pressure-temperature (P-T) conditions of 14-25 GPa and 1400-2400 degrees C. At subsolidus conditions, these experiments demonstrated three phase assemblages, grossular (Gr) + kyanite (Ky) + stishovite (St) at similar to 14 GPa, Gr + calcium-alumino-silicate phase (CAS) + St at similar to 18 GPa, and CAS + CaSiO3-perovskite (CaPv) + St at above similar to 20 GPa, which are related by the reactions Gr + Ky = CAS + St and Gr + St = CAS + CaPv. Following the method of Schreinemakers, we combined our data with the literature data to deduce a P-T phase diagram for a portion of the CaO-Al2O3-SiO2 system at subsolidus conditions, which subsequently helped to solve some long-lasting discrepancies in the high-P behavior of the compositions of anorthite and grossular. The crystal chemistry of the CAS and CaPv solid solutions was examined, and new substitution mechanisms were firmly established. Along the solidus, the melting reaction at similar to 14 GPa is peritectic while that at similar to 22 GPa is eutectic. For both pressures, St is the first phase to melt out and the melt is generally andesitic. For the An composition, its density starts to be significantly higher than the density of pyrolite at similar to 2.5 GPa, a much lower pressure than that for the Or, Ab or Qtz composition (similar to 7.5-10 GPa), so that the An-enriched continental crust material should readily plunge into the upper mantle.