Phonolite – Carbonate interaction in the Somma-Vesuvius plumbing system: experimental insights into the mechanisms of limestone assimilation, crustal limestone melting and Campanian magma differentiation

The interaction of magmas with host-rocks is a common process in magmatic systems. Interaction with carbonate-bearing lithologies is of major interest since the thermometamorphic reactions during magma‑carbonate interaction, may release CO₂ affecting eruptive behaviour. The Somma-Vesuvius volcanic system is built on a km-thick Mesozoic carbonate platform, with evidence of intense interaction in the form of carbonate-xenoliths in juvenile pumices and as skarns. Nevertheless, the timescales and mechanisms of magma‑carbonate interactions remain poorly constrained.Here we present an experimental study on the interaction of a dry Vesuvius' phonolite melt with a high-Ca limestone and a dolomitic limestone. We conducted the experiments in a piston cylinder apparatus at 600 MPa, temperatures of 950 °C and 1200 °C and interaction times of 0–60 min. Bubble nucleation (CO₂ release) has been observed in all experiments. The dolomitic limestone is assimilated via an AFC-process, creating a skarn-like assemblage of periclase-bearing dolomite, forsterite, clinopyroxene and a Ca + Mg-enriched melt. The high-Ca limestone exhibits a partial melting texture, that includes the formation of a Na-K-Ca chloride carbonate melt due to direct diffusive transport of alkalis and chlorine from the dry phonolite melt into the limestone. The melting of limestones at crustal conditions may affect magma differentiation trends in Campanian magmas, especially the Na₂O/K₂O ratio. The reported assimilation timescales of 30–60 min at 1200 °C for both limestones are however maximum estimates, as a dry phonolitic melt was used at pressures exceeding those to be expected in the Vesuvius plumbing system (<200 MPa). The presence of water would greatly accelerate limestone assimilation.