SILICATE-MELT INCLUSIONS IN RECENT VESUVIUS LAVAS (1631-1944) .2. ANALYTICAL-CHEMISTRY

Silicate-melt inclusions in phenocrysts from the recent (A.D. 1631-1944) Vesuvius lavas have been analyzed in order to improve the knowledge on the physico-chemical conditions of the host magmas. Petrographic, SEM-EDS and electron microprobe analyses have been performed. Two kinds of silicate-melt inclusions were found in the phenocrysts. The first and most common type is primary in origin and occurs along the former growth surfaces of the host crystals. The second type is secondary and/or pseudosecondary in origin and occurs along healed fractures. Microprobe analyses on primary silicate-melt inclusions in clinopyroxene show compositional variations between the inclusions in the core and the rim of the host phase. The variation diagrams of major elements versus silica have been used to qualitatively distinguish the phases involved in pre- and post-trapping processes. In particular, the chemical composition of silicate-melt inclusions indicate the fractional crystallization of clinopyroxene + plagioclase +/- Fe-Ti oxides. Based on these data we can suggest that, during the recent activity of Vesuvius, fractional crystallization was the main differentiation process. However, the evolution of Vesuvius magmas may be also subjected to a convective process involving clinopyroxene crystals. Finally, this fractionated environment may be interrupted by inputs of more primitive magmas. The mixing between the fresh, unfractionated magma with the evolved one produced silicate-melt inclusions more basic and reverse and/or oscillatory zonings in clinopyroxenes.