The effects of dissolved CO2 on the density and viscosity of silicate melts: a preliminary study

A potassium silicate liquid with 56.9 mol% SiO2 (KS1.3) has been used as a model system to determine at 1 bar the influence of carbon dioxide on the viscosity and density of magma. For the maximum concentration of 3.5 wt% that could be dissolved, the viscosity decreases by two orders of magnitude near the glass transition at around 750 K. For 1 wt% CO2, the decrease is 1 and only 0.04 log unit at 750 and 1500 K, respectively. Dissolved CO2 has a composition independent partial molar volume of 25.6±0.8 cm3/mol in glasses at room temperature, and does not affect the thermal expansion coefficient of the supercooled liquid which is (12.3±0.3)×10-5 K-1 for CO2 contents varying from 0.6 to 2.2 wt%. Qualitatively, these effects are similar to those of water. However, the density of dissolved CO2 varies from about 1.7 to 1.5 g/cm3 in a 700 K interval above the glass transition, and lowers the density of the glass less than water whose density decreases from 1.5 to 1.2 g/cm3 under the same conditions. The relevance of these results to natural magma is finally pointed out.