Melting of marly limestone from the Khamaryn-Khural-Khiid pyrometamorphic complex in East Mongolia was experimentally studied. The mineral and phase composition of the partly melted marly limestone sample used in the experiments was as follows: calcite (~59 wt %), silicate glasses of various compositions, melilite (gehlenite–akermanite), Al-clinopyroxene, Na–Ca nepheline (davidsmithite), celsian, magnetite, relict grains of sedimentary rock (quartz, albite–anorthoclase, plagioclase, and zircon), and other rarer minerals. Extremely calcium-rich phases were found in the solid experimental products: larnite, a gehlenite-like mineral, calcite, and transformation products of carbonate and carbonate–silicate melts. Carbon content was determined in the carbonate–aluminosilicate glass using electron probe microanalysis (EPMA), and the presence of the CO₃^{2 - } complex was confirmed by Raman spectroscopy. It was established that calcite remains stable in the marly limestone in the temperature range 1250–1300°C and at a CO2 partial pressure of approximately 7–17 MPa. However, the phase state of CaCO3 (molten or crystalline) at the high-temperature stage of the experiments could not be reliably determined from the habit and composition of calcite grains in the experimental products. The physicochemical conditions of synthesis, stability, and liquid immiscibility of carbonate and carbonate–silicate melts obtained in the experiments may be used to evaluate the possibility of their formation during melting of carbonate–silicate sedimentary rocks not only in pyrometamorphic complexes, but also under low-pressure high-temperature crustal conditions.
Chevychelov et al. (Mon,) studied this question.