The problem of rock assimilation by Somma-Vesuvius magma

Mineralogical and chemical compositions of ejected carbonate inclusions of Vesuvian gaseous phase are compared with those of carbonate sediments of the Somma-Vesuvius area. The basement of Vesuvius mainly consists of Mesozoic limestones. Most of these are characterized by extremely low insoluble residues. In this area thick dolomite beds occur in the Triassic system only. A calcareous layer of approximately 100 to 150 meters thickness characterized by high Sr contents (0.19% Sr in the average) is to be found within the Triassic dolostones. Several carbonate ejecta also show high Sr contents (0.1% Sr) but chemical composition of some of these ejecta differs somewhat from that of the Triassic layer high in Sr. Contact with the volcanic volatile phase and melt has produced some alterations in the composition of many ejecta. Magnesian calcites are abundant and periclase, brucite, tremolite, phlogopite and magnesite were found in the ejecta examined. Silicon, manganese, iron, zinc and, to a smaller extent, potassium and copper have been most probably transported from the volcanic gas phase into ejected carbonate inclusions. Owing to metasomatic actions of the volcanic volatile components, rearrangement and alteration of Ca and Mg contents occurred in the carbonate minerals of several ejecta. Under the influence of volcanic pressure and temperature, magnesium content originally in dolomite might be transformed into the calcite structure. \(\frac{{{\text{MgO}}}}{{{\text{CaO + MgO}}}}\) molar ratios of several carbonate ejecta do not correspond with those found in sedimentary limestones and dolostones. It cannot be proved whether the Mg of the mixed calcite-dolomite ejecta has been partially introduced from the volatiles or lost from the carbonate phases.