Compositional Layering and Syn-eruptive Mixing of a Periodically Refilled Shallow Magma Chamber: the AD 79 Plinian Eruption of Vesuvius

Abstract A detailed study of the pyroclastic deposits of the AD 79 ‘Pompei’ Plinian eruption of Vesuvius has allowed: (1) reconstruction of the thermal, compositional and isotopic (87Sr/86Sr) pre-eruptive layering of the shallow magma chamber; (2) quantitative definition of the syn-eruptive mixing between the different magmas occupying the chamber, and its relationships with eruption dynamics; (3) recognition of the variability of mafic magma batches supplying the chamber. During the different phases of the eruption ∼25–30% of the magma was ejected as white K-phonolitic pumice, and 70–75% as grey K-tephri-phonolitic pumice. The white pumice results from the tapping of progressively deeper magma from a body (T = 850–900%C) consisting of two distinct layers mainly formed by crystal fractionation. The grey pumice results from syn-eruptive mixing involving three main end-members: the phonolitic ‘white’ magmas (salic end-member, SEM), mafic cumulates (cumulate end-member, CEM) and a crystal-poor ‘grey’ phono-tephritic magma (mafic end-member, MEM), which was never erupted without first being mixed with ‘white’ magma. Evidence is provided that mixing occurred within the chamber and was characterized by a transition with time from physical mixing at a microscopic scale to chemical hybridization. The MEM magma had a homogeneous composition and constant 87Sr86Sr isotopic ratio, possibly as a result of sustained convection. No unambiguous liquidus phases were found, suggesting that the MEM magma was superheated (T = 1000–1100°C); its very low viscosity was a main cause in the establishment of a physical discontinuity separating the white and the grey magmas. The white-grey boundary layer possibly consisted of a multiply diffusive interface, periodically broken and recreated, supplying the phonolitic body through mixing of moderate amounts of fractionated grey melts with the overlying white magma. The presence of a large overheated mass indicates the young, growing stage of the AD 79 chamber, whose main engine was the periodic arrival of hot mafic magma batches. These were characterized by K-tephritic to K-basanitic compositions, high temperatures (>1150°C), high volatile contents (2·0−2·5% H2O +Cl+F+S), low viscosities [(1+2 × 102 poises)] and relatively low densities (2500–2600 kg/m3). The birth of the Pompei chamber followed the repeated arrival of these batches (on average characterized by 87Sr/86Sr≈0·70729) into a reservoir containing a tephriticphonolitic, crystal-enriched, magma, a residue from the preceding ‘Avellino’ Plinian eruption (3400 BP).In fact, about half of magma ejected during the AD 79 eruption could have been inherited from pre-Avellino times.