Compressed and Heated: A Path from Plagioclase to Diaplectic Glass and Maskelynite

Plagioclase is one of the most abundant minerals in the crust of the Earth, the Moon and Mars. Under high pressures and temperatures produced by shock wave from hypervelocity impact events, the phase changes of plagioclase play a key role in the evolution of planetary surfaces. Diaplectic glass and maskelynite in shocked plagioclase serve as diagnostic features for high grade of shock metamorphism in terrestrial impact craters and meteorites. However, their formation mechanisms remain unclear mainly due to lack of a reliable phase diagram for plagioclase with extended pressure-temperature conditions. We studied intermediate plagioclase labradorite and Ca-rich endmember anorthite at pressure up to 65 GPa and temperature up to 4000 K, using laser-heating diamond anvil cells at GSECARS, Advanced Photon Source. Our experimental results reveal the amorphization, decomposition, and melting of labradorite and anorthite, and new phase diagrams were constructed. The new amorphization boundaries outline the temperature conditions for pressure-induced amorphization of plagioclase to form diaplectic glass, which can be up to 1300-1500 K. We also determined the first melting curve for labradorite and anorthite at high pressures and suggest that maskelynite could be a solid form of melted plagioclase generated at temperature >3000 K. The combination of previous shock Hogoniot data and our new phase diagrams will advance the understanding of the bombardment history on rocky planetary bodies in the solar system.