Characterisation of Pelletal Lapilli in Explosive Melilitite–Carbonatite Eruptions: An Example from Mt. Vulture Volcano (Southern Italy)

Among the volcaniclastic products of melilitite–carbonatite eruptions, pelletal lapilli are often found, resulting in them being particularly useful for characterising the interface between the erupting magma and its volatile component. Pelletal lapilli, which were erupted during the most recent melilitite–carbonatite volcanic activity of the Mt. Vulture volcano, are characterised by a predominantly wehrlitic core with CO2-rich fluid inclusions and a Ca-rich outer portion composed of fine-grained xenocrystic debris of olivine and clinopyroxene, with microcrysts of haüyne and melilite laths (± calcite). The chemical composition of the olivine reflects the interaction with a proto-melilitite–carbonatite melt, which is the main metasomatic agent. The whole-rock analyses of the external portion of pelletal lapilli show values that are comparable with those of extrusive carbonatites. This evidence supports the hypothesis that the primary carbonatite melt was a significant contributor to the CO2-rich magma source that transported the lapilli to the surface. The modelling of the geometric data of the pelletal lapilli structure, together with inferences regarding the role of the CO2 gas phase, the main propellant in an ascending gas-dominated medium, allowed for the reconstruction of a possible scenario where the CO2 expansion and the fluidised spray granulation process are crucial during the volcanic conduit dynamics.