CO2-RICH GLASS, ROUND CALCITE CRYSTALS, AND NO LIQUID IMMISCIBILITY IN THE SYSTEM CAO-SIO2-CO2 AT 2.5 GPA

Following reports that the miscibility gap between silicate and carbon ate liquids located experimentally on feldspar-calcite joins extended to the alkali-free side of the system CaO-Na2O-Al2O3-SiO2-CO2, the mel ting 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-degrees-C. Quartz and rounded calci te crystals were concentrated at the bottom of the capsule, and CO2 wa s distributed in large vapor bubbles in the glass layer and at the top of the capsule. The liquid quenched to transparent glass, which is un usual in carbonate-rich systems. In two-stage reversal experiments, a sample of L + V that was heated to the subsolidus temperature of 1300- degrees-C produced a few rounded calcite grains organized in dendritic patterns; at 1200-degrees-C, dendritic intergrowths of CC + Qz were p roduced with some coarser-grained areas. The glass was found to contai n about 20 wt% CO2 on the basis of the geometry of phase boundaries an d EDS analysis. There was no evidence for immiscible liquids. The roun d calcite crystals are equilibrium mineral phases, not quenched CaCO3 liquids, and surface tension effects control their shapes. Infrared sp ectroscopic studies indicated that (CO3)2- is the dominant CO2 species in the glass, and the silicate structure is partially polymerized, pr obably as a result of interaction between Ca2+ and SiO4 tetrahedra. Th e 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 storage in the mantle and could also yie ld low-SiO2 carbonate-rich magmas under some thermal conditions. Such carbonate-rich melts may be efficient agents for mantle metasomatism.