Woh-Jer Lee, Peter J. Wyllie, George R. Rossman
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, U.S.A.
Following reports that the miscibility gap between silicate and carbonate liquids located experimentally on feldspar-calcite joins extended to the alkali-free side of the system CaO Na2O-Al2O3-SiO2-CO2, the melting of a mixture of calcite (70 wt%) and quartz was investigated at 2.5 GPa. The isobaric reaction, calcite (CC) ± quartz (Qz) = liquid (L) + vapor (V), was reversed at 1350 ºC. Quartz and rounded calcite crystals were concentrated at the bottom of the capsule, and CO2 was distributed in large vapor bubbles in the glass layer and at the top of the capsule. The liquid quenched to transparent glass, which is unusual in carbonate-rich systems. In two-stage reversal experiments, a sample of L ± V that was heated to the subsolidus temperature of 1300 ºC produced a few rounded calcite grains organized in dendritic patterns; at 1200 ºC, dendritic intergrowths of CC + Qz were produced with some coarser-grained areas. The glass was found to contain about 20 wt% CO2 on the basis of the geometry of phase boundaries and EDS analysis. There was no evidence for immiscible liquids. The round calcite crystals are equilibrium mineral phases, not quenched CaCO3 liquids, and surface tension effects control their shapes. Infrared spectroscopic studies indicated that (CO3)2- is the dominant CO2 species in the glass, and the silicate structure is partially polymerized, probably as a result of interaction between Ca2 + and SiO4 tetrahedra. The phase relationships in the CaCO3-SiO2 system, the simplest model for subducted oceanic crust with limestone (or for basalt altered by sea water), show that subducted crust potentially could transport calcite to great depths for long-term stor age in the mantle and could also yield low-SiO2 carbonate-rich magmas under some ther mal conditions. Such carbonate-rich melts may be efficient agents for mantle metasomatism.