Phonolite-Carbonatite Liquid Immiscibility at 3–6 GPa

Abstract

Liquid immiscibility plays an important role in the formation of carbonatites and associated alkaline Si-undersaturated magmas. Experiments in the sodium carbonate-aluminosilicate systems suggest that the carbonate-silicate miscibility gap is limited by crustal and shallow mantle pressures (up to 2.5 GPa). Unlike in the potassium-rich carbonate-aluminosilicate systems, the carbonate-silicate miscibility gap was established at pressures of 3.5–6 GPa. It is therefore interesting to elucidate the immiscibility range under intermediate pressures, corresponding to 100–200 km depths. Here we conducted experiments over 3–6 GPa and 1050–1500 °C in the systems corresponding to immiscible melts obtained by partial melting of carbonated pelite (DG2) at 6 GPa and 1200 °C. We found that partial melting begins with the alkali-rich carbonatite melt, while immiscible phonolite melt appears over 1050–1200 °C at 3 GPa, 1200 °C at 4.5 GPa, and 1200–1500 °C at 6 GPa. As pressure decreases from 6 to 3 GPa, Na becomes less compatible, and the concentration of the jadeite component in clinopyroxene decreases by a factor of 1.5–6. As a result, the compositions of the immiscible phonolite and carbonatite melts evolve from ultrapotassic (K2O/Na2O weight ratio = 10–14) resembling silicic and carbonatitic micro-inclusions in diamonds from kimberlites and placers worldwide to moderately potassic (K2O/Na2O = 1–2), which may correspond to phonolitic and associated carbonatitic melts of the spinel facies of the shallow mantle.