HIGH PRESSURE AND HIGH TEMPERATURE STUDY ON PLAGIOCLASE: A PATH FROM PLAGIOCLASE TO DIAPLECTIC GLASS AND MASKELYNITE

Plagioclase is the most abundant phase in the continental crust and can be found in meteorites. After the cooling of the Moon’s magma ocean, plagioclase crystallized and became an important constituent of lunar crust. Furthermore, it is popularly accepted that diaplectic glass is formed by solid-state transformation of plagioclase, and maskelynite is the product of quenched near-plagioclase composition from melting under high pressure. However, knowledge of stability field of plagioclase is largely unknown at pressure above 25 GPa.

Labradorite (An51) and anorthite (An96) were studied at pressure to about 65 GPa and 3500K using the diamond-anvil cells at GSECARS, Advanced Photon Source. Our in situ high-pressure and high-temperature X-ray diffraction data showed that An51 decomposed into jadeite (Jd) + stishovite (St) + Ca-Al silicate (CAS) + garnet (Gr) at ~15 GPa and >1400K. At higher pressures and temperatures, the phase assembly transformed sequently to St+CAS+CaSiO₃ perovskite (Dvm) + CaFe₂O₄-type (CF) phase + corundum (Crn), St+Dvm+CF+Crn and then St+Dvm+Crn+ new phase. The new phase was observed at above 43 GPa and >1500K but it cannot be retained below 10 GPa during decompression. Eventually, it became amorphous when quenched to ambient conditions. The melting curve of An51 was also determined. For An96, it decomposed to St + kyanite (Ky) + Gr above 15 GPa and >1500 K. Then, this phase assembly transforms into St+CAS+Gr, St+CAS+Dvm and then St+Dvm+Crn at >32 GPa and T >1700 K. Note that St and Crn are the two crystalline phases observed from the quenched samples of plagioclase with compositions of An51-95. The rest of high-pressure phases become amorphous at ambient conditions. Comparing the shock Hugoniot data to the new phase diagrams of An51 and An96, amorphization of plagioclase is suggested to be the phase observed at below 42 GPa and 1400 K and melting will occur above 70 GPa and 2900 K.