Melt inclusions are widely used to recover the pre-eruptive volatiles contents (H2O, CO2, halogens) of magmas. Bubbles are frequently present in melt inclusions and previous studies have highlighted that they store a significant portion of the melt inclusion's CO2, either as fluid or as carbonates microcrystals lining the bubble walls. These carbonates are usually overlooked when quantifying total volatile contents, leading to severe underestimations. Despite their common occurrence in melt inclusion-hosted bubbles, the mechanisms driving carbonate formation remain poorly understood. Here, we present textural and compositional observations made on natural and experimental samples to constrain the origin of carbonates within bubbles. The natural samples are basanitic melt inclusions hosted in olivine phenocrysts from the Bas-Vivarais volcanic province (French Massif Central). The experimental samples are CO2-saturated, olivine-bearing basanitic glasses synthesized in piston-cylinder experiments at 1-2 GPa and 1275-1300 °C, with run durations of 24 hours, and then cooled at different rates. The contents of natural and synthetic bubbles were characterized using Raman spectrometry and focused ion beam-scanning electron microscopy (FIB-SEM). Raman mapping revealed that melt inclusion bubbles contain CO2 (gas and liquid) along with microcrystals, predominantly carbonates with minor sulfides, coating the bubble walls. FIB-SEM analysis identified carbonates associated with a silica-rich phase, both within the melt inclusion bubble and at the olivine-inclusion interface. Bubble-hosted carbonates are micro- to nano-scale crystals or thin carbonate films in natural and experimental samples, though smaller in the latter. The coexistence of carbon- and silica-rich phases suggests that carbonate formation results from a carbonation reaction at the interface between silicate melt and CO₂ bubble during cooling.