Morphologic features of juvenile pyroclasts from magmatic and phreatomagmatic deposits of Vesuvius

Abstract

Three eruptive sequences of historical and recent activity of Vesuvius were carefully studied using scanning electron microscopy analysis techniques. The aim of this study was to characterize and distinguish deposits from magmatic and phreatomagmatic eruption phases. The sample pretreatment methods from previous authors were reviewed and a washing technique was checked, making it possible to obtain easily examinable samples without modifying their morphologic features. In each stratigraphic sequence the single samples were examined and their shape, type of vesiculation, and state of glass (edge modification by abrasion, alteration, presence of aggregates, secondary minerals and coatings) were analyzed and described. Characteristic features were recognized for deposits from different eruptive phases. Samples from phreatomagmatic deposits show the following features: • - glass alteration; • - presence of secondary minerals on the external surfaces or inside the cavities; • - coatings; • - presence of aggregate formed by juvenile and lithic particles. Other features, indicated by many authors as resulting from magma-water interaction, can arise from different mechanisms and cannot discriminate between phreatomagmatic and magmatic nature of deposits. The juvenile clasts from the phreatomagmatic phases of these eruptions of Vesuvius always show primary vesiculation; this observation supports the presence of fragmented magma at the time when interaction occurred. The conclusions from SEM observations are in perfect agreement with the results of granulometric and component analysis on the same eruptive sequences.