Structural controls on the solubility of CO2 in silicate melts: Part I: bulk solubility data

CO2 solubility data are presented for a wide range of melt compositions in the following systems: SiO2–Na2O–Al2O3+CO2 (SNA) at 2.0 GPa and 1600°C; SNA+CaO (SNAC) and SNA+MgO+CaO (MSNAC) at 1.5 GPa and 1275–1400°C; and for several “natural” magma compositions (Mg- and Ca-rich melilitites, andesite and phonolite) at 1.2–2.7 GPa and 1300–1600°C. At a given pressure and temperature, the solubility is found to be a strong function of the “non-bridging oxygen” (NBO) content of the melt, expressed as the NBO/T ratio, where T represents tetrahedral network-forming cations. The NBO/T ratio, calculated from the melt composition, thus provides a useful parameter for expressing and predicting the CO2 solubility. In highly polymerised melts, other dissolution mechanisms involving bridging oxygens become important and NBO/T is no longer the exclusive control on solubility. In Fe-bearing systems, the best correlation between CO2 solubility and NBO/T is found when both Fe3+ and Fe2+ are assumed to be tetrahedral (T), indicating that these cations should be considered in a polymerising role in the melt, with respect to CO2 dissolution. There is also evidence that some fraction of the Mg2+ in the melt should be assigned to a polymerising role.