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° C. At subsolidus conditions, these experiments demonstrated three phase assemblages, grossular (Gr) + kyanite (Ky) + stishovite (St) at ∼14 GPa, Gr + calcium-alumino-silicate phase (CAS) + St at ∼18 GPa, and CAS + CaSiO ₃-perovskite (CaPv) + St at above ∼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-SiO ₂ 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 ∼14 GPa is peritectic while that at ∼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 ∼2.5 GPa, a much lower pressure than that for the Or, Ab or Qtz composition (∼7.5-10 GPa), so that the An-enriched continental crust material should readily plunge into the upper mantle.