The role of the white pumice clasts in the 79 AD Vesuvius post-Plinian PDC deposits

Large explosive eruptions often generate compositionally zoned ignimbrites that provide evidence for the vertical stratification in the magma chamber and insights on magma withdrawal dynamics. In this regard, an excellent case is represented by the AD 79 eruption of Vesuvius, which consisted of two main phases. A first Plinian phase emplaced widespread fallout deposits and minor pyroclastic density currents (PDCs), characterized by a marked compositional variation from white phonolitic pumice clasts at the base, to grey tephriphonolitic pumice clasts at the top. This is commonly interpreted to reflect the emptying of a stratified magma chamber. A post-Plinian phase, emplacing mainly PDC deposits, followed, tapping the lowermost, tephriphonolitic “grey magma” still residing in the magma reservoir. Occasional occurrence of white juvenile clasts in the upper part of the sequence is also reported. We present here new geochemical data on the juvenile clasts from the 79 AD post-Plinian PDC deposits, collected at three type-locations representatives of the proximal and medial facies of the deposits in three different sectors around the volcano. Our dataset testifies the systematic coexistence of both grey and white pumice types at various stratigraphic levels. The grey pumice clasts depict quite regular vertical variations for both major and trace elements, with Fe2O3tot and TiO2 linearly increasing from 4.55 to 5.22 wt.% and from 0.51 to 0.65 wt.%, respectively, while Al2O3 and Zr regularly decrease from 19.2 to 16.9 wt.% and 294 to 255 ppm, respectively. The phonolitic white pumice clasts are much less common and do not display any regular vertical geochemical trend. Two possible interpretations can be proposed for such coexistence: 1) batches of the phonolitic “white magma” had remained trapped in the magma chamber and were sporadically discharged during the eruption of the “grey magma”; 2) the white pumice clasts represent accidental cognate lithic clasts that were scraped off from the basal Plinian fallout deposits by the highly erosive post-Plinian PDCs.