CO_2-rich glass, round calcite crystals, and no liquid immiscibility in the system CaO-SiO_2-CO_2, at 2.5 GPa

Abstract

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 CaONa_2O-Al_2O_3-SiO_2-CO_2, 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 CO_2, 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% CO_2, 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 CaCO_3. liquids, and surface tension effects control their shapes. Infrared spectroscopic studies indicated that (CO_3)^(2-), is the dominant CO_2, species in the glass, and the silicate structure is partially polymerized, probably as a result of interaction between Ca^(2+) and SiO_4 tetrahedra. The phase relationships in the CaCO_3-SiO_2, 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 yield low-SiO_2, carbonate-rich magmas under some thermal conditions. Such carbonate-rich melts may be efficient agents for mantle metasomatism.