Effects of CO<sub>2</sub> on the structure of silicate melts considering the degree of polymerization under pressure

Carbon dioxide (CO2) is a prevalent volatile in Earth’s interior, but its effects on the structural properties of magmas or silicate melts remain insufficiently understood. Previous studies have indicated that the addition of CO2 can decrease the viscosity of silicate melts, but only if they are fully polymerized. In this study, we explored the effects of CO2 considering the degree of polymerization on the structure of silicate melts at high pressures of up to ∼ 5 GPa using in situ synchrotron X-ray diffraction (XRD) measurements and classical molecular dynamics (MD) simulations. The first sharp diffraction peak (FSDP) position of the X-ray structural factor S(Q), which shows the periodicity of an intermediate-range structure, was not affected by the addition of CO2 for partially depolymerized sodium silicate melt (Na2Si3O7). On the other hand, the height of the FSDP for fully polymerized silica melt (SiO2) slightly decreased, indicating that the Si-O network structure was disordered by the addition of CO2. This difference in the behavior of the FSDP may be attributed to the type of carbon species.