The assembly and discharge of the Vesuvius 79 AD eruption magma chamber: new insights from petrological and chemostratigraphic evidence

Following the recent revision of the stratigraphic framework, a detailed petrological and chemostratigraphic investigation of the Vesuvius 79 AD eruption juvenile clasts is presented. This has resulted in an excellent case-study for unravelling the processes that lead to the genesis of zoned pyroclastic sequences, allowing a reassessment of how pre- and syn-eruptive processes, as well as emplacement dynamics, influenced the geochemical variations recorded by the juvenile clasts. The opening pyroclastic density current (PDC) unit C1 and the white pumice lapilli Plinian fall A are dominated by white pumice clasts, much rarer in the following grey pumice lapilli Plinian fall B, intra-Plinian PDC (i-PDC), post-Plinian PDC (p-PDC) deposits. White pumice clasts are strongly vesicular, nearly aphyric, with sanidine, green Al-rich clinopyroxene, garnet, leucite and amphibole, and display a strongly evolved phonolitic composition. Grey pumice clasts, prevailing in the deposits following the fall A, are less vesiculated and slightly richer in crystals, with sanidine, green Al-rich clinopyroxene, biotite, colourless diopsidic clinopyroxene and amphibole, ranging in composition from tephriphonolitic to phonolitic. The late-stage post-Plinian fall (p-f) layers are characterised only by grey pumice clasts, which frequently include “dark patches” with MgO-rich clinopyroxene, olivine and biotite phenocrysts. Chemostratigraphic trends of generally decreasing degree of evolution in the fallout units, from fall A to fall B up to p-f, are thought to reflect compositional trends in the plumbing system. This is interpreted as consisting of an upper vertically stratified “white magma” cap (as indicated by the decreasing degree of evolution with increasing stratigraphic height in fall A), and a lower “grey magma”. The first derives from the differentiation of tephriphonolitic/phonolitic magmas through the removal of alkali feldspar syenite assemblages. The tephriphonolitic/phonolitic magmas, in turn, derive from the prolonged differentiation of tephritic magmas. The grey magma results from mixing tephriphonolitic/phonolitic magmas with i) new arrivals of near-primitive tephritic melts - previously only hypothesised, but here directly evidenced by the dark patches - and ii) melts from alkali feldspar syenite cumulates. This confirmed the common role of cumulate melting processes in the genesis of zoned evolved pyroclastic deposit, although comparisons with literature case studies also highlighted that the factors governing the extent to their involvement, as well as that of the recharge magma, need to be further investigated. As for the PDC deposits, the occurrence of rarer white and more abundant grey pumice clasts at the same stratigraphic levels, both showing large chemostratigraphic oscillations, was observed to be rather common, and not episodic as previously reported. This association is never observed in p-f layers interstratified with p-PDC deposits, suggesting that it is not determined by eruptive mechanisms and/or withdrawal dynamics. The white pumice fragments of the i-PDC and p-PDC are interpreted as clasts eroded from the underlying fall A deposits, then redeposited by the pyroclastic currents. The presence of the two pumice types in the C1 deposit is instead a primary feature, reflecting simultaneous tapping of the white and grey magmas, likely related with a triggering event of magma rejuvenation.