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

Abstract

[1] Multianvil experiments with long experimental durations have been made with the anorthite composition CaAl 2 Si 2 O 8 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-Al 2 O 3 -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.Citation: Liu, X., H. Ohfuji, N. Nishiyama, Q. He, T. Sanehira, and T. Irifune (2012), High-P behavior of anorthite composition and some phase relations of the CaO-Al 2 O 3 -SiO 2 system to the lower mantle of the Earth, and their geophysical implications,

Shock‐induced phase transformation of anorthitic plagioclase in the eucrite meteorite Northwest Africa 2650

“…Although the assemblage grossular + kyanite + silica melt has not been observed below ~9 GPa (Liu et al. ), the inference in this study is not in conflict with the high‐P and high‐T phase diagram of anorthite, in which the stability field of grossular + kyanite + silica melt remains unconstrained. To summarize, the presence of maskelynite and reidite probably indicates a minimum peak shock pressure up to 20 GPa, while the other high‐pressure phases indicate that the shock pressure during the crystallization of shock melt veins might vary from >8 to >2 GPa with a heterogeneous temperature distribution.…”

“…Based on previous experimental results, the assemblage grossular + kyanite + silica has a large range of pressure and temperature (~2–15 GPa and < 2000 °C; Liu et al. ), with various pressure and temperatures corresponding to the species of silica. At relatively low temperature, the silica phase changes from quartz, to coesite, and to stishovite with increasing pressures (Liu et al.…”

“…At relatively low temperature, the silica phase changes from quartz, to coesite, and to stishovite with increasing pressures (Liu et al. ). Silica melt that is coexisting with grossular and kyanite is only present at higher temperatures (up to 1600–2000 °C and 9–15 GPa).…”

“…The temperature should be higher than 1400 °C (Liu et al. ). Although the assemblage grossular + kyanite + silica melt has not been observed below ~9 GPa (Liu et al.…”

“…The presence of assemblage grossular + kyanite + silica melt in NWA 2650 is its first description in shocked meteorites, although grossular and kyanite have been observed in shocked eucrites (Fudge et al 2017;Pang et al 2018b). Based on previous experimental results, the assemblage grossular + kyanite + silica has a large range of pressure and temperature (~2-15 GPa and < 2000°C; Liu et al 2012), with various pressure and temperatures corresponding to the species of silica. At relatively low temperature, the silica phase changes from quartz, to coesite, and to stishovite with increasing pressures (Liu et al 2012).…”