Pelletal lapilli in diatremes-Some inspiration from the old masters

Pelletal lapilli are typically the juvenile component of silica undersaturated, ultramafic and carbonatitic diatreme volcanics; they represent the interface between the erupting magma and the volatile component. In the Monticchio diatremes, Mt Vulture, Italy, unusual pelletal lapilli with shells of melilitite and carbonatite host abundant mantle and not crustal fragments, which indicate that gas exsolution and lapilli formation were dominantly at near mantle depths. The lapilli show a complex history of agglutination and abrasion coupled with consistent change from inner melilitite shells to outer carbonatite shells, which implies an intrinsic process of magma evolution. These features are not consistent with a singular, high-level phreatomagmatic event. The calcite-rich magmatism testifies that the main transporting gas was CO2 and not H2O; pyroclastics show no evidence of water-cooling. More commonly occurring pelletal lapilli lack compositional shells but are cemented by serpentine+diopside in kimberlite pipes, or calcite in melilitite diatremes and probably represent magmatic evolution at a shallower level than the Monticchio examples. Inferred ascent rates for ultramafic, xenolith-carrying magmas require a deep-seated gas-driven fracture-conduit; phreatomagmatic lift-off is too shallow. The poor vesicularity of pelletal lapilli indicates that the initial juvenile volatile content of the magma became concentrated as the fluidising medium.