Vertical geochemical trends in the juvenile clasts from the 79 AD Plinian fallout deposits

The 79 AD Vesuvius eruption was characterized by two main phases: a sustained column Plinian phase, which deposited a thick white to grey pumice lapilli fallout deposit, followed by a column collapse phase, which emplaced several pyroclastic density current deposits. These latter are interstratified with five, minor, thin and lithic-rich fallout layers. The internal structure of the Plinian fallout deposit is weakly stratified, as observed at several studied locations from the Vesuvius slopes up to 20 km from the vent. Here we present chemical analyses for pumice lapilli samples collected at three type-locations representatives of the proximal, medial and distal facies of the deposits. The investigated samples display well-defined variations with respect to the stratigraphic height, both within the entire fallout sequence and within the individual layers recognized. The more evolved phonolitic white pumice clasts show a clear vertical gradient in all major and trace elements with, e.g. MgO, TiO2 and Sr linearly increasing from 0.21 to 0.73 wt.%, 0.22 to 0.35 wt.% and 389 to 605 ppm, while Al2O3, Zr and Nb decrease from 21.8 to 20.6 wt.%, 350 to 312 ppm and 70 to 60 ppm, respectively. The tephriphonolitic grey pumice clasts have a less evolved composition (e.g. MgO 1.65–1.85 wt.%, TiO2 0.56–0.59 wt.%, Al2O3 19.1–19.2 wt.%, Zr 270–297 ppm, Sr 845–915 ppm and Nb 49–50 ppm) and do not display appreciable vertical geochemical trends. These variations indicate a decrease in the degree of magma evolution as eruption proceeded, likely related to the emptying of a stratified, compositionally zoned magma chamber, in line with previous literature. Interestingly, these vertical trends extend to the tephriphonolitic grey pumice clasts from the overlying post-Plinian lithic-rich fallout layers (e.g. MgO increasing from 1.93 to 3.03 wt.%, Nb decreasing from 51 to 45 ppm), analyzed here for the first time. This possibly supports the idea that also the lowermost portions of the magma chamber were compositionally stratified.