A model for the evolution of the Mt. Somma-Vesuvius magmatic system based on fluid and melt inclusion investigations

In this paper, we present compositional data of reheated silicate melt inclusions (MI) from cumulate nodules and from one skarn xenolith trapped by olivine and clinopyroxene crystals. All MIs were analyzed by electron microprobe, while some were selected to be analyzed by secondary ion mass spectrometry (SIMS). Based on petrography and mineral compositions, clinopyroxene in the studied nodules can be divided into three types: cumulus pyroxene; post-cumulus pyroxene and fassaitic pyroxene present in the skarn nodule. The coherent trends displayed by the compositions of cumulus and post-cumulus clinopyroxene suggest that they formed at variable stages of fractionation of a single magmatic system. MIs in nodules display large compositional variations. Their compositions are not consistent with an olivine-clinopyroxene co-crystallization, which is a well-established feature of Vesuvius volcanism. Olivine and clinopyroxene from the nodules may have crystallized from the pre-79 AD Vesuvius magmas. Alternatively, variations in MI compositions may result from post-entrapment re-equilibration with their hosts during residence within the magmatic system prior to eruption. The compositions of MIs in clinopyroxene from the skarn nodule are characterized by anomalously low TiO2 and P2O5 contents and high Na2O/K2O values. FI and MI, in conjunction with data available from previous research, suggest that Mt. Somma-Vesuvius plumbing system is made up of small magma chambers at depths greater than 3.5 km and that possibly a larger chamber exists at or below 12 km. The entire system likely resembles a complex feeding column which is dominated by multiple mush zone environments (small magma chambers), and thus includes a variety of local crystallization environments characterized by contrasting cooling rates and P-T conditions.