High-pressure and high-temperature study of intermediate plagioclase feldspar

Plagioclase feldspar series along anorthite (CaAl₂Si₂O₈) - albite (NaAlSi₃O₈) join are the most abundant constituents of igneous rocks found in the crust of Earth and lunar highland rocks. They can also be seen in lunar and Martian meteorites, and detected on the surfaces of Mars, Venus and Mercury. Knowledge of how intermediate plagioclase response to the variations of pressure and temperature is important for revealing the pressure-temperature history of meteorites and natural impact events on planetary bodies. However, high-pressure and high-temperature behavior of intermediate plagioclase at pressure above 25 GPa is poorly understood and its phase diagram is also unclear.

In this study, we conducted in situ high-pressure and high-temperature synchrotron X-ray diffraction on natural intermediate plagioclase sample at beamline 13ID-D, Advanced Photon Source. When sample was compressed from 15 to 60 GPa and heated up to 1900K, the amorphization of crystalline plagioclase and a series of phase transformations were observed. At 15 - 22 GPa and temperature above 1300 K, jadeite (Jd) appears first from the amorphous sample and then coexists with stishovite (St) + CaAl₄Si₂O₁₁ phase (CAS) + grossular (Gr). At 23 - 27 GPa and temperature above 1300 K, Jd is no longer observable, but a mixture of St + CAS + Ca-perovskite (CaPv) + CaFe₂O₄-type phase (CF)+ corundum (Cr) + Gr is stable. At 28 - 50 GPa and temperature above 1400K, a phase assemblage St + CF + CaPv + Cr was observed. Above 50 GPa and temperature > 1500 K, a new phase appears together with St + CaPv + Cr.

Based on our results, we constructed a new phase diagram of intermediate plagioclase with pressure extended to 60 GPa and the phase boundary of diaplectic glass is also constrained.