Melting in feldspar-bearing systems to high pressures and the structures of aluminosilicate liquids

To test the possibility that aluminosilicate liquids exhibit pressure-induced transformations, particularly involving changes in the coordination of aluminum, we determined melting relationships for the feldspar-bearing systems NaAlSi₃O₈-SiO₂, KAlSi₃O₈-SiO₂, and CaAl₂Si₂O₈-SiO₂ from 1 atm to 25 kbar. Albite and anorthite behave similarly in that they, and presumably liquids of these compositions, transform at high pressures to jadeite, kyanite, corundum, and other structures with aluminum in six-fold coordination, releasing SiO₂ component. This results in a large increase in the activity of SiO₂ component in the liquid (a^l_qz), which is manifested by a significant decrease in the melting-point depression of albite and of anorthite by the addition of quartz at pressures above ∼15 kbar. In contrast, sanidine does not transform to denser phases at pressures below at least 100 kbar, but it melts incongruently to leucite + SiO₂-rich liquid up to ∼ 15 kbar. This produces a relatively large a^l_qz and a small freezing-point depression by quartz below this pressure; the opposite holds above ∼15 kbar. These results support the concept that significant structural changes, including coordination changes in aluminum, occur in magmas in the upper mantle.